We applied different types of fluorescent markers to natural bacterioplankton from different aquatic systems to investigate microscopically the percentage of viable bacteria. To characterise viable bacteria, cell-specific respiration was measured by cyanoditolyltetrazolium chloride (CTC) reduction. Membrane integrity was investigated with 3 'dead cell' stains (SYTOX ® Green, propidium iodide and ethidium homodimer-2). Cellular enzyme activity was detected by artificial substrate analogs with a high cell retention (CellTracker™ Green CMFDA for cellular esterase and 7-amino-4-chloromethylcoumarin L-leucine amide, hydrochloride [CMAC-Leu] for cellular peptidase). The percentage of impermeable, i.e. morphologically intact, cells accounted for 22 to 81% of the total cell number at all locations. Although up to 48% of all bacteria were respiring, they averaged between 10 and 14% in freshwater, estuarine waters and in the Baltic Sea. The portion of esterase-positive cells correlated significantly with the concentrations of dissolved (DOC) and particulate organic carbon (POC) as well as with chlorophyll a (chl a) content. Cellular esterase was shown by this labelling technique in only 9% of freshwater, 12% of estuarine and 5% of Baltic Sea bacteria, . The percentages of bacteria with cellular peptidase were even lower with 6, 5 and 3%, respectively. The different amounts of intact and respiring bacteria as well as those with cellular hydrolytic enzyme activities require not only correct operational definitions of active and viable bacteria, but also the appropriate choice of fluorescent markers regarding the goals of investigation. Fluorescent labels for cellular hydrolytic enzymes will also provide a new tool to localise active cells in aggregates or on sediment particles, where, besides the respiration of organic carbon, hydrolysis of organic substances is an important conversion process.

The implementation of the European Water Framework Directive (WFD) requires the development of ecologically-based classification systems for anthropogenically-induced eutrophication in all types of water bodies. Due to the inherent high temporal and spatial variability of hydrological and geochemical parameters of the coastal waters of the southern Baltic Sea, discrimination between anthropogenic impact and natural variability is necessary. The development of statistical methods for this discrimination was the main aim of this study. These methods were used to derive indicative phytoplankton parameters for different stages of eutrophication for the investigation area. For this purpose, a long-term phytoplankton data series was analysed, which covered a broad salinity and eutrophication gradient. In order to detect eutrophication effects, the analysis was restricted to phytoplankton spring bloom events and to the salinity range between 5 and 10 psu, i.e. superimposing seasonal and hydrodynamic effects were eliminated. An artificial abiotic degradation vector was developed based on four typical water quality parameters. A total of 11 potentially indicative phytoplankton parameters on different taxonomical levels arose from a correlation analysis with this degradation vector. These indicators were then tested for their ability to discriminate between three eutrophication levels. Finally, seven phytoplankton indices could be proposed: total phytoplankton biovolume, the percentage of diatoms and the biovolume of different size ranges of diatoms and one indicative species (Woronichinia compacta).

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 × 10 6 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 × 10 6 bacteria ml ) 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 < 3 × 10 6 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 Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 32: [121][122][123][124][125][126][127][128][129][130][131][132][133][134][135] 2003 organic matter input and decomposition, and subsequently reveal the most important regulating factors of bacterial activity.Besides nutrient availability, grazing as a top down control factor regulates bacterial biomass and species composition (e.g. Berninger et al. 1991). Protozoa, especially heterotrophic nanoflagellates, are an important, or often the dominating, loss factor for bacterial standing stock in pelagic systems (e.g. Jürgens et a...

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