Grazer and nutrient impacts on epilimnetic ciliate communities

The effects of large grazers (Daphnia) and nutrients (nitrogen and phosphorus) on epilimnetic ciliate communities were investigated in eight large in situ enclosures. Contrasts in Daphnia abundance were created by stocking planktivorous fish (Phoxinus eos) in four of the eight enclosures. Contrasts in nutrients were created by adding nitrate and phosphate to two enclosures with fish and to two enclosures without fish. Both the biomass and mean length of total zooplankton were significantly greater in the fish-free enclosures than in the enclosures with fish. Addition of nutrients and fish produced a four-fold variation in concentration of total epilimnetic phosphorus and six-fold variation in algal biomass (Chl a). Duphniu were virtually absent in the enclosures with fish. The abundance of ciliates was one to three orders of magnitude lower when Daphnia were abundant. Addition of nutrients did not increase ciliate abundance in enclosures with Daphnia, but ciliate mean lengths were larger in these enclosures. Ciliate abundance increased with increasing zooplankton biomass when Daphnia were lacking, but it declined rapidly when Daphnia were abundant, producing a hyperbolic pattern. When the data for all the treatments were considered together, there was no relationship between copepod biomass and ciliate abundance, but positive relationships, although not highly significant, cmcrged when the data for enclosures with and without fish were considered separately. Our results suggest that large grazers are the most important factor regulating ciliate communities as well as the responses of ciliates to nutrients and resources. Shifts of zooplankton from copepods to Daphnia dominance may result in large reductions in ciliate abundance, regardless of the trophic status of lake ecosystems. Ciliated protozoans are ubiquitous organisms of variable size (as small as 5 pm to as large as 5 mm) and can constitute an important fraction of zooplankton biomass (Pace and Orcutt 1981; Hunt and Chein 1983; Gates 1984; Car-rick and Fahnenstiel 1990). They recycle phosphorus rapidly (Johannes 1965; Buechler and Dillon 1974; Caron 1991) and may also contribute significantly to diets of other protozoans (Kusch 1993), rotifers (Gilbert 1980; Arndt 1993), copepods (Archbold and Berger 1985; Burns and Gilbert 1993), and cladocerans (Porter et al. 1979; Wickham and Gilbert 1993; Pace and VaquC 1994). One of the most important planktonic grazers in lake ecosystems is Daphnia (for example, Hrbacek et al. 1961; Brooks and Dodson 1965; Mazumder et al. 1990; Mazumder 1994 a, b). Daphnia is a generalized grazer of plankton ranging from bacteria to cells as large as 50 pm (Burns 1968; Bosselmann and Riemann 1986; Pace et al. 1990). They have also been suggested as efficient grazers of ciliates (e.g. Debiase et al. 1990; Wickham and Gilbert 1991), preferentially of ciliates <20 pm (Car-rick et al. 199 1). Considering the size range of plankton grazed by Daphnia, large ciliates Acknowledgments This research was supported by NSERC and FCAR grants to A. .M. Enclosures used in these experiments were financed by Environment Canada through grants to David Lean. Installation of the enclosures was financed by an NSERC (Universite de Montreal) grant to A. M. and by many undergraduate and graduate students and David Lean. We thank the staff at the Station de Biologie de Laurentides for providing the excellent facilities and environment for research. We thank two reviewers and Michael Pace for their comments on an earlier version of this paper. might be expected to have a size refuge from Daphnia (Wickham and Gilbert 1993). Therefore, we may expect to observe a dominance of small ciliates when Daphnia are absent and to observe a dominance of larger ciliates when Daphnia are abundant. Nutrients, notably phosphorus, are another important factor that could directly or indirectly influence ciliate dynamics. In oligotrophic lakes, large algivorous oligotrichs have been found to dominate because there is not enough bacteria to maintain small bacterivorous ciliates (Fenchel 1980; Pace 1982). Ciliate size tends to decrease with increasing lake trophic status (Beaver and Crisman 1982) possibly because bacteria becomes more abundant (Pace 1986). A positive relationship between ciliate abundance and phosphorus has also been observed by Beaver and Crisman (1982), but the relative contribution of ciliates to total planktonic biomass has been found to remain unchanged with lake trophy (Pace 1986). Grazers and nutrients may have antagonistic effects on ciliates. Grazers have a negative impact on ciliate abundance (Gilbert 1989; Pace and Funke 199 1; Wickham and Gilbert 1991, 1993; Wiackowski et al. 1994) and promote a higher mean size, with small ciliates (<20 pm) being preyed upon preferentially to larger ciliates (Carrick et al. 1991; Jack and Gilbert 1993). Increasing nutrients have been associated with an increase in ciliate abundance and a decrease in their mean size (Beaver and Crisman 1982; Pace 1986). Although several previous studies have investigated the effects of nutrients and grazers on ciliate populations, only one study has emphasized both the individual and interactive effects of grazers and nutrients simultaneously (Pace and Funke 1991). Using both grazers and nutrients as factors, Pace and Funke (1991) found that ciliate abundance was more strongly affected by grazers than by nutrients (Pace and Funke 1991).