Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Sobygaard

The impact of fish-mediated changes on the structure and grazingof zooplankton on phytoplankton and bacterioplankton was studiedin Lake Søbygaard during the period 1984–92 bymeans of in vitro grazing experiments (¹⁴C-labelled phytoplankton,³H-labelled bacterioplankton) and model predictions. Measuredzooplankton clearance rates ranged from 0–25 ml l^–1h^–1 on phytoplankton to 0–33 ml l^–1 h^–1on bacterioplankton.The highest rates were found during thesummer when Daphnia spp. were dominant. As the phytoplanktonbiomass was substantially greater than that of bacterioplanktonthroughout the study period, ingestion of phytoplankton was26-fold greater than that of bacterioplankton. Multiple regressionanalysis of the experimental data revealed that Daphnia spp.,Bosmina longirostris and Cyclops vicinus, which were the dominantzooplankton, all contributed significantly to the variationin ingestion of phytoplankton, while only Daphnia spp. contributedsignificantly to that of bacterioplankton. Using estimated meanvalues for clearance and ingestion rates for different zooplankters,we calculated zooplankton grazing on phytoplankton and bacterioplanktonon the basis of monitoring data of lake plankton obtained duringa 9 year study period. Summer mean grazing ranged from 2 to4% of phytoplankton production and 2% of bacterioplankton productionto maxima of 53 and 88%, respectively. The grazing percentagedecreased with increasing density of planktivorous fish caughtin August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundanceseemed highest for bacterioplankton. Accordingly, the percentagecontribution of bacterioplankton to the total ingestion of thetwo carbon sources decreased from a summer mean value of 8%in Daphnia-dominated communities at lower fish density to 0.7–1.1%at high fish density, when cyclopoid copepods or Bosmina androtifers dominated. Likewise, the percentage of phytoplanktonproduction channelled through the bacteria varied, it beinghighest (5–8%) at high fish densities. It is argued thatthe negative impact of zooplankton grazing on bacterioplanktonin shallow lakes is highest at intermediate phosphorus levels,under which conditions Daphnia dominate the zooplankton community.     

Is the midsummer decline of Daphnia really induced by age-0 fish predation? Comparison of fish consumption and MDaphnia mortality and life history parameters in a biomanipulated reservoir

We analysed the temporal pattern of a Daphnia galeata populationand the development of the age-0 Ash community in a long-termbiomanipulated lake (Bautzen reservoir, Germany) during springand early summer of two successive years. In Bautzen reservoir,the age-0 Rsh-Daphnia interaction is a key process within thefood web due to the low abundance of adult zooplanktivorousfish. Daily consumption of daphnids by age-0 fish, as estimatedby a bioenergetics model, was compared to daily mortality ratesof daphnids. In addition, we estimated life history parametersof Daphnia that may indicate predation impact by fish. A midsummerdecline of daphnids occurred only in July of 1996, whereas in1995 the Daphnia biomass remained >4 mg wet weight 1^–1for the entire summer. The percentage of total Daphnia mortalitydue to fish predation before the onset of the midsummer declinewas –2% day^–1. Temporal patterns of individual size,clutch size and size at maturity also indicated that the 1996midsummer decline of daphnids was not the exclusive consequenceof age-0 fish predation. Instead, low reproductive capacityof daphnids also contributed significantly to the decline. Consequently,year-to-year variation of the Daphnia dynamics may be determinedby a fine-tuned ‘timing’ between the period of reducedfecundity and the time of the strongest predation impact byage-0 fish.     

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.     

Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

To gain better insight into the importance of predator and resourcecontrol in New Zealand lakes we surveyed the late summer trophicstructure of 25 shallow South Island lakes with contrastingnutrient levels (6–603 µg TP l^–1) and fishdensities. Total catch of fish per net (CPUE) in multi-meshgillnets placed in the open water and the littoral zones waspositively related with the nutrient level. Trout CPUE was negativelycorrelated with total phosphorus (TP) and total nitrogen (TN).Zooplankton seemed largely influenced by fish, as high fishCPUE coincided with low zooplankton and Daphnia biomass, lowaverage weight of cladocerans, low contribution of Daphnia tototal cladoceran biomass, low ratio of calanoids to total copepodbiomass and low ratio of zooplankton biomass to phytoplanktonbiomass. However, chlorophyll a was only slightly negativelyrelated to Daphnia biomass and not to zooplankton biomass ina multiple regression that included TN and TP. Ciliate abundancewas positively related to chlorophyll a and negatively to Daphniabiomass, but not to total zooplankton biomass, while no relationshipswere found between heterotrophic nanoflagellates and zooplankton.The relationships between fish abundance and nutrients and fishabundance and zooplankton:phytoplankton ratio and between chlorophylla and TP largely followed the pattern obtained for 42 northtemperate Danish lakes. We conclude that fish, including trout,have a major effect on the zooplankton community structure andbiomass in the pelagial of the shallow oligotrophic to slightlyeutrophic New Zealand lakes, but that the cascading effectson phytoplankton and protist are apparently modest.     

Can phaeopigments be used as markers for Daphnia grazing in Lake Constance?

The formation of chlorophyll a degradation products was measuredwith natural phytoplankton from Lake Constance and Daphnia magnaand native Daphnia as grazers in grazing experiments duringspring bloom conditions using high-pressure liquid chromatography(HPLC). Chlorophyll a start concentrations were between 1.2and 16.3 µg l^–1; phaeopigment weights constituted5% of chlorophyll a weight. Only phaeophorbide a was a markerfor Daphnia grazing; concentrations of other phaeopigments (phaeophytina, chlorophyllide a and two unidentified phaeopigments) didnot increase during Daphnia grazing. Conversion efficiencies(chlorophyll a to phaeophorbide a) were between 0 and 43% ona weight basis, and between 0 and 65% on a molar basis. Conversionefficiencies were highest at high grazer density (40 Daphnial^–1) and after a 24 h exposure time. Grazing by microzooplanktonprobably led to the formation of the two unidentified phaeopigments.In Lake Constance, Daphnia density was significantly positivelycorrelated with the phaeophorbide a/chlorophyll a ratio whenit was <5000 Daphnia m^–3. However, when higher Daphniadensities were included in calculations, then Daphnia densitywas positively, but insignificantly, correlated with the phaeophorbidea/chlorophyll a ratio. This suggests that when the level offood per Daphnia is low, then grazing is more efficient withless production of phaeophorbide a and a higher production ofcolourless products.     

Daphnia populations in three interconnected lakes with roach as the principal planktivore

Data on two co-existing Daphnia species, D.cucullata (G.O.Sars)and D.hyalina (Leydig), from three neighboring and interconnectedlakes in northern Poland, revealed an extremely confined rangeof population density fluctuations throughout summer in eachof the two species in each of the three lakes, with D.cucullatabeing an order of magnitude more abundant than D.hyalina (meanof 11.5 ind. l^–1 for D.cucullata and 1.1 ind. l^–1for D.hyalina with 95% confidence limits of ±2.5 and±0.5 ind. l^–1, respectively), in spite of markedlychanging fecundity. There was no apparent phase of decline orincrease that could be related to the distinct phases of lowor high fecundity resulting from different food levels. Analysisof the gut contents of roach (Rutilus rutilus), the dominantplanktivore in the three lakes, revealed identical selectivityfor each of the two Daphnia species, suggesting that the lowerdensity of the D.hyalina population was compensated for by thegreater conspicuousness of individuals of this species. It isconcluded that the population density of these Daphnia speciesremains far below the carrying capacity of the habitat and doesnot depend on food levels. Food availability merely sets therate of population increase, while the actual population densityin the lakes studied reflects the species' vulnerability topredation by planktivorous fish.     

Induction time and reversibility of changes in Daphnia life history caused by the presence of fish

Two experiments were run, to demonstrate how Daphnia life historychanges in the response to fish kairomone, depending on at whichontogenetic stage the simulated presence of fish appears ordisappears. Exposures of Daphnia, starting from different ontogeneticstages, to the fish kairomone showed that the period betweenthe initiation of phenotypic changes induced by the presenceof fish and their adaptive expression in Daphnia varies fromone instar (2–3 days) in the case of size at first reproductionto five instars (about 7 days) in the case of age at first reproduction.Removing the fish kairomone in different ontogenetic stagesshowed, in turn, that if a proximal factor that induced an anti-predatormechanism disappears and is not detected for at least four instars,Daphnia may withdraw from former changes (e.g. maturation whenlarger). It is suggested that the moment of the final ‘decision’regarding size at first reproduction cannot be attributed toa specific instar, but rather to a given moment in the Daphnialife cycle that is not related to any particular developmentalinstar but rather to the instantaneous level of resources thatcan be allocated to the reproduction.     

Size-specific swimming behavior of Daphnia pulex

Body length affects several aspects of the behavior of quietlyswimming Daphnia pulex Swimming and sinking rates were measuredat 0.033 s intervals during the ‘hops’ characteristicof Daphnia swimming behavior Larger animals swim faster, covermore distance, and produce more powerful swimming strokes. LargerDaphnia also sink faster, but the sinking rate scales as lengthto the 0 58 power, far lower than the power of 2 00 predictedby Stokes Law considerations. The number of hops s^–1 wasindependent of body size, although a theoretical analysis predictshopping rate (antennal beat frequency) should increase as bodylength squared. Turning behavior, measured as the ratio of displacementto total distance, during 5 s, is also independent of body sizeIndependence of several parameters of body motion and body sizeimplies that factors other than simple mechanics affect Daphniaswimming behavior     

Environmentally controlled Daphnia spring increase with implications for sockeye salmon fry in Lake Washington, USA

In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka)strongly prefer Daphnia over other prey, switching uniformlyto Daphnia when the threshold abundance of 0.4 Daphnia L^–1is achieved. Using long-term Lake Washington data (1978–2001)and fry trap data (1992–2001) from a major tributary,we examined the following: (i) factors that predict Daphniapulicaria and Daphnia thorata increase to this threshold "switching"abundance, (ii) trends in Daphnia dynamics that may affect sockeyeforaging and (iii) temporal correspondence of Daphnia increaseand fry arrival. The winter abundance of D. pulicaria, in combinationwith basic parameters of spring conditions, was an importantpredictor of the date of D. pulicaria spring increase, indicatinggreater reliance on pelagic population dynamics (versus diapausehatch) than D. thorata exhibited. In addition, D. pulicariawas a more consistent prey than D. thorata, the latter exhibitinglarger population fluctuations. Thus, recently increasing D.thorata prominence could decrease diet consistency for sockeyefry. Additionally, the timing of sockeye arrival to Lake Washingtonand Daphnia’s increase to the switching threshold hasbecome less concordant, so that fry in recent years have hadto rely upon less profitable prey for longer periods. Long-termtrends and species-specific differences in Daphnia phenologymay affect fry through altering diet composition, with additionalimplications for other zooplankton withstanding greater predationpressure in Daphnia’s absence. Recent decades of warmingin Lake Washington are consistent with the warming of lakesworldwide, and complex phenological responses such as thosereported here may be common as the climate continues to change.     

Daphnia invasion: population dynamics of Daphnia assemblages in two eutrophic lakes with particular reference to the introduced alien Daphnia ambigua

The population dynamics of Daphnia species (D.cucullata, D.ambigua,D.galeata) in two small, productive lakes were studied and experimentswith field enclosures were carried out to elucidate the ecologicalniches of the introduced, alien species D.ambigua. Daphnia ambiguocoexisted temporally with the similarly sized D.cucullata, butlived in deeper water. Although both species had the same clutchsizes, similar proportions of ovigerous females, and death rates.D.cucullata was nurnerically dominant over D.ambigua. This suggeststhat the observed difference in abundance is simply a resultof the slower population growth rates of D.ambigua at lowertemperatures in greater depths. The experiments with field enclosuresshowed that when D.culcullata and D.ambigua were forced to livein the same layer of water, D.ambigua was numerically dominantover D.culullata both in the enclosures with and without fish.As the difference between the abundances of the two speciesdecreased in the enclosure with fish, D.cucullata seems to bebetter protected against fish predation than D.ambigua. In theenclosures without fish, D.ambigua and D.cucullata were numericallysurpassed by the larger species D.galeara. The present resultsimply that in some productive lakes with a high fish stock,D.mbigua invaded the niche of D.galeara and that the speciescan become an important component of the native zooplankton.The results also support the opinion that co-existing Daphniacan have very similar dynamics and niches. ¹ Present address: Zoologisches Institut, Christian-Albrechts-UniversitätKiel, Olshausenstrasse 40, D-24098 Kiel, FRG     

Food quality of detritus derived from the filamentous cyanobacterium Oscillatoria limneticafor Daphnia galeata

Detritus derived from the filamentous cyanobacterium ,Oscillatorialimnetica was fed to Daphnia galeataDetritus supported growthand reproduction comparable to that on the green alga Scenedesmusobliquus. The live filaments of O.limnetica were, however, oflower food quality. Biochemical parameters of these food types,thought to be important in Daphnia nutrition, were also determined.It is concluded that detritus can be a good quality food forDaphnia, making eutrophic lakes better food environments thangenerally assumed. ¹Present address: Department of Applied Chemistry and Microbiology,Division of Microbiology, BiocenterViikki, FIN-00014 HelsinkiUniversity, Finland     

In situ filtration responses of Daphnia galeata to changes in food quality

In the stoichiometric study of phyto–zooplankton interactions,a controversy exists about how Daphnia species regulate theirfeeding rate when submitted to low-quality food (i.e. high dietarycarbon:phosphorus [C:P] ratio). In this study, we gathered dataover 3 years on in situ clearance rates of a Daphnia galeatapopulation, by conducting grazing experiments from April 1998to October 2000 in the Esch-sur-Sûre reservoir (Grand-Duchyof Luxembourg). Observed clearance rates (2.5–13.5 mLindividual^–1 day^–1, mean 7.0 mL individual^–1day^–1) were correlated with population and environmentalvariables. Mean body size of Daphnia individuals was the bestpredictor of clearance rate (r² = 0.639), followed by watertemperature (r² = 0.262) and P concentration in the seston (r²     

Multiple predator defence strategies in Daphnia pulex and their relation to native habitat

Daphnia may respond with an array of anti-predator defences(behavioural, morphological and life history) to a chemicalcue (kairomone) exuded by its predators: fish and Chaoborus.Given the wide array of potential responses, it is an interestingquestion whether anti-predator defences are coupled or independentof each other. Since anti-predator responses are costly andeven possessing the genetic information to respond to a certainpredator might involve a cost, clones may only react to predatorsthey co-occur with in nature. In this study, we provide evidencefor an uncoupling of responses by Daphnia pulex in several anti-predatordefences against Chaoborus. We were unable to detect a correlationbetween behavioural (migration), morphological (neck-spine induction)and life history [growth rate, neonate size and size at firstreproduction (SFR)] responses. Furthermore, anti-predator responsesdid not always comply with what is commonly believed. We foundthat Daphnia clones can migrate up or down when exposed to fishor Chaoborus kairomone and that population growth rate, neonatesize and SFR can increase or decrease in response to Chaoboruskairomone. We also show patterns in anti-predator defences thatseem to relate to the habitat from which clones were derived.Daphnia clones that were collected in habitats with Chaoborusas the dominant predator tended to react strongly to Chaoboruskairomone by migrating upward and producing neck-spines. Themigration behaviour against fish kairomone in these clones wasoften an unexpected upward migration. The Daphnia clone thatco-existed with fish predators showed a downward migration inthe presence of fish as well as Chaoborus kairomone. Clonesthat had occurred with either both or no predators had mixedresponses. We sometimes found an upward migration in combinationwith smaller body size as a response to Chaoborus kairomone.This may be interpreted as a behavioural defence against Chaoborusand a life-history defence against fish. Daphnia seem not toexhibit defence behaviour against predators they do not co-occurwith. It might be costly for Daphnia to maintain genetic informationto respond to these predators and protect that information fromgenetic drift.     

Morphological responses of four species of cyclomorphic Daphnia to a short-term exposure to the insecticide carbaryl

Four species of cyclomorphic Daphnia (D.pulex, D.galeata mendotae,D.retrocurva, D.lumholtzi) were exposed to the insecticide carbarylfor a short term (8–4 h) from the final embryonic stageto the first instar. Daphnia pulex formed neckteeth, and theremaining three Daphnia species developed high helmets and longtailspines. The results suggest that the development of suchprotuberant structures (anti-predator morphologies) in responseto the insecticide exposure is a general phenomenon in Daphnia,and that stimuli on the nervous system of Daphnia may inducethe morphological changes, which originally evolved as a responseto predator kairomone. Two clones of D.pulcx were examined anda clone which was more sensitive to the predator Chaobonts kairomonethan another developed more marked neckteeth in response tocarbaryl, suggesting that the sensitivity in morphological responseto the insecticide may be related to the sensitivity to thekairomone. ¹Present and permanent address: Regional Environment Division,National Institute for Environmental Studies Onogawa, Tsukuba,Ibaraki 305, Japan     

Mass occurrence of planktonic oligotrichous ciliates in a bay in southern Norway

A reddish-brown discoloration of the surface water in FlødevigenBay, southern Norway on May 15, 1985 was due to planktonic oligotrichousciliates occurring in densities of 2079 cells ml^–1. Strombidiumreticulatum (Leegaard, 1915) Bush, 1921 and Tintinnopsis beroideaStein, 1867 accounted by numbers for 59.7 and 35.1% respectively.Both the build-up and decline were probably mainly due to wind-inducedhydrographical conditions. No fish mortalities were reported.     

Relative impacts of copepods, cladocerans and nutrients on the microbial food web of a mesotrophic lake

Calanoid copepods, rather than cladocerans, frequently dominatethe zooplankton of lakes in New Zealand. The potential consequencesof this domination for the microbial community of mesotrophicLake Mahinerangi, New Zealand, were determined by field experimentsin which Boeckella and Daphnia were added to in situ enclosuresin the presence and absence of added nutrients. Boeckella hamataat ambient densities (2 and 81^–1) rapidly and severelysuppressed ciliate population growth over 4 days, even whenmicrobial growth was enhanced by added nutrients, but effectsof copepods on other components of the microbial community (bacteria,photosynthetic picoplankton, heterotrophic nano-flagellates,algae) were slight. In contrast, Daphnia carinata at the samedensities (but 3-fold higher biomasses per litre) had a relativelyweak effect on ciliates, suppressing ciliate abundance onlyafter 4 days at 8 Daphnia 1^–1 (330 µg 1^–1);this daphniid density also depressed abundances of large bacterialrods, some photosynthetic picoplankton and the dominant alga,Cyclotella. These results highlight the relative importanceof specific trophic linkages in a microbial food web; they alsosuggest that the dominance of Boeckella in many southern hemispherelakes may account for relatively low ciliate abundances in theselakes.     

Estimation of phosphorus metabolic rates of Daphnia galeata using a three-compartment model

A compartment model to estimate the different phosphorus metabolicrates in Daphnia galeata is presented. The model has three compartments:gut, metabolic pool and structural pool. Existing two-compartmentmodels used for carbon and phosphorus turnover in Daphnia donot allow estimation of ingestion and egestion rates. We extendedexisting two-compartment models with one more compartment, thegut, which allowed us to estimate both the ingestion and theegestion rates. Parameters of the model are estimated from asingle experiment of feeding unlabelled Daphnia with ³²P-labelledScenedesmus obliquus. Separate experiments with juvenile andadult daphnids were carried out in order to compare their metabolisms.This model permits a reliable estimation of the different metabolicrates of Daphrna in a single experiment and discriminates clearlybetween animals of different sizes.     

Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a shallow hypereutrophic lake

The impact of a cyclopoid copepod population on the protozoacommunity (two ciliate categories and Cryptomonas) was assessedweekly during the spring cohort of Cyclops vicinus (one monthduration) in hypereutrophic Lake Søbygård by insitu gradient experiments with manipulation of ambient zooplanktonabundance. As C.vicinus always made up >92% of the zooplanktonbiomass, the response of protozoa is assumed to be a resultof predation by the copepod. Significant effects of copepodbiomass on protozoa net population growth rates were obtainedin the four experiments. Copepod clearance rates were significantlyhigher on oligotrichs than on prostomatids and Cryptomonas butdeclined for all three protozoa categories during the firstthree weeks of the copepod cohort, probably because of the changein developmental instar composition of the copepod population.Grazing impact on protozoa at ambient copepod abundance wasconsiderable (range, 0.05–0.87 day^–1) and could,together with the estimated reproductive potential of protozoans(range, –0.20–0.87 day^–1), account for thedecline in abundance and biomass of protozoa during the cohortdevelopment. Carbon flow from the protozoa to C.vicinus (range,2.8–23.5 µg C l^–1 day^–1) documents thepresence of a trophic link between protozoa and the spring cohortof C.vicinus in Lake Søbygård.     

Zooplankton community grazing in a hypertrophic lake (Hartbeespoort Dam, South Africa)

Seasonal abundance as total biomass and specific densities ofthe main herbivorous zooplankton (>60 µm) in hypertrophicHartbeespoort Dam from 1981 to 1983 are described. After springzooplankton biomass maxima each year there followed a mid-summerdecline in the Daphnia population and a shift to a smaller bodiedcladoceran community dominated by Ceriodaphnia concomitant witha change from largely edible phytoplankton species to abundantcolonial Microcystis. In situ community grazing rates were measuredthroughout 1983 using a ¹⁴C-labelled unicellular alga. Integratedcommunity grazing rates measured in the aerobic water columnwere highest in December (260.2%/day) when Daphnia was abundantand the edible component of the phytoplankton was diminishing.Lowest integrated community grazing rates occurred in January–February(19.8–35.3%/day) and July (28.3%/day) when the phytoplanktonwas composed almost entirely of Microcysris, and Ceriodaphniadominated the zooplankton community whilst food availabilitywas low. Feeding in Ceriodaphnia was not hindered by abundantlarge Microcysris colonies; total biomass specific grazing ratewas high when Ceriodaphnia dominated and low when Daphnia dominatedthe zoo-plankton community. Results indicate that in hypertrophicconditions it is unlikely that large filter-feeders such asDaphnia are able to retard or limit the development of Cyanophyceaeblooms by high grazing pressure.     

Calcium limitation in Daphnia magna

The role of ambient calcium concentrations on survival, moulting,growth and egg production was assessed in the cladoceran Daphniamagna. A threshold for survival was found in the range 0.1–0.5mg Ca l^–1, even when ionic strength of the medium waskept constant. Accumulated length and length specific dry weightwas retarded at low Ca (0.5–1.0 mg Ca l^–1) at foodconcentrations above incipient limiting level. For lower foodlevels, the effect of Ca on growth was less clear. The effectof low Ca on growth rate was most manifest during the firstdays after hatching, reflecting the higher Ca demands of theearly juveniles. Age-specific egg production was strongly reducedat Ca concentrations <10 mg Ca l^–1. This was partlyan indirect effect of reduced growth rates, but could also bean effect of higher energetic costs associated with Ca uptakein a Ca-poor medium. The high Ca demands in D.magna may notbe representative of other Daphnia species. Nevertheless, highspecific Ca content seems to be a common property of Daphniaspp. and Ca deficiency could be a major determinant of speciessuccess and community structure among crustacean zooplankton;it also puts constraints on carbon sequestration in the pelagicfood web of softwater lakes.