Oxygen and sulphur trends in a highly hypertrophic lake: a case study from Salbker South, Germany

Lakes that are supersaturated with nutrients often meet the definition of hypertrophy and draw scientific attention due to their distinctive biogeochemical dynamics. Salbker South, an urban lake located in Magdeburg, Germany, exemplifies such an environment. Since summer 2022, it has been the focus of a high-frequency monitoring that reported total phosphorus (TP) and chlorophyll a (chl a) levels as high as 1.4 mg L^⁻1 and 417 µg L^⁻1, respectively. TP and chl a contribute to substantial organic carbon (OC) accumulation and create a fragile ecosystem where biological communities are under threat from processes such as anoxia induced by OC-mineralisation during thermal stratification between May and September. Anoxic events involve both the hypolimnion, which becomes undersaturated in DO down to 0.0 mg L^-1within a week from the onset of thermal stratification, and the epilimnion where diurnal changes in DO span from 0.0 to > 20 mg L^-1. Additionally, the high sulphate (SO₄) levels in the lake (up to 1.46 mg L^-1) that are derived from the Zechstein Formation and hamper water electrical conductivity to 4 – 5 mS cm^-1 facilitate the production of hydrogen sulphide (H₂S). As a result, H₂S concentrations in the deeper waters have been recorded to exceed 8.2 mg L^-1. These dynamics position Salbker South as a natural H₂S generator. To mitigate these issues and reduce yearly cyanobacterial blooms, targeted restoration programs aimed to lower nutrient concentrations and stabilize lake biogeochemical balances are urgently needed. Our monitoring program that includes nearby groundwater wells, the Elbe River, and Lake Salbker North, is set to continue, aiming to establish this site as a hub for scientific innovation and interdisciplinary collaboration.