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

<p>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.</p>

Geochemical focusing and sequestration of manganese during eutrophication of Lake Stechlin (NE Germany)

Significant sedimentation of manganese (Mn) in form of manganese oxides (MnOx) and the subsequent formation of authigenic calcium-rich rhodochrosite (Mn(Ca)CO3) were observed in the seasonally stratified hard water Lake Stechlin in north-eastern Germany. This manganese enrichment was assumed to be associated with recent eutrophication of the formerly oligotrophic lake. The mechanisms and processes involved were examined by analysing: (i) short sediment cores obtained from seven locations along a depth transect ranging from 69.5 m (the deepest point) to 38 m; (ii) sediment traps located at 20 m and 60 m water depths; (iii) water column profiles; and (iv) porewater profiles at 69.5 m and 58 m depths. Sedimentary Mn enrichment was observed at water depths below 56 m and increased to more than 25 wt% at the deepest site. Between 2010 and 2017, Mn accumulation at the deepest site was 815 g Mn m−2. Transfer of Mn from the shallower towards the deepest parts of the lake was initiated by reductive dissolution of MnOx and diffusion of dissolved Mn from the sediment to the overlying water column. Manganese was then dissipated via turbulent mixing and subsequently oxidised to MnOx before being transported towards the deepest zone. Transformation of the redeposited MnOx to Mn(Ca)CO3 favoured the final burial of Mn. We show that eutrophication and the areal spreading of anoxic conditions may intensify diagenetic processes and cause the spatial redistribution of Mn as well as its effective burial. Contrary to many previous findings, we show that increases of Mn and Mn/Fe can also be used as indicators for increasing anoxic conditions in previously oligotrophic lakes.

Expert Knowledge Content Database Design Suggestions from Validation Aspect

Expert Knowledge Content Database (EKCD) contains small or medium amount of data but its data content is valuable, which can be determined by expert work. Expert work can be a complex measurement, estimation, or data integration. Database design and validation was analyzed in a case study with EKCD Eulakes – Phytoplankton database of Lake Stechlin. Different data quality metrics was calculated and evaluated, and the content based inconsistency was evaluated. In the conclusion a schema is given for helping the design and validation.