Iron from Lignite Mining Increases Phosphorus Fixation in Sediments, but Does Not Affect Trophic States of Lakes Along River Spree (Germany)

To mitigate lake eutrophication, phosphorus (P) availability can be managed by iron (Fe) amendments, which bind P in the water column and settle as Fe oxy-hydroxides. In the fluvial-lacustrine system Spree, Fe oxy-hydroxides enter lakes due to lignite mining in the Lusatian Area (NE Germany). We hypothesized that the amount of P that can be retained from the water column by sediments is positively correlated with their iron content. Column experiments were used to investigate uptake and release of P in the sediments under oxic and anoxic conditions in three downstream lakes (Lake Neuendorfer See, Lake Glower See, Lake Müggelsee) with decreasing mining influence and thus iron loads, and one nearby non-mining-affected lake (Lake Schwielochsee). In lakes interconnected by River Spree, the cumulative P uptake in sediments increased significantly with increasing sedimentary Fe concentrations under both oxic and anoxic conditions. Only the sediments of Lake Glower See had higher P uptake under anoxic than oxic conditions, most likely due to vivianite formation. The net P sedimentation was higher with higher Fe concentration and higher under oxic than anoxic conditions. However, the lakes are classified as eutrophic because although the sediments of the Spree lakes can store further P, this additional P uptake is of little relevance for the P budget in highly P-loaded lakes with short water residence times (10–100 d), as is typical for fluvial-lacustrine systems.