Hydrodynamics and nutrient distribution in bottom sediments of the Archipelago Sea, southwestern Finland

The distribution of nutrients (i.e. water contents, organic carbon, nitrogen and phosphate concentrations) was investigated in the upper 20 cm of coastal marine sediments off southwestern Finland. Bottom sediments were sampled along a transect from the Aurajoki river mouth to the open Archipelago Sea. Towards the Archipelago Sea, sediments showed increasing water and organic carbon contents and decreasing C/N ratios. Phosphate values in the sediments did not show any significant variation along the transect, but increased towards the surface. Most sediments were classified as mud. Resuspension processes can be expected to be dominant in the harbour. Organic carbon contents increase as a result of lateral transport of the lighter fraction towards the more distal stations. Sedimentation is greatest in the small but deep basins in the more distal regions of the transect. The decrease in C/N ratios may be due to the reduced importance of denitrification during the decay of organic matter. Oxygen depletion increases in the sediments because of higher organic carbon contents at the outer stations. Phosphate values increase towards the sediment surface, possibly indicating upward diffusion of phosphorus. Phosphorus depleted surface waters of the Baltic Sea may show greater influence with increasing distance from the harbour. This may be compensated for the higher proportion of fine material which can bind phosphate at the more distal sampling stations.