BACTERIAL PRODUCTIVITY AND PROTISTAN BACTERIVORY IN COASTAL AND OFFSHORE COMMUNITIES OF LAKE ERIE

The importance of protists as bacterivores in a coastal community and an offshore community of Lake Erie was compared during the summer of 1 994. Bacterial density, cell size, and empirical conversion factors fo r bacterial productivity were highly variable at both sites and greate r at the coastal site (P < 0.01). Bacterial productivity at the coasta l site was 25-50 times higher than at the offshore site. Bacterivory w as estimated in situ by fluorescently labeled native bacteria. Per-cel l grazing rate and filtering rate for each taxon were routinely determ ined. Heterotrophic nanoflagellates (HNAN) were the most important pro tistan bacterivores at the offshore site, while HNAN and ciliates were similarly dominant bacterivores at the coastal site. Mixotrophic bact erivory was important only at the offshore site where Dinobryon was th e dominant bacterivore. Bacterial carbon flux through protists was hig her at the coastal site by an order of magnitude. Offshore protists gr azed virtually the entire bacterial production, while coastal protists usually grazed less than half of the bacterial production. These resu lts suggest that coastal and offshore sites differed fundamentally in the significance of protists to carbon flux through the microbial loop to higher trophic levels.