Deep-water oxygen conditions in the Bothnian Sea

The hydrography of the deep waters of the Bothnian Sea (BS) underwent considerable changes during the most recent two decades.Most importantly, slightly worsened deepwater oxygen (O 2 ) conditions followed a gradual increase in the water-column density gradient.Up to the present time, the lowest measured O 2 concentrations fell within the range 3.5-4.5 ml l -1 , 1.4 ml l -1 being used in this work as the upper limit for hypoxia.I investigated the long-term deep-water characteristics of the BS to determine how possible it is for an hypoxic event to occur there in the near future.It appears that there is only a remote probability of this, unless profound changes take place in the hydrographic regime of the BS and/or in the anthropogenic load to the BS.The key factor behind this conclusion is the hydrodynamic regime of the land Sea and the ridge formations around it.This area acts as a buffering/filtration mechanism that prevents the saline and O 2 -poor deep waters of the Northern Baltic Proper from entering the BS without considerable mixing and dilution.A discussion is presented of the consequences of this conclusion, taking into account the past adverse development in the Gulf of Finland, with its internal loading of phosphorus and its accelerated eutrophication.With respect to the Finnish coastal area of the BS, the stable O 2 conditions and the favourable hydrographic setup in the near-coastal area suggest that any O 2 problems will most probably be concentrated in the inner-archipelago areas.