Trends in Total Phosphorus Loadings and Concentrations Regarding Surface Waters of the Baltic Sea, 1968-2007

The Baltic Sea, a large estuarine sea in northern Europe, has for many decades displayed obvious signs of anthropogenic eutrophication. This study relates long-term trends in total phosphorus (TP) loadings and TP concentrations in surface waters regarding five major sub-basins of the Baltic Sea; the Bothnian Bay, the Bothnian Sea, the Baltic Proper, the Gulf of Finland and the Gulf of Riga. The longest time series ever published on these variables was developed and used for this purpose. TP loadings to these waters have decreased greatly and significantly since the 1980s. However, TP concentrations have only decreased slightly in one of the sub-basins while concentrations have increased in the other four basins. Four possible hypotheses about the weak connection between TP loadings and concentrations are 1) increasing TP concentration is a delayed effect from many decades of intensive TP loading, 2) fewer saline inflows of high intensity have decreased sedimentation rates and increased TP concentrations, 3) fewer oxygen-rich saline inflows of high intensity have increased TP diffusion from deep sediments to the whole water column and 4) less ice in the winter has increased the erosion and resuspension of shallow sediments and increased TP concentrations.