The role of sediments in nitrogen cycling in the larger Baltic Sea

Abstract: The eutrophication of coastal areas has become a widespread problem over the last decades. In the Baltic Sea, the input of nitrogen and phosphorus has increased by four and eight times, respectively, since the turn of the century, and this is considered the direct cause of the eutrophication of this ecosystem. Given that nitrogen is the main limiting nutrient for primary production in the Baltic, it is imperative to understand the dynamics of the cycling of this nutrient. Sediments play an important role in this cycle, and may act as either a sink or a source of nitrogen to the water column. This thesis has investigated the role of Baltic sediments in nitrogen cycling.Measurements of nutrient fluxes over the sediment-water interface have been performed in laboratory incubations of sediment cores. In addition, direct measurements of the bacterial denitrification process, which removes nitrogen from the aquatic ecosystem, have also been conducted in sediment cores. All measurements have been made with some spatial, temporal and ecosystem variation. Furthermore, a manipulative experiment investigating the effect of added algal material on sediment mineralisation processes has been carried out.It was seen that Baltic sediments are not an important source of nitrogen to the water column. The net release of inorganic nitrogen could support 0-7.4% of phytoplankton nitrogen demand. Net release of inorganic phosphorus was more variable, and could under certain conditions supply >100% of the phytoplankton phosphorus demand. Denitrification rates were comparatively low, mostly <30 (mol N m-2 h-1, and would remove around 30% of the total anthropogenic nitrogen input to the water column. It is suggested that diffusion of nitrate to the site of denitrification is the ultimate controlling factor of denitrification activity. Further, calculations showed that burial of nitrogen may be equally important as denitrification in nitrogen removal in the Baltic.