Nitrogen cycling in aphotic coastal sandy sediments of the Baltic Sea

Benthic nitrogen (N) cycling in sandy sediments in the stratified aphotic coastal zone (> 15 m) of the Baltic Sea was investigated along a north–south environmental gradient of N loading, trophic status, coastal geomorphology and sediment permeability. The aim was to establish a more comprehensive view of the Baltic Sea coastal N filter, where N transformation processes remove (via denitrification and anaerobic ammonium oxidation) and retain (via dissimilatory nitrate reduction to ammonium) land-derived N and thereby reduce its availability to the open sea; so far these processes have not been quantified in the deeper, aphotic sandy sediments. The main results are that a) not all sandy sediments were permeable enough to experience advective pore-water flow – mass transport in non-permeable sands functions via diffusion and fauna-mediated fluxes only, which simplifies biogeochemical measurement design; b) N removal rates were affected by the availability of labile particulate organic matter as a source of labile organic carbon and N, resulting in higher removal rates in eutrophic than in oligotrophic conditions, as well as similar removal rates in non-permeable sands and muds when also the substrate availability was similar; c) seasonal N removal in the stratified aphotic coastal zone is largely driven by the hydrography-controlled development of bottom water temperature, and differs from the seasonal pattern observed in the mixed photic coastal zone; and d) the role of dissimilatory nitrate reduction to ammonium in the aphotic coastal sandy sediments of the Baltic Sea is presumably more important than previously anticipated. These results indicate that the sandy sediments in the aphotic coastal zone of the Baltic Sea have an important role in N removal and retention, and are thus an integral component of the Baltic coastal N filter. The results further show the strong influence of the local environment on N cycling rates, emphasizing the need for context dependent data analysis, particularly in a diverse coastal setting such as in the Baltic Sea.