Projecting the future ecological state of lakes in Denmark in a 6 degree warming scenario

Lakes are highly sensitive to climate change, and climate warming is known to induce eutrophication symptoms in temperate lakes. In Denmark, climate is projected to cause increased precipitation in winter and increased air temperatures throughout the year by the end of the 21st century. Looking further into the future, the warming trend is projected to continue and likely reach a 6 degrees C increase around the 22nd century (relative to a baseline period of 1986-2005). In the present study, we evaluate the consequences of such extreme changes for temperate Danish lakes. We use a multifaceted modelling approach by combining an eco-hydrological model to estimate future water runoff and catchment nutrient exports with both mechanistic and empirical lake models, describing key biogeochemical indicators in lakes, in order to quantify the effects of future nutrient loads and air temperature on lake ecosystems. Our model projections for the future scenario suggest that annual water runoff will increase (46%), driving also increases in exports of nitrogen and phosphorus (13 and 64%, respectively). Both the mechanistic and empirical modelling approaches suggest that phytoplankton biomass will increase and that potentially toxin-producing cyanobacteria may become a dominant feature of the phytoplankton community from spring. Warming and increased nutrient loads also affect the food webs within the lakes in the direction of higher fish control of algae-grazing water fleas, further reinforcing eutrophication. To be able to mitigate these eutrophication effects, external nutrient loading to the lakes must be reduced considerably.