Abstract Macrozooplankton may affect algal and mi- crobial plankton directly through grazing or predation and indirectly through nutrient regeneration. They may also affect potential prey positively by removing alterna- tive predators. Here, we examined the effects of a cla- doceran (Daphnia) and a calanoid copepod (Eodiapto- mus) on algal and microbial plankton in a Japanese lake using in situ experiments in which we manipulated the nutrient supply and biomass of these macrozooplankton. The response of algal and microbial plankton to macro- zooplankton was diverse and varied depending on the level of nutrient supply. Eodiaptomus seemed to feed mainly on large algae (>20 μm) and microzooplankton, while direct grazing by Daphnia on algae, bacteria, het- erotrophic nanoflagellates (HNF), and microzooplankton (ciliates, heliozoa, and rotifers) was pronounced. Trophic linkages within these microbial plankton was also sug- gested; bacteria were grazed by HNF and these in turn were grazed by microzooplankton. When the nutrient supply was high, both HNF and microzooplankton were exposed to higher amounts of algae and lower bacterial abundance. Moreover, nutrient regeneration by daphnids and Eodiaptomus copepods seemed to differentially stimulate the growth of algae and bacteria. The results suggest that the relationship between macrozooplankton and microbial plankton cannot be fully understood with- out taking into consideration not only the feeding char- acteristics of the macrozooplankton, but also the food web structure, the subsidized algal resource, and nutrient regeneration from the macrozooplankton. Keywords Trophic interactions · Nutrient regeneration · Resource subsidy · Classic and microbial food chains Introduction In pelagic communities, the grazing chain from algae to macrozooplankton such as daphnids and copepods has traditionally been viewed as a major way to sustain high- er trophic levels. However, there is growing evidence that bacteria and protozoan plankton (flagellates and cil- iates) are equally or sometimes more important constitu- ents of pelagic food webs. Via bacteria, this trophic link can transfer substantial amounts of dissolved organic matter to higher trophic levels. This transfer of dissolved organic matter to bacteria and protozoan plankton is called the microbial chain, or the microbial loop (Azam et al. 1983). Because macrozooplankton feed not only on algae but also on a wide range of organisms, including bacteria and protozoans (Hessen et al. 1986; Sanders and Porter 1990; Wickham et al. 1993; Pace and Vaqué 1994), they play a key role in linking the microbial loop to the grazing chain (Porter et al. 1979; Carrick et al. 1991; Jürgens 1994). This, in turn, implies that macro- zooplankton can affect the abundance of organisms in both the microbial and the grazing chains. It is well documented that macrozooplankton affect algal abundance and composition directly through graz- ing and indirectly through nutrient regeneration (Lehman and Sandgren 1985; Sterner 1986; Vanni 1987; Elser and Goldman 1991; Brett et al. 1994; Urabe 1995). In addi- tion, different macrozooplankton affect algal species composition in different ways, primarily because of spe- cies- or taxon-specific characteristics of the feeding mode (Okamoto 1984; DeMott 1986, 1988) and nutrient recycling (Elser and Urabe 1999). Compared with the wealth of knowledge on algal- macrozooplankton interactions, less is known about in- teractions between small heterotrophic plankton and macrozooplankton. A number of studies have examined the grazing effects of macrozooplankton on bacteria (Güde 1988; Jürgens 1994), flagellated protozoans (Riemann 1985; Pace and Funke 1991), ciliated protozo- ans (Pace and Funke 1991; Wickham and Gilbert 1991), and rotifers (Gilbert 1988; Pace and Vaqué 1994). How- T. Yoshida ( ✉ ) · T.B. Gurung · M. Kagami · J. Urabe Center for Ecological Research, Kyoto University, 509-3 Kamitanakami-Hirano, Otsu, Shiga 520-2113, Japan Present address: T. Yoshida, Department of Ecology and Evolutionary Biology, Corson Hall, Cornell University, Ithaca, NY 14853, USA, e-mail: ty59@cornell.edu, Tel.: +1-607-254-4273, Fax: +1-607-255-8088 Oecologia (2001) 129:602–610 DOI 10.1007/s004420100766 Takehito Yoshida · Tek Bahadur Gurung Maiko Kagami · Jotaro Urabe Contrasting effects of a cladoceran (Daphnia galeata) and a calanoid copepod (Eodiaptomus japonicus) on algal and microbial plankton in a Japanese lake, Lake Biwa Received: 15 January 2001 / Accepted: 25 June 2001 / Published online: 31 July 2001 © Springer-Verlag 2001