Oecologia (2002) 133:356–363 DOI 10.1007/s00442-002-1035-0 Abstract To examine both direct and indirect effects of macrozooplankton on phytoplankton species in Lake Biwa, we conducted in situ grazer-gradient experiments under different nutrient levels in summer, when Daphnia galeata dominated, and in autumn, when Eodiaptomus japonicus dominated. The experiments revealed that grazing pressure on phytoplankton was highly dependent on zooplankton species composition. Smaller phyto- plankton species such as Stephanodiscus carconensis were more grazed when D. galeata was abundant, where- as large colonial diatom species such as Aulacoseira granulata were preferentially grazed when E. japonicus dominated. In addition, indirect effect of macrozooplank- ton through nutrient regeneration was suggested, al- though the magnitude of nutrient regeneration effects seemed to differ between D. galeata and E. japonicus. Specifically, growth rates of Sphaerocystis schroeteri were stimulated more by E. japonicus than by D. galeata. Macrozooplankton also enhanced the growth rates of co- lonial cyanobacteria such as Microcystis incerta, proba- bly through decreasing the density of microzooplankton grazers (ciliates and rotifers). The results suggest that the effects of large zooplankton on phytoplankton popula- tions are species-specific and cannot be understood with- out consideration of changes in abundance of other com- ponents of plankton communities. Keywords Grazing · Lake Biwa · Nutrient regeneration · Species-specific response Introduction The abundance and species composition of phytoplank- ton change seasonally according to species-specific dif- ferences in the growth and loss rates in response to tem- poral changes in abiotic (e.g. light, temperature and nu- trient) and biotic (e.g. grazing) factors (Reynolds 1984a). Among these, grazing by large zooplankton such as da- phnids and diaptomus copepods is known to play a cru- cial role in the seasonal succession of phytoplankton populations in lakes and ponds (Sommer et al. 1986; Sterner 1989). However, zooplankton affect phytoplank- ton populations not just through direct grazing alone. They also affect phytoplankton populations indirectly by altering the nutrient conditions through nutrient regener- ation (Lehman and Sandgren 1985; Sterner 1986, 1989; Urabe 1995). Furthermore, since daphnids and copepods influence the ciliates and rotifers negatively through con- sumption, interference, and exploitative competition (Gilbert 1989; Wickham and Gilbert 1991; Pace and Vaque 1994; Brett et al. 1994), they may affect the phytoplankton populations indirectly by modulating the abundance of these small zooplankton grazers. Many studies have examined the effects of large zoo- plankton on phytoplankton abundance and composition in various lakes., Most of them, however, have focused on Daphnia as macrozooplankton grazers (Lynch and Shapiro 1981; Lehman and Sandgren 1985; Bergquist and Carpenter 1986; Elser et al. 1987; Sterner 1989; Elser and Goldman 1991; Elser 1992), because Daphnia are herbivorous and frequently dominate in zooplankton communities. Although diaptomus copepods also are large and herbivorous and are abundant in lakes, their ef- fects on phytoplankton populations have been less well examined. Their effects probably differ from those of daphnids, because the two groups differ greatly in their feeding mode and nutrient-recycling efficiencies (DeMott 1986; Vanderploeg 1990; Hessen 1997; Elser and Urabe 1999). Thus, the results based on daphnids may not be applicable to periods when diaptomus cope- pods dominate (Brett et al. 1994; Burns and Schallen- M. Kagami ( ✉ ) · T. Yoshida · T.B. Gurung · J. Urabe Centre for Ecological Research, Kyoto University, Kamitanakami Hirano-cho 509–3, Otsu, Shiga 520–2113, Japan Present address: M. Kagami, NIOO – Centre for Limnology, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands e-mail: kagami@cl.nioo.knaw.nl Tel.: +31-294-239300, Fax: +31-294-232224 PLANT ANIMAL INTERACTIONS Maiko Kagami · Takehito Yoshida Tek Bahadur Gurung · Jotaro Urabe Direct and indirect effects of zooplankton on algal composition in in situ grazing experiments Received: 2 January 2002 / Accepted: 15 July 2002 / Published online: 13 September 2002 © Springer-Verlag 2002