Alkalinity and nitrate dynamics reveal dominance of anammox in a hyper-turbid estuary

Abstract. Total alkalinity (TA) regulates the oceanic storage capacity of atmospheric CO₂. TA is produced along two general pathways, weathering reactions and anaerobic respiration of organic matter, e.g., by denitrification, the anaerobic reduction of nitrate (NO₃-) to elemental nitrogen (N₂). Anammox, is another anaerobic pathway, yields N₂ as its terminal product via comproportionation of ammonium (NH₄⁺) and nitrite (NO₂-); this is, however, without release of alkalinity as a byproduct. In order to investigate these two nitrate / nitrite respiration pathways and their resulting impact on TA generation, we sampled the highly turbid estuary of the Ems River, discharging into the North Sea in June 2020. We sampled a transect from the Wadden Sea to the upper tidal estuary, five vertical profiles during ebb tide, and fluid mud for incubation experiments in the hyper-turbid tidal river. The data reveal a strong increase of TA and DIC in the tidal river, where stable nitrate isotopes indicate water column denitrification as the dominant pathway. In the fluid mud of the tidal river, the TA data imply only low denitrification rates, with the majority of the N₂ being produced by anammox (> 90 %). The relative abundances of anammox and denitrification, respectively, thus exert a major control on the CO₂ storage capacity of adjacent coastal waters.