The influences of historic lake trophy and mixing regime changes on long-term phosphorus fractions retention in sediments of deep, eutrophic lakes: a case study from Lake Burgäschi, Switzerland

Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short time scales. However, the effects of hypolimnetic redox conditions and eutrophication on long-term sediment P-fraction retention are not clear yet. In this study, we investigated the sediment profiles since the early 1900s from Lake Burgäschi, a deep, eutrophic lake on the Swiss Plateau. The changes of sediment P-fraction retention were assessed with respect to lake trophic evolution (sedimentary green-pigments proxy), hypolimnetic oxygenation regime (Fe / Mn ratio proxy), sediment geochemical characteristics, and lake restoration history. Results showed that long-term retention of total P and labile P-fractions in sediments was predominantly affected by autochthonous Fe and Mn preserved in anoxic sediments, which were controlled by past hypolimnetic redox conditions. By contrast, refractory HCl-P (Ca-P) fraction retention largely resulted from authigenic CaCO3-P precipitation and increased with higher eutrophic levels. The retention of total P and labile P fractions was considerably reduced in surface sediments from 1977–2017 when Lake Burgäschi had the highest eutrophic levels and a persistent anoxic hypolimnion. We attributed the phenomenon to reduced sediment P-binding capacity (Mn and Fe oxyhydroxides) under the eutrophication-induced anoxic hypolimnion and decreased water-P concentrations due to hypolimnetic withdrawal. Our study implies that in seasonally stratified deep lakes like Lake Burgäschi, hypolimnetic withdrawal of P-enriched water can effectively reduce P retention in sediments and potentials of sediment-P release (seen from low P availability after 1977). However, the restoration has not improved lake trophic state, similarly to the findings from lake limnological survey.