Hypoxia and nitrogen processing in the Baltic Sea water column

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water-column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L-1 d(-1)) at the oxic-anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N-2 (up to 810 nmol N L-1 d(-1)) by tens of meters in depth. N-2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium-rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re-establish. Although removal rates can be high, conditions allowing such rates are likely short-lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification.