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

摘要

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 Eckernforde 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. NHJ 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 (%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 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.