Hydrobiologia 515: 161–179, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 161 Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes Soon-Jin Hwang 1 , Ho-Sub Kim 1 , Jae-Ki Shin 1 , Jong-Min Oh 2 & Dong-Soo Kong 3 1 Department of Biological Systems Engineering, Konkuk University, Seoul, Rep. of Korea 143-701 E-mail: sjhwang@konkuk.ac.kr 2 Department of Environmental Science, Kyunghee University, Yongin, Korea 449-701 3 Watershed Management Research Division, National Institute of Environmental Research, Inchon, Korea, 404-170 Received 6 March 2003; in revised form 4 September 2003; accepted 8 September 2003 Key words: grazing, bivalve, zooplankton, phytoplankton, C-flux, food web, water quality management Abstract This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplank- ton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p = 0.002, n = 5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l −1 d −1 ) and to zooplankton (76–264 μgC l −1 d −1 ) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p< 0.01, n = 6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea. Introduction Filter feeding bivalves can play an important role in coupling the pelagic and benthic food webs of a lake, by importing a large portion of phytoplankton primary production from the water column into secondary pro- duction of the sediments (Dame & Dankers, 1988; Loo & Rosenberg, 1989; Heath et al., 1995; Jack & Throp, 2000). In this way, mussels redirect energy flow and matter cycling in the overall lake food web (Noordhius et al., 1992; Yamamuro & Koike, 1993; Heath et al., 1995; Gardner et al., 1995; Dame, 1996; Nakmura & Kerciku, 2000). Intense filtering by mussels also can create food- limiting conditions for planktivorous fish and their predators, and also affect benthic consumer biomass and species composition (Lowe & Pillsbury, 1995; Dermott & Kerec, 1997; Karatayev et al., 1997). Re-