Contrasting effects of a cladoceran (Daphnia galeata) and a calanoid copepod (Eodiaptomus japonicus) on algal and microbial plankton in a Japanese lake, Lake Biwa

Abstract

Macrozooplankton may affect algal and microbial plankton directly through grazing or predation and indirectly through nutrient regeneration. They may also affect potential prey positively by removing alternative predators. Here, we examined the effects of a cladoceran (Daphnia) and a calanoid copepod (Eodiaptomus) 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 macrozooplankton 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, heterotrophic nanoflagellates (HNF), and microzooplankton (ciliates, heliozoa, and rotifers) was pronounced. Trophic linkages within these microbial plankton was also suggested; 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 without taking into consideration not only the feeding characteristics of the macrozooplankton, but also the food web structure, the subsidized algal resource, and nutrient regeneration from the macrozooplankton.

Direct and indirect effects of zooplankton on algal composition in in situ grazing experiments

“…Aliquots of lake water were fixed with 0.4% acid Lugol to determine initial cell numbers of phytoplankton species. Macrozooplankton, mainly D. galeata and E. japonicus, were collected by vertical tows of a conical net with a 300-µm mesh from 15 m depth to the surface, concentrated, rinsed, and used as we report elsewhere (Yoshida et al 2001b).…”

“…For the sample of phytoplankton, 500 ml of the 200-µm filtrate was fixed as above and stored in the dark. Macrozooplankton in the bottles were counted under a dissecting microscope and their biomass was estimated (Yoshida et al 2001b). Phytoplankton in the 500-ml samples were concentrated to 20 ml by sedimentation for at least 4 days, and counted under a microscope in a 1-ml Sedgwick-Rafter chamber at 40-200× magnification.…”

“…Among the phytoplankton, Fragilaria crotonensis, a colonial diatom species, forms large blooms in spring, but Staurastrum dorsidentiferum, a large desmid alga, predominates from summer to autumn. Several studies have examined the effects of zooplankton on phytoplankton in Lake Biwa, including the effects of direct grazing (Okamoto 1984;Kawabata 1987;Urabe et al 1996), nutrient regeneration Elser et al 2001), and both grazing and nutrient regeneration (Yoshida et al 2001b). However, these studies focused mainly on total phytoplankton abundance, as quantified by chlorophyll-a or seston concentration.…”

“…We assessed the effects by incubating plankton with and without nutrient enrichment, as described by Lehman and Sandgren (1985). The results for bacteria and protozoans are reported elsewhere (Yoshida et al 2001b). Here, we show the effects of D. galeata and E. japonicus on phytoplankton.…”

Role of interference from Daphnia and predation by cyclopoid copepods in zooplankton community structure: experimental analysis using mesocosms

“…Gilbert (1985Gilbert ( , 1988b and Schneider (1990) demonstrated that two mechanisms are at work in these interactions: exploitative competition and interference competition. However, it is unlikely that exploitative competition was a factor in our experiment be- Yoshida et al (2001) cause an abundance of C. vulgaris was supplied in the tanks. Therefore, it seems likely that rotifers were out-competed by Daphnia primarily through interference competition (that is, through damage incurred by being swept into the branchial chambers of Daphnia), which apparently is more important than exploitative competition in the interaction between Daphnia and rotifers (Williamson 1987, Gilbert 1988a, Ronneberger et al 1993).…”