Microscale distribution of oxygen and nitrate in sediments inhabited by Nereis diversicolor; spatial patterns and estimated reaction rates

Profiles of oxygen and nitrate were measured with microsensors in sediment surrounding burrow structures of the polychaete Nereis diversicolor in a shallow mudflat in Limfjorden, Denmark. Rates and spatial distribution of reactions involving oxygen and nitrate (oxygen consumption, nitrification and nitrate reduction) in the vicinity of burrow structures and surface sediment were calculated and identified from concentration profiles. The burrow walls of the natural N. diversicolor population increased the surface area of the sediment-water interface several-fold and introduced oxic microniches with high metabolic activity into otherwise anoxic sediment. Nitrification and nitrate reduction were spatially separated processes, with nitrification restricted to the 0.9 to 1.5 mm thick oxic zone and nitrate-reduction occurring as a strictly anaerobic process in the nitrate-containing layers next to the oxic zone. At a density of 2560 individuals m-2, burrow walls of N. diversicolor account for 12 to 40% of bulk sediment oxygen uptake and 50 to 77% and 58 to 82% of bulk nitrification and nitrate reduction, respectively. Thus, burrow structures of macro-infauna are potentially a major sediment compartment involved in nitrogen cycling and may promote loss of bioavailable nitrogen from the sediment. [KEYWORDS: NO3-/NO2- biosensor ; Nitrogen cycling ; Sediments ; Bioturbation ; Nereis diversicolor ; Burrow walls