Regeneration and burial of phosphorus along a lake chain in a complex boreal catchment

Phosphorus is transported laterally within river catchments due to weathering and erosion processes, but may also be retained on floodplains and in lake sediments. The balance between lateral transport and retention on the catchment scale is important in determining downstream impacts of phosphorus loading on water quality. In low-relief boreal environments with positive water balance, river catchments typically consist of complex lake chains connected by short lotic sections. The density of lakes enhances the potential for retention of phosphorus mobilized in upstream areas and thus protection of downstream water quality. However, the morphometry of individual lakes may impact upon retention capacity, through regulating sedimentation and redox conditions and thus also phosphorus regeneration.  Here we studied phosphorus retention in lake sediments in the Siuntionjoki river catchment in southern Finland. The 487 km2 catchment includes 65 lakes of at least one hectare in surface area, draining into the Gulf of Finland to the west of Helsinki. We monitored sedimentary phosphorus accumulation and release in 10 primary lakes along the axis of the main Siuntionjoki river during one annual cycle. In this contribution, we present first results of the project and discuss these in the context of known water quality variability in the catchment.