Vertical distribution and chemical character of sediment phosphorus in two shallow estuaries in the Baltic Sea

The vertical distribution of various phosphorus (P) forms and their relation to physico-chemical properties of estuary sediment material were studied to better understand the potential release and burial of P. Core samples were taken from two dissimilar estuaries in the Baltic Sea: one in the Archipelago Sea (AS) and one in the Gulf of Finland (GoF). The P reserves were characterized by a sequential extraction procedure including the analysis of simultaneously dissolved elements in two extraction steps. The sediment material was also analysed for particle size distribution and total elements. In addition, several environmental variables were determined. The occurrence of the various forms of P varied with sediment depth among different sites. Reductant soluble, iron (Fe) bound P was the most dynamically changing P form in the sediment, while P bound to other metal oxides and apatite-P were the most stable fractions. High sedimentation rate was a dominating factor for sediment P burial. In addition, the content of organic matter, the amount of erosion-transported sorption components, and the oxygen (O-2) conditions in the near-bottom water were important determinants of the behaviour of sediment P. The results indicate that, over the long term, both estuaries have acted as sinks for deposited P and restricted the transport of P to the AS and the open GoF, thereby partly alleviating the eutrophication process.