Production of planktonic bacteria in Lake Michigan’

Bacterial production rates were estimated for the surface waters of a station 100-m deep in southeastern Lake Michigan during 1984. Production was calculated from incorporation of C3Hmethyllthymidine and from empirical conversion factors determined from dilution experiments performed throughout the study. The conversion factors (with typical C.V. ~40%) varied between 4.7 and 18.3 x lo9 cells produced per nanomole of thymidine incorporated into ice-cold trichloroacetic acid extracts. Our estimates yielded bacteria exponential growth rates between 0.05 and 0.24 h-l (C.V. typically ~50%) based on the empirical conversion factors. The growth estimates are much lower (0.004-0.020 h-l) when based on measured 47% thymidine incorporation into DNA and a theoretical conversion factor. The higher growth estimates appear more consistent with estimated grazing losses. Carbon flux estimates are less certain, due to the possible range of bacterial carbon content and growth efficiencies, but most of the higher growth estimates imply a bacterial carbon demand higher than concurrent 14C-based primary production measurements. This may mean that a source other than recent primary production is needed to meet this demand. Recent bacterial production estimates (e.g. Fuhrman and Azam 1982; Kirchman et al. 1982; Newell and Christian 198 1; Jordan et al. 1978) support the hypothesis that bacteria play an important role in the energetics of marine and freshwater ecosystems. Much attention has been given to this heterotrophic production in coastal and open ocean environments (e.g. Klug and Reddy 1984; Hobbie and Williams 1984; Fasham 1984; Ducklow and Hill 198 5a, b). In the few lakes where such measurements have been made, bacterial heterotrophic production appears to rival autotrophic production as a net (albeit de novo) source of particulate carbon available to consumers. For example, net bacterial carbon production in the water column ranges from 12 to 4 1% of total (autotrophic plus bacterial) production in several mesotrophic to eutrophic lakes (Pedros-Alio and Brock 1982; Riemann 1983; Love11 and Konopka 1985a,b; Cole et al. 1984). The precise role of these heterotrophic microbes is unclear. It is not known whether bacterial biomass is consumed and transferred efficiently up the food chain or wheth-er it is metabolized inefficiently, thus becoming an energy sink (Pomeroy 1984). If this “microbial loop” (Azam et al. 1983) is I GLERL Contribution 463. a significant carbon pathway, it provides a route for assimilation of otherwise lost dissolved organic carbon (DOC) into the food chain. If bacterial production is high and the loop is inefficient, then the loop becomes a major agent in nutrient recycling (Sherr and Sherr 1984). This latter role becomes particularly relevant for P-limited environments in light of recent reports of very active bacterial enzyme systems that liberate orthophosphate from 5’-nucleotides (Ammerman and Azam 1985). Recent evidence has suggested the presence of microbial loops for Lake Superior (Fahnenstiel et al. 1986) and Lake Ontario (Caron et al. 1985); however, before their impact can be assessed for Lake Michigan, estimates of bacterial production rates are