Pore water nutrient profiles and dynamics in soft bottoms of the northern Baltic Sea

Chemical profiles of nutrients at the sediment - water interface were measured in the northern Baltic Sea. A whole core squeezer technique capable of mm-scale resolution was used to obtain the vertical profiles of NO3-, NO2-, o-P, NH4+ and Si in the soft bottom sediments. The profiles were compared with nutrient flux and denitrification measurements. In the Gulf of Finland, the profiles revealed a marked chemical zonation in NO3- and NO2- distribution indicating strong potential of nitrification just under the sediment surface followed by a layer of denitrification down to a depth of 30 mm. Below the depth of 20 mm NO3- was usually absent, whereas other nutrients were increasing steadily in concentration. A distinct minimum of NO3- was observed at the sediment - water interface, suggesting NO3- uptake by a microbial biofilm and/or active denitrification at the suboxic microniches usually present in organic-rich sediments. At the deep stations in the Baltic Proper, the NO3- concentration in pore water, as well as denitrification, were very low. The concentrations of NH4+, o-P and Si were usually increasing steadily with depth.