The coastal nitrogen filter in highly dynamic permeable sediments of the southern Baltic Sea

Coastal zones are often described as effective filters for land-derived nutrient loads, referred to as the coastal filter, which applies in particular to bays and estuaries with higher water residence times. Open coastal zones are high energy environments in which the sediments are influenced by waves and currents, and residence time is short. Most open coastal zones are covered by permeable sandy sediments. Contrary to previous assumptions and despite their low organic matter content, permeable sandy sediments contribute to benthic nitrogen cycling and oxygen dynamics. However, there is a lack of understanding of how seasonality and physical processes govern denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in permeable sediments in shallow coastal zones. For this study, monthly field and laboratory experiments were conducted in an annual cycle, combining physical and biogeochemical measurements. Rates of denitrification and DNRA were measured with an adapted revised isotope pairing technique to simulate advective pore water flow during the incubations. Denitrification followed a seasonal cycle with higher N₂-production rates observed in autumn and winter compared to summer. The data suggest that the oxygen penetration depth, activity of benthic primary producers and oxygen respiration are more dependent on prevailing ambient conditions, e.g. wave action, rather than on the season. Our results highlight the largely unknown nitrogen removal potential of permeable sediments. They also demonstrate the importance of including currents, waves, winds and past storms in the analysis of the biogeochemistry in permeable sediments in order to obtain a realistic picture of the prevailing processes.