Heterotrophic Bacterioplankton Dynamics at a Site off the Southern Shore of Lake Superior

Evolution of the paradigm regarding the relative importance of allochthonous and autochthonous sources of organic matter in aquatic systems has rekindled interest in the role of bacteria in energy transfer. The development of material budget calculations characterizing conditions of net heterotrophy requires knowledge of spatiotemporal dynamics in the bacterial community. Here we present results from three years of measurement of bacterioplankton abundance and production at three locations on the south shore of Lake Superior. In general, bacterial numbers (0.63 × 10 6 ± 0.29 × 10 6 cells·mL −1) and production (0.037 ± 0.055 mgC·m −3·hr −1) were comparable to those reported previously for Lake Superior and were consistent with the system's place along the trophic gradient. Interannual differences in abundance and production were apparently related to the timing and magnitude of seasonal phytoplankton dynamics. There was no inter-transect variation or systematic nearshore-offshore gradients in bacterial activity despite substantial differences in proximity to sources of terrigenous materials (between transects) and in phytoplankton biomass (with distance offshore). The most striking signals in bacterial activity were those evidenced by peaks in bacterial production associated with the deep chlorophyll maximum and with the decline in the phytoplankton community with the approach to turnover. It is hypothesized that bacterioplankton activity in Lake Superior exists in a near steady state, fueled by labile organic matter produced through phytoplankton excretion and the photolytic processing of refractory terrigenous materials. Superimposed on this rather stable signal are peaks in bacterial production apparently related to senescence of the phytoplankton community and, perhaps, the generation of nutrients by the grazing community.