Land uplift effects on the phosphorus cycle of the Baltic Sea

Abstract

The isostatic land uplift after the latest glaciation period in northern Europe means that the descending wave base in the eutrophicated Baltic Sea continuously exposes new bottom areas to increasing wind and wave-induced erosion. Erosion adds considerable amounts of phosphorus (P) and clay particles to the water column. This study has used a dynamic mass-balance model to investigate how land uplift affects the whole P cycle in the five major subbasins of the Baltic Sea. The model uses a unitary set of variables and constants for all subbasins with the exception of measurable, basin-specific driving variables. Differences in P concentrations between the subbasins could be quite accurately quantified only when the land uplift gradient was used as a driving variable. The clarifying effect from clay particles was found to be a major reason why those subbasins with the most intensive land uplift rates were also the ones with the lowest P concentrations. Without using the land uplift gradient as a model input, concentration differences could not be quantitatively explained in a meaningful way. Furthermore, simulations showed that clay particle erosion from land uplift has a substantial impact on all major internal P fluxes of the Baltic Sea. At the turn of the millennium, one of the subbasins (the Bothnian Bay) was oligotrophic, whilst the other four major subbasins were mesotrophic. Without the clarifying effect from the clay particles added to the water column during erosion of the rising seafloor, all five major subbasins of the Baltic Sea would probably be substantially more eutrophic.