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Bottom-up and top-down effects of browning and warming on shallow lake food webs
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Austral Centre of Scientific Research, Argentina.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
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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: urn:nbn:se:umu:diva-133326OAI: oai:DiVA.org:umu-133326DiVA: diva2:1086979
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
In thesis
1. Effects of warming and browning on benthic and pelagic ecosystem components in shallow lakes
Open this publication in new window or tab >>Effects of warming and browning on benthic and pelagic ecosystem components in shallow lakes
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
benthic, pelagic, algae, consumers, browning, warming, shallow lakes, food web, dissolved organic matter
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-133327 (URN)978-91-7601-690-9 (ISBN)
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

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