Benthic release of phosphorus and its relation to environmental conditions in the estuarial Gulf of Finland, Baltic Sea, in the early 2000s

Abstract

After the mid-1990s the phosphorus concentrations and budget of the Gulf of Finland have been strongly affected by the enhanced benthic P input caused by reduced conditions at the sediment-water interface. Especially in late summers and autumns of 1996 and 2001 the total areas of reduced bottom sediments were large enough to affect the P concentrations of the whole gulf. The total benthic release of phosphate-P, on an annual level, can be several times higher than the bioavailable P load from the catchment of the gulf. The area of reduced surface sediments varies considerably both spatially and from year to year, especially in the relatively shallow eastern gulf. The intensity of the internal P loading is greatly controlled by vertical stratification, bottom topography , organic matter content of the sediment surface layer, and the abundance of benthic fauna. Evidently the sediment efflux can strongly affect the annual P balance and dynamics of the Gulf of Finland and counteract the decreases in the external P loading. This is verified both by inter-annual concentration changes and experimental results. The only way to reduce the benthic release of nutrients is to decrease primary production and the succeeding sedimentation of organic matter via further cutting external nutrient inputs both directly to the Gulf of Finland and to the Baltic Proper, which is an important source of nutrients for the gulf.