DISTRIBUTION PATTERNS OF DIFFERENT FORMS OF PHOSPHORUS IN SOME SURFICIAL SEDIMENTS OF THE BALTIC SEA

The lateral distribution patterns of different forms of phosphorus (P) in some Baltic Sea sediments were investigated by a sequential extraction procedure. The distribution and amount are mainly related to redox state, grain size, organic content and the chemical characteristics of the sediments. Recalculation from mol P per dry weight (d.w.) to mol P per unit area indicates that coarse sediments typical of transportation bottoms contain more total P than the finer sediments found in accumulation areas. This is applicable for almost all extracted P fractions. The extraction procedure shows that HCl-extractable P, often referred to as apatite-P, is the dominating fraction of almost all sediment types. Oxidized fine-grained sediments of the near-shore area and the archipelago are enriched in the hydrogen sulfide (H2S) extractable P fraction. However, also some transportation bottoms dominated by slightly coarser grain size, i.e. silt instead of clay, have incorporated large amounts of P. A rough estimate indicates that the overall importance of a redox-dependent release of P from the sediments is of secondary importance compared to the total annual load. A predictive calculation of the importance of a hypothetical oxidation of all accumulation bottoms in the Baltic proper indicates that the optimally adsorbed amount of P to these bottoms may represent about twice the annual load. However, this calculation represents an extreme potential uptake. A more realistic adsorption uptake is probably far less than two years' supply. The sorbing capacities of the archipelago and near-shore sediments are today of great importance for the net balance of P in comparison to that of the off-shore sediments. Furthermore, the study also implies that other incorporation mechanisms must occur to balance the total annual load entering the Baltic proper.