The effect of Daphnia interference on a natural rotifer and ciliate community: Short-term bottle experiments

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1 Limnol. Oceanogr., 34(3), 1989, I (0 1989, by the American Society of Limnology and Oceanography, Inc. The effect of Daphnia interference on a natural rotifer and ciliate community: Short-term bottle experiments John J. Gilbert Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire Abstract Two bottle experiments were conducted using water from a small, cutrophic lake to assess the impact of Daphnia interference (encounter) competition on the dynamics of the rotifer and ciliate populations dominating the zooplankton community. Indirect effects of Daphnia exploitative competition were minimized by adding food resources and using short incubation periods. The introduction of Daphnia pulex (16 liter- ) to Cryptomonas-enriched (3-3.5 x lo4 cells ml I) water for 2 d significantly suppressed numbers of the ciliate Campanella sp.; the rotifers Kellicottia bostoniensis, Keratella cochlearis, Keratella crassa, Polyarthra vulgaris, and Synchaeta pectinata; and total rotifers. The presence of Daphnia did not affect numbers of the rotifers Asplanchna girudi, Polyarthra euryptera, and Trichocerca similis. Daphnia-induced death rates of significantly suppressed, common species were highest for K. cochlearis (0.79-l. 14 d-l), intermediate for S. pectinata and Campanella ( d-l), and lowest for K. crassa and P. vufgaris ( d- I). The most marked effects of Daphnia on community structure were 61-77% reductions in the relative abundance of K. cochlearis and up to 100% increases in the relative abundances of K. crassa and T. similis. The observed effects of Daphnia on the rotifer species are consistent with, and largely explicable by, previously conducted laboratory experiments and behavioral observations on interference between Daphnia and these or closely related species. Interference from Daphnia can rapidly reduce the abundance and shift the species structure of rotifer and ciliate assemblages in natural communities. Much direct and circumstantial evidence from field and laboratory studies indicates that rotifers are strongly suppressed by competition from large species of Daphnia (reviewed by Gilbert 1988a). This suppression probably is usually due to a combination of exploitative competition for shared resources (Gilbert 1985~1) and interference or encounter competition in which rotifers swept into Daphnia s branchial chamber are eaten or, more likely, rejected in damaged condition (Gilbert and Stemberger 198 5; Burns and Gilbert 1986a,b; Gilbert 19883). To date, experiments on Daphnia interference have been with laboratory populations of rotifers, usually only one or two species at a time. These experiments showed that rotifer species vary greatly in their susceptibilities to Daphnia interference (Gilbert 19886) and that naturally occurring Acknowledgments Supported by National Science Foundation research grants BSR and BSR 87-l I thank Maxine Bean for technical assistance, A. P. Saunders-Davies for the gift of his MK II compressor, W. Foissner for identifying the ciliate Campanella, and K. L. Kirk, H. J. MacIsaac, P. A. Jumars, R. S. Stemberger, and three anonymous referees for improving the manuscript. densities of Daphnia can impose sufficiently high mortality rates on populations of the most susceptible rotifer species to more than offset even their maximal potential reproductive rates (Burns and Gilbert 1986a; Gilbert 1988a, b). Observations of behavioral interactions between Daphnia and various rotifer species revealed some mechanistic bases for the differential susceptibilities of these species (Gilbert 1987, 1988b). The present study is an attempt to assess the potential of Daphnia interference to affect the species structure of a natural zooplankton community dominated by a variety of rotifer species and a ciliate species. As well as being the first analysis of the impact of Daphnia interference on the dynamics of natural rotifer populations, it is the first analysis of the effect of Daphnia interference on the dynamics of a ciliate population. Since small and soft-bodied rotifers tend to be the most susceptible to 606 ingestion and damage by Daphnia (Gilbert 1988b), similarly sized and, especially, smaller ciliates should also be susceptible to this interference. Large cladocerans are known to eat ciliates (Tezuka 1974; Porter et al. 1979), and they could damage ciliates rejected from their branchial chambers. The

2 Daphnia interference efects 607 SP.. K.b. Ts. K.cr. l?e. C. I Fig. 1. Rotifer and ciliate species in Star Lake (June 1987), with introduced Daphnia. Abbreviations and body lengths (pm) (mean + 1 SD) of preserved (acid Lugol s solution) or live specimens. Preserved: D.p. - Daphnia pulex, 2,750& 100, 2,8 lok90, experiments 1 and 2; Kb. - Kellicottia bostoniensis, 107 f 2 without spines, with spines; K.co. - Keratella cochlearis f. typica, without spines, 119 k 5 with spines; K. cr. -Keratella crassa, 112&4 without spines, 173*6 with spines; P.e. - Polyarthra euryptera, 156 f 8; P. v. - Polyarthra vulgaris, 100&6. Live: A.g.-Asplanchna girodi, 720f 105; C.-Campanella sp., 122*7; S.p. -Synchaeta pectinata, 3 1 O-+ 18; T.s. - Trichocerca similis, without spines and tots, 195 rt 7 with spines and toes. design of the study was to conduct laboratory bottle experiments testing the effect of the addition of Daphnia pulex on the dynamics of the rotifer and ciliate populations from the study lake. Possible effects of exploitative competition from the Daphnia on these microzooplankton were minimized by supplementing lake water in the bottles with algal food and by using short incubations. Materials and methods On two dates in late June 1987, experiments were conducted to determine the effects of Daphnia addition on the zooplankton communities from Star Lake-a small, cutrophic, soft-water lake in Norwich, Vermont, described by Allen (1972). These communities consisted primarily of seven or eight rotifer species and a free-swimming stage of a species of the peritrich ciliate, Campanella (Fig. 1). Colonies of Campa- nella were found attached to aquatic macrophytes. The only crustaceans present were the copepods Tropocyclops prasinus and Diaptomus spatulocrenatus- mostly copepodites and some adults. Populations of the cladoceran D. pulex developed later in the season. Experiment I -Although second chronologically, experiment 1 was the most environmentally controlled and extensively replicated experiment. It began on 29 June. Lake water (2 1.5 C) was collected about 15 cm below the surface in plastic carboys and transported to the laboratory within 1 h. The water was poured through a 363+mmesh nylon screen to remove the larger copepods and then supplemented with Cryptomonas sp. to minimize the potential for exploitative competition for limiting food resources during the incubation period of the experiment. This species of Cryptomo-

3 608 Gilbert nas (N 9 X 1 Oe5 pg cell- I) is efficiently eaten by many species of rotifers and cladocerans (Bogdan and Gilbert 1987) and also has supported long-term population growth in many species of those taxa (Gilbert 1985a, 1988b, unpubl.; Gilbert and Stemberger 1985; Stemberger and Gilbert 1985). Cryptomonas was cultured on sterile-filtered (Millipore GS, 0.22 pm) Woods Hole MBL medium (Nichols 1973) as modified by Stemberger (198 1). A dense culture of this alga (4.4 X lo5 cells ml-l) was added directly to the lake water (1 : 11.5, v : v) to give a final concentration of 3.5 X 1 O4 cells ml- l (m 3.1 pg ml- )-a concentration near or well above that required for Rr,,, in planktonic rotifers at 20 C (Stemberger and Gilbert 1985). The cell density of the Cryptomonas culture was determined with an electronic particle counter (Particle Data Inc.). Narrow-mouthed, screwcapped, 250-ml bottles of clear glass were used for the tests. The exact volumes of all bottles were recorded and ranged from 250 to 252 ml. Such small-volume systems were selected for several reasons. First, the organisms in the community were small and probably unlikely to be adversely affected by the small container size. Second, the bottles could be mounted on a plankton wheel for rotation. Third, the natural population densities of the rotifers and ciliates were sufficiently high so that the population sizes of most species in the bottles were large and quite similar among replicates. Fourth, the population sizes of even the most common species in the bottles were not too large to prohibit counting all individuals. Preliminary experiments showed that subsamples of preserved zooplankton communities from somewhat larger bottles (500 ml) were very heterogeneous because the seston formed large aggregates; the high among-replicate variability of population-size estimates determined from such subsamples decreased sensitivity to treatment effects. Eighteen bottles were completely filled with the well-mixed, 363-pm-mesh-filtered, Cryptomonas-enriched lake water. Six, the initials, were immediately processed to assess the zooplankton community and food resources at the start of the experiment. A sample of about 80 ml was withdrawn through a 25-pm-mesh nylon screen to eliminate the zooplankton and large particles apt to clog the 48-pm-diameter aperture of the electronic particle counter. This sample was stored at 4 C for up to 2 h until it could be analyzed for density and size distribution of particles with equivalent spherical diameters in the 4-l 2-pm range. Particle counts were made on three 0.2-ml subsamples from each of the bottles. The zooplankton in each bottle was then concentrated to a volume of ml, by gently aspirating water through a 25+mmesh screen, and preserved in acid Lugol s solution. The other 12 bottles were for the incubation treatments, control and experimental, designed to determine the effects of introduced D. pulex on the zooplankton community and food resources. The six experimental bottles were each inoculated with four large D. pulex ( mm body length, measured after the incubation) cultured in the laboratory on Cryptomonas sp. after Gilbert (1985a). The six control bottles were left without D. pukex. Each of the 12 bottles was sealed, usually without an air bubble, with a double layer of Parafilm (American Can Co.) underneath the screwcap. They were then grouped into six numbered pairs, each consisting of a treatment and a corresponding control bottle, and placed on the 0.6-m-diameter plankton wheel set in an environmental chamber at 20 C with a photoperiod (15 : 9 L/D, -300 lux). The motor of the wheel, which was connected to an intervalometer (Dimco Gray Co., model 45 l), rotated the wheel at 1 rpm for 80 s every 10 min to prevent stratification of the plankton. After incubation periods of 1.92-l.99 d, the pairs of control and experimental bottles were removed from the plankton wheel and processed in numerical order. Procedures to obtain samples for particle counting and to concentrate and preserve zooplankton were identical to those used for the initial bottles. The D. pulex individuals in the experimental bottles were removed with a pipette before zooplankton concentration and measured alive from head to base of tail spine to the nearest 39 pm with a stereomicroscope. With one exception, rotifers in the

4 Daphnia interference efects 609 preserved zooplankton communities were differentiated to species and enumerated. Individuals of Polyarthra dolichoptera could not be easily distinguished from those of Polyarthra vulgaris at low magnification; they were included in, and constituted < 10% of, the P. vulgaris counts. Body dimensions of the rotifer and ciliate species were determined both from live individuals and from individuals preserved in acid Lugol s solution. Preserved individuals were taken from collections made the day the experiment started and were measured to the nearest 5 pm with a compound microscope. Live individuals were collected at a later date (3 1 July 1987) immobilized without compression in a Saunders-Davies MK II compressor, and measured to the nearest 10 or 19 pm with a stereomicroscope while fully extended. The effect of D. pulex on the zooplankton community was determined by comparing the mean population sizes of the different species in the experimental and control treatments (Student s t-tests, 2-tailed), comparing the frequency distributions of individuals of the different species in the experimental and control treatments, and calculating the Daphnia-induced death rates of significantly suppressed species from their population growth rates in the expcrimcntal and control treatments. The growth rate of a population (r) was calculated from r= In N, - In No t (1) where N1 and No are final and initial population sizes and t is time in days. Six replicate r-values were calculated for each population in both the experimental and control bottles from the six sets of initial, experimental, and control bottles. The mortality rate of a population caused by the presence of Daphnia (d& was calculated from dd = r, - r, (2) where r, and r, are the population growth rates in the control and experimental bottles. Birth rates of the rotifer species and fission rates of the ciliate species were assumed to be the same in the control and experimental bottles. Six replicate values of dr, were obtained for each species from the six pairs of control and experimental r-values. Diflerences in values of dd among populations were analyzed with both parametric ANOVA and a nonparametric equivalent. Differences in mean values of dd between all pairs of species populations were analyzed by calculating the least significant difference (Sokal and Rohlf 198 1). Experiment 2 - Experiment 2 began on 24 June. The zooplankton community was very similar to that in experiment 1, except that Polyarthra euryptera was present and P. dolichoptera constituted a greater proportion of the individuals in the P. vulgaris counts. Procedures for this experiment were similar to those for experiment 1. The major difference resulted from a failure in the refrigeration unit in the environmental chamber. After 6 h, the temperature began to increase. After 10 h, when the temperature had increased to 27 C, the plankton wheel was removed from the chamber and placed at room temperature (2 lo-24 C) for the remainder of the experiment. One experimental and one control bottle burst from thermal expansion, leaving only five replicates for each of these treatments. There were several more minor differences in procedure as well: the lake water (2 1 C) was not filtered through nylon mesh; the final density of the Cryptomonas added to the lake water was 3 x 1 O4 cells ml- 1 (N 2.6 pg ml-- I); the sizes of the D. pulex added to the experimental enclosures ranged from 2.66 to 3.00 mm; and the incubation period ranged from 1.96 to 2.02 d. Results The communities in the experimental bottles incubated with Daphnia were very different from those in the control bottles incubated without Daphnia (Tables 1 and 2). The presence of Daphnia caused significant reductions in population sizes of the ciliate Campanella, total numbers of rotifers, and population sizes of the rotifers Synchaeta pectinata, Keratella cochlearis, Keratella crassa (exp. 1 only), P. vulgaris (exp. 2 only), and Kellicottia bostoniensis (exp. 2 only). The presence of Daphnia had no significant effect on the population sizes

5 610 Gilbert Table 1. Effect of Daphnia pulex addition on the rotifer and ciliate populations of Star Lake. Experiment 1 (29 June 1987). Data are mean population sizes f 1 SD (from total counts of six replicate 250-ml bottles) of rotifer, ciliate, and crustacean species just before the start of the experiment (initial bottles) and after incubation for d at 20 C without Daphnia (control bottles) and with four Daphnia ( mm body Icngth) (experimental bottles). Asterisks indicate that differences in mean population sizes between control and cxpcrimental bottles are significantly different (Student s t-test). Population size (No. per 250 ml) Synchaeta pectinata Keratella crassa Keratella cochlearis Pol.varthra vulgaris Asplanchna girodi Trichocerca similis Kcllicottia bostoniensis Total rotifers Campanella sp. Copepods (copcpoditcs and adults) Daphnia pulex Adults Neonates Initial Control Exptl t-0.4 5l-c f , * ** 67+23*** *** 229f ** 8-t k o of Trichocerca similis, Asplanchna girodi, and P. euryptera. The presence of Daphnia had an impact on the frequency distributions of species in the community. Conversion of the pooled numbers of individuals in each species to percentages of total numbers (Table 3) shows that in both experiments 1 and 2 the experimental communities with Daphnia had much lower proportions of K. cochzearis and somewhat higher proportions of S. pectinata, K. crassa, and T. similis than the control communities without Daphnia. In experiment 1, mean Daphnia-induced death rates of the four significantly affected species ranged from 0.14 d-l (K. crassa) to 0.79 d -l (k. cochlearis) (Table 4, Fig. 2). The four groups of death rates had equal Table 2. Effect of Dahnia pulex addition on the rotifer and ciliate populations of Star Lake. Experiment 2 (24 June 1987). Data are mean population sizes + 1 SD (from total counts of five or six replicate 250-ml bottles) of rotifer, ciliate, and crustacean species just before the start of the experiment (six initial bottles) and after incubation for d at C without Daphnia (five control bottles) and with four Daphnia ( mm body length) (five experimental bottles). Asterisks indicate that differences in mean population sizes between control and experimental bottles are significantly different (Student s t-test). Synchaeta pectinatn Keratella crassa Keratella cochlenris Polyarthra vulgaris Polyarthra euryptera Asplanchna girodi Trichocerca similis Kcllicottia bostoniensis Total rotifers Campanella sp. Copepods (copepodites and adults) Daphnia pulex Adults Neonates Population size (No. per 250 ml) ~~--- Initial C ontrol Exptl t , k36 7k4 1,277*113 19O-t *51 24k6 6k t4 2, *** f20*** 195fl8*** 22a5 2k k2* 1,217&199*** 847& *** t-4 0 4&l

6 Daphnia interference efects 611 Table 3. Frequency distributions (as percentages of total individuals in pooled replicates) of rotifer and ciliate species in bottles incubated for -2 d without Daphnia pulex (control treatment) and with D. pulex (experimental treatment). Zeros indicate presence of species at ~0.5%; dashes indicate abscncc of species. Species abbreviations: S.p. -Synchaeta pectinata; K.cr. - Keratella crassa; K.c. - Keratella cochlearis; P. v. - Polyarthra vulgaris; P.e. - Polyarthra euryptcra; A.g. -Asplanchna girodi; T.s. - Trichocerca similis; K.b. - Kellicottia bostoniensis; C. - Campanella sp. Percentage of total number of individuals Exp. and lrcatmcnt sp. K. cr. K.C. P. v. P. c. /Lg. 7:s. K.b. C. I control expcrimcntal control experimental variances (F,,, = 8.0 with 5 df, P > 0.05) and were normally distributed (Kolmogorov-Smirnov z = 0.1 l-0.22, P I 0.3). There were significant differences among these groups, as judged both nonparametrically (Kruskal-Wallis H = 16.1 with 3 df, P = 0.001) and parametrically (ANOVA F = 25.0 with 3 and 20 df, P < 0.001). Pairwise comparisons revealed that the mean dd values of K. crassa, S. pectinata, and Campanella were significantly lower than the mean & value of K. cochlearis (P < 0.001) but were significantly different from each other (P < 0.05) in only one case (K. crassa and Campanella). The mean values of dr, for S. pectinata, K. cochlearis, and Campanella were similar to the r-values for these species in the control bottles without Daphnia (Table 4). The mean value of dd for K. cochlearis was almost four times the r-value for this species in the control bottles (Table 4). In experiment 2, mean Daphnia-induced death rates of the four, abundant, signihcantly affected species ranged from 0.27 d- 1 (Campanella) to 1.14 d-l (K. cochlearis) (Table 4, Fig. 2). These four groups of death rates had equal variances (Fmax = 2.8 with 4 df, P > 0.05) and apparently normal distributions (Kolmogorov-Smirnov z = , P L. 0.3). There were significant differences among these groups (Kruskal-Wallis H = 11.O with 3 df, P = ; ANOVA FE 79.0 with 3 and 16 df, P < 0.001). Pairwise comparisons showed that the mean dd values of P. vulgaris, S. pectinata, and Campanella were significantly lower than the mean dd value of K. cochlearis (P < 0.001) but were not significantly different from one another (P > 0.05). Mean values of dd for S. pectinata and Campanella were somewhat lower than the r-values for these species in the control bottles without Daphnia (Table 4). The mean dd values of P. vulgaris and K. cochlearis were about seven and five times higher than the r-values for these species in the control bottles (Table 4). In experiment 1, the mean (-+ 1 SD) den- Table 4. Population growth rates (r d I) and Daphnia-induced death rates (d,, d-l) of common rotifer and ciliate species having significantly lower population densities in experimental bottles with Daphnia pulex than in control bottles without D. pulex. All values are means + 1 SD. Six replicates in experiment 1 and five in experiment 2. Replicate values of d,, calculated by subtracting r-value in each experimental replicate from that in paired, control replicate. Exp Synchaeta pectinata Keratclla cochlearis Keratella crassa Pol.yarthra vulgaris Campanella sp. rd I Control Experimental d,,d 0.29kO kO kO kO &O a0.1 I 0.22kO kO &O ko t ko kO kO ko ko ko f-o ko kO kO kO kO. 11

7 612 Gilbert 1.3, I , w = gj E o $ $3 O- 3 s q EXPERIMENT I 6ii EXPERIMENT l- o z w 4- u 5 p. 2- I + o- + k Cryptmonos sp. INITIAL EXPERIMENTAL CONTROL I 1 I, I IO PARTICLE SIZE (pm ESDI Fig. 3. Particle size frequency distributions of a freshly prepared suspension of Cryptomonas sp., lake water in initial bottles, and lake water in experimental (with Daphnia) and control (no Daphnia) bottles after -2 d of incubation in experiment 1. Fig. 2. Mean Daphnia-induced death rates ofabundant rotifer and ciliate species in bottle experiments. Error bars show 1 SD. sities of particles (4-12 pm equivalent spherical diameters) in the six initial bottles and in the six control (no Daphnia) and experimental (with Daphnia) bottles at the end of the incubation period were 4323, , and 22 & 4 particles ~1~ I. The particle density in the control bottles probably did not change during incubation because losses to grazing were offset by cell reproduction. The approximately 50%, and significant (Student s t-test, P < O.OOl), reduction in particle density in the experimental bottles certainly was due to Daphnia grazing. Most of the particles in the initial bottles were the introduced Cryptomonas cells. These algae were added at a density of - 35 cells pl- I. The other 8 particles ~1 -l were those in the natural lake water. The analysis of particle size frequency distributions (Fig. 3) confirms that most of the particles in the initial bottles were the introduced Cryptomonas cells and shows that Daphnia in experimental bottles caused selective depletion of particles in this size range. In experiment 2, the mean (t- 1 SD) densities of particles in the six initial and five control and experimental bottles were 42 -t 5, 32+4, and 7f2 particles ~1~I. The de- creases in particle density during incubation were moderate but significant (Student s t-test, P < 0.01) in the control bottles and very substantial (83%) in the experimental ones. Examination of the particle size frequency distributions and direct microscopic observations at the end of the experiment showed that Cryptomonas cells had been almost completely removed in the experimental bottles with Daphnia but were still abundant in the control bottles. Discussion In two consecutive experiments, introduction of D. pulex to Cryptomonas-enriched Star Lake plankton communities led to significant changes in rotifer and ciliate populations after only 2 d. Comparisons of population sizes in experimental bottles with Daphnia and in control bottles without Daphnia (Tables 1 and 2) showed that the presence of Daphnia caused about 50%, and significant, reductions in the numbers of the ciliate Campanella; about 509 0, and significant, reductions in the total numbers of rotifers; and significant reductions in the numbers of many, but not all, of the component rotifer species. The only rotifer species unaffected by the Daphnia in both experiments were T. similis and A. girodi. Polyarthra euryptera, present only in experiment 2, was also unaffected. Polyarthra vulgaris and K. bostoniensis were significantly suppressed in

8 Daphnia interference efects 613 experiment 2 but not experiment 1. Keratella crassa was significantly suppressed in experiment 1 but not in experiment 2. Synchaeta pectinata and K. cochlearis were significantly suppressed in both experiments. The mean Daphnia-induced death rates ofthe five abundant rotifer and ciliate species significantly suppressed by the Daphnia ranged from 0.14 d-l (K. crassa, exp. 1) to 1.14 d- I (K. cochlearis, exp. 2) (Table 4, Fig. 2) and showed significant among-group differences. In both experiments, the value of d,, for K. cochlearis was significantly greater than those of the other three affected species. The d,, values of these other species were not significantly different from one another in either experiment except in one case (K. crassa and Campanella, exp. 1). The death rates imposed by D. pulex generally were high compared to the ability of the species to reproduce in the absence of Daphnia (Table 4). The disparity was especially large for K. cochlearis and, in experiment 2, for P. vulgaris. The population growth rates of S. pectinata, K. cochlearis, and K. crassa in the control bottles were very similar to laboratory determinations of maximal potential growth rates of these species at about the same temperature (Stemberger and Gilbert 1985). The population growth rates of P. vulgaris in the control bottles were considerably lower than those observed for this species in the laboratory at the same temperature (r,.,, = -0.29) (K. L. Kirk unpubl.). The reproductive potentials of S. pectinata and Ctimpanella in the control bottles of experiment 2 were the only ones high enough to substantially exceed the mortality imposed on them by the Daphnia. In both experiments, the selective suppression of rotifer and ciliate populations by Daphnia caused major changes in the frequency distribution of species in the community. In experiments 1 and 2, the presence of Daphnia reduced the relative abundance of the most vulnerable species- K. cochlearis- by 6 1 and 77% and increased the relative abundance of the very resistant species - T. similis- by 67 and 100% (Table 3). The suppression of some of the rotifer species by D. pulex can be attributed pri- marily to mechanical interference - ingestion and damage of rejected individuals. Exploitative competition from Daphnia may have played some role, especially in experiment 2, but probably was minimized by the addition of Cryptomonas sp. to the bottles at the beginning of the experiments. Cryptomonas is known to be a good food organism for all but one of the rotifer species in the community and for Daphnia (Stemberger 198 1; Stemberger and Gilbert 1985; Gilbert 19883). The diet of T. similis is not known. If this, or any other, rotifer species was unable to utilize Cryptomonas and was dependent on naturally occurring food resources also eaten by Daphnia, it might have been particularly susceptible to exploitative competition from Daphnia. The Trichocerca, however, was not affected by Daphnia in either experiment (Tables 1 and 2). Another factor serving to minimize any effects of exploitative competition from Daphnia was the short duration (-2 d) of the experiments. Even if Daphnia was reducing food resources available to the rotifer populations by the end of the incubation period, there probably would be a lag of about 1 d before this food limitation would be expressed by lowered birth rates. Added Cryptomonas was still present at high densities in the bottles with Daphnia at the end of the incubation period in expcriment 1 but was greatly depleted in experiment 2. Reproductive rates of both S. pectinata and K. cochlearis in the experimental bottles with Daphnia were higher in experiment 2 than in experiment 1, however, suggesting that the lower food supply at the end of the incubation period in experiment 2 did not markedly depress the reproductive potentials of these rotifers. The greater utilization of food resources in experiment 2 probably was due to the higher temperatures, and hence grazing rates, associated with the failure of the refrigeration unit in the environmental chamber. Since the level of interference by Daphnia would increase with grazl rig rate, it is not surprising that the mean & value for K. cochlearis in experiment 2 (1.14 d-l) was significantly higher than that in experiment 1 (0.79 d-l) (Student s t-test, P < 0.01). The mortality rate imposed on K. cochle-

9 614 Gilbert aris by Daphnia interference in this study was about half that predicted by the equations of Burns and Gilbert (1986a) derived from a laboratory study testing the effects of Daphnia size and density on the degree of interference with K. cochlearis. The predicted value of d, for Daphnia of a given size at 20 C can be calculated from d,, d- = ( x 1O--3 L3.1834)N (3) where L is the body length of the Daphnia (in mm) and N is the population density of Daphnia of that size (in ind. liter-l) (R. S. Stembcrger unpubl.). In experiment 1, the temperature was constant at C, the mean length of the D. pulex was 2.75 mm (range = 0.42 mm), and there were 16 D. pulex liter- I. The observed and predicted d, values of K. cochlearis in this experiment were 0.79 and I.56 d-l. The reason for the discrepancy is unknown. Anabaena filaments present in the lake at the time of the experiment are known to inhibit the growth and reproduction of the clone of D. pulex used in the experiment (Gilbert unpubl.) and may have reduced the potential of the Daphnia to interfere with the K. cochlearis and other microzooplankton. The suppression of the ciliate Campanella by Daphnia probably is also primarily due to a combination of ingestion and mechanical damage to rejected individuals. Since the diet of Campanella is unknown, however, the addition of Cryptomonas may not have reduced food limitation resulting from exploitative competition with Daphnia. Preliminary observations of encounters between Campanella and stationary D. pu- Iex showed that ciliates swept into Daphnia s branchial chamber usually were immediately rejected but sometimes were retained for undetermined periods (Gilbert unpubl.). Many of these retained ciliates were rejected, perhaps in damaged condition, and some may have been ingested. Large species of Daphnia are known to eat some ciliates (Tezuka 1974; Porter et al. 1979), and the present study demonstrates that these cladocerans can impose high mortality rates on a ciliate species population in a natural community. Most ciliate species are smaller than the 122~pm-long C ampanella in this study and so should be even more likely to be eaten or damaged by large Daphnia. The density of Daphnia introduced to the experimental bottles (16 ind. liter-l) is high but not unnaturally so. Comparable, and even much higher, population densities of Daphnia are reported from many natural communities (Edmondson and Litt 1982; Ferguson et al. 1982; Shapiro et al. 1982; Cryer et al. 1986). Similar results probably would have been obtained in the present study if lower densities of D. pulex had been incubated in the experimental bottles for a longer time (see Burns and Gilbert 1986b). The differential effects that D. pulex had on the various Star Lake rotifer species in the present study are consistent with previous laboratory experiments and observations on the ability of D. pulex to interfere with different rotifer species. The laboratory studies of Gilbert (1988b) showed that K. cochlearis was much more susceptible to this interference than S. pectinata, Polyarthra remata, and K. crassa and that Asplanchna priodonta and K. bostoniensis were not very susceptible to this interference. A similar pattern of vulnerability occurred in the present study (Fig. 2), although the species of Polyarthra and Asplanchna were different. Although the suppression of K. bostoniensis in experiment 2 was significant (Table 2), the population sizes of this rotifer were so low that its vulnerability to Daphnia is questionable. Direct observations of encounters between various rotifer species and Daphnia (Gilbert and Stemberger 1985; Burns and Gilbert 1986b; Gilbert 1987, 1988b; Stemberger and Gilbert 1987a; Gilbert and Kirk 1988) help to provide mechanistic explanations for the differential susceptibilities of the Star Lake rotifer species to D. pulex. The susceptibility of a rotifer to Daphnia interference generally seems to be a function both of its tendency to be swept into Daphnia s branchial chamber and of its residence time in the chamber if swept inside. The only rotifers that can effectively avoid entry to Daphnia s branchial chamber are those that are too large to be admitted (e.g. Conochilus colonies) or those that can resist the inhalant current by initiating escape responses (e.g. Polyarthra and, to a much lesser extent,

10 Daphnia interference eficts 615 Keratella). If swept into Daphnia s branchial chamber, the rotifers that are most rapidly rejected by the postabdomen are the least likely to be eaten or damaged. Residence time in the branchial chamber generally seems to decrease with increasing rotifer body size, skeletal development, and spination. This trend probably is directly related to the strength of the irritation provided by the rotifer. Thus, large rotifers with rigid and spinous integumentary skeletons may strongly irritate Daphnia s branchial chamber and therefore be rapidly rejected; at the same time, such rotifers would be well protected against damage during their brief residence in the chamber. Polyarthra vulgaris, like P. remata, probably usually escapes entry to Daphnia s branchial chamber by initiating its escape response (Gilbert 19853, 1987; Kirk and Gilbert 1988). Those that do not escape are very vulnerable, however, because of their small size and soft integument (Gilbert 1988b). In experiment 2, P. euryptera was less susceptible than P. vulgaris (Fig. 2), perhaps due to its greater body size (Fig. 1) and hence shorter residence time if caught in Daphnia s branchial chamber. Alternatively, P. euryptera may have been more likely to escape from Daphnia s branchial chamber than P. vulgaris. Keratella crassa was less susceptible than K. cochkearis (Fig. 2), almost certainly because its greater body and spine sizes (Fig. l), and possibly thicker skeleton, resulted in shorter residence times in Daphnia s branchial chamber. The invulnerability of A. girodi (Fig. 2), like A. priodonta studied earlier (Gilbert 19883), certainly is due to its large size (Fig. 1) and rapid rejection from chamber. Additionally, Daphnia s branchial Asplanchna s turgidity when contracted may protect it from damage in Daphnia s branchial chamber, as well as from predation by copepods (Stemberger and Gilbert 198 7b). Trichocerca similis probably was safe from Daphnia interference (Fig. 2) because it is well protected from damage while inside Daphnia s branchial chamber. This rotifer is relatively large and spinous (Fig. l), and its cylindrical skeleton may be thick, like that of Trichocerca rattus (Amsellem and Clement 1977; Clement 1977), and hence highly resistant. Preliminary observations of encounters betwcen T. similis and D. pulex showed that all rotifers swept into Daphnia s branchial chamber were rejected, apparently unharmed, even though their average residence time in the chamber was rather long (m 11 s) (Gilbert unpubl.). The rodlike body of T. similis may easily become tangled in Daphnia s appendages and be difficult to reject. tt is curious why the ciliate Campanella was no more susceptible to D. pulex interference than the much larger rotifer S. pectinata (Figs. 1 and 2). Campanella sp. from Star Lake, formerly misidentified as Rhabdostyla sp., is known to be distasteful to both Asplanchna (Gilbert 1980) and the copcpod Mesocyclops edax (Williamson 1980). Thus, D. pulex may reject this ciliate from its branchial chamber more rapidly than it would a similarly sized but more palatable ciliate. In conclusion, it is clear that interference from D. pulex, and likely other large species of this genus, can severely suppress susceptible rotifer and ciliate species in natural communities. This suppression can cause rapid reductions in the population sizes of individuals in these two taxa and thus rapid shifts in the species structure of microzooplankton assemblages toward resistant species. Under natural conditions, large Daphnia should often suppress rotifers and ciliates through a combination of both mechanical interference and exploitative competition for shared food resources. Together, these interactions should have a much more dramatic, and probably also more general, suppressive effect on rotifer and ciliate populations than interference competition by itself. The probable effects of these two types of Daphnia competition on rotifer communities have been considered in detail elsewhere and can explain a generally observed negative correlation between the abundances of rotifers and large Daphnia in natural communities (Gilbert 1988a, b). The present study suggests that a similar, negative correlation should also exist between the abundances of ciliates and large Daphnia. Some short-term (24 h) enclosure experiments (Hamilton and Taylor 1987) showed that ciliates were suppressed by

11 616 Gilbert crustacean zooplankton, but this suppression could have been due to predation by the numerically dominant copepods, to exploitative and interference competition from cladocerans, or to both. Only a few other experimental studies have directly examined the effect of Daphnia on rotifer populations in natural communities. They involved larger enclosures and longer incubations than those use@ in the present study and did not attempt to separate the effects of exploitative and interference competition. Neil1 (1984, 1985) showed that removal of Daphnia rosea from Gwendoline Lake enclosures caused marked increases in the fertilities and population densities of planktonic rotifers, mostly K. cochlearis and Kellicottia longispina. Vanni (1986) found that adding D. pulex did not significantly reduce populations of K. cochlearis and P. vulgaris in Larimore Pond and Dynamite Lake enclosures but did significantly suppress the Trichocerca multicrinis population in the latter enclosures. The absence of a significant Daphnia effect on most of his rotifer populations may be explained, however, by low and declining rotifer densities in both control and expcrimental enclosures at both sites, by quite low Daphnia densities in the experimental enclosures in Dynamite Lake, and by limi ted replication. References ALLEN, H. L Phytoplankton photosynthesis, micronutrient interactions, and inorganic carbon availability in a soft-water Vermont lake. Am. Sot. Limnol. Oceanogr. Spec. Symp. 1: AMSELLEM, J., AND P. CLEMENT COrrelatiOIIS bctwecn ultrastructural features and contraction rates in rotiferan muscle. 1. Preliminary observations on longitudinal retractor muscles in Trichocerca rattus. Cell Tissue Res. 181: BOGDAN, K. G., AND J. J. GILBERT Quantitative comparison of food niches in some freshwater zooplankton. A multi-tracer-cell approach. Occologia 72: 33 I-340. BURNS, C. W., AND J. J. GILBERT. 1986a. Effects of daphniid size and density on interference between Daphnia and Kcratella cochlcaris. Limnol. Oceanogr. 31: , AND b. Direct observations of the mechanisms of interference between Daphnia and Keratella cochlearis. Limnol. Oceanogr. 31: CLEMENT, P Ultrastructural research on rotifers. Ergeb. Limnol. 8: CRYER, M.,G. PIERSON,AND~. R. TOWNSEND Reciprocal interactions between roach, Rut&s rutilus, and zooplankton in a small lake: Prey dynamics and fish growth and recruitment. Limnol. Oceanogr. 31: EDMONDSON, W. T., AND A. H. LITT Daphnia in Lake Washington. Limnol. Oceanogr. 27: FERGUSON, A. J. D., J. M. THOMPSON, AND C. S. REYNOLDS Structure and dynamics of zooplankton communities maintained in closed systems, with special reference to the algal food supply. J. Plankton Res. 4: 523-,543. GILBERT, J. J Observations on the susceptibility of some protists and rotifers to predation by Asplanchna girodi. Hydrobiologia 73: a. Competition between rotifers and Daphnia. Ecology 66: 1943-l b. Escape response of the rotifer Polyarthra: A high-speed cinematographic analysis. Oecologia 66: p The Pofyarthra escape response: Defense against interference from Daphnia. Hydrobiologia 147: a. Suppression of rotifer populations by Daphnia: A review of the evidence, the mccha- nisms, and the effects on zooplankton community structure. Limnol. Oceanogr. 33: Susceptibilities of ten rotifer species to interference from Daphnia pulex. Ecology 69: 1826-t , AND K. L. KIRK Escape response of the rotifer Keratella: Description, stimulation, fluid dynamics, and ecological significance. Limnol. Oceanogr. 33: ,AND R.S. STEMBERCER Controlof Keratella populations by interference competition from Daphnia. Limnol. Oceanogr. 30: HAMILTON, D.T., AND W.D. TAYLOR Shortterm effects of zooplankton manipulations on phosphate uptake. Can. J. Fish. Aquat. Sci. 44: KIRK, K. I,., AND J. J. GILBERT The escape behavior ofpolyarthra in response to artificial flow stimuli. Bull. Mar. Sci. 43: in press. NEILL,, W. E Regulation of rotifer densities by crustacean zooplankton in an oligotrophic montane lake in British Columbia. Oecologia 6 1: The effects of herbivore competition upon the dynamics of Chaoborus predation. Ergeb. Limnol. 21: NICHOLS, H. W Growth media-freshwater, p Zn J. R. Stein led.], Handbook of phycological methods. Cambridge. PORTER, K.G.,M.L. PACE,AND J.F. BATTEY Cililate protozoans as links in freshwater planktonic food chains. Nature 277: SHAPIRO, J., AND OTHERS Experiments and experiences in biomanipulation. U.S. EPA 600/ SOKAL, R. R., AND F. J. ROHLF. I98 1. Biometry, 2nd ed. Freeman.

12 Daphnia interference eflects 617 STEMBERGER, R. S A general approach to the culture of planktonic rotifers. Can. J. Fish. Aquat. Sci. 38: , AND J. J. GILBERT Body size, food concentration, and population growth in planktonic rotifers. Ecology 66: 115 l-l , AND a. Multiple-species induction of morphological defcnscs in the rotifer Keratella testudo. Ecology 68: , AND Defenses of planktonic rotifers against predators, p In W. C. Kerfoot and A. Sih [eds.], Predation. Direct and indirect impacts on aquatic communities. New England. TEZUKA, Y An experimental study on the food chain among bacteria, Paramecium and Daphnia. Int. Rev. Gesamten Hydrobiol. 59: VANNI, M. J Competition in zooplankton communities: Suppression of small species by Daphniapulex. Limnol. Oceanogr. 31: WILLIAMSON, C. E The predatory behavior of Mesocyclops edax: Predator preferences, prey defenses, and starvation-induced changes. Limnol. Oceanogr. 25: Submitted: 20 January 1988 Accepted: 21 November 1988 Revised: 3 January 1989