Production of planktonic bacteria in Lake Michigan

Abstract

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 [3H‐methyl]thymidine 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 × l09 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‐1 (C.V. typically <50%) based on the empirical conversion factors. The growth estimates are much lower (0.004–0.020 h‐1) when based on measured 47% thymidine incorporation into DNA and a theoretical conversion factor. The higher growth estimates appear more consistentwith 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.