As an extreme consequence of declining oxygen concentrations in marine systems, coastal anoxic events characterized by an accumulation of toxic hydrogen sulfide (H₂S) have increased in occurrence over the past decades. H₂S is thought to mainly originate from the sediment and is generally contained within the anoxic water mass by microbial sulfide oxidation processes at the oxic-anoxic interface. While oxygen is an important oxidant here, recent studies point to the additional involvement of nitrate and nitrite. Aiming at a deeper understanding of pathways, interactions, and controls, we studied nitrogen and sulfur cycling in the seasonally anoxic Mariager Fjord, Denmark. H₂S accumulated from the oxic-anoxic interface at ~14 m to reach ~400 µM near the bottom at 26 m depth. Sulfate reduction was detected at rates of 0.02 – 0.12 µM d^-1 through the anoxic water column, corresponding to a depth-integrated rate of 0.6 mmol m^-2 d^-1 or approx. 25% of the estimated flux of H₂S to the oxic-anoxic interface. Thus, while rarely quantified, pelagic sulfate reduction appears to contribute substantially to H₂S accumulation in this system. Anoxic incubations with ¹⁵N-labeled nitrate demonstrated a high potential for nitrogen dependent sulfide oxidation with ~1^st-order dependence of nitrate reduction rates on H₂S concentrations up to 50 µM. N₂ was the main product of nitrite reduction, thus emphasizing H₂S as the primary driver of fixed nitrogen removal at the oxic-anoxic interface. Indeed, pelagic nitrogen removal rates may exceed benthic rates in the sediments surrounding the basin.

Primary Presenter: Bo Thamdrup, University of Southern Denmark (bot@biology.sdu.dk)

Authors:

Bo Thamdrup Thamdrup, University of Southern Denmark  (bot@biology.sdu.dk)

Laura Bristow, University of Gothenburg  (laura.bristow@gu.se)