Diatom-inferred increase in limnetic phosphorus concentration and the associated changes in sedimentary phosphorus fractions in Valkjärvi, a lake in Kärkölä, Finland

A diatom-based total phosphorus (TP) model developed for southern Finland is used to infer past epilimnetic TP concentrations in Valkjärvi, a lake located in Kärkölä, Finland; and the results are compared with sedimentary phosphorus fractions. The diatom-based models accurately predicted epilimnetic TP concentrations as indicated by monitoring data available since 1979. Altogether, concentrations have increased from 15 ug P l–1 in the 1940s to the present level of ~65 ug P l–1. After an initial slow increase, the lake became eutrophicated rapidly in the 1960s as a result of municipal wastewater from the Kärkölä commune. These nutrient inputs resulted in increased sedimentary phosphorus concentrations. The NaOH-extracted phosphorus fraction in particular increased in the profundal sediments. The changes in sedimentary phosphorus nevertheless occurred later than the increase in diatom-inferred limnetic TP concentration.