title: Ciliates are the dominant grazers on pico- and nanoplankton in a shallow, naturally highly eutrophic lake.
abstract: 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 Võrtsjärv, which has a large open surface area (average 270 km2) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-microm-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 (<or=4.2% of total grazing). Ciliates were relatively abundant (<or=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-microm-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 microm) 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.
authors.affiliation: Institute of Agricultural and Environmental Sciences, Võrtsjärv Centre for Limnology, Estonian University of Life Sciences, Rannu, 61101, Tartu County, Estonia.
authors.forename: Priit
authors.initials: P
authors.lastname: Zingel
authors.forename: Helen
authors.initials: H
authors.lastname: Agasild
authors.forename: Tiina
authors.initials: T
authors.lastname: Nõges
authors.forename: Veljo
authors.initials: V
authors.lastname: Kisand
mesh_terms: D000818:Animals; D001419:Bacteria / Q000254:growth & development*; D018533:Biomass; D016798:Ciliophora / Q000502:physiology*; D017753:Ecosystem; D056890:Eukaryota / Q000254:growth & development / Q000502:physiology; D005068:Eutrophication; D020387:Food Chain; D005618:Fresh Water / Q000382:microbiology / Q000469:parasitology*; D010933:Plankton / Q000254:growth & development*