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

Symptoms of eutrophication are increasingly evident in remote clearwater lakes. To identify the sources and dynamics of phosphorus release, we measured total phosphorus (TP) in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment–water interface at 54 locations in a deep temperate lake. Once renowned for its clear waters, Lake Stechlin has experienced a fourfold increase in water column TP over the past decade. SRP fluxes from sediments generally increased with water depth across all three lake basins, although there were significant variations in SRP concentrations, up to threefold, among sampling locations at the same depth. Notably, the lake´s total mean SRP flux in June (1.12 mg m^-2 day^-1) was higher than that determined in October (0.74 mg m^-2 day^-1). This result can be attributed to the substantial contribution (about 31 % of total SRP release) of shallower sediments (0-20 m), which are not affected by by seasonal anoxia. Our findings highlight a notable spatial variability of SRP fluxes and underscore the importance of considering often overlooked shallow sediments when assessing P dynamics in lakes.