Growth and production of bacterioplankton in a deep mesohumic boreal lake

Vertical distribution of bacterial biomass and production, as well as phyto plankton production, was studied in a deep, mesohumic lake in Southern Finl and. In summer 1992 32-56 % of bacterial production but only ca. 17 % of bi omass was found in the euphotic zone (0-5 m), which consists of 20 % of the : total lake volume. In the deep central area of the lake, the production o f bacteria varied between 63 and 121 mg C m(-2) d(-1) and that of phytoplan kton 57-158 mg C m(-2) d(-1). High bacterial production to primary producti on ratio (>0.5) indicates that the phytoplankton production alone was insuf ficient to sustain the total bacterial production, and that bacterioplankto n utilised other carbon sources, such as allochthonous organic compounds. F actors controlling growth of bacteria in the aphotic zone (allochthonous an d autochthonous dissolved organic matter (DOM), PO4-P, temperature and graz ing) were studied in a laboratory experiment. The most pronounced increase in bacterial growth was achieved when allochthonous humic water originating from the catchment was added or when it was added together with PO4-P. Fur thermore, PO4-P addition together with autochthonous organic carbon, result ed in higher bacterial growth rates than the addition of autochthonous DOC alone. The results suggest that despite the high concentration of dissolved organic carbon (DOC >10 mg C/L) in the lake, the quality of DOM and availa bility of PO4-P are the principal factors controlling the growth of bacteri oplankton.