Spatial patterns of sediment phosphorus contents and release in a deep clearwater lake undergoing rapid eutrophication

Eutrophication symptoms are increasingly observed in remote clearwater lakes. A notable example is Lake Stechlin, a deep, endorheic lake in north-eastern Germany. Between 2010 and 2020, the total phosphorus (TP) concentration in the lake increased from ~15 to over 60 µg L^-1, accompanied by a gradual decline in hypolimnetic oxygen concentration. We determined TP concentrations in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment water interface at 54 sites in the lake to determine spatial patterns of sediment P and release dynamics. SRP fluxes strongly increased with depth across all three basins of the lake, despite considerable variation among sites. Littoral sediments played a significant role in the overall phosphorus (P) release, with higher average SRP fluxes observed in June (1.11 mg m²/day) compared to October (0.75 mg m²/day). In contrast, profundal sediments showed higher release rates in October, as anticipated. Notably, about 46% of the surface sediment area is located in water depths of less than 20 meters, underscoring the importance of littoral sediment zones. Importantly, SRP fluxes from profundal sediment depended less on absolute water depth than on the position of the sampling site relative to the oxycline, which differed greatly among lake basins. Overall, our findings point to considerable spatial variability of SRP sediment concentrations and fluxes to the water column, underscoring the importance of considering littoral sediments, which are often overlooked when assessing P release dynamics in lakes.