BACTERIOPLANKTON IN A SMALL POLYHUMIC LAKE WITH AN ANOXIC HYPOLIMNION

Bacterioplankton biomass and dark fixation of inorganic carbon were me asured in the highly humic (water colour up to 550 mg Pt l-1) and acid ic lake, Mekkojarvi. Strong thermal and chemical stratification develo ped in the water column early in spring and led rapidly to anoxia in t he 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 hig her than in the epilimnion. These differences are thought to be the re sult of different grazing pressure from zooplankton in the two zones. In late summer a high concentration of bacteriochlorophyll d in the up per hypolimnion indicated a high density of photosynthetic bacteria. B acterial biomass was similar to that of phytoplankton in the epilimnio n, but 23 times higher in the whole water column. In August, dark fixa tion of inorganic radiocarbon in the anaerobic zone was 51% of the tot al C-14-incorporation and the contribution of light fixation was only 5.4%. In the polyhumic Mekkojarvi, bacterioplankton was evidently a po tentially significant carbon source for higher trophic levels, but bac terioplankton production could not be supported by phytoplankton alone . Allochthonous inputs of dissolved organic matter probably support mo st of the bacterial production.