Horizontal distribution of photo- and heterotrophic microorganisms on the coastal area of the northern Baltic Sea--a case study

The horizontal distribution of bacteria (BACT), bacterial production(FDC), picoplanktonic cyanobacteria (CYANO), and photosynthetic(PNF) as well as heterotrophic (HNF) nanoflagellates was studiedat the end of June on the SW coast of Finland, the Baltic Sea.Samples were taken at 0 and 10 m depths from three transects500 and 800 m apart. The distance between sampling points was20 m. Cell counts of BACT, CYANO, PNF, HNF and the FDC showed3- to 5-fold deviations in the whole data set. Differences betweenthe duplicate counts within the subsamples were 10–20%.Within the transects, the maximum values were 1.2–3.0times higher than the minimum values. Differences between thetransects were statistically significant for all variables exceptFDC. Deviations in cell numbers between the surface and 10 mwere quite irregular. but often statistically significant. Inmost cases, the organism groups did not correlate significantlywith each other. The horizontal distribution of pelagic microbeswas sufficiently uniform, on a meter scale. for one samplingpoint to give a reliable estimate of microbial densities inthe area under investigation. The differences between the surfaceand the lower stratum of the mixed layer (10 m) were in mostcases statistically significant.     

Bacterioplankton in a small polyhumic lake with an anoxic hypolimnion

Bacterioplankton biomass and dark fixation of inorganic carbon were measured in the highly humic (water colour up to 550 mg Pt l^?1) and acidic lake, Mekkojärvi. Strong thermal and chemical stratification developed in the water column early in spring and led rapidly to anoxia in the hypolimnion, which extended to less than 1.0 m from the surface. In the epilimnion only small bacteria were abundant. In the anoxic zone both the abundance and the mean size of bacteria were considerably higher than in the epilimnion. These differences are thought to be the result of different grazing pressure from zooplankton in the two zones. In late summer a high concentration of bacteriochlorophyll d in the upper hypolimnion indicated a high density of photosynthetic bacteria. Bacterial biomass was similar to that of phytoplankton in the epilimnion, but 23 times higher in the whole water column. In August, dark fixation of inorganic radiocarbon in the anaerobic zone was 51% of the total ¹⁴C-incorporation and the contribution of light fixation was only 5.4%. In the polyhumic Mekkojarvi, bacterioplankton was evidently a potentially significant carbon source for higher trophic levels, but bacterioplankton production could not be supported by phytoplankton alone. Allochthonous inputs of dissolved organic matter probably support most of the bacterial production.