Sediment internal phosphorus loading before and after excavation of a shallow lake

Mankind is taking advantage of numerous services by small shallow lakes such as drinking water supply, irrigation, transportation, recreational function and aesthetic values; however, eutrophication drives many of them to a deteriorated state. Internal loading of phosphorus (P) may often be the main reason why shallow lakes are unresponsive to watershed nutrient management programs. In this case, removal of sediments enriched in P is a possible method to reduce the internal P loading.
In our study, we examined the changes in sediment P mobility and internal phosphorus loading after excavation of a 1-ha highly eutrophicated lake in Viljandi, Estonia. For that, we measured redox potential of the surficial sediments, sediment pore water P concentrations and calculated diffusive fluxes in deepest location before excavation in 2015 and in three locations (close to the inflow, close the outflow, and in the center of the lake) after excavation in 2017. Additionally, total phosphorus (TP), iron (Fe), and fractions of P were determined from the sediments before and after excavation.
The P fractionation analyses revealed an increase in P retention capacity after lake excavation, indicated by an increase in Fe/P ratio of the sediments. Moreover, we observed a decrease in the concentrations of potentially mobile forms of P, as labile-P and Fe-bound P at all sampling sites. However, we observed notable spatial differences. At the station that displayed the highest Fe/P ratio, the redox potential of the surficial sediments was highest and the pore water SRP concentrations had lowest value. The diffusive fluxes were found to be highest at a sampling site that displayed anoxia in the near-bottom water layer. The lake will be monitored also in 2018 for the changes in P mobility and water quality.