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Bacterial variables are expected to respond differently to eutrophication. This was investigated along the eutrophication gradient in the narrow Schlei fjord (northern Germany). Bacterial extracellular enzyme activities (peptidase, a-and P-glucosidase, chitinase) were measured together with a large number of autotrophic and heterotrophic biological variables. Increases of values measured along the eutrophication gradient were generally higher for bacterial substrate uptake and growth than for bacterial counts and enzyme activities. Annual patterns of activities (per volume of water) obtained from stations with different degrees of eutrophication were clearly different from each other. In contrast, annual patterns of activities per bacterial cell at the stations with different degrees of eutrophication were not clearly different from each other, Indicating that they did not depend on eutrophication. Size fractionation of enzyme activities revealed that most of the peptidase activity was generally associated with free-living bacteria (<0.2 to 3 pm, average 57 % of total). Chitinase (average 54 % of total) and a-/P-glucosidases activities during summer were mainly associated with the > 3 pm particle size class (43 and 5 2 % , respectively). Free enzyme activities (

Bacterial response to the rare event of solid ice cover in the western Baltic Sea (Kiel Bight) was investigated from February to March 1996. Samples (ice cores, brine and water) were taken at a shallow, near-shore station at irregular time intervals. Bacterial abundance, biomass and production were measured in brine and the underlying water as were the concentrations of NOa, NOa, NH,, PO, and SiO,. Vertical distributions of bacterial abundance, biomass, morphotypes and size classes and chlorophyll a and nutrients were investigated within sea ice. A bacterial growth experiment with brine bacteria was carried out to measure bacterial carbon production via total incorporation of [3H]thymidine (TTI) and [ 3~] l e u c i n e (TLI). During February the abundance, biomass and production of bacteria within brine exceeded values from under-ice water, whereas the opposite was observed in March. High N o 3 and NH4 concentrations in ice and under-ice water of up to 112 pM and 55 pM, respectively, resulted in N:P ratios of 18 to 330. Algae and bacteria were considered to benefit from that nutrient supply. For bacteria this was supported by TT1 and particularly high TLI rates during the ice situation, with TL1:TTI ratios of 25 to 213. The high TLI rates were due to a large degree of unspecific labeling by leucine and characterised the bacleria during winter 1996 as extremely active. Bacterial production (based on TTI) rose in water from 0.021 1-19 C 1-' h-' at the beginning to 0.909 pg C 1-' h-' at the end of the investigation, and in brine from 0.122 to 0.235 pg C 1-' h-'. Abundance of bacteria in brine increased from 1.7 X 106 cells ml-' initially to 2.8 X 10' cells ml-' in March. The average cell volume of these bacteria was 0.2 pm3 whereas the bactena in water reached only 0.08 pm3 The bacterial assemblage in the ice was dominated by large rods and in the water by small rods and cocci. Bacterivorous activity within sea ice was assumed to be reduced due to the speclfic vertlcal distribution of the different bacterial size classes. This was further supported by a good correlation between the development of the bacterial standing stock and the potential blomass, in sea ice as well as in the underlying water, calculated from generation times towards the end of the investigation. Low grazing pressure, high standing stocks of algae and sufficient substrate supply accounted for bacterial biornass within the ice and the underlying water that exceeded that from former winters by far A comparison with Arctic and Antarctic sites demonstrated that the bacterial community withln the sea ice showed many similarities to those found in sea ice of polar regions.

Phytoplankton and bacteria are sensitive indicators of environmental change. The temporal development of these key organisms was monitored from 1988 to the end of 2007 at the time series station Boknis Eck in the western Baltic Sea. This period was characterized by the adaption of the Baltic Sea ecosystem to changes in the environmental conditions caused by the conversion of the political system in the southern and eastern border states, accompanied by the general effects of global climate change. Measured variables were chlorophyll, primary production, bacteria number, -biomass and -production, glucose turnover rate, macro-nutrients, pH, temperature and salinity. Negative trends with time were recorded for chlorophyll, bacteria number, bacterial biomass and bacterial production, nitrate, ammonia, phosphate, silicate, oxygen and salinity while temperature, pH, and the ratio between bacteria numbers and chlorophyll increased. Strongest reductions with time occurred for the annual maximum values, e.g. for chlorophyll during the spring bloom or for nitrate during winter, while the annual minimum values remained more stable. In deep water above sediment the negative trends of oxygen, nitrate, phosphate and bacterial variables as well as the positive trend of temperature were similar to those in the surface while the trends of salinity, ammonia and silicate were opposite to those in the surface. Decreasing oxygen, even in the surface layer, was of particular interest because it suggested enhanced recycling of nutrients from the deep hypoxic zones to the surface by vertical mixing. The long-term seasonal patterns of all variables correlated positively with temperature, except chlorophyll and salinity. Salinity correlated negatively with all bacterial variables (as well as precipitation) and positively with chlorophyll. Surprisingly, bacterial variables did not correlate with chlorophyll, which may be inherent with the time lag between the peaks of phytoplankton and bacteria during spring. Compared to the 20-yr averages of the environmental and microbial variables, the strongest negative deviations of corresponding annual averages were measured about ten years after political change for nitrate and bacterial secondary production (~ −60%), followed by chlorophyll (−50%) and bacterial biomass (−40%). Considering the circulation of surface currents in the Baltic Sea we interpret the observed patterns of the microbial variables at the Boknis Eck time series station as a consequence of the improved management of water resources after 1989 and – to a minor extent – the trends of the climate variables salinity and temperature