Regime shifts in freshwater ecosystems exposed to multiple stressors by increasing temperature, fertilizers and pesticides

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Shallow freshwater habitats provide vital ecosystem functions but are threatened by multiple stressors acting at different spatial and temporal scales. While a response to global climate change might be gradual, abrupt changes are possible when critical thresholds by additional effects of local stressors are exceeded. The difficulty in analysing effects of multiple stressors is to account for complexity, as stressors may act additive, synergistic or antagonistic. CLIMSHIFT aims for a mechanistic understanding of stressor interactions acting on shallow aquatic systems, which are especially vulnerable to climate warming and agricultural run-off due to their high surface to water ratios, large riparian interface and groundwater connectivity. The complex interactions between different functional groups of benthic and pelagic primary producers and associated consumers result in the occurrence of stable regimes, and multiple stressors may trigger non-linear shifts between those regimes, with far-reaching effects on crucial ecosystem processes and functions. Our main hypothesis is that increased temperature will enhance negative effects of agricultural run-off, containing nitrates, organic pesticides and copper. Submerged plants, periphyton and phytoplankton as primary producers will be combined with the second trophic level, consumers, composed of the snail Lymnaea, consuming periphyton and plants, and benthic and pelagic phytoplankton filter-feeders, Dreissena and Daphnia. We will apply different exposure scenarios at two different spatial scales to understand effects at the individual, community and ecosystem level. We start with investigations in microcosms at laboratory scale, and will upscale to larger, outdoor mesocosms. Throughout the project, we will use modelling to simulate potential outcomes and critical thresholds, and predict stressor interactions. Model development will be conducted in close collaboration with all work packages to identify the most appropriate modelling approach, integrate empirical results, link different spatial and temporal scales, generalize and extrapolate results, and develop and test hypotheses. We expect that combined stressors will lead to sudden shifts in community structure in these highly coupled systems. Submerged macrophytes are expected to be replaced by phytoplankton or benthic algae, with major consequences for important ecosystem functions. The strength of our proposal is that common ecotoxicological stress indicators such as growth and biomarkers of the different organisms will be combined with functional community/ecosystem approaches looking at ecosystem metabolism and dynamics. The combined expertise of 5 laboratories with complementary expertise and all necessary facilities will ensure the project feasibility. The outcome of our project can help defining safe operating spaces for a sustainable agriculture and management of shallow aquatic systems in a changing world.