Chaoborus predation and delayed reproduction in Daphnia: a demographic modeling approach

I develop a demographic model that examines the impact of Chaoborus predation on the population dynamics and life history of Daphnia. Predation effects are determined through analysis of the various components of the predator-prey interaction (encounter, attack, strike efficiency), and are integrated into a stage-classified matrix population model. The model is parameterized with data from interactions between D. pulex and fourth-instar C. americanus. I test this model with two laboratory experiments that examine population growth of D. pulex under the influence of five different levels of Chaoborus predation. With the exception of a single predation treatment in each experiment, the model accurately predicted the observed reduction in Daphnia numbers with increasing Chaoborus predation. I then use this model to investigate the evolution of delayed reproduction in D. pulex that are exposed to Chaoborus. I ask whether delayed reproduction may evolve in Daphnia that are subjected to Chaoborus predation as a trade-off for the benefits of larger body size. The model predicts that the effectiveness of such a life history trade-off depends on the relative sizes of predator and prey. In some interactions between Chaoborus and Daphnia, increasing Daphnia body length by as little as 5% from base growth trajectories sufficiently increases fitness (by reducing vulnerability to Chaoborus predation) to compensate for the cost of delayed reproduction. In other interactions, however, increased body length provides no benefit to Daphnia (and may even reduce fitness), and selection would act against the evolution of delayed reproduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chaoborus predation and the function of phenotypic variation in Daphnia

To investigate the role of helmet formation in defense against predation, laboratory experiments were used to analyze the effects of morphological changes in Daphnia on susceptibility to Chaoborus predation. Behavioral observations of Chaoborus preying on helmeted and non-helmeted Daphnia suggest pre-contact advantages for helmeted prey but post-contact advantages for non-helmeted prey. Helmeted Daphnia are better at evading capture by Chaoborus but may also be more easily handled by the predator. Swimming behavior of the prey, which is influenced by the presence of a tailspine, may affect Chaoborus strike distance. These results re-emphasize the potential hydromechanical importance of body shape changes in defense against predation.     

Predation and refugia: implications for Chaoborus abundance and species composition

1. Previous studies have suggested that the occurrence of larval Chaoborus in lakes may be affected by fish predation, pH, elevation, temperature, nutrient level, water transparency and interspecific competition, but so far, a detailed statistical evaluation of these findings has not been performed. 2. The aim of this study was to apply regression and ordination techniques to a large data set of 56 lakes in order to test which variables related to lake morphology, water chemistry, and fish predation determine (1) the abundance of individual Chaoborus species and (2) their species composition. 3. Individual Chaoborus species were influenced by very different sets of environmental factors. Nutrient levels positively affected the largest species, Chaoborus americanus, which was restricted to fishless lakes. Abundance of the smallest and most transparent species, C. punctipennis, seemed to be controlled more by the larger Chaoborus species than by fish. Larger chaoborids required low water clarity in order to co‐exist with fish, probably to increase refuge availability. Generally, small lakes (for C. flavicans/C. trivittatus) and shallow lakes (for C. punctipennis) supported higher abundances of Chaoborus.     

Habitat segregation and interactive effects of multiple predators on a prey assemblage

1. In a field experiment we examined the interactive effects of two common predators of zooplankton, bluegill sunfish (Lepomis macrochirus) and Chaoborus spp. on the growth rate and habitat use of three congeneric prey species (Daphnia). Bluegill and Chaoborus both consume Daphnia, but bluegill also prey on Chaoborus. The prey species, Daphnia pulicaria, D. rosea and D. retrocurva, differed in body size and vertical distribution. We expected the largest species, D. pulicaria, to be most vulnerable to fish predation and the smallest species, D. retrocurva, to be most vulnerable to Chaoborus predation. 2. As we expected, the population growth rate of D. pulicaria was significantly reduced by fish. However, Chaoborus also significantly reduced the growth rate of this species. No significant interaction effect was detected, indicating that the effect of these predators was additive. The growth rates of D. rosea and D. retrocurva were significantly reduced by Chaoborus, but a significant interaction effect indicated that the effect of Chaoborus was stronger in the absence of fish than when fish were present. Therefore the impact of Chaoborus and fish on D. rosea and D. retrocurva was non-additive. The interactive effect of the two predators on D. retrocurva was greater in magnitude than on D. rosea. 3. In the absence of predators, the three Daphnia species showed no differences in mean habitat depth between day and night. Both predators significantly affected diel habitat use of D. pulicaria and D. rosea. Fish caused both of these Daphnia species to move deeper during the day, whereas Chaoborus caused Daphnia to move into shallower water at night. Daphnia retrocurva tended to migrate upwards at night in all predator treatments, but no significant differences in migration were observed among the predator treatments. The effects of predators on habitat use were not interactive for any prey species. 4. Our results suggest that body size, habitat use and the diel migratory response to predators are important factors mediating the interactive effects of multiple predator types on zooplankton.     

Influence of benthic and interstitial processes on nutrient changes along a regulated reach of a large river (Rhône River,France)

An enclosure study was conducted in Ranger Lake in south-central Ontario, Canada from 4 July to 5 August 1997 to determine predation effects of the larvae of the phantom midge fly Chaoboruson the zooplankton community. Zooplankton assemblages were established in 12 enclosures (2 m in diameter, 7.5 m deep). Three densities of fourth-instar Chaoborus trivittatus (0 l^–1, 0.1 l^–1 and 0.5 l^–1) were introduced as predator treatments to the enclosures. Temperature, dissolved oxygen and zooplankton community composition were monitored for six weeks. To determine if the zooplankton community composition changed, a repeated measures multivariate analysis was performed on percent biomass of Bosmina and calanoid copepods. There were no significant differences in mean taxon percent biomass among predator treatments. There were significant differences in mean taxon percent biomass between water layers (epilimnion and metalimnion). There were also significant differences in lengths of Bosmina and calanoid copepods among predator treatments at the end of the experiment. Crop content analysis of C. trivittatusshowed that Bosmina constituted 88–98% of the prey items found in the crops. These results demonstrate that the use of deep enclosures, a Chaoborus species which vertically migrates, and lower natural densities of Chaoborus may provide prey with an important natural refuge from predation and so allow a more accurate determination of the predation impact of Chaoborus trivittatusin temperate lakes where fish control Chaoborus densities.     

Spatial avoidance of littoral and pelagic invertebrate predators by <Emphasis Type="Italic">Daphnia</Emphasis>

Studies on spatial avoidance behaviour of predators by prey often ignored the fact that prey typically face multiple predators which themselves interact and show a spatial pattern in abundance and predation rates (PRs). In a series of laboratory experiments, we investigated predation risk (PRI) and horizontal migration of the cladoceran Daphnia magna between open water and vegetation in response to two important invertebrate predators with a contrasting spatial distribution: pelagic Choaborus and vegetation-associated Ischnura. As expected, PRI by Chaoborus was higher in open water due to higher numbers and higher PRs of Chaoborus, while for Ischnura, PRI was highest in the vegetation due to higher densities, despite lower PRs of Ischnura. In accordance with this, Daphnia moved into the vegetation in the presence of the pelagic Chaoborus alone. In the presence of Ischnura alone, however, Daphnia showed no response. We hypothesize this may be the result of a constitutive behaviour of Daphnia to avoid pelagic fish, which impedes a response to the open water. In the combined predator treatment, Daphnia migrated to the open water zone. The increased risk of predation in the vegetation, due to a facilitating effect of Chaoborus on Ischnura PRs is believed to have caused this migration of the Daphnia. This response of Daphnia declined through time and Daphnia moved toward the vegetation. A decline in the activity of the Ischnura larvae through time may have switched the risk balance in favour of the vegetation environment.     

The extant Chaoborus assemblage can be assessed using subfossil mandibles

1. Assemblages of Chaoborus were examined in 80 thermally stratified southern central Canadian Shield lakes to explore whether subfossil mandibles could be useful in assemblage‐level studies of Chaoborus. 2. Chaoborus (Sayomyia) (probably Chaoborus punctipennis in this study region) and Chaoborus flavicans were the most common taxa recorded, while Chaoborus trivittatus was rarer. Chaoborus americanus was not recorded in subfossil assemblages, because no fishless lakes were included in this study. Chaoborus flavicans had higher relative abundances (%) in lakes with higher dissolved organic carbon (DOC), probably because of reduced fish predation in less transparent water. 3. Results from logistic regression indicate that patterns of presence/absence for Chaoborus mandibles in the study lakes were influenced primarily by hypolimnetic oxygen concentration, probably because of the presence or extent of a hypolimnetic refugium from fish predation. 4. Chaoborus species richness in lakes, derived from subfossil assemblages, did not differ significantly from species richness estimates derived from plankton sampling with a net. Patterns of dominance and coexistence [e.g. the widespread co‐occurrence of C. flavicans and C. (Sayomyia)] determined from subfossil assemblages agreed with previous studies of the contemporary living assemblage. 5. These results suggest that subfossil assemblages may be used as an alternative to nocturnal plankton sampling to carry out research on the community ecology of Chaoborus. 6. We propose a hierarchical conceptual model of assemblage‐level patterns of Chaoborus in temperate lakes. Chaoborus americanus dominates in fishless lakes, whereas in lakes with fish Chaoborus is typically absent where there is no anoxic hyplimnion. In lakes with anoxic strata, C. trivittatus tends to dominate in lakes with few fish; in the remaining lakes, C. flavicans and C. (Sayomyia) dominate, although C. flavicans is more relatively abundant in lakes with lower water clarity (higher DOC).     

Feeding selectivity of a tropical Chaoborus population

SUMMARY. The Chaoborus population of Lake Lanao, Philippines, was sampled weekly over a 65-week period. Specimens representing all four instars, all times of the year, and two points in the daily migration cycle were dissected for determination of gut contents. Major items in the diet of Lanao Chaoborus include Bosmina, Diaphanosoma, and the copepodid and adult stages of cyclopoid and calanoid copepods. Items that are available but seldom eaten include nauplii and rotifiers except Keratella. Feeding rate averages 2.5% of body weight per day in instars 3–4. Predator size affects but does not fully explain prey selection. Electivity values are much higher for Bosmina and Diaphanosoma than for copepods, even though these food items overlap in size. Bosmina, which has the highest electivity value of any prey, is virtually identical in size to the calanoid nauplius, which has one of the lowest electivity values. This and other similar data suggest that prey of the same size differ greatly in palatability or vulnerability. There is a marked variation in feeding rate and food composition from day to night. The smallest Chaoborus feed more during the day than at night, but the opposite is true for Chaoborus of moderate to large size. Large Chaoborus switch from a daytime emphasis on copepod copepodids and adults to a nighttime emphasis on Cladocera. Diurnal variation between instars in food composition cannot be explained simply on the basis of the vertical distributions of predators and prey. An interaction of vertical distribution with prey density and predator selectivity does explain the overall Chaoborus feeding pattern, however. During the day, larger Chaoborus move deep into the water column where food is scarce. Their daytime feeding rate is lower due to low prey density at great depths. Low prey density is partially compensated by relaxation of preference. At night, upward migration of large Chaoborus into an area of high prey density permits a resumption of marked selectivity and high feeding rates. Small Chaoborus do not descend deep into the water column during the day, as their lower hunting efficiency apparently requires higher food density and use of visual cues to sustain adequate feeding rates. Prey density thus affects both the vertical distribution and feeding selectivity of the Chaoborus population.     

Chaoborus americanus predation on various zooplankters; Functional response and behavioral observations

Summary Predation rates for Chaoborus americanus on different types of noncyclomorphic zooplankton prey were determined in the laboratory as a function of both prey species and density. The sequence of events leading to consumption of prey was also observed, and probabilities determined for the various components.Predation rates generally reached an upper limit as the density of prey increased to high levels. Larger prey were always less vulnerable to Chaoborus predation. Among the species of zooplankton prey examined Holopedium gibberum suffered the lowest rates of predation, while those of the other plankters (Daphnia pulex, D. rosea, Moina hutchinsoni, Diaptomus birgei, D. leptopus, Cyclops vernalis) were approximately equivalent for any given size category.Observation of the predatory sequence indicates that for all the plankters except H. gibberum there were relatively high rates of evasion of attack by C. americanus and low rates of escape after capture. For H. gibberum the situation was reversed, and escape after capture was particularly important in determining the predation rate.The gelatinous envelope of Holopedium gibberum seems to operate as an effective anti-predator device which reduces the likelyhood of retention by Chaoborus after contact has been made.     

Density-dependent predation ofChaoborus flavicans onDaphnia longispina in a small lake: the effect of prey size

Functional response curves of fourth instar larvae ofChaoborus flavicans preying on two size classes ofDaphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parametersa (attack rate) andTh (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm)Daphnia. In 1:1 mixture of these prey size classes the predation rates ofChaoborus on smallDaphnia at prey densities above 20 l^–1 were greater than predicted from the single size-class experiments. The observed predation rates on largeDaphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference ofChaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated byD. longispina)Chaoborus preference forDaphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions ofDaphnia. When medium-sizedDaphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.     

Estimating predation risk in zooplankton communities: the importance of vertical overlap

In order to estimate predation risk in nature, two basic components of predation need to be quantified: prey vulnerability, and density risk. Prey vulnerability can be estimated from clearance rates obtained from enclosure experiments with and without predators. Density risk is a function of predator density, and the spatial and temporal overlap of the predator and prey populations. In the current study we examine the importance of the vertical component of overlap in making accurate estimates of predation risk from the invertebrate predator Mesocyclops edax on rotifer versus crustacean prey. The results indicate that assumptions of uniform predator and prey densities cause a significant underestimation of predation risk for many crustacean prey due to the coincident vertical migration of these prey with the predator. The assumption of uniformity is more reasonable for estimating predation risk for most rotifer prey.     

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.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

Combined effect of the insecticide carbaryl and the Chaoborus kairomone on helmet development in Daphnia ambigua

The cladoceran Daphnia ambigua was exposed to both the insecticide carbaryl and the kairomone released from the predator Chaoborus simultaneously, and its morphological changes were analyzed. Daphnia developed helmets in response to the kairomone, but not in response to carbaryl at low (sublethal) concentrations (1–3 µg 1^–1). However, the carbaryl enhanced the development of high helmets and prolonged the maintenance period of the helmets over instars in the presence of the kairomone. These results suggest that sublethal concentrations of the insecticide alter predator-prey interactions by inducing helmet formation in Daphnia, which may reduce vulnerability of the Daphnia to predation.     

Neck spine protects Daphnia pulex from predation by Chaoborus, but individuals with longer tail spine are at a greater risk

We tested the prey preference of Chaoborus sp. on Daphnia pulexwith different defensive morphologies. The protective functionof inducible morphological defences, such as neck spine andlonger tail spine, was evaluated Second instar D.pulex individualsof two clones differing in their strength of neck spine inductionwere offered as prey to both Chaoborus obscuripes and Chaoborusflavicans. We used logistic regression analysis to evaluatethe effect of morphometry on the vulnerability of Daphnia. Thepresence of a neck spine and increased total length protectedD.pulex from Chaoborus predation. However, individuals witha longer tail spine were more vulnerable to Chaoborus predationChaoborus obscuripes was able to eat daphnids with a neck spinedue to the larger gape size of this chaoborid. The smaller speciesC.flavicans almost always ate prey with no neck spine.     

Morphological defenses induced in situ by the invertebrate predator Chaoborus : comparison of responses between Daphnia pulex and D. rosea

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gr?fenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l^–1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m³) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term ”maximum size for neckteeth formation” (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures. Received: 10 April 1999 / Accepted: 6 April 2000     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA     

Enhanced growth at low population density in Daphnia: the absence of crowding effects or relief from visual predation?

1. It has been suggested that chemical information from crowded populations of an animal such as Daphnia carries a cue indicating imminent food limitation, and we suggest that in the presence of fish kairomones, it may also convey a hint of the need to enhance antipredation defences. 2. We performed two‐factorial experiments with Daphnia grown in flow‐through plankton chambers in medium containing high levels of Scenedesmus food plus chemical information on either low or high population density levels and in the presence or absence of fish chemical cues (kairomones) and recorded (i) the effects on Daphnia growth rate and reproduction, and (ii) the effects on Daphnia depth selection. Further depth‐selection experiments were performed to test the reaction of Daphnia to crowding information at different Daphnia concentrations and to test its effect on daytime and night‐time depth selection by different Daphnia instars in the presence of kairomones. 3. The effects of crowding information alone (in the absence of kairomones) were weak and were not significantly strengthened by the addition of kairomones. The effects of kairomones alone (in the absence of crowding information) were much stronger and were increased by the presence of crowding chemicals: Daphnia selected greater depths in daylight (the later the instar and the larger its body size, the greater the depth), their body growth was slower and daily reproductive investment reduced, compared with Daphnia grown in the absence of crowding information. This suggested that crowding chemicals carry a cue indicating the need to invest more into antipredation defences. 4. The adaptive significance of these effects was confirmed by the differential vulnerability to predation of the Daphnia when offered as prey to live roach after being grown for 6 days either in the presence (higher vulnerability) or in the absence (lower vulnerability) of information on high density. 5. The strong interaction between crowding information and fish kairomones may be explained either as the reaction to a cue indicating impending food stress or as the reaction to a signal of increased predation risk. While the former scenario is already known from crowding studies, the latter is a novel idea that stems from the old concept of ‘low‐density anti‐predation refuge’. The two scenarios are not mutually exclusive: each stems from the need to invest in survival rather than in growth and reproduction [Corrections were made to this paragraph after first online publication on 4 April 2012].     

Effects of pH and predation by Chaoborus larvae on the plankton of a shallow and acidic forest lake

1. Eutrophic acid lakes are not common. Delamere Lake in Cheshire, U.K. is shallow and acid (mean pH 4.5) with a very high phytoplankton crop (mean 290 μg chlorophyll a L^?1), dominated by Dictyosphaerium pulchellum. Rotifers were dominant in the pelagic waters but small cladocerans (Alona guttata, Chydorus sphaericus and Scapholeberis mucronata) were occasional in the littoral waters. Chaoborus flavicans larvae were the top predators in this fishless lake. Two mesocosm experiments were carried out in which pH and Chaoborus populations were manipulated. 2. Progressively higher concentrations of D. pulchellum were maintained in the elevated pH treatments (pH 6 and 8; P < 0.001) with increased amounts of a Chlamydomonas species at the end of the experiment. Highest species richness was seen at ambient pH. Thus the low pH of Delamere Lake alone did not control the structure of the phytoplankton community. Keratella quadrata showed significantly higher abundance at pH 6 than in other pH treatments (P < 0.001). Species richness of rotifers was unaffected by pH. 3. Most Cladocera were C. sphaericus. Although never seen in the open lake, Daphnia pulex appeared in all the pH treatments. Low pH did not control small Cladocera abundance in Delamere Lake, but probably hampered reproduction in Daphnia. Negative correlations between chlorophyll a concentrations and Daphnia in the mesocosms (r² = 0.215, P < 0.05), however, indicated the potential of large‐bodied daphniids in controlling phytoplankton. 4. Neither different combinations of Chaoborus instars (none, instars 1 and 2 and instars 3–5) nor different densities of instars 3–5 (0.15, 0.5 and 1.0 L^?1) had a negative impact on Cladocera. Daphnia pulex remained unaffected in the experiment, perhaps because of its large size, and C. sphaericus because of its high reproductive rate compensating predatory losses. 5. Very low pH in Delamere Lake might suppress Daphnia by hampering its reproduction. Consequently, Daphnia may be vulnerable to invertebrate predation even at low predator density in the lake.