The influences of historic lake trophy and mixing regime changes on long-term phosphorus fraction retention in sediments of deep eutrophic lakes: a case study from Lake Burgaschi, 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 timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgaschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe/Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe/Mn-P fraction comprised similar to 50% of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca-P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca-P fraction became the primary P fraction in sediments (representing similar to 39% of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgaschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones.