This report describe SMHls environmental monitoring in the open sea areas around Sweden, from the Skagerrak to the northem part of the Bothnian Sea A great part of the results from the project described here are also reported as a national Swedish contribution to the Baltic Monitoring Programme (BMP) of the Helsinki Commission (HELCOM). Physical, chemical and biological parameters are studied in water samples, zoobenthos is studied in sediment grab samples and samples of fish and mussels are collected for the determination of harmful substances in the Kattegat, the Sound (Öresund), the Baltic Proper and the Gulf of Bothnia.
The temperature of the sea surface layer was higher than normal most part of the year, just as in 1989. In the entire sea area the temperatures were 1- 3 °C above the long term mean value for the period 1979 - 1989. The oxygen conditions in the south eastern Kattegat once again were unfavorable in late summer and early autumn. However, despite the unusually calm autumn with only weak mixing of the water masses the oxygen concentrations increased again and the situation never was serious. Also in the Baltic Proper the bad oxygen situation in the deep water prevailed in several basins the whole year. No major inflow of oxygen rich water occurred to ventilate the deep basins. In the Bornholm Basin hydrogen sulphide was present under the halocline (with an extension into the Hanö Bight) the whole year with the exception of the August cruise. In the Gdansk Basin there is no longer any stratification and oxygen is mixed into the water from the atmosphere. This has improved the conditions in the bottom water. In the Eastern Gotland Basin hydrogen sulphide has been present in the bottom water for more than 13 years continuously. The Northem Central Basin, including the Landsort Deep, and the Western Central Basin were free from hydrogen sulphide the whole year. Nutrient conditions did not show any remarkable or unexpected changes during 1990. During the winter period phosphate and nitrate were present in about normal concentrations, which decreased to near detection limit during the production period in the spring and early summer and then increased again during autumn. The petroleum hydrocarbons showed during the spring roughly the same concentrations as during the previous years, whereas the autumn values were normal in the surface water only and all other values were high or very high. The reason for this has not been identified.