Denitrification in the water column of the central Baltic Sea

Removal of fixed nitrogen in the water column of the eastern Gotland Basin, central Baltic Sea, was studied during two cruises in September 2008 and August 2010. The water column was stratified with anoxic sulfidic bottom water meeting oxic nitrate containing water at the oxic-anoxic interface. Anammox was never detected whereas denitrification was found in all incubations from anoxic depths and occurred immediately below the oxic-anoxic interface. Sulfide (H ₂S+HS ^-+S ^2-) was in most cases the only electron donor for denitrification but, in contrast to previous findings, denitrification was in some situations driven by organic matter alone. Nitrous oxide (N ₂O) became an increasingly important product of denitrification with increasing sulfide concentration and was >80% of the total N gas formation at 10μM sulfide. The potential rates of denitrification measured in incubations at elevated NO3- or sulfide concentrations were converted to in situ rates using the measured water column concentrations of NO3- and sulfide and the actual measured relations between NO3- and sulfide concentrations and denitrification rates. In situ denitrification ranged from 0.24 to 15.9nMN ₂h ^-1. Assuming that these rates were valid throughout the anoxic NO3- containing zone, depth integrated in situ denitrification rates of 0.06-2.11mmol Nm ^-2d ^-1 were estimated. The thickness of this zone was generally 3-6m, which is probably what can be maintained through regular turbulent mixing induced by internal waves at the oxic-anoxic interface. However, layers of up to 55m thickness with low O ₂ water (<10μM) were observed which was probably the result of larger scale mixing. In such a layer nitrification may produce NO3- and once the O ₂ has been depleted denitrification will follow resulting in enormous rates per unit area. Even with an active denitrification layer of 3-6m thickness the pelagic denitrification per unit area clearly exceeded sediment denitrification rates elsewhere in the Baltic Sea. When extrapolated to the entire Baltic Proper (BP) denitrification in the water column was in the range of 132-547ktonNyr ^-1 and was thus at least as important as sediment denitrification which has recently been estimated to 191ktonNyr ^-1. With a total external N-input of 773ktonNyr ^-1 it is clear that denitrification plays a significant role in the N-budget of the BP.