The origin of the hypertrophic state of a shallow boreal shield lake

The trophic development and external reasons for changes in the geochemical environment were evaluated for a period covering the last 200 years in the now hypertrophic Lake Köyliönjärvi, SW Finland. The geochemistry and chironomid remains indicate the activation of several interconnected mechanisms favoring phosphorus postdepositional mobility since the 1940s. The geochemical indicators include sedimentary biogenic silicon concentration, P fractions, Fe:P and Fe:Mn ratios, sulfides and CH4 convection, since 1961 also several water quality parameters. The most important external reasons for this development have probably been the lowering of the water level of the lake in 1938–40 and the following intensive use of P fertilizers on fields in the catchment area. The present great extent of summertime methane convection in the top sediments of Lake Köyliönjärvi highlights the importance of this P transfer mechanism. The results show that under favorable conditions the sedimentary geochemical record can be used to trace the past magnitude of P internal loading.