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Effects of warming and browning on benthic and pelagic ecosystem components in shallow lakes
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.ORCID iD: 0000-0002-2327-9716
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The majority of lakes on Earth are shallow, unproductive and located at high latitudes. These lakes are experiencing big changes due to climate change, where two environmental drivers operate simultaneously, browning and warming. How they affect lake ecosystems is not well understood. Here, I addressed this issue by using a theoretical and an experimental approach. In particular, I generated model predictions and compared them with the results of a realistic large-scale experiment, where browning and warming were manipulated in a factorial design. In addition, model outcomes were compared with data from 12 unproductive lakes sampled along a gradient of browning. Another novelty of my thesis is that it integrates benthic and pelagic food web components in the model and experimental approaches. I found that browning affected the resources availability for benthic and pelagic producers in the model and in the experiment. With browning, benthic primary producers became increasingly light limited and declined, while pelagic producers became less nutrient limited and increased. Pelagic nutrient limitation was alleviated by two non-exclusive mechanisms. Browning directly enriched the water with nutrients, and browning indirectly increased the nutrient flowing from the sediment to the pelagic habitat via suppression of benthic producers. To tease apart these two mechanisms I applied structural equation modeling (SEM). The indirect evidence by SEM suggests that both mechanisms contributed equally to the pelagic nutrient concentration in the experiment. Interestingly, a model food web with only primary producers shows similar qualitative behavior as a food web with grazers and carnivores included. This happens because carnivorous fish exert strong top-down control in the more productive habitat, which relaxes grazing pressure on primary producers and increases resource limitation in the adjacent habitat. Biomass of benthic and pelagic consumers followed the same pattern as their resources. The lake data were largely congruent with model expectations and supported the findings of the experiment. Furthermore, the model also predicted a negative relationship between total phosphorus and both primary and fish production, which was observed across the 12 lakes. Warming effects were more complex. The model predicts that warming effects should depend on browning and are expected to be strongest in the more productive of the two (benthic and pelagic) habitats. For example, at low levels of browning the biomasses of benthic algae and fish are expected to decline with warming, which was observed in the experiment. In contrast, observed warming effects at high levels of browning deviated from model expectations. The mechanisms by which browning and warming interactively affect lake food webs are still poorly understood. This thesis offers a conceptual foundation for their further study through the integration of within- and between-habitat interactions.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2017. , 23 p.
Keyword [en]
benthic, pelagic, algae, consumers, browning, warming, shallow lakes, food web, dissolved organic matter
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-133327ISBN: 978-91-7601-690-9 (print)OAI: oai:DiVA.org:umu-133327DiVA: diva2:1087023
Public defence
2017-05-04, KBE301, Lilla Hörsalen (KBC), Umeå Universitet, Umeå, 09:30 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2011-3908Swedish Research Council, 621-2014-5238The Kempe FoundationsKnut and Alice Wallenberg FoundationEcosystem dynamics in the Baltic Sea in a changing climate perspective - ECOCHANGE
Available from: 2017-04-13 Created: 2017-04-05 Last updated: 2017-04-11Bibliographically approved
List of papers
1. Asymmetrical competition between aquatic primary producers in a warmer and browner world
Open this publication in new window or tab >>Asymmetrical competition between aquatic primary producers in a warmer and browner world
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2016 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 97, no 10, 2580-2592 p.Article in journal (Refereed) Published
Abstract [en]

In shallow lakes, pelagic and benthic producers engage in spatially asymmetrical resource competition. Pelagic producers intercept the flux of light to the benthic habitat and benthic producers intercept the flux of sediment-derived nutrients to the pelagic habitat. In boreal and subarctic regions, climate change is affecting this interaction both directly through warming and indirectly through increased loading with colored dissolved organic matter (cDOM) from the catchment (brownification). We use a dynamical ecosystem model to explore the consequences of these changing environmental conditions for lake primary production and compare model predictions with the results of an experiment in which we manipulated water temperature and cDOM supply in a 2x2 factorial design. The experiment was performed in field mesocosms large enough to harbor reproducing fish populations and was run over an entire growing season. In agreement with model predictions, benthic algal production and biomass declined and pelagic algal production and biomass increased with browning. Pelagic nutrient concentrations diverged over time between low and high cDOM treatments, suggesting that browning alleviated pelagic algal nutrient limitation by shading benthic competitors and preventing them from intercepting the release of nutrients from the sediment. Warming considerably reduced benthic and pelagic algal production as well as pelagic algalbiomass and total phosphorus. The warming results are only in partial accordance with model expectations, but can be explained by an indirectly inferred, positive response of macrophyte production (which was not included in the model) to warming. Our study suggests that lake ecosystem responses to climate change are mediated by cross-habitat feedbacks between benthic and pelagic producers.

Keyword
asymmetry, benthic, boreal, brownification, pelagic, resource competition, shallow lake, warming
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-127720 (URN)10.1002/ecy.1487 (DOI)000386088000007 ()27859128 (PubMedID)
Available from: 2016-12-15 Created: 2016-11-18 Last updated: 2017-04-05Bibliographically approved
2. Terrestrial organic matter reverses competition between aquatic primary producers by altering within-lake, cross-habitat resource fluxes
Open this publication in new window or tab >>Terrestrial organic matter reverses competition between aquatic primary producers by altering within-lake, cross-habitat resource fluxes
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Flows of energy and matter across habitat boundaries can be major determinants of the functioning of recipient ecosystems. It is currently debated whether terrestrial dissolved organic matter (tDOM) is a resource subsidy or a resource subtraction in recipient lakes. In a long-term field experiment, pelagic phosphorus concentration and aquatic primary production increased with increasing tDOM input, suggesting that tDOM acted primarily as a direct nutrient subsidy. Piecewise structural equation modeling supports, however, an approximately equally important role for a second mechanism: colored tDOM acted also as a resource subtraction by shading benthic algae, preventing them from intercepting nutrients released across the sediment-water interface. Inhibition of benthic algae by colored tDOM thus indirectly promoted pelagic algae and whole-ecosystem primary production. We conclude that cross-ecosystem terrestrial DOM inputs can modify light and nutrient flows between aquatic habitats and alter the outcome of resource competition between benthic and pelagic producers. These results are particularly relevant for shallow northern lakes, which are facing increased tDOM runoff with climate change.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-133325 (URN)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, 621-2011-3908Swedish Research Council, 621-2014-5238Ecosystem dynamics in the Baltic Sea in a changing climate perspective - ECOCHANGEThe Kempe Foundations
Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2017-04-11
3. Bottom-up and top-down effects of browning and warming on shallow lake food webs
Open this publication in new window or tab >>Bottom-up and top-down effects of browning and warming on shallow lake food webs
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The productivity and trophic structure of aquatic ecosystems is the result of an interplay between bottom-up and top-down forces that operate both within and across the benthic and pelagic compartments of lake food webs. Contemporary and projected climate changes urge the question how this interplay will be affected by increasing inputs of terrestrial derived, dissolved organic matter (‘browning’) and warming. We addressed this issue by exploring how browning and warming affect the behavior of a relatively simple, conceptual model of a shallow lake food web that is compartmentalized into, dynamically coupled, benthic and pelagic components (abiotic resources, primary producers, grazers, and carnivores). We compared model expectations with the results of a factorial manipulation of browning and warming in a replicated, large-scale field experiment. Both the model and the experiment suggest that browning affects the food web from the bottom-up by reducing light supply to the benthic habitat and increasing nutrient supply to the pelagic habitat, with concomitant decreases of benthic and increases of pelagic primary and secondary production. The model also predicts that warming effects should primarily operate via relaxed top-down control by top consumers in the more productive of the two habitats. The latter was only partially supported by the experimental data, possibly because the model still lacks one or two important trophic links, such as the one from pelagic producers to benthic deposit feeders. We propose that our coupled benthic-pelagic food web model provides a useful conceptual starting point for future theoretical and empirical studies of the impacts of environmental changes on shallow lakes.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-133326 (URN)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, 621-2011-3908Swedish Research Council, 621-2014-5238Ecosystem dynamics in the Baltic Sea in a changing climate perspective - ECOCHANGEThe Kempe Foundations
Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2017-04-11
4. Resource and consumer control of cross-habitat trophic interactions in shallow lakes
Open this publication in new window or tab >>Resource and consumer control of cross-habitat trophic interactions in shallow lakes
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(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-111297 (URN)
Available from: 2015-11-12 Created: 2015-11-12 Last updated: 2017-04-05

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