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Lower bacterial production and phytoplankton edibility reduces crustacean zooplankton biomass at low light
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Institutionen för vatten och miljö, Sveriges Lantbruksuniversitet, Uppsala.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Anthropogenic changes in the nitrogen (N), phosphorus (P), and carbon (C) cycles have altered nutrient concentrations and the light climate in freshwaters globally. These factors influence phytoplankton (PPr) and bacterial production (BP), which constitute the basal energy resource for higher trophic levels in the pelagic zone of lakes. The light:nutrient hypothesis (LNH) predicts that although basal production will decrease at low light, seston C:nutrient ratios also decrease, thus increasing food quality for crustacean zooplankton, which tend to have relatively fixed C:nutrient ratios. We tested the LNH in a mesocosm experiment by examining the effects of N, P and C additions and shading on PPr, BP, seston C:nutrient ratios and zooplankton biomass in an oligotrophic clear water lake. We found that zooplankton biomass was strongly reduced in shaded treatments. Although PPr was unaffected by shading, BP decreased with shading. Bacteria can be an important energy and P source for zooplankton when mobilized through intermediate trophic levels, and correlations between BP, bacterial biomass, ciliates and zooplankton support this. Seston C:nutrient ratios were not affected by shading, possibly due to a high abundance of mixotrophic phytoplankton across treatments. Shading shifted the phytoplankton community towards low light adapted, but potentially less edible phytoplankton species, such as colony forming Dinobryon (Chrysophyta) and gymnoid (Dinoflagellata) taxa, which were negatively correlated with zooplankton biomass. Thus, the LNH may be inadequate when predicting changes in crustacean zooplankton biomass in response to light and nutrients in oligotrophic systems, where Daphnia is rare and mixotrophic phytoplankton are abundant.

URN: urn:nbn:se:umu:diva-43466OAI: diva2:414056
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved
In thesis
1. Productivity and carbon transfer in pelagic food webs in response to carbon, nutrients and light
Open this publication in new window or tab >>Productivity and carbon transfer in pelagic food webs in response to carbon, nutrients and light
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Some of the major problems we face today are human induced changes to the nitrogen (N), phosphorus (P) and carbon (C) cycles. Predicted increases in rainfall and temperature due to climate change, may also increase dissolved organic matter (DOM) inflows to freshwater ecosystems in the boreal zone. N, P, C and light, are essential resources that most often limit phytoplankton (PPr) and bacterial production (BP) in the pelagic zone of lakes. PPr and BP not only constitute the total basal C resource for the pelagic aquatic food web, but also influence ecosystem function and biogeochemical cycles.

In this thesis I studied how N, P, C and light affect the relative and absolute rates of PPr and BP, along a wide latitudinal and trophic gradient using published data, and in two in situ mesocosm experiments in a clear water oligotrophic lake. In the experiments I manipulated bottom-up drivers of production and top-down predation to examine how these factors interact to affect pelagic food web structure and function.

The most important predictors of PPr globally (Paper I) were latitude, TN, and lake shape. Latitude alone explained the most variation in areal (50%) and volumetric (40%) PPr. In terms of nutrients PPr was primarily N-limited and BP was P-limited. Therefore bacteria and phytoplankton were not directly competing for nutrients. BP:PPr was mostly driven by PPr, therefore light, N, temperature and other factors affecting PPr controlled this ratio. PPr was positively correlated with temperature, but not BP, consequently, higher temperatures may reduce BP:PPr and hence the amount of energy mobilised through the microbial food web on a global scale.

In papers II and III interaction effects were found between C-additions and top-down predation by young-of-the-year (YOY) perch. Selective predation by fish on copepods influenced the fate of labile C-addition, as rotifer biomass increased with C-addition, but only when fish were absent. Interaction effects between these top-down and bottom-up drivers were evident in middle of the food web, which is seldom examined in this type of study. Although the energy pathway from bacteria to higher consumers is generally longer than from phytoplankton to higher trophic levels, increased BP still stimulated the biomass of rotifers, calanoid copepods and YOY fish. However, this appeared to be mediated by intermediate bacterial grazers such as flagellates and ciliates.

Light was an important driver of crustacean zooplankton biomass (paper IV), but the light:nutrient hypothesis was inadequate to predict the mechanisms behind the decrease in zooplankton biomass at low light. Instead, it appeared that reduced edibility of the phytoplankton community under low light conditions and reduced BP most strongly affected zooplankton biomass. Thus, the LNH may not apply in oligotrophic lakes where PPr is primarily N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are abundant.

N, P, C and light manipulations have very different effects on different parts of the pelagic food web. They influence the relative rates of PPr and BP, affect phytoplankton community composition, alter the biomass of higher trophic levels and change pathways of energy transfer through the pelagic food web. This thesis adds valuable information as to how major changes in these resources will affect food web structure and function under different environmental conditions and future climate scenarios.

Place, publisher, year, edition, pages
Umeå: Institutionen för ekologi, miljö och geovetenskap, Umeå universitet, 2011. 33 p.
bacterial production, phytoplankton production, mesocosms, food webs, carbon, nutrients, light
National Category
Research subject
urn:nbn:se:umu:diva-43467 (URN)978-91-7459-191-0 (ISBN)
Public defence
2011-05-26, KBC-huset, Stora Hörsalen, Umeå Universitet, Umeå, 10:00 (English)
Lake ecosystem response to environmental change
Available from: 2011-05-05 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved

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Faithfull, CarolynBergström, Ann-Kristin
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