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.