Vertical distribution of microbiological and hydrographic-chemical parameters in different areas of the Baltic Sea

During 2 cruises with RV 'Poseidon in August 1982 and August 1986 in the Baltic Sea, a series of vertical profiles between the Fehmarn Belt and the northern Gulf of Bothnia was investigated. Among the bacteriological variables determined were total bacteria number, bacterial biomass, saprophyte count on different media, turnover rate and maximum uptake velocity of glucose, and, for some stations, extracellular enzyme activity. Highest bacteria numbers and biomass values were always found in the photic zone. In the colder waters below the mixed layer all microbiological values decrease. In the boundary layer of the central Gotland Sea, especially between the oxic and anoxic hydrogen sulfide-containing zone bacteriological values increase markedly. In the water layer above the sediment another increase of bacterial stock and activity measurements was occasionally determined. In the Gulf of Bothnia, where the whole water column is sufficiently aerated, only in bottom waters may an increase in bacterial values occur. Depending on the oxygen regime, bacterial nitrogen conversion is very different in the individual areas of the Baltic Sea. Comparison of nitrite and nitrate concentrations shows that nitrification in the Gotland Deep occurs down to the chernocline. Below this a narrow denitrification zone (about 10 m) was found in which nitrate almost completely disappears. In the Gulf of Bothnia, as a result of the favorable oxygen conditions, the n~triflcation zone extends down to the sea bottom