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. In heterotrophic temperate estuaries, anaerobic respiration of organic matter, e.g., by denitrification, can be an important source of TA. Denitrification is the anaerobic reduction of nitrate (NO3-) to elemental nitrogen (N2). By contrast, anammox yields N2 as its terminal product via comproportionation of ammonium (NH4+) and nitrite (NO2-); however, this occurs without release of TA as a byproduct. In order to investigate these two nitrate and 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. During ebb tide, a transect was sampled from the Wadden Sea to the upper tidal estuary, where we additionally sampled fluid mud for incubation experiments and five vertical profiles in the hyper-turbid tidal river. The data reveal a strong increase of TA and dissolved inorganic carbon (DIC) in the tidal river, where stable nitrate isotopes indicate water column denitrification as the dominant pathway. However, in the fluid mud of the tidal river, the measured TA and the N2 incubation experiments 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.