Significance of lacustrine groundwater discharge for the rapid eutrophication of formerly oligotrophic Lake Stechlin

Lake Stechlin was an oligotrophic lake but experienced rapid eutrophication in the last 12 years. The speed of the deterioration is surprising, especially since the lake is located in a nature reserve, its catchment is almost entirely forested, there is no agriculture and there are only two small settlements with a total of less than 1000 inhabitants. Undoubtedly, groundwater is a crucial component of the water and compound balances of Stechlin, as there are no surface inflows. However, if one considers the total amount of groundwater entering the lake and the maximum compound loads possible with it, groundwater alone cannot explain the rapid increase in P concentrations in the Stechlin. Thus, internal P cycling, i.e. the mobilization of previously imported P, is an important process in the biogeochemistry of the lake. However, even if it is true that most of the P is remobilized by internal processes, it can still originate originally from groundwater. And groundwater might be the decisive trigger of the eutrophication in recent years. We provide evidence that not only P import from groundwater is the trigger, but also changes in loads of compounds that are closely coupled to the P cycle, namely calcium, manganese, iron and sulphate. While there is some evidence of elevated concentrations in the aquifer below the two settlements, the main P import into the Stechlin probably originates from the eutrophic Lake Dagow, which drains into Lake Stechlin via the aquifer.