Community structure, picoplankton grazing and zooplankton control of heterotrophic nanoflagellates in a eutrophic reservoir during the summer phytoplankton maximum

Abstract

An intensive 5 wk study was conducted to investigate the role of protists, especially heterotrophic nanoflagellates (HNF), in microbial food webs during the summer phytoplankton bloom in the epilimnion and metalimnion of the eutrophic h m o v reservoir (South Bohemia. Czech Republic). On average, protists consumed -90 % of bacterial production in both layers. The community composition of HNF and the relative importance of different HNF groups as picoplankton consumers were determined. Small HNF ( < 8 pm), as chryson~onads, bodonlds and choanoflagellates, usually accounted for <30% of total HNF biomass but numerically dominated the community in both layers They consumed most of (-70 to 85 %) the bacterioplankton as well as autotrophic picoplankton (APP, exclusively cyanobacteria) production in the reservoir, with the rest consumed by ciliates. Both ciliates and HNF had higher clearance rates on APP than on bactena and their grazing was likely responsible for a sharp decrease in APP abundance (from 3-4 X 105 to <2 X 103 ml-') and a very constant size structure of bacterioplankton in which short rods in the size class of 0.4 to 0.8 pm constituted 55 to 80':') of the total bacterial biomass in both layers. The proportion of HNF to total picoplankton biomass in the epilimnion indicated that the picoplankton biomass was sufficiently high to support HNF growth for most of the study. Uptake of picoplankton by less numerous, but larger, HNF (kathablepharids, Goniomonas sp., and Streptomonas sp.) was negligible, while their biomass, especially in the metalimnion, exceeded that of smal.1 HNF and the total biomass of picoplankton. This suggested that food items other than picoplankton were consumed to meet their carbon requirements. Analyzing potential bottom-up and top-down factors controlling HNF numbers and biomass, we dld not find a tight relationsh~p between HNF and the concentration of baclerid dnd chlorophyll. Variab~iity of HNF abundance and biomass in the epilimnion could largely be explained by cladocerans or by pooled abundances of all potential crustacean consumers of HNF. In the metalimnion, the mean cell volume of HNF was positively linked to chlorophyll but negatively to the abundance of Cyclopidae and to the pooled abundances of Ceriodaphnia quadrangula and Diaphanosoma brachyurum.