Microbial foodweb comparison of the Laurentian Great Lakes during the summers of 2001–2004

Abstract

A structural and functional assessment of the microbial foodweb of Lakes Superior, Huron, Erie and Ontario was undertaken during late summer (August) between 2001 and 2004. One lake was sampled in each year. Our analysis included microscopic enumerations of bacteria, autotrophic picoplankton, phytoplankton, heterotrophic nanoflagellates and ciliates coupled with radioisotope tracer measurements of primary productivity (14C) and bacterial growth (3H). Phytoplankton biomass was highest on average in Lake Erie (≈1.9 g m−3) and lowest in Lake Ontario (≈0.2 g m−3), whereas microbial loop biomass was highest in Lake Ontario (≈2.4 g m−3) and lowest in Lake Huron (≈0.1 g m−3). The organic carbon pool was found to be predominantly autotrophic (>80%) in Lakes Superior (≈280 mg C m−3), Huron (≈195 mg C m−3) and Erie (≈660 mg C m−3) and smaller picoplankton had the highest carbon turnover rates (≈0.4–1.5 d−1). However, in Lake Ontario (≈335 mg C m−3) the carbon pool was about 75% heterotrophic and larger net plankton had the highest carbon turnover rates (≈6.8 d−1). Despite differences in trophic state, the microbial foodwebs of Lakes Superior, Huron and Erie appeared to function in a similar and efficient manner. In contrast, the microbial foodweb of Lake Ontario appeared to be unhealthy with autochthonous production being sequestered by heterotrophic nanoflagellates. More detailed work is needed to understand the foodweb linkages both within the Great Lakes and among them.