The relevance of groundwater-lake interactions for the rapid eutrophication of Lake Stechlin

Until recently, Lake Stechlin was one of the few oligotrophic lakes in North-Eastern Germany. Lake Stechlin is located in a nature reserve and its catchment is nearly completely forested, there is no agriculture and only one small settlement. About 10 years ago there were the first indications in the lake’s hypolimnion for changes of the trophy. In the last 3 years the lake is experiencing a rapid eutrophication and phosphorus (P) concentrations quadrupled compared to the concentrations 10 years ago. It is generally agreed that the origin of this P is internal P cycling which is a self-reinforcing process. However, the trigger that started the intense internal P cycling is still unknown. There are several different hypotheses and we focused on investigating the role of groundwater for the eutrophication of Lake Stechlin. Groundwater is a crucial component of the water balance of Lake Stechlin because there are basically no surface inflows and outflows, i.e. besides precipitation and evaporation, both lacustrine groundwater discharge and infiltration of lake water into the aquifer are the only other relevant terms of the water balance. Anthropogenic and climate change-induced alterations in groundwater inflow and outflow might have triggered the rapid eutrophication by different processes and we present a conceptual model of the involved processes. Main findings are (1) At a few locations we measured P concentration in the aquifer which were up to two orders of magnitude above the P concentrations of the lake water. (2) Due to several years of low precipitation in a row, the volume of lacustrine groundwater discharge decreased and with that the input of important P binding agents decreased, thus influencing the lake's internal P cycling. (3) Warmer average annual temperatures increase evaporation and simultaneously lead to a concentration of phosphorus in the lake. Local reversals of groundwater flow directions could also prevent lake water and with it P from leaving the lake. Thus, groundwater might be an important factor for the degradation of Lake Stechlin.