AME 32:121-135 (2003)  -  doi:10.3354/ame032121

ABSTRACT: Various freshwater, estuarine and coastal stations of the Southern Baltic Sea were comparatively studied to evaluate pelagic bacterial performance. Inner coastal waters (so-called bodden or lagoons) are highly productive systems and dominate the coast of the Southern Baltic Sea. Due to high nutrient loads up to the 1990s in combination with an enhanced primary production, increased amounts of particulate (POC) and dissolved organic carbon (DOC) accumulated in these waters. In the Darß-Zingst bodden chain, POC < 16 and DOC < 13 mg C l^-1, and C:N ratios of 9 to 11 in particulate matter were measured in winter and spring samples. Due to high POC concentrations, the average ratio of DOC:POC was 1.1:1, which is very low compared to other aquatic systems. Bacterial abundance and activities were rather high and reached 24 x 10⁶ ml^-1 and 18 μg C l^-1 h^-1, respectively. Although 2 of the 3 investigated freshwater systems were classified as eutrophic, the highest measured POC concentrations, bacterial abundance and production were much lower (1.6 mg C l^-1, 11 x 10⁶ bacteria ml^-1, 4.4 μg C l^-1 h^-1) than in the bodden. In contrast to that, the DOC load was as high as in these inner coastal waters (<12 mg C l^-1). The coastal stations of the Baltic Sea, classified as mesotrophic, were not severely loaded with organic matter and bacteria (POC < 0.8, DOC < 5.5 mg C l^-1, bacteria < 3x 10⁶ ml^-1). Bacterial production again was lower than at all other stations; however, levels did reach an exceptional 4.6 μg C l^-1 h^-1, which is comparable to values of the freshwater systems. Compared to the other investigated marine and freshwater systems, the bodden were heavily loaded with organic matter, especially particulate organic matter (POM). The origin of this material is assumed to be mainly autochthonous as it is known not to be transported by rivers into these estuaries. Although dissolved inorganic nitrogen (DIN) concentrations were high at least in winter, POM was of poor quality; this was reflected by high C:N ratios and a low contribution of phytoplankton carbon to POC. However, this is particularly surprising, because nitrogen should be readily available at all bodden sites by resuspension from the sediment caused by frequent winds in these very shallow systems of <2 m depth.

KEY WORDS: Organic matter · Bacterial production · Hydrolytic enzymes · Marine systems · Brackish systems · Freshwater systems