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Ecosystem resistance to climate change: feeding and survival strategies in the face of global warming

Final Activity Report Summary - ENDURE (Ecosystem resistaNce to climate change: feeDing and sUrvival stRategies in the facE of global warming)

Climate change is a threat to ecosystems, both aquatic and terrestrial. Shallow aquatic systems apparently exist in one of two alternative states, both of which are resistant to change due to multiple buffering mechanisms. Much research has gone into elucidating under what conditions and how will these mechanisms be overcome as a result of climate change. The zooplankton is a key link in trophic cascades of shallow lakes, since they provide the food base of fish through planktivory, and the limit to phytoplankton crops, through herbivory. Shifts from a clear to a turbid state in shallow lakes typically involve an increase or decrease in their capacity to control algal biomass and therefore water transparency. This pivotal role of zooplankton could be affected by global warming, with cascading effects on algal dominance and lake ecosystem state.

Specifically, global warming may influence the survival and feeding strategies of the zooplankton in shallow lakes. Project ENDURE has examined zooplankton-related processes behind the switch to turbid conditions in shallow lakes occurring in response to global warming through the study of survival and feeding strategies of lake zooplankton. A combination of empirical and mechanistic approaches have been used, through a) inter-seasonal variations and time series analyses of contemporary data, b) novel analyses of zooplankton populations and their egg banks and c) in situ experiments with artificial plants.

Winter free-swimming zooplankton populations and the resting egg ('ephippia') bank act as inocula for populations developing in spring and summer. Weather-driven changes in the relative importance and magnitude of these winter survival strategies could thus influence the seasonality and size of the grazer populations and their impacts on phytoplankton crops and lake state. Winter is thought to be particularly sensitive to warming trends. The global climate warming trend reflected in warmer water temperatures in winter could increase the size of winter planktonic populations and reduce ephippial production. Little is known on the relative importance of potential physical and biological cues inducing cladoceran ephippia production, and therefore predictions of potential effects of the warming trend are fairly unreliable. To contribute to a better understanding of these cues, field data on the seasonality and magnitude of the ephippial peaks of the cladoceran species were collected in open-water and littoral areas of a small shallow lake in Denmark. Ephippial production was found to be concentrated at the end of the growing season (from mid-October), though the length of the production period varied both among years and in areas of the lake (pelagial or littoral), suggesting photoperiod is not the exclusive cue for ephippial production for these species. For some species, production was very intense coinciding with fish reproduction.

Analyses of a large database of lakes across Denmark on various biological and physical variables demonstrated a new mechanism for climate change effects on zooplankton populations and community structure, and thereby on lake state particularly during summer. By this mechanism, climate change may strongly impact the shallow lakes in northern latitudes, as grazers are not released from fish predation by winter conditions when winters become more mild and fish kills, less intense.

The combination of a fully replicated factorial experiment with detailed monitoring of macroinvertebrates, zooplankton, fish and food types (periphyton and planktonic algae), as well as background limnological parameters, has provided a unique opportunity for understanding how particularly fish determine lake state through their effects on their invertebrate prey in a complex plant-structured environment.