Rates and regulation of nitrogen cycling in seasonally hypoxic sediments during winter (Boknis Eck, SW Baltic Sea): Sensitivity to environmental variables

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 (O 2−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 (NH 4 +), nitrate (NO 3 −) and nitrite (NO 2 −) 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 NO 3 − (−0.35 mmol m −2 d −1 of NO 3 −), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO 3 − reduction to NO 2 − by denitrifying microorganisms. NH 4 + fluxes were high (1.74 mmol m −2 d −1 of NH 4 +), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO 2 − fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO 3 − + NO 2 − + NH 4 +) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH 4 +, thus reducing the capacity for nitrification of NH 4 +. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m −2 d −1 of N 2, 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 NO 3 − in the bottom water (NO 3 − BW). Higher O 2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N 2 production (%R amx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %R amx and water depth in the seminal study of Dalsgaard et al. (2005). 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.