Zooplankton grazing on bacteria and phytoplankton in a regulated large river (Nakdong River, Korea)

Zooplankton grazing on bacteria and phytoplankton was evaluated at monthly intervals, from March 1998 to March 1999, in the lower Nakdong River, Korea. We quantified bacterial and phytoplankton carbon contents, and measured carbon ingestion rates (CIRs) by two size classes of zooplankton: (i) microzooplankton (MICZ), ranging in size from 35 to 157 μm and including rotifers and nauplii, but protists were excluded; and (ii) macrozooplankton (MACZ), of a size larger than 157 μm and including cladocerans and copepods. Two types of laboratory grazing experiments were carried out to quantify zooplankton grazing on bacteria and phytoplankton. Species-specific and community filtering rates were measured in the feeding experiments with representative fluorescent microspheres (FM): 0.75 μm FM for bacteria and 10 μm FM for phytoplankton. CIRs were measured using natural bacterial and phytoplankton communities in the zooplankton density manipulation experiments. Bacterial carbon was considerably lower (average ± SD: 36 ± 24 μg C l–1, n = 25) than phytoplankton carbon (383 ± 274 μg C l–1, n = 25). Total zooplankton carbon (236 ± 520 μg C l–1) was usually dominated (>65%) by the MICZ fraction. Rotifers were the dominant taxonomic group. Bacterial carbon was positively related to both MICZ and MACZ carbon (P < 0.05) seasonally, but phytoplankton carbon was not. The community filtering rates (CFRs; ml l–1 day–1) and biomass grazing rate (G; % day–1) of MICZ, on both bacteria and phytoplankton, were always higher than those measured for MACZ. MICZ CIRs on bacteria (average 5.3 ± 5.5 μg C l–1 day–1) and phytoplankton (<35 μm in size) (average 63 ± 28 μg C l–1 day–1) were ~twofold higher than MACZ CIRs. On average, MICZ accounted for 70 and 83% of total zooplankton grazing on bacteria and phytoplankton, respectively. Considering the total zooplankton community, MICZ generally were more important than MACZ as grazers of bacteria and phytoplankton. Rotifers, in particular, played an important role in transferring both bacterial and phytoplankton carbon to higher trophic levels in the lower Nakdong River ecosystem.