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This article focuses on the ecological role of benthic macrofauna on nutrient dynamics and benthic-pelagic coupling in the Baltic Sea with relation to eutrophication. Generally, benthic macrofaunal activities have large effects on sediment biogeochemistry and often with stimulatory effects on processes that counteract eutrophication, i.e., denitrification and increased phosphorus retention of the sediment. The degree of faunal impact on such processes varies depending on faunal density and functional group composition. The effect of macrofaunal activities on sediment nutrient dynamics can also result in a higher nitrogen : phosporus ratio of the sediments efflux compared with sediments without macrofauna. Increased internal nutrient loading during eutrophication-induced anoxia is suggested to be caused both by altered sediment biogeochemical processes and through reduced or lost bioturbating macrofauna and thereby a reduced stimulatory effect from their activities on natural purification processes of the Baltic Sea ecosystem. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]