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Even large inflows of oxygen-rich seawater to the Baltic Proper have in recent decades given only short-lived relief from oxygen deficiency below the halocline. We analyse long-term changes in oxygen deficiency, and calculate the "total oxygen debt" [Formula: see text]OD, the oxygen required to oxidize the hydrogen sulphide (H 2 S) and ammonium (NH 4 ) that builds up during stagnation periods. Since the early 1990s, oxygen below 65m has gradually decreased during successive stagnation periods, and the [Formula: see text]OD has increased, with NH 4 more important than previously recognised. After the major inflow in 2014, the Baltic Proper [Formula: see text]OD has reached its highest level so far. The gradual shift of the [Formula: see text]OD to shallower sub-halocline waters in the western and northern basins has increased the risk of periodic coastal hypoxia and export of hypoxic water to the Bothnian Sea. The potential for inflows large enough to more than eliminate the [Formula: see text]OD seems limited in the near term.

How oxygen deficiency in the Baltic Sea proper has spread and worsened: The role of ammonium and hydrogen sulphide