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.     

How Daphnia cope with algae selected for inedibility in long- running microcosms

Macro video records of restrained feeding Daphnia enabled usto measure simultaneously carapace gape, claw rejection rate,filter limb beat rate, and mandible movement rate. We comparedthe effects of high and low concentrations of highly ediblealgae and of inedible algae, the latter selected by long-termDaphnia grazing in oligotrophic microcosms. Inedible algae slowedthe filtering process and influenced the carapace gape (wideningat low concentration and narrowing at high), but did not affectthe rejection rate.     

Fixation of Elemental Nitrogen by Marine Blue-green Algae

Three blue-green algae, Calothrix scopulorum, Nostoc entophytum,and Oscillatoria brevis, isolated from the upper littoral andsupralittoral fringe of the sea-shore were obtained in pureculture and tested for fixation of elemental nitrogen. Appreciablefixation by Calothrix and Nostoc was detected, a proportionof the total nitrogen fixed being liberated into the culturemedium. There was no evidence of fixation by Oscillatoria. Thisappears to be the first evidence that blue-green algae isolatedin pure culture from marine habitats fix nitrogen.     

Nutritional value of algae: a critical control on the stability of Daphnia-algal systems

In Daphnia–algal systems, the effect of nutrient enrichmenton stability is an important ecological issue. Here I considera system of Daphnia and two potential prey; one prey termedprimary algae, which are preferentially consumed, and the othersecondary algae, which yield less nutrition and are more resistantto the grazer. The presence of secondary algae is a key to thestability, but their nutritional value has not been clearlydefined in the previous theory and the actual value varies.Here I use a simple mathematical model defining explicitly thenutritional values of algae and examine the stability of thesystems as a function of phosphorus enrichment. Whether or notall three species can stably coexist depended on the combinationof the algal species used for simulation. In systems where allthe species coexist in a stable manner, in which enrichmentdoes not necessarily lead to destabilization, there is alwaysa critical nutritional value of the secondary algae. Empiricalwork supports the possibility that the unknown nutritional valueof secondary algae takes a value close to the critical one.Furthermore, at the critical nutritional value, the populationresponse in the systems to enrichment is consistent with theobserved trend in natural systems. This suggests that Daphnia–algalsystems in nature can maintain stability in the face of enrichment,without requiring specific assumptions such as spatial heterogeneity.     

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodiedDaphnia pulicaria have a different effect from smaller-bodiedDaphnia galeata mendotae on the composition of summer algalassemblages in eutrophic lakes. Three separate cubitainer experimentswere run for 5 days in a replicated factorial design utilizingtwo algal community types and the two Daphnia species. Inorganicphosphorus and nitrogen were added to prevent nutrient limitationof the algae. Both edible and inedible size fractions of chlorophylla increased in cubitainers without Daphnia spp. Grazer additionusually resulted in a reduction in edible chlorophyll; reductionswere greater in D.pulicaria cubitainers. Grazing by Daphniaspp. on presumed inedible chlorophyll was variable. Algal sizewas not always a good predictor of grazeability. The resultsof this study indicate that D.pulicaria, because of its greaterfiltration potential and ability to ingest larger particles,provides a stronger control on inedible-sized algae when comparedto equal numerical densities of D.g.mendotae. However, Aphanizomenonincreased as a response to heavy grazing pressure by D.pulicariaon other algal species. This suggests that biomanipulation effortsthat promote large-bodied Daphnia may not produce desirableresults if nutrient inputs remain high.     

Differences of the primer-independent phosphorylase isozyme in thermophilic and mesophilic algae

Kinetic studies of the primer-independent, aa phosphorylaseisozyme of two thermophilic algae, Cyanidium caldarium and Oscillatoriaprinceps, and two mesophilic algae, Spirogyra setiformis andChlorella pyrenoidosa, indicate that the isozyme of thermophilicspecies is more resistant to denaturation by heat, and has lessaffinity for the glucose-1-phosphate substrate than the sameisozyme of mesophilic algae. This points out the possibilitythat this enzyme's structure is more rigid in thermophiles andmay lack the flexible conformation found in the mesophilic isozyme.The evolutionary significance of thermophily as a primitivecharacter rather than an adaptive trait is discussed in thelight of studies with a low temperature variant strain of thethermophilic alga, Oscillatoria princeps, LTV. (Received November 15, 1972; )     

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.     

Electrophoresis of algal glucans

The ability of reactive dyes based on chloro-s-triazine to reactwith the primary and secondary hydroxyl groups of glucose, formingcovalent bonds, has been used to dye the storage polyglucosidesof the algae, Cytmidium caldarium, Oscillatoria princeps, Rhodymeniapertusa and Spirogyra setiformis, prior to electrophoresis oncellulose polyacetate strips in borate buffer. The techniqueresolves mixtures of glucans and defines some of the parametersof the internal structure of these glucans. The similarity ofthe glucans from Cyanidium caldarium and Oscillatoria princepsis evident using this method. The two components of the "starch"from Spirogyra setiformis have been studied and identified bytheir reactions with the dye and the mobilities of the dyedcomponents during electrophoresis. Inferences as to the biphyleticevolutionary pathways leading from the Cyanophyceae to the Rhodophyceaeand the Chlorophyceae can be made based on the data. (Received June 27, 1973; )     

Effects of Daphnia longispina on rotifer populations in a natural environment: relative importance of food limitation and interference competition

To test the relative importance of exploitative and interferencecompetition from Daphnia limiting rotifer populations in Bermejalesreservoir, birth rates of rotifers, food abundance, zooplanktonbiomass and size distribution of Daphnia were analysed fromhigh frequency sampling at two depths. A depletion in food abundanceby Daphnia feeding should be reflected in the birth rates ofrotifers susceptible to exploitative competition, while interferenceshould produce direct negative relationships between the numberof individuals of the interacting species. Results show thatsignificant relationships between food (chlorophyll-a) and birthrates of rotifers are found when the ratio chlorophyll-a/herbivorouszooplankton biomass is used as a measure of food availability.In our study, Anuraeopsis fissa was only affected through exploitativecompetition by Daphnia, while only Synchaeta oblonga showedevidence of being suppressed through interference competitionby this cladoceran.     

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.     

Consumer-dependent responses of lake ecosystems to nutrient loading

The nutrient loading concept proposes that algal biomass, waterclarity and the processes of lake eutrophication are a functionof nutrient loading. We hypothesized that grazers play an importantrole in determining the impacts of nutrient loading on algalbiomass and water clarity, and the overall eutrophication process.To test how the contrasting grazer communities modify the fateof nutrients, we added nutrients (nitrate and phosphate) ata known loading rate to four large enclosures, but in two ofthe four enclosures large cladoceran grazers (Daphnia >1mm mean length) were allowed to develop by removing the planktivorousfish. In the remaining two enclosures, the development of largeDaphnia was prevented by adding planktivorous fish. The concentrationsof epilimnetic total phosphorus (TP) increased at a similarrate in all four enclosures. However, the daily accumulationof added phosphate into the participate or planktonic forms,especially into plankton <20 µm, was three times fasterwhen large Daphnia were absent than when large Daphnia wereabundant. In the enclosures with large Daphnia, added phosphatewas accumulated in the dissolved pool instead. At a constantnutrient loading, algal biomass (chlorophyll a) increased fourtimes faster in the enclosures without large Daphnia than inthose with large Daphnia. Similarly, Secchi depth declined from3.5 to <1 m when Daphnia were absent, but did not declinewhen Daphnia were common. Our results demonstrate that the samenutrient loading and the resultant increase in epilimnetic TPdo not produce the same trophic conditions, as indicated byalgal biomass and water clarity, if the grazers of the majorassimilators of nutrients (the fraction of plankton edible toDaphnia) are different. We suggest that stratified lake ecosystemshaving functionally dominant large grazer communities may beless prone to eutrophication than those lacking large grazers.Consistent with the nutrient loading concept, epilimnetic concentrationsof phosphorus increase proportionately with increased loadingof phosphorus, but the trophic conditions of ecosystems indicatedby algal biomass and water clarity do not follow the same patternsunder contrasting conditions of grazer communities. We suggestthat models predicting algal biomass from loading rates shouldaccount for the role of grazers.     

Structure and dynamics of zooplankton communities maintained in closed systems, with special reference to the algal food supply

Seasonal changes in the structure of the zooplankton communitiesof two large (18 500 m³) experimental enclosures (‘LundTubes’) are reviewed. The diets of the two most prominentcrustaceans, Daphnia hyalina and Diaptomus gracilis, were investigatedby direct analyses of gut contents and in a series of shortterm experimental manipulations of communities isolated in 6m³ bags, placed in one of the enclosures. The relative abundanceof Daphnia hyalina generally coincided with warmer water temperaturesbetween late spring and late summer, but its numbers fluctuatedwithin that period, apparently in response to the availabilityof smaller algae (<10⁴ µm³ unit volume: chiefly cryptomonads,chlorellids and Ankyra). Daphnia numbers declined steadily throughthe summer in the enclosure overwhelmingly dominated by Microcystis,although it did feed on smaller colonies (< 10⁵ µm³in volume). Diaptomus gracilis and most rotifers were also prominentin the first half of the year, but peak abundances of Keratellaquadrata followed the presence of large populations of small(<10³ µm³) algae. Under conditions of limiting foodavailability, mortality was greatest amongst the smaller speciesand the juvenile instars of the large Crustacea. ¹Present address: Anglian Water Authority, North Street, Oundle,Peterborough, UK     

Horizontal distribution of Daphnia pulex in response to toxic and non-toxic algal extracts

We studied the horizontal distribution of Daphnia pulex as aresponse to gradients of toxic extracts of two cyanobacteria,Microcystis aeruginosa and Oscillatoria agardhii and non-toxicextracts of the green alga Scenedesmus obtusiusculus. The experimentwas performed in the dark using a flow through system with fivechambers. Daphnia pulex aggregated at the inlet of Scenedesmusextract, but showed similar responses to extracts of toxic cyanobactenaand food-free medium. Chemical cues seem to affect food search,but do not effect Daphnia pulex's avoidance of toxic algae.     

Preferential feeding behaviour of Daphnia magna

Preferential feeding behaviour by Daphnia magna was shown when Daphnia were fed on a mixture of ¹⁴C-labelled algae (Chlorella vulgaris or Scenedesmus quadricauda) and ³H-labelled bacteria (Escherichia coli). Daphnia consumption of bacteria was equal or higher in the presence of algae. On the other hand, in the presence of bacteria, algal consumption decreased by 40–70% compared to algal consumption in the absence of bacteria. ¹⁴C radioactive uptake was in good agreement with the chlorophyll content and demonstrates the preferential feeding behaviour of Daphnia.     

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     

How<Emphasis Type="Italic"> Daphnia</Emphasis> copes with excess carbon in its food

Animals that maintain near homeostatic elemental ratios may get rid of excess ingested elements from their food in different ways. C regulation was studied in juveniles of Daphnia magna feeding on two Selenastrum capricornutum cultures contrasting in P content (400 and 80 C:P atomic ratios). Both cultures were labelled with ¹⁴C in order to measure Daphnia ingestion and assimilation rates. No significant difference in ingestion rates was observed between P-low and P-rich food, whereas the net assimilation of ¹⁴C was higher in the treatment with P-rich algae. Some Daphnia were also homogeneously labelled over 5 days on radioactive algae to estimate respiration rates and excretion rates of dissolved organic C (DOC). The respiration rate for Daphnia fed with high C:P algae (38.7% of body C day^-1) was significantly higher than for those feeding on low C:P algae (25.3% of body C day^-1). The DOC excretion rate was also higher when animals were fed on P-low algae (13.4% of body C day^-1) than on P-rich algae (5.7% of body C day^-1) . When corrected for respiratory losses, total assimilation of C did not differ significantly between treatments (around 60% of body C day^-1). Judging from these experiments, D. magna can maintain its stoichiometric balance when feeding on unbalanced diets (high C:P) primarily by disposing of excess dietary C via respiration and excretion of DOC.     

Influence of different sized Daphnia species on chlorophyll concentration and summer phytoplankton community structure in eutrophic Wisconsin lakes

We conducted field observations on 13 eutrophic Wisconsin lakesdominated by either the larger bodied Daphnia pulicaria or thesmaller bodied Daphnia galeata mendotae. While Daphnia numericaldensities were not significantly different between groups oflakes, pulicaria lakes had much higher Daphnia biomasses andfiltration potentials than galeata lakes. Although we foundsignificant differences in chlorophyll (Chl) a between bothgroups of lakes during June, on a seasonal basis populationsof different sized Daphnia were not associated with significantdifferences in Chl a. Filtration potential per se was the majordeterminant of Chl a, regardless of which Daphnia species dominated.However, in pulicaria lakes, the clear-water phase started earlier,lasted longer, and was usually characterized by greater Secchidisc readings than in galeata lakes. For large blue-green algaesuch as Aphanizomenon, D.pulicaria appeared to delay bloom conditions,but ultimately did not prevent the alga from growing. Our resultssuggest that high densities of large-sized Daphnia are a desirablegoal of biomanipulation because they can attain filtration potentialshigh enough to increase summer water clarity in eutrophic lakes.     

Chemical interference by Daphnia on Keratella: a life table experiment

Daphnia can suppress rotifers through mechanical interference(interference competition) and exploitative competition. Totest chemical interference (interference competition) by Daphniaon the rotifer Keratella, life history characteristics wereanalysed for Keratella cochlearis in the presence and absenceof Daphnia-mediated chemicals. The presence of Daphnia-mediatedchemicals induced a lower fecundity and a decrease in the intrinsicrate of population growth. This type of interference competitionshould affect the composition of natural rotifer communitiesregardless of mechanical interference and exploitative competitiondue to Daphnia.     

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free‐swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.     

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.