Ciliates are the dominant grazers on pico- and nanoplankton in a shallow, naturally highly eutrophic lake

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Vortsjarv, which has a large open surface area (average 270 km(2)) and highly turbid water (Secchi depth < 1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-mu m-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (< 50 cells mL(-1)) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (<= 4.2% of total grazing). Ciliates were relatively abundant (<= 158 cells mL(-1)) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-mu m-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3-6 mu m) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top-down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.