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Abstract: This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck – a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O2−BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH4 +), nitrate (NO3 −) and nitrite (NO2 −) were simulated using a benthic model that accounted for transport and biogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO3 − (−0.35 mmol m−2 d−1 of NO3 −), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO3 − reduction to NO2 − by denitrifying microorganisms. NH4 + fluxes were high (1.74 mmol m−2 d−1 of NH4 +), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO2 − fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO3 − + NO2 − + NH4 +) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH4 +, thus reducing the capacity for nitrification of NH4 +. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m−2 d−1 of N2, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO3 − in the bottom water (NO3 − BW). Higher O2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N2 production (%Ramx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %Ramx and water depth in the seminal study of . Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently. [Copyright &y& Elsevier]

Benthic fluxes and water column distributions of dissolved inorganic nitrogen (DIN) and total dissolved phosphate (PO 4 3 − ) were measured in situ at 7 sites across a redox gradient from oxic to anoxic bottom waters in the Eastern Gotland Basin (Baltic Sea). The study area was divided into the oxic zone (60 to ca. 80 m water depth, O 2 > 30 μM), the hypoxic transition zone (HTZ, ca. 80 to 120 m, O 2 < 30 μM) and the deep anoxic and sulfidic basin (> ca. 120 m). Sediments in the HTZ were covered by mats of vacuolated sulfur bacteria. Ammonium (NH 4 + ) fluxes in the deep basin and the HTZ were elevated at 0.6 mmol m − 2 d − 1 and 1 mmol m − 2 d − 1 , respectively. Nitrate (NO 3 − ) fluxes were directed into the sediment at all stations in the HTZ and were zero in the deep basin. PO 4 3 − release was highest in the HTZ at 0.23 mmol m − 2 d − 1 , with a further release of 0.2 mmol m − 2 d − 1 in the deep basin. Up-scaling the benthic fluxes to the Baltic Proper equals 109 kt yr − 1 of PO 4 3 − and 266 kt yr − 1 of DIN. This is eight- and two-fold higher than the total external load of P (14 kt yr − 1 ) and DIN (140 kt yr − 1 ) in 2006 (HELCOM 2009b). The HTZ makes an important contribution to the internal nutrient loading in the Baltic Proper, releasing 70% of P (76 kt yr − 1 ) and 75% of DIN (200 kt yr − 1 ) despite covering only 51% of area. [ABSTRACT FROM AUTHOR]