Dynamics in bacterioplankton production in a shallow, temperate lake (Lake Neusiedl, Austria): Evidence for dependence on macrophyte production rather than on phytoplankton

Abstract

The seasonal dynamics in bacterioplankton abundance and production were studied at 3 characteristic stations (open water, large pond within the reed belt and within the reed Phragmites australis) in the shallow Lake Neusiedl, Austria, and related to phytoplankton primary production and dissolved organic carbon (DOC). DOC concentrations ranged from about 1 to 2.5 mmol l-1 with humic DOC contributing between 40% during the winter and 55% during the summer. Phytoplankton production was highest in the pond within the reed belt, where the attenuation was lowest, reaching 110 mg C m-2 d-1 during a distinct phytoplankton bloom in August. Bacterial abundance ranged from 2 x 106 cells ml-1 during winter to about 10 x 106 cells ml-1 during summer. Bacterial production calculated by thymidine (TdR) and leucine (leu) incorporation, respectively, were in good agreement at the stations in the reed belt, but bacterial production based on leu incorporation was significantly lower than bacterial production based on TdR incorporation at the open water station. Based on a bacterial growth yield of 16% determined in an earlier study, bacterioplankton carbon demand was always at least 1 order of magnitude higher than carbon production of phytoplankton, indicating that bacterioplankton metabolism in Lake Neusiedl is heavily dependent on non-phytoplankton sources of DOC. The bacterial carbon demand (ranging from 225 to 870 mg C m-2 d-1 depending on the sampling site and substrate used) could be matched by the production of the reed P. australis amounting to 750 to 4510 mg C m-2 d-1. Since there is no major allochthonous organic matter input from other sources, this macrophyte production is obviously channeled to the pelagic food web via the bacterioplankton.