January Inflow may not have been enough to flush out the Baltic Sea

The January Storm over Scandinavia, that significantly raised sea-levels in the Baltic Sea, caused an inflow of much needed saline water from the Atlantic, but may not have a significant clean-out effect on the stagnant bottom waters of the deep basins in the sea.

According to information provided to HELCOM by the Swedish Meteorological and Hydrological Institute (SMHI), measurements during 1 - 14 January 2005 indicate an inflow through the Sound (Öresund, between Sweden and Denmark) of about 35 cubic kilometres. This means that the total inflow, including the larger amount of water that was likely to have flowed through the Belt Sea, may have reached 140 cubic kilometres.

Scientists say that there is a possibility that this water could reach the bottom waters of the southwest Baltic Proper and may, in case it contains high levels of oxygen, have a temporary positive effect in this area.

Inflows of saline, oxygen-rich water are the only possibility to renew the deep water in the central basins of the Baltic Sea and to improve the oxygen situation there. Oxygen depletion leads to the extinction of benthic life, including fish, and the release of oxygen-consuming phosphorus from the sea bottom.

Deep water renewal processes in the Baltic Sea depend on specific meteorological circumstances, which force substantial amounts of seawater, enriched with salt and oxygen, from the Kattegat through the Danish Straits into the Western Baltic. From there, it slowly moves as a thin bottom layer into the central Baltic basins, replacing aged water masses there. To make this happen, easterly winds have to blow continuously for about 10 days to lower the Baltic fill factor, followed by a sudden turn to westerly gale winds, which again need to last for about 10 days or longer in order to cause the fill factor rising to its maximum.

Before about 1980, such events were relatively frequent and could be observed on average once a year. In the last two decades, however, they have become scarce, with the last two major inflows having taken place in 1993 and 2003. The major Baltic inflow in 2003 terminated a long-lasting stagnation period which lasted since 1995. It transported approximately 200 cubic kilometres of saline, cold and oxygenated water into the Baltic Sea and renewed most of the deep water. The 2003 inflow takes rank 25 in the list of the 97 major Baltic inflows since 1897. Unfortunately the effects of this clean-out did not last, and in the middle of 2004 the near-bottom water in the Bornholm and eastern Gotland Basin returned back to anoxic conditions.

Contacts

HELCOM Secretariat

Mr Juha-Markku Leppänen
Professional Secretary
Tel: +358 9 62202227
Fax: +358 9 6220 2239

Mr Nikolay Vlasov
Information Secretary
Tel: +358 9 6220 2235
Fax: +358 9 6220 2239