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  • 1. Anderson, K. E.
    et al.
    Nisbet, R. M.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Spatial scaling of consumer-resource interactions in advection-dominated systems2006In: American Naturalist, Vol. 168, no 3, p. 358-372Article in journal (Refereed)
    Abstract [en]

    Ecologists studying consumer-resource interactions in advection-dominated systems such as streams and rivers frequently seek to link the results of small-scale experiments with larger-scale patterns of distribution and abundance. Accomplishing this goal requires determining the characteristic scale, termed the response length, at which there is a shift from local dynamics dominated by advective dispersal to larger-scale dynamics dominated by births and deaths. Here, we model the dynamics of consumer-resource systems in a spatially variable, advective environment and show how consumer-resource interactions alter the response length relative to its single-species value. For one case involving a grazer that emigrates in response to high predator density, we quantify the changes using published data from small-scale experiments on aquatic invertebrates. Using Fourier analysis, we describe the responses of advection-dominated consumer-resource systems to spatially extended environmental variability in a way that involves explicit consideration of the response length. The patterns we derive for different consumer-resource systems exhibit important similarities in how component populations respond to spatial environmental variability affecting dispersal as opposed to demographic parameters.

  • 2. Anderson, K. E.
    et al.
    Nisbet, R. M.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Cooper, S. D.
    Scaling population responses to spatial environmental variability in advection-dominated systems2005In: Ecology Letters, Vol. 8, no 9, p. 933-943Article in journal (Refereed)
    Abstract [en]

    We model the spatial dynamics of an open population of organisms that disperse solely through advection in order to understand responses to multiscale environmental variability. We show that the distance over which a population responds to a localized perturbation, called the response length, can be characterized as an organisms average lifetime dispersal distance, unless there is strong density-dependence in demographic or dispersal rates. Continuous spatial fluctuations in demographic rates at scales smaller than the response length will be largely averaged in the population distribution, whereas those in per capita emigration rates will be strongly tracked. We illustrate these results using a parameterized example to show how responses to environmental variability may differ in streams with different average current velocities. Our model suggests an approach to linking local dynamics dominated by dispersal processes to larger-scale dynamics dominated by births and deaths.

  • 3. Behl, Stephan
    et al.
    de Schryver, Vera
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stibor, Herwig
    Trophic transfer of biodiversity effects: functional equivalence of prey diversity and enrichment?2012In: Ecology and Evolution, ISSN 2045-7758, Vol. 2, no 12, p. 3110-3122Article in journal (Refereed)
    Abstract [en]

    Producer diversity is frequently assumed to be detrimental to herbivores, because less edible taxa are more likely to dominate diverse communities. Many producers are, however, complementary in their resource use, and primary production is often positively related to producer diversity. We performed an experiment with microalgae and a generalist herbivore to explore the hypothesis that such positive effects are transferred up the food chain and are functionally comparable to effects of enrichment with a limiting resource. In both absence and presence of grazers, primary production was positively affected by both light supply and producer diversity. Survival, reproduction, and biomass of herbivores were also positively affected by light supply and producer diversity, with both factors contributing equally to grazer performance. We conclude that producer diversity can indeed have similar positive effects on secondary production as enrichment with a limiting resource and discuss conditions under which such positive effects are likely to dominate over negative ones.

  • 4. Berger, S. A.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Kunz, T. J.
    Albrecht, D.
    Oucible, A. M.
    Ritzer, S.
    Light supply, plankton biomass, and seston stoichiometry in a gradient of lake mixing depths2006In: Limnology and Oceanography, Vol. 51, no 4, p. 1898-1905Article in journal (Refereed)
    Abstract [en]

    We derive from a dynamic model that light availability, phytoplankton density, and the carbon : nutrient ratio of phytoplankton biomass should all be negatively related to mixed surface layer depth, whereas the areal standing stock of phytoplankton should show a unimodal, and total and dissolved nutrients a horizontal or increasing, relationship to mixing depth. These predictions agree closely with data from 65 central European lakes during summer stratification. In addition, zooplankton biomass was strongly negatively related to mixing depth in a subset of lakes. A decrease in mixing depth is thus a form of enrichment with light of the mixed surface layer, the effects of which could propagate to higher trophic levels.

  • 5. Berger, S. A.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Stibor, H.
    Trommer, G.
    Ruhenstroth, M.
    Wild, A.
    Weigert, A.
    Jager, C. G.
    Striebel, M.
    Water temperature and mixing depth affect timing and magnitude of events during spring succession of the plankton2007In: Oecologia, Vol. 150, no 4, p. 643-654Article in journal (Refereed)
    Abstract [en]

    In many lakes, the most conspicuous seasonal events are the phytoplankton spring bloom and the subsequent clear-water phase, a period of low-phytoplankton biomass that is frequently caused by mesozooplankton (Daphnia) grazing. In Central European lakes, the timing of the clear-water phase is linked to large-scale climatic forcing, with warmer winters being followed by an earlier onset of the clear-water phase. Mild winters may favour an early build-up of Daphnia populations, both directly through increased surface temperatures and indirectly by reducing light limitation and enhancing algal production, all being a consequence of earlier thermal stratification. We conducted a field experiment to disentangle the separate impacts of stratification depth (affecting light supply) and temperature on the magnitude and timing of successional events in the plankton. We followed the dynamics of the phytoplankton spring bloom, the clear-water phase and the spring peak in Daphnia abundance in response to our experimental manipulations. Deeper mixing delayed the timing of all spring seasonal events and reduced the magnitudes of the phytoplankton bloom and the subsequent Daphnia peak. Colder temperatures retarded the timing of the clear-water phase and the subsequent Daphnia peak, whereas the timing of the phytoplankton peak was unrelated to temperature. Most effects of mixing depth (light) and temperature manipulations were independent, effects of mixing depth being more prevalent than effects of temperature. Because mixing depth governs both the light climate and the temperature regime in the mixed surface layer, we propose that climate-driven changes in the timing and depth of water column stratification may have far-reaching consequences for plankton dynamics and should receive increased attention.

  • 6.
    Berger, Stella A.
    et al.
    Univ Munich, Dept Biol 2, Planegg Martinsried, Germany; Univ Georgia, Skidaway Inst Oceanog, Savannah, GA USA.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Univ Munich, Dept Biol 2, Planegg Martinsried, Germany.
    Stibor, Herwig
    Univ Munich, Dept Biol 2, Planegg Martinsried, Germany.
    Sebastian, Patrizia
    Univ Munich, Dept Biol 2, Planegg Martinsried, Germany.
    Scherz, Antonia
    Univ Munich, Dept Biol 2, Planegg Martinsried, Germany.
    Separating effects of climatic drivers and biotic feedbacks on seasonal plankton dynamics: no sign of trophic mismatch2014In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 59, no 10, p. 2204-2220Article in journal (Refereed)
    Abstract [en]

    Climate change may impact most strongly on temperate lake plankton communities in spring, when light availability and water temperature change rapidly due to thermal stratification. Effects of changing light and temperature on one food-web component transfer to other components, producing a complex interplay between physical drivers and biotic feedbacks. Understanding this interplay is important, because altered climate regimes could result in phenological mismatch between the phytoplankton spring bloom and the timing of maximum food requirements of grazers. To separate direct effects of light and temperature on spring plankton dynamics from effects mediated through micro- and mesograzer feedbacks, we manipulated water temperature, stratification depth and presence/absence of the mesograzer Daphnia in lake mesocosms. In early spring, stratification depth and water temperature directly influenced the light supply to phytoplankton and the growth rates of all plankton groups. Subsequently, indirect effects, including light-dependent food supply to grazers and temperature-dependent grazing pressure, became increasingly important. Phytoplankton and Daphnia peaked earlier in warmer treatments and reached higher peaks when stratification depth was shallower. Ciliates responded positively to increased food density and higher temperature and subsequently affected the taxonomic composition, but not the total biomass, of phytoplankton. In the absence of Daphnia, phytoplankton did not enter a distinct clear water phase. When present, Daphnia caused an extended clear water phase, maintaining phytoplankton and ciliates at low levels throughout early summer and suppressing all direct effects of physical drivers on these plankton groups. Our Daphnia treatments mimicked the high and low fish predation settings of the largely descriptive, recently revised Plankton Ecology Group (PEG) model of seasonal plankton succession and explored their responses to climate change scenarios. The results largely support the PEG model, but attribute greater importance to early season temperature effects and later season grazing effects of Daphnia. In warmer treatments, the timing of phytoplankton and zooplankton peaks tended to be more closely coupled, and temperature did not affect the height of zooplankton peaks. In line with other experiments, these results do not support the widely held concern that warming may create a trophic mismatch between phytoplankton and zooplankton and reduce spring zooplankton production.

  • 7. Berger, Stella A
    et al.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stibor, Herwig
    Trommer, Gabriele
    Ruhenstroth, Miriam
    Water temperature and stratification depth independently shift cardinal events during plankton spring succession2010In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 16, no 7, p. 1954-1965Article in journal (Refereed)
    Abstract [en]

    In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification-dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.

  • 8. Chase, J. M.
    et al.
    Abrams, P. A.
    Grover, J. P.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Chesson, P.
    Holt, R. D.
    Richards, S. A.
    Nisbet, R. M.
    Case, T. J.
    The interaction between predation and competition: a review and synthesis2002In: Ecology Letters, Vol. 5, no 2, p. 302-315Article in journal (Refereed)
    Abstract [en]

    This review discusses the interface between two of the most important types of interactions between species, interspecific competition and predation. Predation has been claimed to increase, decrease, or have little effect on, the strength, impact or importance of interspecific competition. There is confusion about both the meaning these terms and the likelihood of, and conditions required for, each of these outcomes. In this article we distinguish among three measures of the influence of predation on competitive outcomes: short-term per capita consumption or growth rates, long-term changes in density and the probability of competitive coexistence. We then outline various theoretical mechanisms that can lead to qualitatively, distinct effects. of predators,. The qualitative effect of predators can depend both on the mechanism of competition and on the definition of competitive strength/impact. In assessing the empirical literature, we ask: (1) What definitions of competitive strength/impact have been assumed? (2) Does strong evidence exist to support one or more of the possible mechanisms that can produce a given outcome? (3) Do biases in the choice of organism or manipulation exist, and are they, likely, to have influenced the conclusions reached? We conclude by discussing several unanswered questions, and espouse a stronger interchange between empirical and theoretical approaches to this important question.

  • 9. Cooper, S. D.
    et al.
    Barmuta, L.
    Sarnelle, O.
    Kratz, K.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Quantifying spatial heterogeneity in streams1997In: Journal of the North American Benthological Society, Vol. 16, no 1, p. 174-188Article in journal (Refereed)
    Abstract [en]

    Although theoretical and empirical studies show that spatial heterogeneity has important effects on the dynamics of populations and the structure of communities, there has been little rigorous quantification of terms like ''patchiness'' or ''spatial heterogeneity'' in studies of lotic systems. In order to compare the spatial heterogeneity of different systems and understand the causes and consequences of that heterogeneity, we must first be able to quantitatively measure it. Spatial heterogeneity has many aspects that change with the scale of our observations, so we need a battery of descriptive measures that explicitly consider the scale-dependence of ecological pattern Response variables exhibiting similar frequency distributions (i.e., similar overall variability) can have very different spatial distributions; consequently, descriptions of spatial heterogeneity require spatial data, i.e., data related to geographic locations (maps). We review statistical techniques for quantitatively describing aspects of heterogeneity in spatial data, emphasizing the decomposition of heterogeneity into different scales of variation (trends, overall variability and spatial dependence or autocorrelation). Gradients in spatial data can be evaluated using trend analyses (e.g., regressions), whereas the spatial structure of variation around trends can be evaluated using geostatistical methods. The central concept of geostatistics is spatial dependence, which is the degree to which values of a response variable differ as a function of the distance (lag) between sampling locations. Semivariograms plot variation among samples separated by a common lag Versus lag, and can be objectively decomposed by piece-wise regression techniques to estimate the strength and scales of spatial dependence. A variety of other methods can be used to quantify spatial heterogeneity from categorical and numerical maps depending on the question of interest and the underlying structure of the spatial data (e.g., methods derived from fractal geometry and information theory, nearest neighbor analysis, spectral analysis, Mantel's test). Spatial heterogeneity in stream organisms is driven by local variation in environmental conditions, by interactions between individuals of the same or different species, and by the effects of organisms on their abiotic environment. By applying geostatistical methods to spatial data collected from field experiments, stream ecologists can evaluate the effects of biotic and abiotic factors on the spatial arrangement of organisms in streams. We present examples of data obtained from experiments examining how consumers affect, and respond to, spatial heterogeneity in their resources. The results indicate that consumer-resource feedbacks should be considered when modeling the causes and consequences of spatial heterogeneity in streams.

  • 10. Declerck, Steven A. J.
    et al.
    Malo, Andrea R.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Waasdorp, Dennis
    Lemmen, Kimberley D.
    Proios, Konstantinos
    Papakostas, Spiros
    Rapid adaptation of herbivore consumers to nutrient limitation: eco-evolutionary feedbacks to population demography and resource control2015In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 18, no 6, p. 553-562Article in journal (Refereed)
    Abstract [en]

    Humans alter biogeochemical cycles of essential elements such as phosphorus (P). Prediction of ecosystem consequences of altered elemental cycles requires integration of ecology, evolutionary biology and the framework of ecological stoichiometry. We studied micro-evolutionary responses of a herbivorous rotifer to P-limited food and the potential consequences for its population demography and for ecosystem properties. We subjected field-derived, replicate rotifer populations to P-deficient and P-replete algal food, and studied adaptation in common garden transplant experiments after 103 and 209days of selection. When fed P-limited food, populations with a P-limitation selection history suffered 37% lower mortality, reached twice the steady state biomass, and reduced algae by 40% compared to populations with a P-replete selection history. Adaptation involved no change in rotifer elemental composition but reduced investment in sex. This study demonstrates potentially strong eco-evolutionary feedbacks from shifting elemental balances to ecosystem properties, including grazing pressure and the ratio of grazer:producer biomass.

  • 11.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    DIRECT AND INDIRECT EFFECTS OF OMNIVORY IN A LITTORAL LAKE COMMUNITY1995In: Ecology, Vol. 76, no 6, p. 1727-1740Article in journal (Refereed)
    Abstract [en]

    In spite of the ubiquity of omnivory in nature, its consequences for population dynamics have received little attention from theoretical and experimental ecologists. Having three direct consumer-resource links, three indirect numerical links, and a potential for indirect effects mediated by size structure and/or behavioral flexibility, a three-species omnivory system may exhibit complex population dynamics. In 2 x 3 m field enclosures in the littoral zone of a lake, I manipulated the dominating native benthivorous fish (greater than or equal to 2yr-old perch, Perca fluviatilis, omnivorous top consumer) and the dominating native benthic invertebrate predators (Sialis lutaria, Megaloptera, and odonates, intermediate consumers) in three gradients: increasing densities of perch in the near absence of Sialis and odonates, and increasing densities of Sialis and odonates in either the presence or absence of perch. The densities of their common prey (mainly chironomids) were left unmanipulated. Macroinvertebrate abundance, biomass, and size structure, as well as gut contents of perch and Sialis were sampled monthly over a 3-mo summer period. In the absence of perch, the experimental gradient of Sialis densities remained unchanged over time. In the presence of perch, Sialis decreased by about half at high initial densities, but remained unchanged at low initial densities. Perch also had a negative effect on odonates. In the near absence of Sialis and odonates, perch had a strong, negative effect on chironomids. Compared to enclosures without perch, chironomid abundance was strongly reduced at the lowest perch density, but leveled off with further increases in perch density. Sialis and odonates did not affect chironomid abundance when perch were absent. In contrast, chironomid abundance was positively affected by Sialis and odonates when perch were present. The overall effect of perch on chironomid abundance in the presence of Sialis and odonates was negative. The combined predatory and competitive effects of perch on Sialis and odonates raise the issue how Sialis and odonates coexist with perch. The lack of effect of perch on Sialis and chironomids at low densities of these prey suggests that prey refuges contribute significantly to their persistence in natural systems. The indirect positive effect of Sialis and odonates on chironomids indicates density dependence in per capita interaction coefficients and is most likely to be explained by a behavioral response of perch or chironomids to Sialis and odonates. I suggest that the theoretical and empirical investigation of model systems (such as three-component omnivory systems), which are sufficiently simple to be analytically and experimentally tractable, but still display a richness of common indirect effects, will contribute to our understanding of the dynamics of more complex food webs. Their study will benefit from experimental manipulations of more than one population at more than two densities.

  • 12.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    FISH PREDATION AND BENTHIC COMMUNITY STRUCTURE - THE ROLE OF OMNIVORY AND HABITAT COMPLEXITY1992In: Ecology, Vol. 73, no 5, p. 1646-1661Article in journal (Refereed)
    Abstract [en]

    In population models, omnivorous predation (i.e., predation on > 1 trophic level) generally has a destabilizing influence, whereas habitat heterogeneity tends to stabilize both predatory and competitive interactions. I experimentally evaluated the impact of a native omnivorous predator and of habitat structural complexity on the structure of a freshwater littoral macroinvertebrate community. Perch (Perca fluviatilis) were stocked at three densities (no fish, and low and high natural densities) into pond enclosures with dense submerged vegetation present or absent. Macroinvertebrate abundance, biomass, size structure, and diversity as well as gut contents of perch were monitored over a 2-mo summer period. Perch consumed both predatory and nonpredatory invertebrates except molluscs. In vegetation, perch consumed macroinvertebrates almost exclusively during the whole experiment, whereas in the absence of vegetation, perch successively increased their consumption of microcrustaceans. The biomass of predatory invertebrates and the abundance of the dominant predatory invertebrate Sialis lutaria (Insecta: Megaloptera) decreased with increasing perch density in both vegetation treatments. In contrast, perch density had no effect on the biomass of nonmolluscan herbivores and detritivores. Vegetation increased the biomass of all macroinvertebrates and tended to delay the negative effect of perch on predatory invertebrates. Perch density had a negative effect on the mean size of total nonmolluscan macroinvertebrates, but not of predatory invertebrates and nonmolluscan herbivores/detritivores considered separately. While perch density had an effect on the diversity (Shannon evenness and Shannon diversity) of only nonmolluscan macroinvertebrates (i.e., potential prey), vegetation had a positive effect on species richness (but not on evenness) of all macroinvertebrates. The results suggest that perch disproportionally preyed on predatory invertebrates, but that characteristics other than size per se were responsible for their higher vulnerability to perch. Perch may have affected nonmolluscan herbivores/detritivores simultaneously through direct consumption and through indirect release from invertebrate predation, which could explain the lack of a perch effect on nonmolluscan herbivores/detritivores. Complex interactions between the omnivorous predator, intermediate consumers, and resource species (macroinvertebrate herbivores and detritivores) may explain the relationship between perch density and macroinvertebrate diversity. The reduction in foraging efficiency of perch in submerged vegetation, and the observed diet shift of perch towards microcrustaceans when predatory invertebrates were still fairly abundant, suggest that relatively large predatory invertebrates were still fairly abundant, suggest that relatively large predatory invertebrates can coexist with fish in the littoral of lakes to a higher extent than in the pelagic zone. Predatory invertebrates may thus play a more important role in food web dynamics of littoral than of pelagic communities.

  • 13.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Paradoxes of enrichment: Effects of increased light versus nutrient supply on pelagic producer-grazer systems2007In: American Naturalist, Vol. 169, no 6, p. E173-E191Article in journal (Refereed)
    Abstract [en]

    Energy-based plant-herbivore models produce the "paradox of enrichment," a destabilizing influence of enrichment on population dynamics. Because many plants change their carbon : nutrient stoichiometry in response to the light : nutrient supply ratio, enrichment with light can cause a mismatch between the elemental compositions of plants and their herbivores. Herbivore growth rates may then decrease with increased light supply, which is termed the "paradox of energy enrichment." I present a stoichiometric phyto-plankton-grazer model that accounts for the dynamical vertical light gradient and explore how algal and grazer densities, mineral nutrient concentration, algal nutrient stoichiometry, and system stability respond to enrichment with light ( through changes in irradiance, background turbidity, and water column depth) versus enrichment with nutrients. Parameterized for Daphnia, the model produces several "unusual" phenomena: multiple equilibria ( with grazers extinct in spite of high algal biomass at one equilibrium), inconsistent light enrichment effects on stability ( light enrichment first destabilizes and then stabilizes), and the paradox of energy enrichment. These phenomena are restricted to the low end of realistic nutrient supplies except in very shallow systems, where high sedimentation rates effectively deplete the water column of nutrients. At higher nutrient supplies, light enrichment produces the classical paradox of enrichment, leading first to an increase in grazers at a stable equilibrium and then to algae-grazer oscillations.

  • 14.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Phytoplankton, light, and nutrients in a gradient of mixing depths: Theory2002In: Ecology, Vol. 83, no 2, p. 386-398Article in journal (Refereed)
    Abstract [en]

    The depth of the well-mixed surface layer of lakes and oceans fundamentally affects phytoplankton populations. Specific nutrient supply, specific algal production, and specific sinking losses all decrease with increasing mixing depth. I use a dynamical model to investigate how phytoplankton biomass, light availability, and the distribution Of nutrients among various pools vary along a mixing depth gradient, and how the relationships of these variables to mixing depth depend on algal sinking velocity, abiotic light absorbents, nutrient enrichment, and the mode of nutrient supply (closed system with recycling vs. open system with external input). If phytoplankton is dominated by sinking algae, the primary causes of biomass limitation shift with increasing mixing depth from sinking loss limitation to nutrient limitation to light limitation. Consequently, algal biomass in the mixed layer (expressed per volume or area) and sedimented nutrients are unimodally related to mixing depth, whereas dissolved inorganic and total water column nutrients show the inverse pattern. Compared to closed systems, the maximum in the biomass concentration-mixing depth relationship occurs at much shallower depths in open systems without recycling of sedimented nutrients (such as mixed surface layers on top of stratified water columns). With increased algal sinking velocity, algal biomass decreases, and light penetration and dissolved nutrients both increase, whereas sedimented and total water column nutrients may increase or decrease. Increased abiotic turbidity reduces light penetration, algal biomass, and sedimented nutrients but increases dissolved and total water column nutrients. Finally, with nutrient enrichment, algal biomass and all nutrient compartments increase, whereas light penetration decreases. I use a graphical isocline approach to show that increasing external light supply, decreasing abiotic turbidity, and decreasing mixing depth represent three conceptually different forms of enrichment with light. Of those, decreasing abiotic turbidity is conceptually similar to enrichment with a mineral nutrient.

  • 15.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    The evolution and maintenance of omnivory: Dynamic constraints and the role of food quality2003In: Ecology, Vol. 84, no 10, p. 2557-2567Article in journal (Refereed)
    Abstract [en]

    In several taxa, the ability to feed omnivorously from more than one trophic level has evolved from more specialized feeding habits. A prerequisite for this evolutionary scenario is initial coexistence (on the same or different trophic levels) of the consumers subsequently involved in omnivory. Evolution of omnivory should also be favored by prior asymmetries in consumer interactions. I use a dynamic model to explore conditions for coexistence of two consumers depending on a single biotic resource in simple. food webs of increasing asymmetry between consumers: (1) pure resource competition, (2) resource competition with unidirectional interference, (3) omnivory, and (4) pure predation. If consumers are energy limited, omnivory is unlikely to evolve from a competitive system, because stable coexistence of two consumers on a single resource is impossible, even if there is a trade-off between resource exploitation and interference abilities. Initial coexistence of two consumers is easier to achieve, if the species are arranged in a food chain. If costs of diet generalization are low, selection should then favor the inclusion of the resource in the top consumer's diet. In systems with high resource carrying capacity, the intermediate consumer will then, however, be frequently driven to extinction-in which case omnivory is not maintained. Low nutritional quality of the resource can stabilize the omnivory system and allow three-species coexistence at high resource carrying capacities. Low nutritional quality of the resource may also create conditions for the evolution of omnivory from a competitive system. If the content of an essential nutrient in resource biomass is dynamic, stable coexistence of two competitors becomes possible if both are limited by the nutrient content of the resource. In this case, selection may favor the, evolution of omnivory, because competitor biomass is usually of higher nutritional quality than resource biomass. An explicit treatment of flexible resource stoichiometry thus broadens the possibilities for the evolution of omnivory.

  • 16.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Berger, S.
    Ptacnik, R.
    Wild, A.
    Phytoplankton, light, and nutrients in a gradient of mixing depths: Field experiments2002In: Ecology, Vol. 83, no 2, p. 399-411Article in journal (Refereed)
    Abstract [en]

    We studied the effects of water column mixing depth and background turbidity on phytoplankton biomass, light climate, and nutrients in two field enclosure experiments designed to test, predictions of a dynamical model. In 1997 and 1998, we created gradients of mixing depth by enclosing the 100-mum-filtered phytoplankton community of a phosphorus-deficient lake in cylindrical plastic bags of varying depth (1.5-15 m) which were continuously mixed. To mimic different levels of background turbidity, we surrounded the transparent enclosure walls with a layer of opaque white (1997) or black (1998) plastic. The experiments were run for 4 wk (1997) and 6 wk (1998). The results supported two key assumptions of the model: specific production and specific sedimentation losses both decreased with increasing mixing depth. At all mixing depths, fast-sinking diatoms dominated (lie communities. In accordance with model predictions, algal biomass concentration and standing stock (summed over the mixed layer) showed a unimodal relationship to mixing depth when background turbidity was high (1998). When background turbidity was lower (1997), only the ascending limbs of the corresponding relationships were found, which support.,; the prediction that the mixing depth at which biomass peaks (i.e., becomes predominantly limited by light) increases with decreasing background turbidity. Also in accordance with predictions, light intensity at the bottom of the mixed layer decreased with increasing mixing depth and with increasing background turbidity. Finally, the data supported only the ascending limbs of the predicted inverse unimodal relationships among mixing depth and dissolved inorganic and total water column phosphorus. The absence of descending limbs in these relationships at low mixing depths was probably due to deviations of the experimental systems from two model assumptions. First, the remineralization rate of sedimented phosphorus may have been too slow to equilibrate with sedimentation losses over the experimental periods. Second, biomass yield per unit nutrient (the ratio of seston carbon to phosphorus) was not constant, but decreased with increasing mixing depth. To our knowledge, these are the first field experiments in which the effects of mixing depth on phytoplankton and its resources have been investigated systematically along a large gradient.

  • 17.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Berger, S.
    Wohrl, R.
    Flexible nutrient stoichiometry mediates environmental influences, on phytoplankton and its resources2005In: Ecology, Vol. 86, no 11, p. 2931-2945Article in journal (Refereed)
    Abstract [en]

    The elemental composition of plants, which affects ecosystem processes such as energy transfer to consumers and nutrient recycling, varies with the supply of nutrients and light. In contrast to most terrestrial systems, aquatic plants "compete" with abiotic light absorbents. Light supply to pelagic producers and, consequently, algal carbon to nutrient stoichiometry are therefore affected by background turbidity (light absorption by non-algal components, K-bg) and mixing depth (vertical extension of the mixed water column, z). Still, light as a dynamic variable has been neglected in models exploring the ecological consequences of flexible algal stoichiometry. In addition, there are hardly any field experiments exploring effects of light supply on planktonic systems. We present a dynamical model that accounts for the simultaneous dependence of algal production on light supply and internal algal nutrient stores and derive predictions on how a suite of state variables (algal biomass, light climate, algal nutrient stoichiometry, dissolved nutrient concentration, and nutrients in sediment) should be affected by z and K-bg. We compare these predictions with results of an enclosure experiment in a phosphorus-deficient lake, in which we manipulated z and K-bg. Algal biomass decreased at higher K-bg and showed a unimodal response to z (being limited by sinking losses at shallow z and by light at deep z). The biomass peak shifted toward lower z with increased K-bg. The seston carbon: phosphorus ratio decreased with increasing z and K-bg. Dissolved mineral phosphorus was undetectable in all treatments. Total water column phosphorus and sedimented phosphorus increased with increasing z but were unaffected by K-bg. These data are in almost complete qualitative congruence with model predictions over the examined range of mixing depths (1-15 m). The model thus provides a useful framework for the continued mechanistic exploration of how environmental drivers influence producer stoichiometry in pelagic environments.

  • 18.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Cooper, S. D.
    Kratz, K. W.
    Nisbet, R. M.
    Roll, S. K.
    Wiseman, S. W.
    Jenkins, T. M.
    Effects of multiple, predator-induced behaviors on short-term producer-grazer dynamics in open systems2000In: American Naturalist, Vol. 156, no 3, p. 293-313Article in journal (Refereed)
    Abstract [en]

    We investigated the population consequences of multiple behavioral responses of grazers to a foraging return-predation risk trade-off in an open system consisting of primary producers, grazers, and predators. Using a dynamical model where grazers adjust their foraging activity and emigration rate to the densities of predators and producers, we explored how changes in control variables (predator density, grazer immigration, and producer immigration and carrying capacity) affect the dynamics of producers and grazers at temporal scales shorter than consumer and predator reproduction. The model predicts that producer biomass increases and that both the density of foraging grazers and the feeding rate of predators decrease with predator density. These predictions hold although total (foraging + nonforaging) grazer density may actually increase with predator density. The latter will occur whenever the benefit of higher resource density outweighs the increased risk of predation. In this case. per capita grazer emigration decreases with predator density, which might be misinterpreted as a direct "freezing" response to predators. Increased grazer immigration is predicted to result in decreased producer density and increased densities of both foraging and total grazers, as well as increased grazer emigration and predator feeding rates. Increased producer immigration or carrying capacity should increase producer and grazer densities and predator feeding rate but decrease per capita grazer emigration. Manipulation of predator (trout) densities in a set of nine large (50 m(2)) stream channels produced results in broad agreement with model predictions. Most notably, a positive effect of trout on benthic algal biomass was mainly mediated through grazer behavior (changes in the use of epibenthic surfaces and in emigration rare) rather than through consumptive reductions of grazer numbers by trout.

  • 19.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Eklov, P.
    EFFECTS OF PISCIVORE-MEDIATED HABITAT USE ON RESOURCES, DIET, AND GROWTH OF PERCH1995In: Ecology, Vol. 76, no 6, p. 1712-1726Article in journal (Refereed)
    Abstract [en]

    We investigated experimentally how presence or absence of different piscivores influenced habitat use, diet, and individual growth of two size classes of juvenile perch (Perca fluviatilis) and abundances of juvenile perch resources in different habitats. Pond enclosures encompassed 3 X 6 m of dense vegetation and 9 X 6 m of open habitat, and were stocked with 75 young-of-year and 30 1-yr-old perch and with either three piscivorous perch, three northern pike (Esox lucius), or no piscivores. Northern pike were both larger and possessed a larger gape than piscivorous perch. To isolate behavioral responses of juvenile perch to piscivores, we replaced consumed young-of-year perch. Prey fish mortality was lowest without piscivores and highest with northern pike. Young-of-year perch increased their use of vegetation in the presence of both piscivores, whereas 1-yr-old perch increased their use of vegetation only with northern pike. For both age classes of prey fish, increased use of the vegetation led to reduced individual growth, owing to two complementary mechanisms. First, the physical complexity of submerged macrophytes likely interfered with the benthic feeding of perch, Second, increased use of the (relatively small) vegetated habitat increased the mean density experienced by prey fish. Piscivore-induced changes of prey fish densities in the two habitats had substantial effects on the biomass of prey fish resources in the open habitat, but only minor effects in the vegetation. Sialis lutaria, the major predatory macroinvertebrate (approximate to 50% of total macroinvertebrate biomass in the open habitat), and total predatory macroinvertebrates were positively affected by piscivores in the open habitat, but not in the vegetation. Chironomids (<3% of total macroinvertebrate biomass in the vegetation) and the sizes of nonpredatory macroinvertebrates were negatively affected by piscivores in the vegetation, but not in the open habitat. Biomass of nonpredatory macroinvertebrates, Cladocera, and Copepoda did not differ among treatments in either habitat. From our review of field experiments, vulnerable prey fish always change their habitat use in the presence of piscivores. Behaviorally mediated indirect effects of piscivores on individual growth rates and prey fish resources were just as frequently observed as direct effects of piscivores on prey fish survival.

  • 20.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Feissel, M.
    Intraguild prey suffer from enrichment of their resources: A microcosm experiment with ciliates2001In: Ecology, Vol. 82, no 11, p. 2977-2983Article in journal (Refereed)
    Abstract [en]

    Predation among species that compete for resources is widespread. Theory suggests a counterintuitive consequence of such intraguild predation: enrichment of the shared resource, for which the intraguild prey must be the superior competitor, reduces the population size of the intraguild prey and may even cause its extinction. Experiments with bacteria (shared resource) and the ciliates Tetrahymena (intraguild prey) and Blepharisma (intraguild predator) confirmed these expectations. Tetrahymena was the better resource competitor. Grown separately, it persisted at lower levels of enrichment and drove bacterial densities to lower levels than did Blepharisma. When grown together with Blepharisma, Tetrahymena declined in density with increasing enrichment, whereas bacteria and Blepharisma increased. The data thus support the theoretically suggested mechanism as the cause of this pattern: with enrichment, increasing densities of bacteria sustained an increasing predation pressure from Blepharisma on Tetrahymena, which overrode the direct positive effect of bacteria as a food source of Tetrahymena. Generalizing from these results, we speculate that staggered invasions of omnivores followed by extinctions of intermediate prey should contribute to the frequently observed unimodal or neutral responses of species richness and food chain length to increasing productivity.

  • 21.
    Diehl, S.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Kornijow, R.
    Influence of submerged macrophytes on trophic interactions among fish and macroinvertebrates1998In: Structuring Role of Subbmerged Macrophytes in Lakes, 1998Chapter in book (Other (popular science, discussion, etc.))
  • 22.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of habitat structure on resource availability, diet and growth of benthivorous perch, perca-fluviatilis1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 67, no 3, p. 403-414Article in journal (Refereed)
    Abstract [en]

    I experimentally evaluated the impact of habitat structural complexity on the interactions between a generalist predator and a benthic macroinvertebrate prey assemblage in a freshwater pond. Benthivorous perch (Perca fluviatilis) were stocked over a range of natural densities (no fish, low, and high densities) into enclosures with or without dense submerged vegetation. The biomass of macroinvertebrate prey decreased over time in the presence of perch and was always higher in enclosures with vegetation present than in enclosures lacking vegetation. The increase in mass of perch was positively related to the abundance of macroinvertebrate prey and negatively related to perch density and the density of vegetation. In the treatments lacking vegetation, the proportion of zooplankton in the diet of perch increased, and the growth rate of perch decreased over time. In the vegetation treatments, the proportion of zooplankton in the diet was low throughout the experiment and the growth rate of perch was constant over time. As a consequence, initial increase in mass was considerably higher in the treatments lacking vegetation than in the vegetation treatments, whereas no such pattern was observed in the second half of the experiment. In the absence of vegetation, perch are apparently able to forage efficiently, but this may reduce the availability of macroinvertebrate prey to the extent that perch are forced to include less profitable zooplankton prey into their diet. In vegetated habitats, the foraging efficiency of perch is reduced, which possibly prevents over-exploitation of macroinvertebrate prey and consequently may allow for a moderate, but relatively constant, consumption of macroinvertebrates by perch. The density-dependence of growth rates in both vegetated and unvegetated habitats can only partly be explained by resource competition, which suggests the presence of an additional mechanism of density-dependence. In natural lake communities, efficient predation from benthivorous fish should keep the biomass of macroinvertebrate prey in structurally simple habitats below the high levels initially present in my experiment. In these communities, submerged vegetation may be an equally profitable habitat for juvenile perch as are open areas. Through its effects on the feeding efficiencies of juvenile perch and other benthivorous fish, submerged vegetation may affect individual growth rates and the size structure of perch populations, which may contribute to explain differences in fish community structure among lakes differing in submerged vegetation cover.

  • 23.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Relative consumer sizes and the strengths of direct and indirect interactions in omnivorous feeding relationships1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 68, no 1, p. 151-157Article in journal (Refereed)
    Abstract [en]

    Omnivory (the consumption of resources from more than one trophic level) is widespread in nature and has the potential to produce a richness of indirect effects. Nevertheless, its effects on population dynamics have received very little attention. In its simplest case, omnivory involves a top consumer, an intermediate consumer, and a resource that is common to both consumers. Simple models predict that the intermediate consumer can only coexist with the top consumer if the former is more efficient in exploiting the common resource, which would imply a net positive effect of the top consumer on the equilibrium density of resources (compared to the situation where only the intermediate consumer is present). Among 22 experimental manipulations of omnivorous top consumers I found only 2 studies in which top consumers had significant positive effects on resources. This discrepancy between experimental results and model predictions is, at least partly, related to deviations of the experimental systems from model assumptions. However, considerations of relative body sizes of intermediate and top consumers suggest, that top consumers having negative net effects on the basic resource should be common in nature. I argue that in systems where intermediate consumers and basic resources are relatively similar in size, but both are much smaller than omnivorous top consumers (e.g. vertebrate omnivores feeding on benthos, soil invertebrates, terrestrial insects etc.), the direct negative effect of top consumers on basic resources should not be outweighed by indirect positive effects, and that other mechanisms (e.g. prey refuges) must be invoked to explain the persistence of intermediate consumers in many natural systems. I further argue that a better knowledge of the population dynamical consequences of omnivory and the role of relative consumer sizes is necessary to improve our understanding of the-trophic dynamics of different kinds of communities.

  • 24.
    Diehl, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Biology II, Ludwig-Maximilians-University, Germany.
    Berger, Stella A.
    Soissons, Quentin
    Giling, Darren P.
    Stibor, Herwig
    An experimental demonstration of the critical depth principle2015In: ICES Journal of Marine Science, ISSN 1054-3139, E-ISSN 1095-9289, Vol. 72, no 6, p. 2051-2060Article in journal (Refereed)
    Abstract [en]

    Sverdrup's critical depth hypothesis, which has had an almost canonical status in biological oceanography, has recently been challenged as a universal explanation for the formation of oceanic spring blooms, and several alternative hypotheses have been proposed. Arguments pro and contra alternative explanations have so far relied on theoretical considerations and purely observational data. In this paper, we propose that mesocosm experiments with natural plankton communities could make important contributions to the resolution of the issue. We first briefly review the foundations of the critical depth concept and derive an approximate relationship that relates optically scaled critical depth (="critical optical depth", i.e. the product of the light attenuation coefficient and the critical depth) to light-dependent phytoplankton production in the mixed surface layer. We describe how this relationship can be used to scale experimental mesocosms such that they reproduce ambient light conditions of natural water columns from the surface down to the critical depth and beyond. We illustrate the power of the approach with a mesocosm study in which we experimentally controlled the onset of the spring bloom of a lake plankton community through the manipulation of optically scaled mixed-layer depth. This experiment may be the first experimental demonstration of the critical depth principle acting on a natural plankton community. Compensation light intensity (=minimum average mixed-layer light intensity required to trigger a bloom of the ambient plankton community) could be constrained to be somewhat above 3.2 moles PAR m(-2) d(-1), corresponding to a critical optical depth of 10.5. We compare these numbers to estimates from marine systems and end with a discussion of how experiments could be designed to (i) more accurately determine the critical depth in a given system and (ii) resolve among competing hypotheses for vernal bloom onset.

  • 25.
    Diehl, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Berger, Stella
    Uszko, Wojciech
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Stibor, Herwig
    Stoichiometric mismatch causes a warming-induced regime shift in experimental plankton communitiesManuscript (preprint) (Other academic)
  • 26.
    Diehl, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Lundberg, Peter A
    Gardfjell, Hans
    Oksanen, Lauri
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Daphnia-phytoplankton interactions in lakes: is there a need for ratio-dependent consumer-resource models?1993In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 142, no 6, p. 1052-1061Article in journal (Refereed)
  • 27.
    Diehl, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Integrated Science Lab (IceLab), Umeå University, Sweden.
    Thomsson, Gustaf
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Kahlert, Maria
    Guo, Junwen
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Liess, Antonia
    Inverse relationship of epilithic algae and pelagic phosphorus in unproductive lakes: Roles of N-2 fixers and light2018In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 63, no 7, p. 662-675Article in journal (Refereed)
    Abstract [en]

    Phosphorus (P) often limits the biomass of primary producers in freshwater lakes. However, in unproductive northern lakes, where anthropogenic nitrogen (N) deposition is low, N instead of P can limit primary producers. In addition, light can be limiting to primary producers at high concentrations of coloured dissolved organic matter (cDOM), as cDOM is the major determinant of light penetration in these lakes.

    To address resource limitation of epilithic algal biomass, we repeatedly sampled epilithon (periphyton on stony substrata) in 20 lakes covering a large, correlated cDOM and N‐deposition gradient across boreal and subarctic Sweden. Across these lakes, pelagic total N (TN) and total P (TP) were positively correlated, and benthic light supply was negatively correlated, with cDOM. Microscopically determined algal biovolume and epilithic carbon (C), N and P were subsequently regressed against benthic light supply and pelagic TN and TP.

    Patterns in epilithic biovolume were driven by N2‐fixing cyanobacteria, which accounted for 2%–90% of total epilithic biovolume. Averaged over the growing season, epilithic algal biovolume, C and N were negatively related to TP and positively to TN, and were highest in the clearest, most phosphorus‐poor lakes, where epilithon was heavily dominated by potentially N2‐fixing cyanobacteria.

    A structural equation model supports the hypothesis that cDOM had two counteracting effects on total epilithic algal biovolume: a positive one by providing N to algae that depend on dissolved N for growth, and a negative one by shading N2‐fixing cyanobacteria, with the negative effect being somewhat stronger.

    Together, these findings suggest that (1) light and N are the main resources limiting epilithic algal biomass in boreal to subarctic Swedish lakes, (2) epilithic cyanobacteria are more competitive in high‐light and low‐nitrogen environments, where their N2‐fixing ability allows them to reach high biomass, and (3) epilithic N increases with N2 fixer biomass and is—seemingly paradoxically—highest in the most oligotrophic lakes.

  • 28.
    Diehl, Sebastian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Thomsson, Gustaf
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wickman, Jonas
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics.
    Vasconcelos, Francisco R.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Uszko, Wojciech
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ask, Jenny
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Byström, Pär
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Resource and consumer control of cross-habitat trophic interactions in shallow lakesManuscript (preprint) (Other academic)
  • 29. Effenberger, M.
    et al.
    Engel, J.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Matthaei, C. D.
    Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment2008In: Freshwater Biology, Vol. 53, no 5, p. 996-1011Article in journal (Refereed)
    Abstract [en]

    1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood-prone German stream. 2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m(2)) to a depth of 15-20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near-bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM). 3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study. 4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near-bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters. 5. Our results suggest that several 'direct' initial effects of local disturbance history on the invertebrates were subsequently replaced by 'indirect' effects of disturbance history (via disturbance-induced changes in habitat parameters such as current velocity or food).

  • 30. Effenberger, Michael
    et al.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gerth, Maximilian
    Matthaei, Christoph D
    Patchy bed disturbance and fish predation independently influence the distribution of stream invertebrates and algae2011In: Journal of Animal Ecology, ISSN 0021-8790, E-ISSN 1365-2656, Vol. 80, no 3, p. 603-14Article in journal (Refereed)
    Abstract [en]

    1. The identification of factors determining the patchy distribution of organisms in space and time is a central concern of ecology. Predation and abiotic disturbance are both well-known drivers of this patchiness, but their interplay is still poorly understood, especially for communities dominated by mobile organisms in frequently disturbed ecosystems. 2. We investigated the separate and interactive influences of bed disturbance by floods and predation by fish on the benthic community in a flood-prone stream. Electric fields excluded fish predators from half of 48 stream bed patches (area 0·49 m(2) ) with contrasting disturbance treatments. Three types of bed disturbance were created by either scouring or filling patches to a depth of 15-20 cm or by leaving the patches undisturbed, thus mimicking the mosaic of scour and fill caused by a moderate flood. Benthic invertebrates and algae were sampled repeatedly until 57 days after the disturbance. 3. Disturbance influenced all ten investigated biological response variables, whereas predation affected four variables. Averaged across time, invertebrate taxon richness and total abundance were highest in stable patches. Algal biomass and densities of five of the seven most common invertebrate taxa (most of which were highly mobile) were higher in fill than in scour patches, whereas two taxa were more abundant in scour and stable than in fill patches. Furthermore, two common invertebrate grazers were more abundant and algal biomass tended to be reduced in fish exclusion patches, suggesting a patch-scale trophic cascade from fish to algae. 4. Our results highlight the importance of patchy physical disturbance for the microdistribution of mobile stream organisms and indicate a notable, but less prevalent, influence of fish predation at the patch scale in this frequently disturbed environment. Disturbance and predation treatments interacted only once, suggesting that the observed predation effects were largely independent of local bed disturbance patterns.

  • 31. Eklov, P.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    PISCIVORE EFFICIENCY AND REFUGING PREY - THE IMPORTANCE OF PREDATOR SEARCH MODE1994In: Oecologia, Vol. 98, no 3-4, p. 344-353Article in journal (Refereed)
    Abstract [en]

    In predator-prey interactions, the efficiency of the predator is dependent on characteristics of both the predator and the prey, as well as the structure of the environment. In a field enclosure experiment, we tested the effects of a prey refuge on predator search mode, predator efficiency and prey behaviour. Replicated enclosures containing young of the year (0+) and 1-year-old (1+) perch were stocked with 3 differentially sized individuals of either of 2 piscivorous species, perch (Perca fluviatilis), pike (Esox lucius), or no piscivorous predators. Each enclosure contained an open predator area with three small vegetation patches, and a vegetated absolute refuge for the prey. We quantified the behaviour of the predators and the prey simultaneously, and at the end of the experiment the growth of the predators and the mortality and habitat use of the prey were estimated. The activity mode of both predator species was stationary. Perch stayed in pairs in the vegetation patches whereas pike remained solitary and occupied the corners of the enclosure. The largest pike individuals stayed closest to the prey refuge whereas the smallest individuals stayed farthest away from the prey refuge, indicating size-dependent interference among pike. Both size classes of prey showed stronger behavioural responses to pike than to perch with respect to refuge use, distance from refuge and distance to the nearest predator. Prey mortality was higher in the presence of pike than in the presence of perch. Predators decreased in body mass in all treatments, and perch showed a relatively stronger decrease in body mass than pike during the experiment. Growth differences of perch and pike, and mortality differences of prey caused by predation, can be explained by predator morphology, predator attack efficiency and social versus interference behaviour of the predators. These considerations suggest that pike are more efficient piscivores around prey refuges such as the littoral zones of lakes, whereas perch have previously been observed to be more efficient in open areas, such as in the pelagic zones of lakes.

  • 32.
    Englund, Göran
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Öhlund, Gunnar
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hein, Catherine L
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Temperature dependence of the functional response2011In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 14, no 9, p. 914-21Article in journal (Refereed)
    Abstract [en]

    The Arrhenius equation has emerged as the favoured model for describing the temperature dependence of consumption in predator-prey models. To examine the relevance of this equation, we undertook a meta-analysis of published relationships between functional response parameters and temperature. We show that, when plotted in lin-log space, temperature dependence of both attack rate and maximal ingestion rate exhibits a hump-shaped relationship and not a linear one as predicted by the Arrhenius equation. The relationship remains significantly downward concave even when data from temperatures above the peak of the hump are discarded. Temperature dependence is stronger for attack rate than for maximal ingestion rate, but the thermal optima are not different. We conclude that the use of the Arrhenius equation to describe consumption in predator-prey models requires the assumption that temperatures above thermal optima are unimportant for population and community dynamics, an assumption that is untenable given the available data.

  • 33. Haas, K.
    et al.
    Kohler, U.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Kohler, P.
    Dietrich, S.
    Holler, S.
    Jaensch, A.
    Niedermaier, M.
    Vilsmeier, J.
    Influence of fish on habitat choice of water birds: A whole system experiment2007In: Ecology, Vol. 88, no 11, p. 2915-2925Article in journal (Refereed)
    Abstract [en]

    It is notoriously difficult to study population interactions among highly mobile animals that cannot be meaningfully confined to experimental plots of limited size. For example, migratory water birds are believed to suffer from competition with resident fish populations for shared food resources. While observational evidence in support of this hypothesis is accumulating, replicated experiments addressing this issue at the proper spatial scale are lacking. Here, we report from a replicated whole-system experiment in which we stocked large (0.07 km(2)), shallow (<= 2.5 m deep), highly eutrophic ponds in the bird protection area "Ismaninger Speichersee mit Fischteichen'' with different densities of carp and assessed the responses of water birds and their food resources during summer over several years. In all years, the biomasses of benthic macroinvertebrates, macroalgae, and macrophytes as well as the densities of herbivorous, carnivorous, and omnivorous water birds were reduced in carp ponds compared to fishshless ponds. The negative effects of carp on food resources and on the numbers of water birds feeding in carp ponds increased over the season (May-September) and were stronger at high than at low stocking densities of carp. Consequently, differences in resource densities between ponds with and without carp increased, and the ranking of ponds with respect to resource densities became more predictable over the season. These factors may have contributed to a seasonal improvement of the birds' abilities to track resource densities across ponds, as suggested by tight correlations of bird numbers on ponds with resource densities late, but not early, in the season.

  • 34. Hall, S. R.
    et al.
    Shurin, J. B.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Nisbet, R. M.
    Food quality, nutrient limitation of secondary production, and the strength of trophic cascades2007In: Oikos, Vol. 116, no 7, p. 1128-1143Article in journal (Refereed)
    Abstract [en]

    Recent meta-analyses confirm that the strength of trophic cascades (indirect positive effects of predators on plant biomass through control of herbivores) varies among ecosystem types. In particular, most terrestrial systems show smaller cascades than most aquatic ones. Ecologists still remain challenged to explain this variation. Here, we examine a food quality hypothesis which states that higher quality plants should promote stronger trophic cascades. Food quality involves two components: digestion resistance of plants and magnitude of stoichiometric imbalance between plants and herbivores (where stoichiometry involves ratios of nutrient: carbon ratio of tissues). Both factors vary among ecosystems and could mediate conversion efficiency of plants into new herbivores (and hence control of plants by herbivores). We explored the food quality hypothesis using two models, one assuming that plant stoichiometry is a fixed trait, the other one allowing this trait to vary dynamically (but with a minimal nutrient: carbon ratio of structural mass). Both models produce the same suite of results. First, as expected, systems with more easily digested plants promote stronger cascades. Second, contrary to expectations, higher (fixed or minimal) nutrient: carbon ratio of plants do not promote stronger cascades, largely because of the net result of ecosystem feedbacks. Still, the model with dynamic stoichiometry permits positive correlations of realized plant nutrient: carbon ratio and cascade strength (as predicted), mediated through digestion resistance. Third, lower nutrient: carbon ratio of herbivores promotes stronger cascades. However, this result likely cannot explain variation in cascade strength because nutrient: carbon stoichiometry of herbivores does not vary greatly between terrestrial and aquatic ecosystems. Finally, we found that predation promotes nutrient limitation of herbivores. This finding highlights that food web processes, such as predation, can influence stoichiometry-mediated interactions of plants and herbivores.

  • 35.
    Henriksson, Anna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wardle A., David
    Swedish University of Agriculture Sciences, Department of Forest Vegetation Ecology.
    Trygg, Johan
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Englund, Göran
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Strong invaders are strong defenders: implications for the resistance of invaded communities2016In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 19, no 4, p. 487-494Article in journal (Refereed)
    Abstract [en]

    Many ecosystems receive a steady stream of non-native species. How biotic resistance develops over time in these ecosystems will depend on how established invaders contribute to subsequent resistance. If invasion success and defence capacity (i.e. contribution to resistance) are correlated, then community resistance should increase as species accumulate. If successful invaders also cause most impact (through replacing native species with low defence capacity) then the effect will be even stronger. If successful invaders instead have weak defence capacity or even facilitative attributes, then resistance should decrease with time, as proposed by the invasional meltdown hypothesis. We analysed 1157 introductions of freshwater fish in Swedish lakes and found that species' invasion success was positively correlated with their defence capacity and impact, suggesting that these communities will develop stronger resistance over time. These insights can be used to identify scenarios where invading species are expected to cause large impact.

  • 36. Jager, C. G.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Matauschek, C.
    Klausmeier, C. A.
    Stibor, H.
    Transient dynamics of pelagic producer grazer systems in a gradient of nutrients and mixing depths2008In: Ecology, Vol. 89, no 5, p. 1272-1286Article in journal (Refereed)
    Abstract [en]

    Phytoplankton grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton-grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon:nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the "paradox of energy enrichment" (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.

  • 37. Jenkins, T. M.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Kratz, K. W.
    Cooper, S. D.
    Effects of population density on individual growth of brown trout in streams1999In: Ecology, Vol. 80, no 3, p. 941-956Article in journal (Refereed)
    Abstract [en]

    Some studies suggest that lotic populations of brown trout (Salmo trutta) are regulated through density-dependent mortality and emigration to the extent that mean growth rates of resident survivors are unrelated to trout densities. To test this, we studied the relationship between density and growth, mortality, and emigration of brown trout in two alpine streams and a set of stream channels in eastern California. We sampled trout at the scale of "segments" (5-31 m long riffles, runs, and pools) and "sections" (340-500 m in length) of Convict Creek over a 3-yr period. Trout were also sampled during 6 yr in seven 90-m sections of Mammoth Creek. For 2 yr, we manipulated trout densities in Convict Creek by removing trout from two sections and adding trout to two other sections. We also manipulated densities in seven 50-m stream channels, using a natural size distribution of trout in one year and underyearlings only in a second year. In both streams, average size (body length or mass) of underyearlings in fall was negatively related to trout density and was furthermore affected by sampling location and year. The strong, negative relationship between individual mass and density of trout could be detected at the spatial scale of whole sections, but not at the scale of individual segments. The Convict Creek and stream channel experiments also revealed strong negative effects of density on average mass of underyearlings in fall, and on proportional mass increase of yearling and older trout from spring to fall. In contrast, mortality and emigration were unrelated to initial stocking densities in the channels. In all our data, the negative effects on growth were most pronounced at densities <1 trout/m(2) and the growth-density relationships were well described by negative power curves. Large individuals were always less affected by increasing trout density than were small individuals, suggesting a competitive advantage of large over small trout that increased with density. We conclude that individual growth of brown trout in streams can be affected by trout density to an extent that suggests a substantial influence on population regulation. Results from our multiyear, multiscale, and experimental study indicate that density dependence in the growth of stream salmonids will be difficult to detect in purely observational data, especially in systems with relatively high fish densities (where the growth-density relationship has a flat slope), when data are collected and analyzed at small spatial scales, and when insufficient information is collected to assess the contribution of interannual variation in growth.

  • 38.
    Jäger, Christoph G.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Resource competition across habitat boundaries: asymmetric interactions between benthic and pelagic producers2014In: Ecological Monographs, ISSN 0012-9615, E-ISSN 1557-7015, Vol. 84, no 2, p. 287-302Article in journal (Refereed)
    Abstract [en]

    In shallow aquatic systems, benthic and pelagic primary producers typically compete for light and nutrients along opposing vertical supply axes: pelagic algae shade the benthic habitat; conversely, benthic algae intercept the nutrient flux from the sediment to the pelagic habitat. We present a general framework for analyzing such spatially asymmetric resource competition across habitat boundaries using a mechanistic, dynamical model. We visualize the mechanisms determining the outcome of these cross-habitat interactions using zero-net-growth isoclines, resource supply points, and resource consumption vectors. In extensive invasion analyses, we characterize the abiotic and competitive persistence boundaries of pelagic and benthic primary producers, which are set by environmental factors determining nutrient and light supply and are modified by resource use by the competitor in the respective other habitat. We note several qualitative differences between cross-habitat and "classical" within-habitat resource competition. First, coexistence of cross-habitat competitors is facilitated by, but does not require niche differentiation with respect to, the utilization of resources. Because each species has a competitive edge for the resource that is supplied from "its" side of the system, a competitor that is inferior in utilizing both resources can sometimes coexist with, or even exclude, a superior competitor. Second, increasing the external supply of one resource (the nutrient) may initially favor both competitors, until a breakpoint is reached where the benthic producer goes abruptly extinct. Finally, whether a given pair of cross-habitat competitors coexist or shows alternative states may depend on the environment. Specifically, benthic and pelagic algae may coexist at low nutrient and light supply but produce alternative states at high nutrient and light supply. Alternative states are, in turn, promoted by any algal trait combination that increases the spatial asymmetry in resource consumption, i.e., leads to a higher nutrient consumption in the benthic habitat and/or a higher light consumption in the pelagic habitat. In a first empirical application, we show that predictions from our model give a good fit to published data on benthic and pelagic primary production in temperate and arctic lakes spanning a broad range of nutrient environments.

  • 39.
    Jäger, Christoph G
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Diehl, Sebastian
    Emans, Maximilian
    Physical Determinants of Phytoplankton Production, Algal Stoichiometry, and Vertical Nutrient Fluxes2010In: The American naturalist, ISSN 1537-5323, Vol. 175, p. E91-E104Article in journal (Refereed)
    Abstract [en]

    Abstract: Most phytoplankters face opposing vertical gradients in light versus nutrient supplies but have limited capacities for vertical habitat choice. We therefore explored a dynamical model of negatively buoyant algae inhabiting a one-dimensional water column to ask how water column depth and turbulence constrain total (areal) phytoplankton biomass. We show that the population persistence boundaries in water column depth-turbulence space are set by sinking losses and light limitation but that nutrients are most limiting to total biomass in water columns that are neither too shallow or too weakly mixed (where sinking losses prevail) nor too deep and turbulent (where light limitation prevails). In shallow waters, the most strongly limiting process is nutrient influx to the bottom of the water column (e.g., from sediments). In deep waters, the most strongly limiting process is turbulent upward transport of nutrients to the photic zone. Consequently, the highest total biomasses are attained in turbulent waters at intermediate water column depths and in deep waters at intermediate turbulences. These patterns are insensitive to the assumption of fixed versus flexible algal carbon-to-nutrient stoichiometry, and they arise irrespective of whether the water column is a surface layer above a deep water compartment or has direct contact with sediments.

  • 40.
    Jäger, Christoph G.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Schmidt, Gertraud M.
    Influence of water-column depth and mixing on phytoplankton biomass, community composition, and nutrients2008In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 53, no 6, p. 2361-2373Article in journal (Refereed)
    Abstract [en]

    We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) andwater-column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a fieldenclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphoticdepth of only 3.7 m. All enclosures were placed in a well-mixed water bath to equalize temperature acrosstreatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, whichwas highest in the shallowest, least light-limited water columns where dissolved mineral phosphorus subsequentlybecame strongly limiting. As a consequence, the depth-averaged mineral phosphorus concentration increased andthe seston carbon (C) : phosphorous (P) ratio decreased with increasing water-column depth. Low turbulenceenclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixedenclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation ratesin the mixed enclosures over the 35-d experimental period. Low turbulence enclosures showed pronouncedvertical structure in water columns .4 m, where diversity was higher than in mixed enclosures, suggesting verticalniche partitioning. This interpretation is supported by a primary production assay, where phytoplanktonoriginating from different water depths in low-turbulence treatments had the relatively highest primaryproductivity when incubated at their respective depths of origin.

  • 41. Krivan, V.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Adaptive omnivory and species coexistence in tri-trophic food webs2005In: Theoretical Population Biology, Vol. 67, no 2, p. 85-99Article in journal (Refereed)
    Abstract [en]

    The commonness of omnivory in natural communities is puzzling, because simple dynamic models of tri-trophic systems with omnivory are prone to species extinction. In particular, the intermediate consumer is frequently excluded by the omnivore at high levels of enrichment. It has been suggested that adaptive foraging by the omnivore may facilitate coexistence, because the intermediate consumer should persist more easily if it is occasionally dropped from the omnivore's diet. We explore theoretically how species permanence in tri-trophic systems is affected if the omnivore forages adaptively according to the "diet rule", i.e., feeds on the less profitable of its two prey species only if the more profitable one is sufficiently rare. We show that, compared to systems where omnivory is fixed, adaptive omnivory may indeed facilitate 3-species persistence. Counter to intuition, however, facilitation of 3-species coexistence requires that the intermediate consumer is a more profitable prey than the basal resource. Consequently, adaptive omnivory does not facilitate persistence of the intermediate consumer but enlarges the persistence region of the omnivore towards parameter space where a fixed omnivore would be excluded by the intermediate consumer. Overall, the positive effect of adaptive omnivory on 3-species persistence is, however, small. Generally, whether omnivory is fixed or adaptive, 3-species permanence is most likely when profitability (= conversion efficiency into omnivores) is low for basal resources and high for intermediate consumers. (C) 2004 Elsevier Inc. All rights reserved.

  • 42. Kunz, T. J.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Phytoplankton, light and nutrients along a gradient of mixing depth: a field test of producer-resource theory2003In: Freshwater Biology, Vol. 48, no 6, p. 1050-1063Article in journal (Refereed)
    Abstract [en]

    1. Variation in depth of the mixed surface layer of temperate lakes should affect phytoplankton dynamics because, with increasing mixing depth, average light intensity in and specific sedimentation losses out of the mixed layer both decrease. 2. Our aim was to test a recent dynamic model which relates phytoplankton biomass and the availability of production-limiting resources (light and dissolved mineral nutrients) to mixing depth and nutrient supply from external sources. 3. During summer stratification we sampled the mixed layers of 30 dimictic, phosphorus-limited, oligo- to mesotrophic, mostly non-humic lakes north of the Alps. 4. The results agree well qualitatively with model expectations. Algal concentration in the mixed layer was negatively related to mixing depth or its surrogate log-transformed lake area. Light intensity at the bottom of the mixed layer decreased whereas the concentration of available, inorganic phosphorus increased with increasing mixing depth. Across all depths, higher total phosphorus content was accompanied by higher phytoplankton biomass, lower light availability, and higher inorganic phosphorus concentration. 5. Our data match the predicted shift with increasing mixing depth from predominantly nutrient limitation towards increased light limitation of algal biomass.

  • 43. Liess, A.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Effects of enrichment on protist abundances and bacterial composition in simple microbial communities2006In: Oikos, Vol. 114, no 1, p. 15-26Article in journal (Refereed)
    Abstract [en]

    We experimentally investigated effects of nutrient enrichment and trophic structure in a microbial food web consisting of mixed bacteria, two bacterivorous ciliates (Tetrahymena sp. and Colpidium sp.) and an omnivorous ciliate (Blepharisma sp.) feeding on both trophic levels. We assembled all possible food webs including one or more of the ciliate species and cross-classified them with four levels of enrichment of the bacterial medium. The qualitative outcome of food web interactions was independent of enrichment and always the same: Tetrahjrmena strongly depressed or excluded Colpidium, and Blepharisma strongly depressed or excluded both bacterivores. Consequently, in all sub-webs only the dominant ciliate species responded positively to enrichment. The total density of bacteria increased with enrichment irrespective of food web composition. In contrast, the response of singlecelled bacteria to enrichment depended on food web composition and was only weakly positive in most food webs with the omnivore. Enrichment had a positive effect on the relative success of (presumably more defended) bacterial aggregates. The outcome of interspecific interactions among ciliates could not be predicted from monoculture experiments and deviated from earlier experiments in which each bacterivore coexisted separately with the omnivore. As a potential explanation we suggest that changes in experimental protocol reduced spatial heterogeneity and increased attack rates. A simple, dynamical model shows that increased attack rates can indeed greatly decrease the upper limit and range of enrichment over which intermediate consumers can coexist with omnivores.

  • 44. Nisbet, R. M.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Wilson, W. G.
    Cooper, S. D.
    Donalson, D. D.
    Kratz, K.
    Primary-productivity gradients and short-term population dynamics in open systems1997In: Ecological Monographs, Vol. 67, no 4, p. 535-553Article in journal (Refereed)
    Abstract [en]

    We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer reproduction, and at the spatial scales typically employed in field experiments. These models incorporate flexible behavioral responses of organisms to their predators and resources in spatially heterogeneous environments that are open to immigration and emigration. The basic models include passive immigration at all trophic levels, producer growth rates and losses to grazer consumption, grazer emigration rate as a behavioral response to producer and predator densities, grazer losses to predator consumption, and predator emigration as a function of grazer density. We model this system as: (1) a set of ordinary differential equations (''well-mixed model''); (2) a set of partial differential equations describing a population of discrete grazers foraging on discrete patches of primary producers (''discrete-grazer model''); and (3) a set of simulation rules describing the movement and foraging of individual grazers and the growth of primary producers on discrete patches in explicit space (''individual-based model''). The ordinary differential-equation models produced similar results to individual-based models with well-mixed producers, and the discrete-grazer and individual-based models produced similar results when grazers possessed a long-term memory of patch reward rates. The well-mixed and discrete-grazer models thus represent specific, limiting cases of the general individual-based model. Multiple equilibria and sustained oscillations are possible but are less likely in the discrete-grazer and individual-based models than in the well-mixed model, because localized foraging of discrete grazers leads to the rapid development of spatial heterogeneity in producer biomass and, hence, to a decrease in overall primary production. All models predict that stable equilibrium densities of all trophic levels increase with enrichment, provided grazers increase their emigration rates as predator density increases. If increasing predator density leads to decreasing grazer-emigration rates, predator and grazer densities increase, but producer biomass may increase or decrease with enrichment. These results contrast with predictions from models that assume ideal free distributions of grazers and/or predators with respect to their resources. Our models also predict that densities at all trophic levels will increase with increasing producer immigration, and that producer density will decline with increasing grazer immigration and increase with increasing predator immigration. Our qualitative findings on enrichment are used to interpret an experiment dealing with the short-term dynamics of a stream community open to grazers and predators.

  • 45.
    Persson, L.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    MECHANISTIC INDIVIDUAL-BASED APPROACHES IN THE POPULATION COMMUNITY ECOLOGY OF FISH1990In: Annales Zoologici Fennici, Vol. 27, no 2, p. 165-182Article in journal (Refereed)
  • 46. Persson, L.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Johansson, L.
    Andersson, G.
    Hamrin, S. F.
    TROPHIC INTERACTIONS IN TEMPERATE LAKE ECOSYSTEMS - A TEST OF FOOD-CHAIN THEORY1992In: American Naturalist, Vol. 140, no 1, p. 59-84Article in journal (Refereed)
    Abstract [en]

    In a comparative study covering 11 temperate lake ecosystems of low to intermediate productivity, we tested the predictions of the food chain model developed by L. Oksanen et al. The pelagic habitat of the most unproductive lakes lacked secondary carnivores (piscivores), whereas secondary carnivores inhabited the more productive lakes in agreement with the assumption that food chain length is limited by the productivity of the system. The observed relationships between planktivores (primary carnivores), zooplankton (herbivores), and phytoplankton (primary producers) were in agreement with the predictions of the Oksanen et al. model for three- and four-trophic-link systems. Piscivore relationships were significant only when the biomass of piscivorous perch only was used as the independent variable (piscivorous perch amounted to greater-than-or-equal-to 97% of total piscivore biomass in five out of six four-link systems). The results suggest that gross patterns of food web dynamics in temperate lakes may be collapsed into food chain dynamics. However, as the higher trophic levels of most lakes were dominated by the same two species, an alternative hypothesis is that the simplicity of the food webs studied allowed food chain dynamics to approximate food web dynamics. The food chain model tested gave accurate predictions of trophic-level relationships within the rather narrow productivity range studied. However, previous data suggest that, due to size-structured interactions, secondary carnivores (piscivores) play a minor role in highly productive systems, a circumstance that may explain the postulated increase in mortality rate of zooplankton with increasing productivity. The observed increase in number of trophic levels with productivity does not necessarily imply a causal relationship between productivity and food chain length, as lake productivity is also correlated with structural complexity (in the form of submerged vegetation and an oxygenated hypolimnion). Habitat heterogeneity may thus influence food chain length, and in general it is likely that food chain length will depend on more than one environmental variable.

  • 47. Persson, L
    et al.
    Johansson, L
    Andersson, G
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hamrin, SF
    Density dependent interactions in lake ecosystems: whole lake perturbation experiments1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 66, no 2, p. 193-208Article in journal (Refereed)
    Abstract [en]

    Density dependent interactions between higher and lower trophic levels were studied in two consecutive whole lake experiments in a highly productive lake. In the first experiment, the zooplanktivorous fish species, roach (Rutilus rutilus) (primary carnivore), was reduced to 27% of its previous population size (53%) of biomass) by a selective rotenone treatment. In the second experiment, zander (Stizostedion lucioperca) (secondary carnivore), were stocked in the lake resulting in a reduction in the population size of roach to 51% of its previous population size (31% of biomass). In both experiments, seasonal average biomass of Daphnia cucullata) increased in the two years with the lowest roach biomasses. but no shift to larger zooplankton forms took place. In the years with increased Daphnia biomass, phytoplankton biomass and transparency showed an increased seasonal variation (measured as coefficient of variation). In contrast to manipulations in which planktivorous fish were totally removed, no effect on seasonal average phytoplankton biomass was observed. Due to the high reproductive capacity of roach, the system returned to previous conditions within a year or two after the perturbations. Under unperturbed conditions, the lake showed few indications of instability as suggested by the paradox of enrichment hypothesis. This lack of instability can. among other things, be related to high zooplanktivore predation pressure present in highly productive lakes preventing overexploitation of primary producers by grazers. In addition, the omnivorous feeding characteristics of roach (feeding on both zooplankton and algae/detritus) and interactions between the open water and the detritus nutrient pool are likely to increase the stability of the system. The two perturbation experiments plus a previous natural experiment (winter fish kill), which all were pulsed perturbations, provide no evidence for the presence of alternative stable states in highly productive lakes. Possibly, a sustained perturbation over several years may shift highly productive lakes from a dominance of phytoplankton production to a dominance of macrophyte production leading to an alternative state.

  • 48. Ptacnik, R.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Berger, S.
    Performance of sinking and nonsinking phytoplankton taxa in a gradient of mixing depths2003In: Limnology and Oceanography, Vol. 48, no 5, p. 1903-1912Article in journal (Refereed)
    Abstract [en]

    According to a recent dynamical model, the depth of a well-mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon-specific sinking velocity and decrease with increasing background turbidity. In two field-enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100-mum-screened phytoplankton community of a clear, unproductive, but silica-rich lake to a gradient of mixing depths (1.5-15 m) during 4-6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon-specific sinking velocity and negatively related to background turbidity.

  • 49. Roll, S. K.
    et al.
    Diehl, S.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Cooper, S. D.
    Effects of grazer immigration and nutrient enrichment on an open algae-grazer system2005In: Oikos, Vol. 108, no 2, p. 386-400Article in journal (Refereed)
    Abstract [en]

    After disturbance, recovery dynamics of local populations depend on arrival rates of immigrants and local growth conditions. We studied the effects of herbivore immigration rates and nutrient enrichment on the dynamics of grazing insect larvae, benthic microalgae, and filamentous macroalgae recovering from low local densities in an open stream system. The two types of algae approximate a trade-off between capabilities for growing at low resource levels and resisting herbivory. Many microalgae achieve relatively high growth rates at low nutrient levels but are vulnerable to grazers, whereas many macroalgae require high nutrient levels for growth but become increasingly defended with filament growth. We hypothesized that macroalgae should benefit more strongly than microalgae from increasing nutrient levels and decreasing grazer immigration rates, because both conditions increase macroalgal chances to grow into a size refuge from herbivory. We created a gradient of nutrient concentrations and manipulated drift immigration rates of macroinvertebrates. Macro- and microalgal biomass and the relative contribution of macroalgae to total algal biomass increased with increasing nutrient enrichment and decreased with increasing grazer immigration. Grazer densities responded positively to nutrient enrichment. The densities of large baetids responded positively to higher immigration rates of large baetids, whereas small baetids and chironomid larvae showed the opposite response. Per capita emigration of small baetids decreased with increasing algal biomass. The data suggest that large baetids negatively affected algal biomass and that small baetid and chironomid densities tracked resource levels set by nutrient enrichment and large baetids. Our experiments highlight the prospects of integrating disturbance with nutrient supply, immigration rates and local trophic interactions (determining recovery trajectories) into conceptual models of open system dynamics. We suggest that recovery trajectories towards micro- or macroalgal dominated states may depend on the spatial scale of disturbance relative to the movement ranges of migrating grazers and to nutrient supply.

  • 50. Sebastian, Patrizia
    et al.
    Stibor, Herwig
    Berger, Stella
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of water temperature and mixed layer depth on zooplankton body size2012In: Marine Biology, ISSN 0025-3162, E-ISSN 1432-1793, Vol. 159, no 11, p. 2431-2440Article in journal (Refereed)
    Abstract [en]

    Ecological consequences of global warming include shifts of species ranges toward higher altitudes and latitudes as well as temporal shifts in phenology and life-cycle events. Evidence is accumulating that increasing temperature is also linked to reduced body size of ectotherms. While temperature can act directly on body size, it may also act indirectly by affecting the timing of life-cycle events and the resulting population age and size structure, especially in seasonal environments. Population structure may, in turn, be influenced by temperature-driven changes in resource availability. In a field mesocosm experiment, we investigated how water temperature and mixed surface layer depth (a temperature-dependent determinant of light availability to phytoplankton) affected population dynamics, population age and size structure, and individual size at stage (size at first reproduction) of Daphnia hyalina during and after a phytoplankton spring bloom. Mixed layer depth was inversely related to the magnitudes of the phytoplankton spring bloom and the subsequent Daphnia peak, but had no effect on the body size of Daphnia. Conversely, temperature had no effects on abundance peaks but strongly affected the timing of these events. This resulted in at times positive, at other times negative, transient effects of temperature on mean body size, caused by asynchronous changes in population size structure in cold versus warm treatments. In contrast to mean body size, individual size at stage consistently decreased with increasing temperature. We suggest that size at stage could be used as an unbiased response parameter to temperature that is unaffected by transient, demographically driven changes in population size structure.

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