Direct and indirect impact of two common rotifer species (Keratella spp.) on two abundant ciliate species (Urotricha furcata, Balanion planctonicum)

1. We investigated the potential competition and feeding impact of the common rotifer species, Keratella cochlearis and K. quadrata , on the abundant prostomatid ciliates, Urotricha furcata and Balanion planctonicum, in laboratory batch culture experiments. All four species have similar feeding preferences, co-occur in many freshwater environments, and are thus potential competitors for the same algal food.
2. Two small Cryptomonas species served as food for the ciliates and the rotifers in the experiments. Growth rates of each ciliate species were measured when they grew alone and when they were paired with one of the rotifer species.
3. Both rotifer species reduced the growth rate of U. furcata , probably primarily by direct feeding on the ciliates. Growth rate of B. planctonicum was unaffected by K. cochlearis , but was drastically reduced by grazing and/or mechanical interference of K. quadrata .
4. These results suggest niche partitioning of the sympatric ciliates with respect to their rotifer competitors/predators.     

The susceptiblity of Keratella cochlearis to interference from small cladocerans

SUMMARY. 1. Even at high population densities (300–2000 ind. 1⁻¹), only one of five small cladoceran species (adult body length <1 mm) significantly suppressed population growth of the rotifer Keratella cochlearis through interference (encounter) competition. At 500 ind. H, adults of D. ambigua (0.96 mm body length) imposed an instantaneous per capita death rate of 0.21 day⁻¹ on this rotifer. These short-term experiments may have underestimated cladoceran interference because newborn rotifers were rarely present.
2. Newborn rotifers (<12 h old) were much more susceptible than adult rotifers (> 24 h old) to interference from Ceriodaphnia dubia. All of the small cladoceran species tested were very much less likely than large Daphnia (body lengths >1.2 mm) to interfere with K. cochlearis , but perhaps at high population densities they could suppress population growth of susceptible rotifer species by damaging, and possibly eating, relatively small and soft-bodied newborn individuals.
3. K. cochlearis of the tecta form, without a posterior spine, produced offspring of the typica form, with a posterior spine, in the presence of C. dubia. This developmental response is stimulated by at least several, and possibly all, cladocerans and probably reduces the susceptibility of the rotifer to cladoceran interference.     

Effects of Daphnia on microzooplankton communities

Intact phytoplankton and microzooplankton communities from eutrophicStar Lake were incubated for 4 days with and without Daphniapulex, Daphnia galeaia mendotae, or a natural assemblage ofDaphnia species. They were sampled at the onset and terminationof the experiment for bacterial, phytoplankton, ciliate, rotifer,copepod and cladoceran densities. The cladocerans had variedeffects on the rotifers, ranging from significant suppressionof most rotifer species (Keratella cochlearis, Polyarthra remata,Keratella crassa) in the D.pulex jars, to the suppression ofone (K.crassa) or no species in the D.galeata mendotae and StarLake Daphnia assemblage jars, respectively. Small ciliates (<30µm, longest dimension), such as Strobilidium sp. and Pseudo-cyclidiumsp., were adversely affected by most of the cladoceran treatments,while several larger ciliates (>81 µm) were unaffectedin all such treatments. Ciliates were not consistently morevulnerable to cladoceran suppression than similarly sized rotifers.The suppression of ciliates and rotifers was attributable toboth direct effects (predation, interference, or both) and indirecteffects (e.g. resource competition) of the cladocerans. ¹Present address: Department of Biology, University of Louisville,Louisville, KY 40292, USA     

Evidence for biochemical limitation of population growth and reproduction of the rotifer Keratella quadrata fed with freshwater protists

The biochemical factors that determine the food quality of protistsfor rotifers are poorly understood. We evaluated populationgrowth rates and egg production of the rotifer Keratella quadratafed with four protist species growing on either an algal ora bacterial diet. The cryptomonad Cryptomonas phaseolus, consideredas a good quality prey, and assays without prey served as controls.Population growth rates and egg numbers of K. quadrata werecorrelated with single biochemical compounds (fatty acids, aminoacids and sterols) of the protists. Feeding on the alga C. phaseolusor the algivorous ciliates resulted in enhanced population growthrates and high egg production by K. quadrata, whereas feedingon bacterivores supported only moderate egg production but nopopulation growth. The rotifers’ egg production was correlatedwith the protist biochemical composition, including polyunsaturatedfatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanoland the amino acid leucine. No significant relationships wereobserved between population growth rates of the rotifers andthe protists’ biochemistry, suggesting that populationgrowth and reproduction of K. quadrata may have different nutritionalrequirements. To our knowledge, this is one of the first studiesto analyze a large variety of biochemical compounds to determinethe food quality of protists for a zooplankton predator andthe first study to analyze the biochemical quality of protiststo a rotifer species.     

rDNA ITS sequences among morphotypes of Keratell cochlearis, Keratell quadrata and Brachionus forficula (Rotifera)

Morphological variation in rotifers is affected by environmental conditions, making it hard to identify some rotifer taxa. We examined the rDNA ITS sequences of 10 unspined (KCU1-KCU10) and 17 spined (KCS1-KCS17) Keratell cochlearis clones, 26 two-spined (KQT1-KQT26), 18 single-spined (KQS1-KQS18) and 9 unspined (KQU1-KQU9) K. quadrata clones, and 17 long-spined (BL1-BL17) and 11 short-spined (BS1-BS11) Brachionus forficula clones collected from Lake Tingtang in Wuhu city, China. Molecular phylogenetic trees were constructed by neighbor-joining, maximum-likelihood, maximum parsimony, and Bayesian inference methods using B. calyciflorus as an outgroup. The K. cochlearis clones included 20 haplotypes, the K. quadrata clones included 37 haplotypes, and the B. forficula clones included 25 haplotypes. Different morphotypes of each rotifer species had shared haplotypes. Sequence divergences were 0.1-8.9% among different K. cochlearis haplotypes, and 8.1-8.9% between KCHAP1 (KCU1 and KCU10), KCU3, KCU4 and KCU6, and the other haplotypes. Sequence divergences were 0.1-14.5% among different K. quadrata haplotypes, and 11.9-14.5% between KQS17 and the other haplotypes. Sequence divergences were 0.1-11.7% among different B. forficula haplotypes, 11.0-11.7% between BL15 and the other haplotypes, 9.3-10.1% between BS3 and the other haplotypes, and 11.7% between BL15 and BS3. The four phylogenetic trees all supported that KCHAP1, KCU3, KCU4, KCU6 and the other 16 haplotypes among the 20 K. cochlearis haplotypes, KQS17 and the other 36 haplotypes among the 37 K. quadrata haplotypes, and BL15, BS3 and the other 23 haplotypes among the 25 B. forficula haplotypes all belonged to their own isolated clades. The morphological variation of the three rotifer species was attributed mainly to phenotypic plasticity.     

Effects of rotifers and ciliates on the growth and survival of Daphnia

Daphnia can suppress ciliates and rotifers through predationand interference competition, but it is not known whether thisproduces any direct benefit to Daphnia. We conducted survivorshipand cohort lifetable experiments to determine whether Daphniacan utilize ciliates and rotifers as food. Three species ofoligotrich ciliates (Halteria grandinella, Strobilidium gyransand Strobilidiumvelox) and one rotifer (Keratella cochlearis)were used. Lifetable experiments were conducted with a basallevel of algae (Cryptomonas sp.), plus either ciliates or rotifers,while survivorship experiments had only the rotifers or ciliates.Densities of 30 H.grandinella ml^–1, 50 S.gyrans ml^–1and 15 S.velox ml^–1 enhanced Daphnia pulex's populationgrowth rate 35–50% over controls with only algae. TenS.gyrans ml^–1 did not produce a significant change inDaphnia's growth rate. Densities of 100 and 300 K.cochlearis^–1 increased Daphnia population growth rates by II and10%, respectively. Both 10 and 50 S.gyrans ml^–1 enhancedDaphnia's survivorship compared to starved controls, but neither100 nor 300 K.cochlearis l^–1 enhanced its survivorship.The amount of enhancement of Daphnia growth rates by rotifersand ciliates is roughly proportional to the death rates imposedby Daphnia. The death rate imposed by Daphnia on rotifers isa function of both algal density and Daphnia size. Per unitbiomass, neither ciliates nor Keratella appear to be as nutritiousfor Daphnia as is Cryptomonas.     

Rotifers as predators on components of the microbial web (bacteria,heterotrophic flagellates,ciliates) — a review

Recent investigations have shown that processes within the planktonic microbial web are of great significance for the functioning of limnetic ecosystems. However, the general importance of protozoans and bacteria as food sources for rotifers, a major component of planktonic habitats, has seldom been evaluated. Results of feeding experiments and the analysis of the food size spectrum of rotifers suggest that larger bacteria, heterotrophic flagellates and small ciliates should be a common part of the food of most rotifer species. About 10–40 per cent of rotifers' food can consist of heterotrophic organisms of the microbial web. Field experiments have indicated that rotifer grazing should generally play a minor role in bacteria consumption compared to feeding by coexisting protozoans. However, according to recent experiments regarding food selection, rotifers should be efficient predators on protozoans. Laboratory experiments have revealed that even nanophagous rotifers can feed on ciliates. Preliminary microcosm and chemostat experiments have indicated that rotifers, due to their relatively low community grazing rates compared to the growth rates of bacteria and protozoans, should generally not be able (in contrast to some cladocerans) to suppress the microbial web via grazing, though they may structure it. Filter-feeding nanophagous rotifers (e.g. brachionids) seem to be significant feeders on the smaller organisms of the microbial web (bacteria, flagellates, small ciliates), whereas grasping species (e.g. synchaetids and asplanchnids) seem to be efficient predators on larger organisms (esp. ciliates). Another important role of rotifers is their feedback effect on the microbial web. Rotifers provide degraded algae, bacteria and protozoans to the microbial web and may promote microbial activity. Additional experimental work is necessary for a better understanding of the function of rotifers in aquatic ecosystems.     

Age-specific Chaoborus predation on rotifer prey

SUMMARY. 1. This is the first study to examine predator-prey interactions between Chaoborm instars and rotifer prey. The predatory behaviour of instars I–III of Chaoborus pimctipennis and the diet selectivity of instars I—IV feeding on rotifers were examined in the laboratory. Prey used in direct observations of predatory behaviour included a variety of rotifers (Symhacta pectlnata, S. ohUmga, Polyarthra remata, Asplanchna girodi, Keratella crassa, spined and unspined forms of Keratella cochlearis) and two crustaceans (Bosmitia longirostris, Mesocyclops edax nauplii. 2. In general, strike efficiencies (percentage of strikes resulting in inges- tion) increased in successive instars I—III. Early instar (I and II) strike efficiencies were low when compared with other invertebrate predators. For a given instar. mean prey handling times varied among prey species more than strike efficiencies. Mean handling times for small, soft-bodied rotifers were lowest and those for wide, hard-bodied prey were highest. 3. Instar I exhibited significantly greater selectivity for the small, soft- bodied S. obUmga than for the larger S. pectinata, hard-bodied K. crassa, and spined and unspined forms of K. cochlearis. Instars II—IV positively selected both the large and small Symhaeta species over all Keratella species. The relationship between Chaobortts selectivity and prey value (weight of prey per unit handling time) can be described by a power function. Ingestion rates of rotifers by older instars (III and IV) are among the highest reported for invertebrate predators. 4. Rotifer vulnerability to Chaoborus predation probably depended on rotifer cuticle texture, body width, and hydrodynamic disturbances. Spined rotifers were not necessarily protected from Chaoborus predation because Chaohorus can manipulate and swallow them. Giguere et al.'s 1982) encounter rate model must be modified to predict encounter rates of slow-moving rotifer prey with Chaohorus.     

Seasonal changes in the abundance and distribution of planktonic rotifers in Grasmere (English Lake District)

SUMMARY. The planktonic rotifers of Grasmere, a small lake in the English Lake District, were studied from August 1969 to December 1972. Twenty-four species were recorded but five were very rare. The remaining species were divided into three groups according to their seasonal occurrence: spring to autumn species ( Keratella quadrata. K. cochlearis, Gastropus stylifer, Asplauduia priodonta. Kellicottia Umgispina, Conochilus hippocrepis )., spring to early summer species ( Polyarthra dolichoptera, Synchaeta tremula, S. pectinata, S. stylata, S. oblonga, C. unicornis ), summer to autumn species ( Polyarthra vulgaris, P. major. Filinia terminalis, S. grandis, Ploesoma hudsoni, Trichocerca capucina, T. similis ). The months in which each species was abundant are given.
Keratella quadrata and Filinia terminalis were most abundant in the deepest stratum, Kellicottia longispina and Conochilus spp. were most abundant in the upper and middle strata. Trichocerca spp. showed no pronounced vertical distribution, and all the remaining species were most abundant in the upper stratum. Keratella cochlearis and Kellicottia longispina attained their highest densities over a wide range of temperature and oxygen concentration. Filinia terminalis, Conchihis spp., Asplanchna priodonta. K. quadrata , and Polyarthra spp., attained their highest densities within a narrow range of temperature and oxygen concentration. Optimum ranges are given for each species.
Major changes occurred between 1971 and 1972 when Asplanchna priodonta. Kellicottia longispina, Conochilus unicornis and Filinia terminalis increased in abundance, whilst Keratella quadrata. Gastropus stylifer, C. hippocrepis, Polyarthra spp., Synchaeta spp. and Ploesoma hudsoni decreased in abundance. These changes are discussed in relation to the temperature and oxygen requirements of each species and also to the probable enrichment of the lake after the opening of a new sewage works in June 1971.     

Spring bloom development of microbenthic algae and associated invertebrates in two reaches of a small lowland stream with contrasting sediment stability

SUMMARY. 1. Population growth rates and relative competitive abilities of the rotifer Keratella cochlearis f. tecta and the small-bodied cladoceran Daphnia ambigua were studied under different schedules of food addition but equal total food quantity (per 4-day interval). The initial population growth rate of Keratella was significantly affected by the feeding schedule and by the presence of competitors, while that of Daphnia was affected by neither factor. Population densities of both species tended to increase as the frequency of food addition increased.
2. Daphnia suppressed and excluded Keratella from mixed-species cultures when food was provided intermittently at a high concentration, but it failed to exclude the rotifer when food was provided in a near-continuous supply at low concentration. Keratella had only a minor suppressive effect on Daphnia in all mixed-species treatments.
3. Starvation experiments indicate that Daphnia is able to withstand food shortages for significantly longer periods of time than Keratella . These and other results indicate that the outcome of interspecific competition between these species may be influenced by me frequency and concentration at which food is supplied. Daphnia ambigua is competitively superior to K. cochlearis when food is concentrated or 'pulsed', but much less so when ambient food levels are chronically low. Patterns of food availability may have important effects in determining the relative abundance of rotifers and small cladocerans in natural zooplankton communities.     

Seasonal development of planktonic rotifers in Slapy Reservoir (Czech Republic)

The rotifer community of the canyon shaped Slapy Reservoir was studied over four years, from 2000 to 2003. A total of 38 rotifer taxa were identified in the pelagial zone. The dominant species, Keratella cochlearis, K. quadrata, Polyarthra dolichoptera, P. vulgaris, Synchaeta lakowitziana and Pompholyx sulcata, were similar to other reservoirs in the same area. Tychoplanktic species, which are typical for communities of upstream reservoirs, are reduced in the Slapy Reservoir. The seasonal fluctuation of rotifer abundance is characterized by a significant spring peak, clear water depression, summer peak and sometimes a lower peak in autumn. Fluctuation of the rotifer species shows a highly unified trend through the years with very low variation through the years. It results from the input of hypolimnetic water from an upstream reservoir, which erased a large part of the effect of year to year climatic fluctuations.     

Regulation of rotifer species by invertebrate predators in a hypertrophic lake: selective predation on egg-bearing females and induction of morphological defences

Temporal changes in the biomass of rotifer plankton were examinedin a shallow hyper trophic lake during 1 month (April/May),when the predatory rotifer Asplanchna brightwelli develops.The abundance of herbivorous rotifers was regulated by predationfrom A.brightwelli and from the copepod Acanthocyclops robustus.The densities and fecundity rates of Keratella cochlearis andKeratella quadrata were negatively related with the biomassof predators. Stomach analyses showed that Asplanchna fed selectivelyon reproductive females of K.cochlearis, reducing the fecundityof this species. Predators induced longer caudal spines in K.quadrata,which were negatively related to the fecundity of this rotifer,suggesting a reproductive cost associated with spine production.In contrast, spine length of K.cochlearis was not related topredators, but to temperature. These results showed that predatorscan reduce rotifer densities through increasing mortality andthrough decreasing rotifer fecundity rates regardless of phytoplanktonbiomass. We also show that morphological defences of K.cochlearisand K.quadrata are induced in different ways.     

The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experimentwas performed in the mesotrophic Schöhsee (Plön, FRG)in order to assess the impact of crustacean zooplankton on therotifer and phytoplankton community. Among the crustacean plankton,calanoid and cyclopoid cope-pods were abundant, but Daphnia‘longispina’ reached the highest densities in thisexperiment. The colonial rotifer Conochilus unicomis was notaffected by crustacean plankton. The two most abundant species,Synchaeta peclinata and Keratella cochlearis, increased exponentiallywhen macrozooplankton had been excluded from the enclosures,but did not increase when crustaceans were present. Birth anddeath rates of K.cochlearis could be reliably determined inthis field experiment, suggesting that this rotifer specieswas mainly controlled by exploitative competition rather thanby mechanical interference or predation. Daphnia ‘longispina’generally grazed selectively on the smaller ciliates and algae,thus depriving the rotifers of their phytoplankton resources.The dominant alga, the chrysophycean Dinobryon, increased, whethercrustaceans were present or not, but appeared to be grazed uponto a certain extent despite its considerable cell size and colonialorganization.     

Rotifers as indicators of lake types in Estonia

Data on the pelagic rotifer fauna in 10 Estonian lakes accumulated for 20 years, as well as published data are analysed. It is possible to distinguish three main indicator groups among rotifers: (1) for oligo- and mesotrophic lakes (Ploesoma hudsoni, Keratella serrulata, Synchaeta grandis, Asplanchna herricki, Ascomorpha ovalis, Gastropus stylifer, Conochilus hippocrepsis); (2) for meso-and eutrophic lakes (Trichocerca capucina, Filinia longiseta, F. limnetica, Keratella quadrata, K. cochlearis tecta, K.c. hispida, Polyarthra euryptera, Keratella hiemalis, Trichocerca porcellus, T. pusilla); (3) for eutrophic lakes (Brachionus spp., Anuraeopsis fissa, Pompholyx sulcata, P. complanata, Trichocerca cylindrica, Hexarthra mira).     

Contribution of rotifers to the diet and fitness of Boeckella (Copepoda: Calanoida)

1. The calanoid copepods, Boeckella triarticulata Sars and Boeckella hamata Brehm, are major components of the freshwater zooplankton of New Zealand. It was not known whether these copepods ingest rotifers, nor whether the inclusion of rotifers in their diets might improve their fitness. The present study aimed to identify rotifer taxa which are eaten by each copepod species, and to examine the fitness consequences of the inclusion of one species of rotifer in the diet of B. triarticulata .
2. In feeding experiments using natural rotifer assemblages, both species of copepod ingested the rotifer Anuraeopsis fissa (0.4–4% of daily carbon intake), and B. triarticulata also ingested Polyarthra dolichoptera (6–30% of daily carbon intake) and Keratella cochlearis tecta (1% of daily carbon intake).
3. The contribution of rotifers to the fitness of adult female B. triarticulata was assessed by comparing survival and reproduction among five diets that contained varying densities of algae ( Cryptomonas sp.) and/or Polyarthra dolichoptera (4 μg C L⁻¹). Boeckella triarticulata produced fewer clutches on a rotifer-only diet than on a solely algal diet, and addition of rotifers to a threshold algal diet did not affect copepod fitness relative to the solely algal diets. The present results suggest that Polyarthra at 4 μg C L⁻¹ is not a high-quality food for B. triarticulata .     

Feeding ecology of pelagic fish species in the Gulf of Riga (Baltic Sea): the importance of changes in the zooplankton community

The feeding ecology of four pelagic fish species was studied in relation to their prey availability in the Gulf of Riga (Baltic Sea) during the summer 1999-2006. The zooplankton community was dominated by the cladoceran Bosmina longispina, rotifers Keratella cochlearis and K. quadrata and the copepod Eurytemora affinis, with the highest interannual variability in abundance recorded for B. longispina. The last influenced the diet of adult sprat Sprattus sprattus, juvenile smelt Osmerus eperlanus and three-spined stickleback Gasterosteus aculeatus as these were strongly selecting for B. longispina. The fish feeding activity did not match the abundance dynamics of their preferred prey, suggesting that fishes may switch to consume other prey in case the preferred diet was limited. A considerable dietary overlap indicated high potential competition between pelagic fish species. While herring Clupea harengus membras and G. aculeatus were relying on very different food, the diets of young O. eperlanus and G. aculeatus were very similar. Interannual variability in zooplankton composition and abundance significantly affected the diet composition of fishes, but those changes were insufficient to exert a consistent influence upon fish feeding activity and total amounts of zooplankton consumed.     

Spine development in the rotifer Keratella cochlearis: induction by cyclopoid copepods and Asplanchna

SUMMARY. 1. Spine development in a clone of unspined Keratella cochlearis was promoted by a water soluble factor released by the copepods Tropocyclops prasinus and Mesocyclops edax and by the predatory rotifer Asplanchna priodonta. Between 9 and 55% of K. cochlearis populations cultured in predator-conditioned media responded to the inducing chemical.
2. The K. cochlearis form possessing a posterior spine of medium length was much less susceptible to predation by small Tropocylops and Asplanchna than the form lacking the posterior spine. These predators consistently selected the unspined form over the spined form when offered equal densities of each. However, both spined and unspined forms were equally susceptible to predation by large Mesocyclops.
3. The phenotypic variation of offspring produced from induced unspined Keratella females encompassed much of the variation reported for the taxon in North America. These morphotypes are similar to the variation of forms reported for the K. cochlearis tecta series known from Europe. We suggest that the North American morphotypes be identified in studies of this species because the presence of the posterior spine can greatly affect predator selectivity.     

Evaluation of bacterivory of Rotifera based on measurements of in situ ingestion of fluorescent particles, including some comparisons with Cladocera

Bacterivory of pelagic rotifers and cladocerans in eutrophicLake Loosdrecht (The Netherlands) was determined by microscopicobservation of in situ tracer particle uptake. Ingestion ratesof rotifer species using 0.51 µm microspheres or fluorescentlylabelled bacteria as tracers differed, with one exception. Theingestion rates depended on both the species and the tracertype. For cladocerans, fluorescently labelled bacteria seemedto underestimate grazing, presumably due to rapid digestionof tracer cells. Comparing results obtained with 0.51 µmmicrospheres, rotifers were much more important grazers on bacteriathan cladocerans in the study period (April-September). Basedon microspheres, the rotifer populations with the highest uptakeof bacteria were Filinia longiseta (May-July) andAnuraeopsisfissa (June-September). According to the uptake of fluorescentlylabelled bacteria, Conochilus unicornis had the highest uptakein June and A.fissa in July.     

The effect of fish on planktonic rotifers

The dynamics, community structure, and productivity of planktonic rotifers were studied during 3 years in two lakes near St. Petersburg (Russia). One lake was repeatedly stocked with larvae of the fish Coregonus peled; the other contained no fish. Fish addition led to a shift in plankton community structure. Population densities of some rotifer species (Keratella cochlearis, K. irregularis, Asplanchna priodonta) increased as a result of the elimination of large crustaceans by fishes during summer and autumn. An inverse relationship was found between the biomass of rotifers and Daphnia.     

Relative vulnerabilities of natural rotifer and ciliate communities to cladocerans: laboratory and field experiments

SUMMARY. 1. Two experiments with plankton communities from Storrs Pond (NH), one conducted in the laboratory and one in field enclosures, assessed the impact of different cladocerans on rotifers and ciliated protozoa.
2. The smallest cladoceran, Bosmina longirostris , did not depress rotifer or ciliate growth rates while the intermediate sized dadoceran, Daphnia galeata mendotae , reduced ciliate growth rates in the enclosure experiment but had only a marginal effect in the jar experiment. D. galeata mendotae had no effect on any of the rotifers in either experiment.
3. In both experiments the largest cladoceran, Daphniapulex , depressed the growth rates of ciliates and those rotifers known to be vulnerable to interference competition. Polyarthra vulgaris , previously shown to be resistant to cladoceran interference, was the only rotifer unaffected by D. pulex in the field experiment but was depressed by the much higher densities of this cladoceran in the laboratory experiment.
4. Cladocerans did not affect phytoplankton or bacterioplankton abundance in either experiment. Therefore the mechanism most likely to be responsible for the suppressive effect of cladocerans on rotifers and ciliates in these experiments is direct mechanical interference or predation, rather than exploitative competition.