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Bacteria in sea ice and underlying brackish water at 54°26'5"N (Baltic Sea, Kiel Bight)

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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 NO3, NO2, NH4, PO4 and SiO4. 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 [3H]leucine (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 NO3 and NH4 concentrations in ice and under-ice water of up to 112 µM and 55 µM, 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 TTI and particularly high TLI rates during the ice situation, with TLI:TTI ratios of 25 to 213. The high TLI rates were due to a large degree of unspecific labeling by leucine and characterised the bacteria during winter 1996 as extremely active. Bacterial production (based on TTI) rose in water from 0.021 µg Cl-1 h-1 at the beginning to 0.909 µg Cl-1 h-1 at the end of the investigation, and in brine from 0.122 to 0.235 µg Cl-1 h-1. Abundance of bacteria in brine increased from 1.7 x 106 cells ml-1 initially to 2.8 x 106 cells ml-1 in March. The average cell volume of these bacteria was 0.2 µm3 whereas the bacteria in water reached only 0.08 µm3. 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 specific vertical 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 biomass, 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 biomass 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 within the sea ice showed many similarities to those found in sea ice of polar regions.

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