Biodiversity and tipping points in future freshwater ecosystems (LIMNOTIP)

The accelerating loss of global biodiversity has affected species in all biomes and ecosystem types. Eutrophication is and continues to be a major and severe environmental threat both within and outside Europe, which causes abrupt regime shifts, i.e. systems that reach "tipping points" and change from clear-water states to turbid conditions with dense algal blooms. Reaching such tipping points generally results in a dramatic decline in biodiversity. Using algae-zooplankton feeding experiments, we will investigate how changes in algal biodiversity along a productivityemperature gradient affect the amount of essential dietary nutrients (including lipids and their omega- 3 fatty acids) available for higher trophic levels. We hypothesize that, a) algal biodiversity increases with increasing phosphorus (P; limiting nutrient) concentrations and temperature, but decreases once P concentrations and temperature keep increasing (identification of P- and temperature-induced tipping points), and, b) increasing algal biodiversity initially increases the amount of omega-3 fatty acids per unit biomass, but levels off with further biodiversity increase. These tests will enable us to assess the `tipping point` at which the biochemical, dietary quality of algal biodiversity decreases. As a consequence, we predict that reduced algal biodiversity will lead to lower nutritional values of e.g. crustaceans, which constitute major dietary energy for fish. Hence, changes in algal biodiversity due to climate change may affect processes determining the resilience and induce tipping points for biodiversity and social-ecological systems.

The rationale to launch LIMNOTIP as an international research project was that despite freshwater ecosystems constitute a tremendous resource of great economic, scientific, cultural and recreational value for human society, they are heavily affected by human activities throughout Europe. In shallow lakes, one of the most important effects is abrupt regime shifts i.e. where lakes reach a tipping point and change from a clear-water state with lush stands of submerged macrophytes to turbid conditions with dense algal blooms. Reaching such a tipping point generally results in a dramatic decline in biodiversity and, further, the value of the lakes as providers of goods and services decreases. Knowledge about early warning signals before a tipping point will be of immense value in the development of tools for proper restoration and sustainable management. However, it was as yet not clear how deep, oligotrophic lakes, typical for the European Alps, will be affected by climate change? Our part of LIMNOTIP in Austria has thus focused on predicting future effects of climate warming and other large-scale environmental perturbations, such as increased run-off of humic substances from the catchments (brownification). We have addressed those issues in experimental scenarios and performed long-term mesocosm studies and showed that increases in water temperature caused smaller and faster-growing plankton to develop, whereas additional browning of water had very little effect on plankton. Overall, we conclude that the planktonic biodiversity decreased with increasing water temperature and that the nutritional quality of plankton decreased concurrently.