Trophic relations between cyclopoid copepods and ciliated protists: Complex interactions link the microbial and classic food webs.

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1 Notes 1173 DOLLAR, S. J., AND R. W. GRIGG Impact of a kaolin clay spill on a coral reef in Hawaii. Mar. Biol. 65: DREW, E. A The biology and physiology of algae-invertebrate symbioses. 2. The density of symbiotic algal cells in a number of hermatypic hard corals and alcyonarians from various depths. J. Exp. Mar. Biol. Ecol. 9: DUBINSKY, Z., P. G. FALKOWSKI, AND K. Wm Light harvesting and utilization by phytoplankton. Plant Cell Physiol. 27: 1335-l , AND OTHERS The effect of nutrient resources on the optical properties and photosynthetic efficiency of Stylophora pistillata. Proc. R. Sot. Lond. Ser. B 239: 23 l-246. FALKOWSKI, P. G Light and shade adaptation in marine phytoplankton, p. 99-l 19. In P. G. Falkowski [ed.], Primary productivity in the sea. Plenum. -, AND Z. DUBINSKY Light-shade adaptation of Stylophora pistillata, a hermatypic coral from the Gulf of Elat. Nature 289: , L. MUSCATINE, AND J. W. PORTER Light and bioenergetics of a symbiotic coral. Bioscience 34: , L. McCXoSKEY, L. MUSCATINE, AND Z. DUBINSKY Population control in symbiotic corals. Bioscience 43: GLYNN, P. W El Nino-Southern Oscillation 1982-l 983: Near-shore, population, community and ecosystem responses. Annu. Rev. Ecol. Syst. 19: ~ Coral reef bleaching: Ecological perspectives. Coral Reefs 12: , AND L. D CROZ Experimental evidence for high temperature stress as the cause of El Nino-coincident coral mortality. Coral Reefs 8: 18 l-l , R. I-1, K. SAKAI, Y. N-0, AND K. YAMAzATO Experimental response of Okinawan (Ryuku Islands, Japan) reef corals to high sea temperature and UV radiation, p Proc 7th Int. Coral Reef Symp. Guam. GOREAU, T. F Mass expulsion of zooxanthellae from Jamaican reef communities after hurricane Flora. Science 145: , AND A. H. MACFAFUANE Reduced growth rate of Montastrea annularis following the coralbleaching event. Coral Reefs 8: HOEGH-GULDBERG, O., AND G. J. SMITH Influence of the population density of zooxanthellae and supply of ammonium on the biomass and metabolic characteristics of the reef corals Seriatopora hystrix and Stylophora pistillata. Mar. Ecol. Prog. Ser. 57: JAPP, W. C., AND J. WHEATON Observation on Florida reef corals treated with fish collecting chemicals. Fla. Mar. Res. Publ. 10: JEFFREY, S. W., AND G. F. HUMPHREY New spectrophotometric equations for determining chlorophylls a, b, c 1 and c2 in higher plants, algae and natural phytoplankton. Biochem. Physiol. Pflanz. 167: 19 l-l 94. Jo-, R. E., AND W. J. WIEBE A method for determination of coral tissue biomass and composition. Limnol. Oceanogr. 15: MANDEL, J The statistical analysis of experimental data. Interscience. MUSCATINE, L., P. G. FALKOWSKI, Z. DUBINSKY, P. A. COOK, AND L. MCCLOSKEY The effect of nutrient resources on the population dynamic of zooxanthellae in a reef coral. Proc. R. Sot. Lond. Ser. B 236: 3 1 l-324. PRÉZELIN, B. B Light reactions in photosynthesis, p. l- 43. In Physiological bases of phytoplankton ecology. Can. Bull. Fish. Aquat. Sci SCHONWALD, H., Y. ACHITUV, AND Z. DUBINSKY Differences in the symbiotic interrelation between algae and host in light- and dark-coloured colonies of the hydrocoral Millepora dichotoma. Symbiosis 4: 17 l-l 84. SZMANT, A. M., AND N. J. GASSMAN The effects of prolonged bleaching on the tissue biomass and reproduction of the reef coral Montastrea annularis. Coral Reefs 8: Submitted: 15 September I993 Accepted: 23 March 1995 Amended: I May 1995 Limnol. Oceanogr., 40(6), 1995, , by the American Society of Limnology and Oceanography, Inc. Trophic relations between cyclopoid copepods and ciliated protists: Complex interactions link the microbial and classic food webs Abstract-Two field experiments examined the effects of cyclopoid copepods on ciliates. The presence or absence of Cyclops abyssorum, Cyclops kolensis, and zooplankton ~64 pm was manipulated to determine the relative importance of direct cyclopoid predation on protists vs. indirect effects mediated through cyclopoid predation on other metazooplankton. In the second experiment, presence or absence of C. abyssorum was cross-classified with five concentrations of the metazooplankton community. Cyclopoid effects on ciliates were dependent on predator and prey species and on the abundance of alternate prey for cyclopoids. A trophic cascade was also observed, but only for two small ciliates, and only with the larger C. abyssorum. C. abyssorum had a stronger predation effect on oli- gotrich ciliates when metazooplankton had been removed, and this effect appeared at a lower metazooplankton concentration with a larger ciliate, compared to a smaller species of the same genus. These results suggest that for cyclopoid+iliate interactions, switching behavior in the predator may be at least as important as a trophic cascade. The concept of a trophic cascade structuring aquatic food webs has become a dominant theme in aquatic ecology. Planktivorous fish have been shown to reduce the abundance of herbivorous zooplankton, thereby reducing

2 1174 Notes grazing by zooplankton on algae, which results in increased algal biomass (HrbaCek 1962; Carpenter et al. 1985; Kerfoot 1987). Sprules and Bowerman (1988) however, have suggested that omnivory-feeding on multiple trophic levels-is common in aquatic communities. Omnivory has the potential to reduce the strength of trophic cascades, if, for example, a zooplankter consumes both algae and other algal grazers. Increased abundance of the omnivore could either increase or decrease algal biomass, depending on the relative strength of the interactions between the omnivore and herbivores, omnivores and algae, and herbivores and algae. Cyclopoid copepods are omnivorous predators that feed in a selective, raptorial fashion and are known to prey on rotifers, cladocerans, Calanoid copepods, and copepod nauplii (Brand1 and Fernando 1975; Williamson 1980; Stemberger 1985). Cyclopoids are usually described as selecting smaller prey items from the available prey size spectrum (Brand1 and Fernando 1975; Gliwicz and Umana 1994). However, factors such as predator hunger, prey shape, hardness, and behavior, and the availability of alternate prey are at least as important as prey size (Li and Li 1979; Williamson 1980; Stemberger 1985). In general, cyclopoids select for soft-bodied species that lack defensive behaviors, and they become more selective when they are satiated. There is also evidence that at least some adult cyclopoids ingest algae, primarily diatoms (Adrian 1991). Cyclopoids are also capable of preying on ciliated protists (Williamson 1980; Wiackowski et al. 1994; Wickham in press). Although maximal predation rates can be as high as 180 ciliates copepod- l h- l, such ingestion rates are seen only at very high ciliate densities ( cells ml-r), which are rare in nature. Total planktonic ciliate abundance is usually ~20 cells ml-l (Pace and Orcutt 198 1; Pace 1986; Beminger et al. 1993) and at these concentrations cyclopoid predation rates are considerably lower (l-l 0 ciliates copepod- h- ; Wickham in press). The copepod nauplii, cladocerans, and some rotifers that cyclopoids prey upon are themselves capable of preying on ciliates (see Sanders and Wickham 1993). Given the selective and omnivorous nature of cyclopoid predation, it is unclear whether the direct predation impact of cyclopoids on ciliates is offset by a trophic cascade where cyclopoids prey on other metazooplankton, which then releases ciliates from predation pressure. Ciliates can be major herbivores, and metazooplankton predation on ciliates may be an important link between the classic and microbial food webs. The microbial food web consists of bacteria and autotrophic picoplankton, preyed upon by heterotrophic flagellates and ciliates, which then remineralize nutrients that are reutilized by algae and bacteria (Azam et al. 1983; Stockner and Porter 1988). At times, ciliates may have a grazing impact on phytoplankton equivalent to that of metazooplankton, while also being the major consumers of heterotrophic flagellates (Weisse et al. 1990). Flagellates are often the major consumers of bacteria (Sanders et al. 1989; Pace et al. 1990), so strong flagellate-ciliate and ciliate-metazooplankton links have the potential to display a trophic cascade from metazooplankton to bacteria. In this study, I examined the direct and indirect effects of cyclopoid copepods on ciliates, and whether these effects would be transmitted to the rest of the microbial food web. Two experiments were conducted in Schohsee, a moderately eutrophic lake in northern Germany, during fall The experimental containers were 2-liter PVC bottles, incubated in situ at 2-m depth. In both experiments, the cyclopoids used in the experiments were obtained from the lake the day before the experiment and incubated overnight in filtered lake water at 11 C. Only gravid females were used to ensure that animals were of the same sex, life stage, and roughly the same physiological condition, but all egg sacs were removed to prevent reproduction during the experiment. In both experiments, cyclopoid treatments had 10 cyclopoid liter- l. The experimental design in the first experiment was three levels of a cyclopoid treatment (either Cyclops abyssorum, Cyclops kolensis, or no cyclopoids), cross-classified with two levels of a zooplankton > 64-pm treatment (presence or absence). There were three replicates per treatment combination, giving a total of 18 bottles (3 copepod treatments x 2 zooplankton treatments x 3 replicates). Both C. abyssorum and C. kolensis are known to consume rotifers, copepod nauplii and copepodites, and cladocerans. C. kolensis is a small species (mean metasome length of adult females used in the experiment, mm; SE = 0.035; n = 104). C. kolensis has been shown to consume algae in addition to metazoan prey, and algae may constitute as much as 57% of its diet (Adrian 199 1). C. abyssorum is a larger species (mean metasome length of adult females used in the experiment, 1.20 mm; SE = ; n = 94) and is highly predatory, consuming rotifers, copepod nauplii and copepodites, and cladocerans at rates as high as 20 individuals d- l, about twice the predation rate found for C. kolensis (Adrian 199 1; van den Bosch and Santer 1993). To begin an experiment, I pumped water from 2-m depth into a 90-liter container with a hand-operated bilge pump. Some of the water was then poured through a 64- pm mesh into a second container in order to obtain water for the treatments lacking zooplankton >64 pm. Bottles were filled in random order, adding copepods into the appropriate bottles. The first experiment ran for 4 d (l- 5 October 1993), with a final water temperature of 12.3 C. Three initial samples of water with and without zooplankton >64 pm were taken at the beginning, middle, and end of filling the experimental bottles by filling 2-liter bottles in the same manner as the experimental bottles. Initial and final samples were processed in the same manner. Bacteria and flagellated protists were enumerated by fixing 5-ml samples in 2% (final concn) glutaraldehyde, filtering DAPI-stained cells onto 0.2-pm black polycarbonate filters, and counting them with epifluorescence microscopy. Autotrophic flagellates were differentiated from heterotrophs by their autofluorescence. Ciliates were counted by scanning all of 50-ml settled, Bouin sfixed samples on an inverted microscope. Chlorophyll a was determined spectrophotometrically after filtering 250 ml of water onto GF/F filters and using ethanol extraction

3 Notes 1175 (Nusch and Palme 1975). Metazoan zooplankton were sampled by passing the remaining 1.7 liters through a 30- pm mesh, fixing in sucrose Formalin, and then settling and counting on an inverted microscope. Data were analyzed in a 3 x 2 factorial ANOVA. Rather than testing the hypothesis that there was no difference between the three levels of the cyclopoid treatment (the cyclopoid main effect), the differences between the nocyclopoid control and each of the two cyclopoid treatments were tested in two contrasts. To test whether cyclopoid effects were independent of the concentration of other metazooplankton, I used two preplanned contrasts which tested the cyclopoid-metazooplankton interaction separately for each copepod. Specifically, the contrasts had the null hypotheses that (C. abyssorum, zooplankton present) - (no cyclopoids, zooplankton present) = (C. abyssorum, no zooplankton) - (no cyclopoids, no zooplankton) and (C. kolensis, zooplankton present) - (no cyclopoids, zooplankton present) = (C. kolensis, no zooplankton) - (no cyclopoids, no zooplankton), where the text within parentheses represents the organisms contained in the different treatment combinations. A second experiment was designed to determine whether there was a certain threshold zooplankton concentration at which C. abyssorum s effect on ciliate abundance changed from direct predation to enhancement through a trophic cascade. The experiment was also run in 2-liter bottles in the Schijhsee, incubated at 2-m depth. Five concentrations ofzooplankton >64 pm (0,0.25,0.5,0.75, or 1 times the natural concn) were cross-classified with the presence or absence of C. abyssorum. Where it was present, C. abyssorum was added at 10 liter-l. Two replicates were used per treatment combination, giving a total of 20 bottles. The experiment was run for 6 d (27 October-2 November 1994) with a final water temperature of 8.4 C. Bottles were filled in a manner similar to the first experiment. Water was pumped from 2-m depth into a 90-liter container. In random order, appropriate volumes of whole water and 64-pm filtered water were added to each bottle to obtain the correct dilutions, adding copepods to half the bottles. Two initial samples were taken for each dilution. Initial and final samples were processed in the same manner as in the first experiment. The data were analyzed in a 5 x 2 factorial ANOVA (five zooplankton dilutions vs. cyclopoid presence or absence). Differences in treatments with and without C. abyssorum at the different zooplankton dilutions were ascertained by conducting five preplanned contrasts. These were essentially the same as t-tests between the two cyclopoid levels at each zooplankton dilution, but using the pooled, experiment-wise, measure of variance in the denominator (the mean square error). Linear regression was used to test whether the dilutions had any effect on ciliate and metazooplankton initial abundances. The ciliate community in the C. abyssorum-c. kolensis experiment comprised 12 species, of which six made up, on average, 87% of initial and 94% of final total abundance. The mean initial, total ciliate abundance was 3.0 ciliates ml- l. Removing zooplankton with the 64-pm mesh did not significantly affect the initial ciliate densities (P = 0.18). Only four species (Strobilidium sp. 1, Strobilidium sp. 2, Strobilidium velox, and Urotricha sp.) had mean abundances > 1 cell ml-l in any treatment at the end of the experiment. The metazoan zooplankton community (metazooplankton) was comprised primarily of a Calanoid copepod, Eudiaptomus sp., copepod nauplii, the rotifer Keratella cochlearis, and low numbers of the cladocerans Bosmina longirostris and small (< 1.5 mm) Daphnia galeata (cladoceran maximum final abundance of 2 liter-l). Numbers of Calanoid copepods, copepod nauplii, and cladocerans were significantly reduced by screening the water through the 64-pm mesh (P < 0.05). Although the abundance of K. cochlearis was reduced from 10.5 to 5.6 ind. liter- by filtering water through the 64-pm mesh, this reduction was not significant (P = 0.12), due to high variance. Removal of zooplankton > 64 pm generally resulted in higher ciliate numbers, compared to metazooplanktonpresent treatments (Fig. 1). In treatments without metazooplankton (ignoring cyclopoid effects), total ciliate abundance was 1.6 times higher than in treatments with metazooplankton ( 15.6 vs. 9.7 ciliates ml- 1 ). If only treatments without cyclopoids are examined, in five of the six major ciliate taxa present, treatments without metazooplankton had higher numbers of ciliates than treatments with metazooplankton. However, significant (P < 0.05) main effects of metazooplankton on ciliates were found for only three species: Strobilidium spp. 1 and 2 and Urotricha. The paucity of significant main effects is due to interactions between metazooplankton and cyclopoid treatments. The two cyclopoid species had clear effects on the ciliate community, but these effects depended on the ciliate species, the cyclopoid species, and whether other metazooplankton were present. C. abyssorum had a similar impact on two of the oligotrich ciliate species (Strobilidium sp. 1 and Strombidium sp.) and to a lesser extent on a third oligotrich, Strobilidium velox. All three ciliates are relatively large. Strobilidium sp. 1 and Strombidium sp. both pm in diameter, while S. velox is somewhat larger, pm. When other metazooplankton were not present, C. abyssorum had a clear negative impact on oligotrich abundance compared to the no-cyclopoid controls (Fig. 1). However, when other metazooplankton were present, C. abyssorum had no impact on these three ciliate species. The C. abyssorum-zooplankton interaction was significant for Strombidium sp. and Strobilidium sp. 1 (P < 0.05) while a similar but nonsignificant trend was seen for S. velox (P = 0.10, Table 1). C. abyssorum had an effect on two small ciliate species, Urotricha sp. and Strobilidium sp. 2 (both ~20 pm in diameter), that was also dependent on the presence or absence of other metazooplankton. When metazooplankton were absent, there was no difference in ciliate abundance between treatments with and without C. abyssorum (Fig. 1). When metazooplankton were present, however, treatments with C. abyssorum had greater numbers of these two ciliates than did treatments without C. abyssorum. The effect was significant for Strobilidium sp. 2 (P = ), and while the effect was not significant (P

4 Notes L 3.5 E 3.0 e 2.5 % 2.0 z 1.5 -? g 0.5 z 0.0 -: F % $ absent Zooplankton present > 64 pm -s g 0.15 P El G : ":-i LO.4 ; 0.3 j 0.2 Ko.1 v _ absent Zooplankton.-...,... present > 64 pm Fig. 1. Final ciliate abundance in treatments with the presence or absence of Cyclops abyssorum, Cyclops kolensis, and metazooplankton. No cyclopoids added-cl---o; C. abyssorum present-a- - -A; C. kolensis present Vertical bars represent 1 SE. = 0.11) for Urotricha, the trend was similar and the probability of seeing a significant effect, if one were present, was low (power = 0.25). The effect of C. kolensis on the ciliate community was considerably simpler than that of C. abyssorum. C. ko- lensis reduced the abundance of two species, Strombidium sp. and S. velox, either significantly or nearly significantly (P = and 0.086, respectively). This reduction was independent of whether other metazooplankton were present or absent (P > 0.8, Fig. 1 and Table 1). In all other ciliate species except for the small Strobilidium sp. 2, C. kolensis had no significant impact on the ciliates, and the presence or absence of metazooplankton had no effect on this (Fig. 1, Table 1). The effect of C. kolensis on Strobilidium sp. 2 is rather puzzling: C. kolensis had a negative effect on Strobilidium sp. 2 when metazooplankton were present, but a positive effect when metazooplankton are absent. The same, though nonsignificant, trend (P = 0.11) was also seen with the larger Strobilidium sp. 1. There were also differences in the h;yclopoid impact on metazooplankton. The larger C. abyssorum had a much stronger impact than did the smaller C. kolensis. C. abyssorum s impact on copepod nauplii and Keratella was dependent on the metazooplankton concentration (P < 0.05, Fig. 2). The impact of C. abyssorum on nauplii and Keratella was proportionately greater when these animals were abundant (in the whole-water treatments) than when they were rare (in the 64-pm-screened treatments). The same trend was seen for Daphnia and calanoids, but numbers of these organisms were low and variance was high, resulting in nonsignificant effects (Fig. 2, Table 1). C. kolensis had no effect on calanoids or copepod nauplii (P > 0.05) and had a weak effect on Keratella and Daphnia (P = and P = , respectively; Fig. 2). Cyclopoid effects did not extend beyond metazooplankton and ciliates. Abundance of bacteria, autotrophic flagellates, and heterotrophic flagellates was not affected by either cyclopoid species or by metazooplankton (P > 0.1). Autotrophic flagellate abundance was slightly lower in treatments with cyclopoids than in noncyclopoid controls (2.67 x lo4 ml-l vs x lo4 ml-l), but even in a nonprotected posthoc test, the difference was at best Table 1. P-values of effects in the Cyclops abyssorum-cyclops kolensis-metazooplankton experiment. The C. abyssorum and C. kolensis main effects represent contrasts testing for significant differences between the means of no-cyclopoid treatments and each of the cyclopoid treatments. The cyclopoid-metazooplankton interactions are from the contrasts described in the text. Dependent variable Strombidium sp Strobilidium velox Strobilidium sp Strobilidium sp Urotricha sp Cyclidium sp Copepod nauplii <o.ooo 1 Calanoid copepods Daphnia Keratella cochlearis < Cyclopoid-metazoo- Main effects plankton interactions C. C. C. c. abyssorum kolensis Zooplankton abyssorum kolensis < <

5 Notes z5 $4 E3,:.,,. &.,,.,,j 62-0,/. 51.,.. H..,> I 30 ii,,. 7 u! G 5.0 E z absent present absent present Zooplankton > 64 pm Zooplankton > 64 pm Fig. 2. Final abundance of the major metazooplankton groups in the Cyclops abyssorum-cyclops kolensis-metazooplankton experiment. Symbols as in Fig. 1. marginally significant (P = 0.105). Chlorophyll a levels were also independent of the cyclopoid treatment but not of metazooplankton concentration. Chl a concentrations were slightly, but significantly, higher in treatments where metazooplankton were present (P = ). Treatments with metazooplankton had 3.7 pg Chl a liter-l; Chl a in treatments without metazooplankton was 3.07 pg liter-l. Ciliate abundance was somewhat lower in the second, C. abyssorum-metazooplankton abundance experiment, both at the beginning and in the peak final abundance. Initial total ciliate abundance was 2.9 ciliates ml-l, and there was no initial difference across treatments (P = 0.48). As designed, there was a positive linear relationship between initial metazooplankton densities and the proportion diluted (linear regression of metazooplankton groups vs. dilution proportion: P < 0.05, R2 = 0.5 l-0.94). In this experiment too, the effect of C. abyssorum on ciliates was dependent on the ciliate species and the presence or absence of metazooplankton. With only two replicates per treatment combination, variance was relatively high and the power to see real differences low. Nevertheless, strong effects were observed. Both Strobilidium sp. 1 and the larger S. velox showed differences in abundance between C. abyssorum-present and C. abyssorumabsent treatments that were dependent on the metazooplankton concentration (Fig. 3, Table 2). The metazooplankton concentration at which C. abyssorum no longer reduced ciliate abundance compared to no-cyclopoid treatments was different for the two ciliates. C. abyssorum had a strong effect on Strobilidium sp. 1 only when there were no other metazooplankton present; at 0.25 or more times the natural metazooplankton concentration, the difference between the C. abyssorum-present and C. abyssorum-absent treatments disappeared (Fig. 3, Table 2). In contrast, C. abyssorum depressed the abundance of the -: ; L. E 0.6 d 0.5 : 0.4 s g 0.1 Co o.oa : Zooplankton > 64 pm Zooplankton > 64 pm (proportion of natural community) (proportion of natural community) 0.5, z E 0.3 g b!!skl i5 i Fig. 3. Final ciliate abundance in the Cyclops abyssorummetazooplankton concentration experiment. C. abyssorum present-o; C. abyssorum absent-o. The x-axis is the proportion of unfiltered water (64 pm) used in the treatment. Vertical bars represent 1 SE. larger ciliate species, S. velox, at all concentrations of other metazooplankton except the highest. The treatment effects on the other four abundant ciliate taxa were less clear. C. abyssorum had no effect on Strombidium sp. or Cyclidium sp. (P > 0.24), although increasing metazooplankton concentration resulted in lower ciliate numbers relative to the no-metazooplankton treatment (P = and P = 0.017, respectively; Fig. 3). Urotricha and the small Strobilidium sp. 2 also had lower abundances with increasing metazooplankton concentration (P < 0.03). However, Strobilidium sp. 2, and to a lesser extent Urotricha, had higher abundances at the lowest two metazooplankton concentrations when C. abyssorum was present, rather than when it was absent. This effect was similar to that seen with Cyclidium in the first (C. abyssorum-c. kolensis) experiment. C. abyssorum had clear effects on K. cochlearis and copepod nauplii, but not on Calanoid copepodites and adults or on cladocerans (Fig. 4). When naupliar abundance exceeded 5 ind. liter-l (at 0.5 times natural abundance), C. abyssorum suppressed nauplii relative to non- C. abyssorum treatments. A similar effect was seen with K. cochlearis. Only when K. cochlearis levels in control

6 1178 Notes Table 2. P-values of effects in the Cyclops abyssorum-metaozoplankton concentration experiment. The cyclopoid-metazooplankton contrasts are from the contrasts described in the text. The proportion of natural community is the proportion of unfiltered water (64 pm) used in the treatment. Dependent variable Strombidium sp Strobilidium velox < Strobilidium sp Strobilidium sp < Urotricha sp Cyclidium sp Copepod nauplii <o.ooo 1 Calanoid copepods Daphnia Keratella cochlearis Main effects Cyclopoid-metazooplankton contrasts -Proportion of natural community c. Zooplankabyssorum ton ooo ooo ooo treatments exceeded - 5 ind. liter- 1 (at times natural abundance) was there a significant difference between control and C. abyssorum treatments (Fig. 4, Table 2). In contrast, C. abyssorum had no affect on Calanoid copepods at any of the abundances seen in the experiment. Calanoid abundance increased with the increase in the proportion of unfiltered water used in the treatment, but Calanoid abundances in treatments with and without C. abyssorum at any given dilution were nearly identical. As in the first experiment, C. abyssorum also had no affect on cladocerans (Bosmina and small Daphnia galeata), but cladoceran abundance was much lower than Calanoid abundance, and variance was much higher. In none of the treatments did Daphnia abundance exceed 1.5 ind. liter - 1. These experiments show that cyclopoids can affect cil k 121 T Zooplankton > 64,um Zooplankton > 64 pm (proportion of natural community) (proportion of natural community) Fig. 4. Final abundance of the major metazooplankton groups in the Cyclops abyssorum-metazooplankton concentration experiment. Symbols as in Fig. 3. iate communities, and that these effects depend not only on the cyclopoid and ciliate species, but also on the presence of alternate prey for cyclopoids. Depending on these factors, ciliate response to cyclopoid addition was consistent with suppression due to direct cyclopoid predation (e.g. C. abyssorum-s. velox when no other metazooplankton were present), enhancement through cyclopoid predation on other predators of ciliates (e.g. C. abyssorum- Urotricha), or no response at all (e.g. C. kolensis-urotricha). For C. abyssorum, the direct predation effects were most pronounced with the larger ciliates. If only those treatments in which other metazooplankton had been screened out are considered, in the first experiment, the reduction in ciliate numbers with the addition of C. abyssorum was greatest for Strobilidium sp. 1, S. velox, and Strombidium (all between 30 and 50 pm in size) and small to nonexistent for Strobilidium sp. 2 and Urotricha (both pm), while Cyclidium (- 20 pm long) abundance was somewhat higher when C. abyssorum was added. This size-dependent predation is consistent with the observations of Wiackowski et al. (1994) who found that Diacyclops imposed greater mortality on larger ciliates, rather than on smaller ones. In contrast, Wickham and Gilbert (199 1, 1993) and Jack and Gilbert (1993) found that filter-feeding cladoceran zooplankton had their greatest affect on small ciliates. The difference is likely to be due to a combination of cladocerans being relatively nonselective predators, especially in comparison to cyclopoids, and the larger ciliates being large enough to be difficult for cladocerans to ingest. Burns (1968) regression of maximum particle diameter cleared vs. Daphnia size predicts that a 2-mm Daphnia could clear a particle no larger than 49 pm. Although this was calculated with rigid beads, and soft-bodied ciliates may be easier to ingest, it is nonetheless plausible that larger ciliates suffer proportionately less predation than smaller ciliates from suspension-feeding cladocerans, while smaller ciliates are less vulnerable to cyclopoid predation. However, Carrick et al. (199 1) found that when the natural zooplankton com-

7 Notes 1179 munity was made up of Calanoid copepods, small protists were grazed at a higher rate than large protists. Therefore, small size may be a refuge only from cyclopoid predation and not from copepod predation in general. There was some evidence for a trophic cascade from cyclopoids to ciliates through metazooplankton, but this was present for only two small ciliate species, Urotricha and Strombidium sp. 2, and only when C. abyssorum was the top predator. For these two species, presence of C. abyssorum had no affect when other metazooplankton were absent, but C. abyssorum enhanced ciliate abundance when other metazooplankton were present. This combination of effects is also consistent with C. abyssorum confining its direct predation effects to larger ciliates and metazooplankton. It would seem that a cyclopoidmetazooplankton-ciliate trophic cascade only appears when the ciliates are small enough to be relatively immune from cyclopoid predation but vulnerable to predation by other metazooplankton. For three of the other common species (Strombidium, S. velox, and Strobilidium sp. I), the addition of C. abyssorum when other predators on ciliates were present did not have the positive effect expected from a trophic cascade. Instead, C. abyssorum had no net effect when other metazooplankton were present and a strongly negative impact in treatments where zooplankton >64 pm had been largely screened out (Fig. 1). The second experiment suggests that when this change from no impact to a negative impact occurs, the switch is at a higher metazooplankton density for larger ciliates. C. abyssorum continued to depress S. velox abundance relative to no-cyclopoid controls at all metazooplankton concentrations less than the natural concentration. In contrast, it was only at the lowest proportion of the natural metazooplankton community (where only K. cochlearis was present at an abundance > 1 liter-l) that C. abyssorum significantly depressed the abundance of the much smaller Strobilidium sp. 1. The change in the impact of C. abyssorum on ciliates, depending on the abundance of other metazooplankton, was consistent with switching behavior by C. abyssorum. Marine Calanoid copepods are known to switch between algal and metazoan prey, depending on the relative abundance of the two (Landry 198 l), but C. abyssorum in these experiments seems to switch between ciliate and metazoan prey. In the first experiment, C. abyssorum had a much greater impact on copepod nauplii, Daphnia, and Calanoid copepods when they were abundant compared to when they were rare. Clearance rates can be calculated from the differences in the natural logarithms of the final abundances in the Cyclops and no-cyclops treatments, and from clearance rates, electivities can be calculated (Chesson 1983). Electivities range from - 1 to + 1 and are a measure of whether a prey item is grazed in proportion to its relative abundance. Although some caution must be used when interpreting clearance rates or electivities calculated from low numbers of prey, the data do support a switching hypothesis. In the whole-water treatments, where metazooplankton were abundant, C. abyssorum had a negative electivity for Strombidium, Strobilidium sp. 1, and S. velox (electivity = -0.6, -0.7, and -0.3), indicating that these prey were grazed at rates less than would be expected from their relative abundance. In the 64-pm-screened treatments, where metazooplankton were rare, C. abyssorum grazed these three ciliates at rates higher than expected by their relative abundance (electivity = 0.3, 0.2, and 0.4). In contrast, the electivity for nauplii, Daphnia, and calanoids was negative in the 64-pm-screened treatments, where they were rare (electivity = -0.4, -0.3, and - l), but in the whole-water treatments, where metazooplankton were abundant, electivity for Daphnia was positive (electivity = 0.4) and it was neutral for nauplii and calanoids (electivity = 0). Thus, it would seem that C. abyssorum captures ciliate prey when metazooplankton are rare, but largely ignores them when metazooplankton are abundant. In the second experiment, the alternate prey for C. abyssorum appeared to be copepod nauplii and K. cochlearis. K. cochlearis has been described as being selected against by Diacyclops (Stemberger 1985), but in that study the alternate prey was Synchaeta, a soft-bodied rotifer that is considerably larger than the ciliates seen in my study. Although Keratella may not be an optimal prey item for C. abyssorum, choices of alternate prey were limited. Had there been higher abundances of small cladocerans and rotifers such as Synchaeta, that are known to prey on ciliates but are also vulnerable to cyclopoid predation, then it is possible that a trophic cascade would have been more evident. However, the evidence from these experiments suggests that while the metazooplankton present were capable of reducing ciliate abundance, simply adding an invertebrate predator is not necessarily enough to induce a trophic cascade. Ciliate size alone is clearly not sufficient to explain all the differences in the impact of cyclopoids on ciliates, even when there were no differences in the metazooplankton community. Cyclidium in the first experiment and Urotricha and Strobilidium sp. 2 in the second experiment are more abundant when C. abyssorum is present and metazooplankton are absent than when both C. abyssorum and metazooplankton are absent. All three of these species are small (5 20 pm), but this effect is consistent with neither direct predation nor a trophic cascade. These three ciliates are likely to be capable of preying on bacteria as well as flagellates (Fenchel 1980; Sherr et al. 1986) and may have benefited from some combination of nutrient regeneration enhancing bacterial and flagellate production and a reduction in other, larger predators on flagellates. However, the design of the experiments was such that these simultaneous effects, if they were occurring, could not be ascertained. In general, the smaller C. kolensis did not have an impact on either ciliates or other metazooplankton that was strongly dependent on metazooplankton concentration. C. kolensis had moderate affects on the abundance of Keratella and Daphnia, but these effects were independent of the relative abundance of these two species.

8 1180 Notes Similarly, the effect of C. kolensis on Strombidium and S. velox was independent of other metazooplankton, and there was no affect at all on Cyclidium or Urotricha. However, C. kolensis reduced the abundance of Strombidium sp. and S. velox by about the same amount when these two species were abundant as when they were relatively rare (Fig. 1). Therefore, while the ingestion rates may be equal, the clearance rate for these two ciliates was higher when they were less abundant. Similarly, Strobilidium spp. 1 and 2 were slightly more abundant when both C. kolensis and metazooplankton were absent, but slightly reduced relative to no-cyclopoid treatments when only metazooplankton were present (Fig. 1). In contrast to C. abyssorum, C. kolensis seems to ingest ciliates at a rate that is largely independent of either alternate prey or ciliate abundance. Both cyclopoid species had strong impacts on ciliates and metazooplankton, but this did not cascade down as far as flagellates or bacteria. A trophic cascade extending as far as bacteria seems to occur only when there are high abundances of Daphnia. Moderate changes in Daphnia density or changes in metazooplankton other than Daphnia does not result in changes in bacterial abundance, even where there are changes in Protist abundance (Pace and Funke 199 1; Wickham and Gilbert 199 1, 1993). In my experiments, Daphnia abundance was low, and addition of cyclopoids did not produce large changes in their abundance. The more abundant calanoids and copepod nauplii ingest flagellates at considerably lower rates than do Daphnia (reviewed by Sanders and Wickham 1993) and are unlikely to be major bacterial grazers (Sanders et al. 1989). There was also no trophic cascade from the cyclopoids through the other metazooplankton, leading to an increase in chlorophyll a. Instead, in the first experiment, chlorophyll a values were marginally (0.63 hg liter-, or 17%) but significantly lower when metazooplankton had been reduced and ciliate abundance enhanced. This suggests that the approximate 6 cells ml- 1 reduction in ciliate numbers between treatments with and without metazooplankton has a greater impact on algal biomass than the change in cladoceran, rotifer, and copepod numbers. Ciliate clearance rates are quite variable, but 6 ciliates ml- I, each clearing 5 ~1 h- l, would clear 3% of the water column per hour (Jonsson 1986). This estimated impact is consistent with the findings of Weisse et al. (1990), who reported ciliates to be the major herbivore group during the early part of the spring phytoplankton bloom in Lake Constance. In the second experiment, the addition of C. abyssorum did not produce increased chlorophyll a levels, but maximum total ciliate abundance was only 59% of that in the first experiment, so there was less scope for increased ciliate herbivory. In both experiments, however, metazooplankton numbers were low, increasing the relative importance of ciliate herbivory and reducing the potential for a strong trophic cascade. Although trophic cascades may structure many aquatic communities, Sprules and Bowerman (1988) have documented the prevalence of omnivory in aquatic systems. In this study, the cyclopoids seemed to switch their prey preferences among ciliates and metazooplankton, depending on relative abundances. At least some of the metazooplankton were capable of preying on algae, ciliates, and flagellates. Ciliates were likely to have preyed on algae, flagellates, and bacteria. Given that the Cyclops in these experiments had such varied effects on different herbivores, it is perhaps not surprising that there is no clear cascade from carnivorous cyclopoids through other metazooplankton and ciliates, resulting in an increase in algae or bacteria. Max-Planck-Institut Postfach Plijn Germany References ftir Limnologie Stephen A. Wickham ADRLAN, R The feeding behaviour of Cyclops kolensis and C. vincinus (Crustacea, Copepoda). Int. Ver. Theor. Angew. Limnol. Verh. 24: AZAM, F., AND OTHERS The ecological role of watercolumn microbes in the sea. Mar. Ecol. Prog. Ser. 10: BERNINGER, U.-G., S. A. WICKHAM, AND B. J. FINLAY Trophic coupling within the microbial food web: A study with fine temporal resolution in a eutrophic freshwater ecosystem. Freshwater Biol. 30: BRANDL, Z., AND C. H. FERNANDO Food consumption and utilization in two freshwater cyclopoid copepods (Mesocyclops edax and Cyclops vicinus). Int. Rev. Gesamten Hydrobiol. 60: 47 l-494. BURNS, C. 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Acknowledgments I thank Dietmar Lemke for the chlorophyll measurements, Barbara Santer for help in copepod identification, and Uhike Beminger, William DeMott, and Winfried Lampert for comments on an earlier version of the manuscript.

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