Different effects of planktonic invertebrate predators and fish on the plankton community in experimental mesocosms

Ann. Limnol. - Int. J. Lim. 50 (2014) 71–83 Ó EDP Sciences, 2014 DOI: 10.1051/limn/2014001 Available online at: www.limnology-journal.org Different effects of planktonic invertebrate predators and fish on the plankton community in experimental mesocosms Michal Šorf1*, Zdeněk BrandlX1, Petr Znachor1,2 and Mojmı́r Vašek2 1 2 Faculty of Science, University of South Bohemia, Branišovská 31, CZ-370 05, České Budějovice, Czech Republic Institute of Hydrobiology, Biology Centre of the Academy of Sciences of the Czech Republic, Na Sádkách 7, CZ-370 05, České Budějovice, Czech Republic Received 26 June 2013; Accepted 24 December 2013 Abstract – The impact of fish and cyclopoid copepod predation on zooplankton communities was evaluated using large-volume mesocosms (depth 9.5 m; volume 13 m3) in the Řı́mov reservoir (Czech Republic). Two yearling roach and perch individuals introduced into mesocosms represented the fish treatment, which was compared to cyclopoid copepods (initial abundance of 2 ind.Lx1) and a control with no initial addition of predators. Our results clearly support the hypothesis that planktivorous fish feeding leads to the suppression of large-bodied cladocerans. In the presence of fish, the cladoceran community changed from a dominance of large-bodied Daphnia spp. at the beginning to dominance by the smaller Bosmina longirostris at the end of the experiment. Chlorophyll-a concentration and rotifer abundances increased in the absence of daphnids. In the absence of fish, the presence of large-bodied cladocerans resulted in decreasing chlorophyll-a concentration. Although no significant differences were observed between cyclopoid abundances in treatments stocked with cyclopoids and the control, the proportion of large cladocerans clearly showed the effect of the manipulation. The similar trends in both of these treatments did not confirm the importance of cyclopoid predation in our experiment. The overall strong effect of fish over cyclopoid predation suggests the main role of fish predation in the forming of zooplankton communities and in turn impacting phytoplankton biomass in mesocosms. Key words: predation / zooplankton / perch / roach / mesocosms Introduction Predation is one of the important factors determining food webs in the aquatic environment. The combination of vertebrate and invertebrate predation forms zooplankton assemblages in natural lakes (Blumenshine and Hambright, 2003) as well as in reservoirs (Hansson et al., 1998). There are great differences between the effect of fish and that of invertebrate predators (IP) on zooplankton species composition, biomass and size structure (e.g., Wojtal et al., 2004 or Manca et al., 2008). The reason for such dissimilarities lies in the different way they find and obtain suitable prey. Although most zooplanktivorous fish (and predatory cladocerans) forage visually (Werner and Hall, 1974), planktonic IP, such as cyclopoid copepods, look for food by means of mechanoreceptors and/or chemoreceptors (Dussart and Defaye, 2001). Generally, the encounter rate increases with prey body size. Large prey is more visible and therefore more susceptible to *Corresponding author: XDeceased visually foraging predators. Furthermore, planktivorous fish benefit from feeding on larger prey which exceeds the energy cost for foraging (Persson and Greenberg, 1990). Unlike fish, planktonic predators such as cyclopoid copepods or predatory cladocerans cannot consume the whole prey, but tear the prey into parts. Hence, feeding on small or medium-sized prey seems to be more effective for IP, as predicted by the optimal foraging theory (Werner and Hall, 1974). The structuring role of fish on zooplankton assemblage has been revealed by many authors since the 1960s. The early observations of the dramatic impact of fish on zooplankton abundance and size structure (e.g., Hrbáček and Hrbáčková-Esslová, 1960; Hrbáček et al., 1961; Hrbáček, 1962) resulted in the postulation of the size-efficiency hypothesis (Brooks and Dodson, 1965), which greatly influenced further studies on predator–prey interactions. Afterwards, recognition of subsequent effects of fish predation on lower levels of freshwater food webs led to the concept of cascading trophic interactions (Carpenter et al., 1985; see also the review by Brett and Goldman, 1996). This concept suggests that the promotion of efficient Article published by EDP Sciences 72 M. Šorf et al.: Ann. Limnol. - Int. J. Lim. 50 (2014) 71–83 planktonic herbivores through decreased biomass of planktivores by piscivores can reduce the biomass of phytoplankton. This general pattern can be modified by altered life histories or other interactions among particular species such as defence mechanisms in prey (Jeffries, 1988; Kolar and Wahl, 1998; Straile and Halbich, 2000). These findings are widely used during food web manipulation with the ultimate goal to improve water quality management (i.e., biomanipulation techniques) (Berg et al., 1997; Beklioglu, 1999; Ha et al., 2013). Although the role of fish in aquatic food webs is widely accepted, the effect of planktonic IP on lower trophic levels might be even higher than that of fish (e.g., Blumenshine and Hambright, 2003). Benndorf et al. (2000) reported high abundance of IP when planktivorous fish were reduced in numbers by very strong pressure from piscivorous fish. Increasing water turbidity also gives preference to IP over fish (Horppila and Liljendahl-Nurminen, 2005). On the contrary, Hansson and Tranvik (1996) concluded that IP have only a marginal effect compared with vertebrate predation pressure on zooplankton. The statement of Carpenter and Kitchell (1992) that a “rigorous re-examination of accumulated results is beneficial” seems to be valid even after 20 years. In this study, large-volume mesocosms were employed in a manipulation experiment aimed at evaluating the separate effects of vertebrate and invertebrate predation in a man-made lake. The scope of the experimental manipulation lies in the evaluation of the separate effects of vertebrate and planktonic IP on a plankton community. The pelagic zone of the meso-eutrophic Řı́mov reservoir (Czech Republic) was simulated in the field mesocosm experiment where top predators of zooplankton were manipulated. Besides predatory cladocerans Leptodora kindtii, which occur lately in the season, and Polyphemus pediculus inhabiting inshore habitats, cyclopoid copepods are the most abundant IP in the reservoir (Devetter and Sed’a, 2006) though their impact on zooplankton assemblage compared to fish predation is largely unknown. Based on the above mentioned facts on differences between fish and invertebrate predation, and given the situation in the Řı́mov reservoir, we hypothesised that (a) IP (cyclopoid copepods) would prominently affect both rotifers and smaller cladocerans in terms of their abundances; (b) fish predation, eliminating larger cladocerans, would increase phytoplankton biomass and (c) phytoplankton would facili (...truncated)