Interannual variability in a predator-prey interaction: climate, chaetognaths and copepods in the southeastern Bering Sea

The interaction of the chaetognath Sagitta elegans with thecopepod community of the southeast Bering Sea middle shelf wasexamined in relation to environmental conditions during 1995–1999.Predation impact was estimated for 2 years, 1995 and 1997, usinggut content analysis, experimentally derived digestion time(DT) and abundances of chaetognaths and prey. Pseudocalanusconcentrations correlated with water temperature and Calanusmarshallae with sea ice extent. Sagitta elegans were less abundantbut individuals were larger in 1995, when C. marshallae predominated,compared to 1997, when Pseudocalanus and Acartia were the primaryprey. Predation by S. elegans removed <1% standing stockday^–1 of Pseudocalanus or C. marshallae in 1995 and 1.7to 2.3% of Pseudocalanus in 1997. The percent of the copepodcommunity biomass required by chaetognaths was estimated tobe <1% in 1995 compared with 8–12% in 1997. Calanusmarshallae may be more vulnerable than Pseudocalanus to cumulativepredation effects because of its reproductive strategy. Theeffect of chaetognath predation on the copepod community dependson which copepod species is predominant and its susceptibilityto cumulative predation effects, as well as on daily predationimpact, both of which varied between years with different climaticconditions.     

Copepod feeding behavior and the measurement of grazing rates in vivo and in vitro

In this paper some aspects of the use of the gut fluorescence method for estimating ingestion rates have been, examined. One assumption is that gut turnover time in feeding copepods is equal to the gut clearance time in filtered seawater. When arctic Pseudocalanus were pre-fed on Thalassiosira weisflogii, and then given a trace addition of the same C¹⁴-labelled culture, or were transferred to filtered seawater, results suggested that this assumption was probably justified. In another experiment in which Pseudocalanus were fed at the same concentration of either melted ice algae, or pelagic under ice algae, there were significant differences in both gut clearance times and gut pigment levels in the two cases.Pigment: biogenic silica ratios in epontic algae were higher than those in faecal pellets produced by Pseudocalanus feeding on the algae, suggesting that pigment destruction was occurring during grazing. In a 28 hr time course experiment ingestion rates determined by rate of disappearance of particulate chlorophyll were higher than those simultaneously determined by the gut fluorescence method, which also supports the idea of pigment destruction in copepods guts.     

Predator-limited population growth of the copepod Pseudocalanus sp.

The impact of predators on population growth of Pseudocalanussp. was investigated in Dabob Bay, Washington. Mortality ofPseudocalanus sp. was determined from stage-specific survivorship,from seasonal changes in mortality rates of adult males andfemales and from incidence of injuries to adult copepods. Theprincipal predators of adult Pseudocalanus were identified asthe predatory copepod Euchaeta elongata, the omnivorous euphausiidEuphausia pacifica and the chaetognath Sagirta elegans. Predatorattack rates - and prey mortality rates - are highly density-dependentand thus sensitive to prey dispersion in the water column, particularlyto layering in the vertical plane. Predation rates by the threeprincipal predators exceeded 100% of the recruitment rate toadult Pseudocalanus sp. beginning in early summer, thus restrictingpopulation growth. Planktivorous fish predation (by adult three-spinestickleback, Gasterosteus aculeatus, and juvenile chum salmon,Oncorhynchus keta) on Pseudocalanus sp. adults was estimatedto be two orders of magnitude lower than consumption rates bypredatory zooplankton, at a deep water station in July. Analysisof seasonal changes in prey ingested by Sagitta elegans revealedthat Pseudocalanus sp. was the major prey item of S. elegansin April (61.0% of prey) and in June (67.0% of prey), thereafterdeclining seasonally in importance. Predation by S. elegansvaried seasonally with changes in chaetognath stage structure,vertical distribution and diapause, not size structure alone.Although chaetognath recruitment and population growth appearto be directly coupled to the abundance of Pseudocalanus sp.,predation by S. elegans has little reciprocal impact on Pseudocalanussp. population growth; hence asymmetries may occur in the interaction of planktonic prey and predators.     

Changes in the structure of a zooplankton community during a Ceratium (dinoflagellate) bloom in a eutrophic fishless pond

The community structure of zooplankton was studied in a eutrophic,fishless Japanese pond. The ecosystem was dominated by a dinoflagellate,Ceratium hirundinella, two filter-feeding clado-cerans, Daphniarosea and Ceriodaphnia reticulata, and an invertebrate predator,the dipteran Chaoborus flavicans. The midsummer zooplanktoncommunity showed a large change in species composition (theDaphnia population crashed) when a heavy Ceratium bloom occurred.It is shown that (i) the rapid density decline of D.rosea inmid-May was mainly caused by a shortage of edible phytoplankton,which was facilitated by the rapid increase in Chirundinellaabundance; (ii) the low density of D.rosea in June-July wasconsidered to be mainly caused by the blooming of Ceratium hirundinella(which may inhibit the feeding process of D.rosea), while predationby Cflavicans larvae, the changing temperature, the interspecificcompetition and the scarcity of edible algae were not judgedto be important; (iii) the high summer biomass of the planktonicCflavicans larvae was maintained by the bloom of C.hirundinella,because >90% of the crop contents of C.flavicans larvae wereC.hirundinella during this period. The present study indicatesthat the large-sized cells or colonies of phytoplankton arenot only inedible by most cladocerans, but the selective effectof the blooming of these algae can also influence the compositionand dominance of the zooplankton community, especially for thefilter-feeding Cladocera, in a similar way as the selectivepredation by planktivorous fish. The large-sized phytoplanktoncan also be an important alternative food for ominivorous invertebratepredators such as Chaoborus larvae, and thus may affect theinteractions between these predators and their zooplanktonicprey. In this way, such phytoplankton may play a very importantrole in regulating the dynamics of the aquatic food web, andbecome a driving force in shaping the community structure ofzooplankton.     

Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Experiments with the copepods Acartia clausi, Centropages hamatusand Pseudocalanus sp. were performed to assess the species-specificeffect of these copepods on the development of monospecificalgae (Nephroselmis pyriformis) and ciliate communities (Strombidiumvestitum, Strombidium conicum, Strombidium sp. and Lohmanniellaoviformis). It was hypothesized that potentially switching copepodslike A. clausi will stabilize the algal community by switchingbetween ciliate and algal food, in contrast to copepods withstereotypic filter feeding behaviour (Pseudocalanus sp.). Intreatments with Pseudocalanus sp. and C. hamatus, all ciliatespecies were wiped out in 2 days, resulting in blooms of N.pyriformis. In treatments with A. clausi, two of the ciliatespecies were able to persist, but the combined ciliate and copepodcommunity was not able to control the algal bloom. Ciliatesbecame abundant in control treatments without copepods, butonly S. vestitum and S. conicum seemed able to establish grazingcontrol. Hence, when evaluating the role of ciliates in foodwebs, their actual numbers and species composition should betaken into account. Likewise, the species composition of copepodsmay be crucial; these experiments demonstrate that small filterfeeding copepods may have tremendous impact on ciliate numbers.     

Seasonality, substrate perference and attachment sites of epizoic diatoms on cladoceran zooplankton

Cladocerans from three lakes in northwest Iowa were examinedfor attached diatoms over a period of 1 year to assess the seasonality,substrate preference and attachment site specificities of epizoicdiatoms. Over 90% of the cladocerans had attached diatoms immediatelyfollowing ice thaw, when the lakes were not yet thermally stratified,surface water temperatures were low(10–15°C)and watertransparency was at its annual high. Mean maximum density ofdiatoms on cladocerans averaged among lakes at this time was3000 cells animal^–1. The dominant epizoic diatom was Synedracyclopum Brutschy, which was found most often on large, pelagicDaphnia pulicaria and Daphnia galeata mendotae. Heavily infestedanimals had diatoms concentrated on the second antennae andanterior ventral margin of the carapace. Animals with <5epizoic diatoms were more likely to have cells attached to thepostabdomen or tail spine. In addition to living epizoically,S.cyclopum was found free living in the plankton, exhibitingtemporal fluctuations in densities similar to epizoic counterpartsHowever, the densities of the free-living cells seldom exceededepizoic densities, suggesting that S.cyclopum is adapted forgrowth on motile cladoceran substrates. ¹Present address: Savannah River Ecology Lab, Drawer E, Aiken,SC 29802, USA ²Present address: Department of Fisheries and Aquatic Sciences,7922 NW 71st St, Gainesville, FL 32606, USA     

Use of dried algae as a food source for zooplankton growth and nutrient release experiments

Zooplankton growth and nutrient recycling are key processes in the operation of pelagic food webs. Most studies investigating these processes rely on complex methods and often require extensive laboratory facilities. Here we introduce a technique for preserving algae by rapid drying for later use in laboratory- or field-based growth and nutrient recycling experiments. Chemostat-grown Scenedesmus acutus was rapidly dried for later experiments evaluating its nutritional composition, suitability for animal growth and potential for use in nutrient release experiments. Reconstituted dried algae had slightly lower nitrogen (N), Phosphorus (P) and protein content (% dry weight) than fresh algae, but lipid content did not differ and elemental ratios were in the range considered to indicate favorable food quality. These elemental and biochemical differences did not appear functionally important, as Daphnia magna grew identically on fresh and dried food. Freeze-dried S.acutus did not work as an alternative to oven drying as it resulted in 100% mortality. NH4 and PO4 concentrations did not change over 24 h when dried algae were resuspended in normal media or boiled lake water. However, concentrations of PO4 decreased over 24 h, suggesting chemical adsorption of PO4 to the dried algae and reinforcing the need for animal-free controls in nutrient release experiments using this approach. N and P release rates for D.magna and natural zooplankton communities were estimated using dried algae, and values were comparable to published ones. Thus, dried algae may be a useful, simple technique for studying food quality and nutrient release in environments where maintaining active algal cultures may not be practical and a constant supply of consistent quality food is needed.      

Influence of temperature and food concentration on body size,weight and lipid content of two Calanoid copepod species

Two species of Copepoda Temora longicornis (Müller) and Pseudocalanus elongatus (Boeck) were cultured continuously in the laboratory. Four and three generations, respectively, were raised at 16 different combinations of temperature and food concentration. Prosome length and ash-free dry weight were measured in the adult stage and in Pseudocalanus also in copepodite stage I, and the relation between length and weight was established. In Pseudocalanus also the relative amount of lipid was estimated.Prosome length and length-specific body weight (condition factor) were positively correlated with food concentration. Lipid content in Pseudocalanus was also strongly affected by the concentration of food.Prosome length was negatively correlated with temperature. However, length-specific body weight in Temora was positively correlated with temperature. Therefore, at higher temperature Temora was smaller, but heavier per unit body length. In Pseudocalanus a similar but less significant influence of temperature on length-specific weight was found; lipid content was not significantly influenced by the temperature.Females of Temora had larger length-specific weights than males. In Pseudocalanus the opposite was found, coinciding with a higher lipid content in males than in females.It is discussed that at natural concentrations of phytoplankton body size and weight of copepods are reduced in the North Sea during the major part of the growing-season.     

The production and ingestion of faecal pellets by nauplii of marine calanoid copepods

The downward transport of organic matter as zooplankton faecalmaterial is influenced by copepods which fragment, ingest andrecycle some of the pellet contents. Most of this activity hasbeen attributed to the later copepodite stages and the adults,but little is known about the role of nauplii. Stage-relateddefaecation rates during the naupliar development of two speciesof copepod, Calanus helgolandicus and Pseudocalanus elongatus,were quantified in a series of laboratory experiments. The productionof faecal material commenced soon after the appearance of theNIII in both species and increased throughout naupliar development.The causes of the increase were the formation of larger pelletsby later stages in Calanus and an increased rate of productionby Pseudocalanus. Calanus nauplii, when supplied with algalfood at concentrations that would support full naupliar development,ingested or broke up the pellets of the smaller Pseudocalanusspecies at rates of 1.15 pellets nauplius^–1 h^–1This consumption increased to 2.96 pellets nauplius^–1h^–1 when the concentration of algal food was reduced toa limiting level. Pseudocalanus was not able to consume thepellets of Calanus. Ingestion of Pseudocalanus faecal pelletsby Calanus could supply a nutritional benefit to a food-limitednauplius.     

Dependency of Antarctic zooplankton species on ice algae‐produced carbon suggests a sea ice‐driven pelagic ecosystem during winter

How the abundant pelagic life of the Southern Ocean survives winter darkness, when the sea is covered by pack ice and phytoplankton production is nearly zero, is poorly understood. Ice‐associated (“sympagic”) microalgae could serve as a high‐quality carbon source during winter, but their significance in the food web is so far unquantified. To better understand the importance of ice algae‐produced carbon for the overwintering of Antarctic organisms, we investigated fatty acid (FA) and stable isotope compositions of 10 zooplankton species, and their potential sympagic and pelagic carbon sources. FA‐specific carbon stable isotope compositions were used in stable isotope mixing models to quantify the contribution of ice algae‐produced carbon (α_Ice) to the body carbon of each species. Mean α_Ice estimates ranged from 4% to 67%, with large variations between species and depending on the FA used for the modelling. Integrating the α_Ice estimates from all models, the sympagic amphipod Eusirus laticarpus was the most dependent on ice algal carbon (α_Ice: 54%–67%), and the salp Salpa thompsoni showed the least dependency on ice algal carbon (α_Ice: 8%–40%). Differences in α_Ice estimates between FAs associated with short‐term vs. long‐term lipid pools suggested an increasing importance of ice algal carbon for many species as the winter season progressed. In the abundant winter‐active copepod Calanus propinquus, mean α_Ice reached more than 50% in late winter. The trophic carbon flux from ice algae into this copepod was between 3 and 5 mg C m^?2 day^?1. This indicates that copepods and other ice‐dependent zooplankton species transfer significant amounts of carbon from ice algae into the pelagic system, where it fuels the food web, the biological carbon pump and elemental cycling. Understanding the role of ice algae‐produced carbon in these processes will be the key to predictions of the impact of future sea ice decline on Antarctic ecosystem functioning.     

Distribution and species-specific egg production of Pseudocalanus in the Gulf of Alaska

Pseudocalanus species are important contributors to the secondaryproduction of the northern hemisphere mid- to high-latitudeoceans. In the coastal Gulf of Alaska, Pseudocalanus are presentyear round and are represented by three species. In 2001, Pseudocalanusmimus was the dominant Pseudocalanus species on the shelf duringspring and summer, comprising 30–100% of the total, whilePseudocalanus newmani dominated in Prince William Sound (10–90%).Pseudocalanus minutus were only abundant in Prince William Soundduring early spring. Egg production (by number and volume) wasa function of female prosome length and decreased from springto summer; however, significant variability was attributableto regional influences that were independent of size. For thesame sized female, P. newmani produced more eggs per clutchthan P. mimus. Pseudocalanus mimus, however, tended to havea larger mean egg size than P. newmani. Consequently, clutchvolumes of the two species were indistinguishable. Pseudocalanusegg production rates (EPRs) (eggs female^–1 day^–1)were lower in July and August (ca. 2–4) than April andMay (ca. 1–9), but total egg production by the population(eggs day^–1) was nearly equivalent for the two time periodsdue to higher female concentrations in summer.     

The Contributions of Sea Ice Algae to Antarctic Marine Primary Production

The seasonally ice-covered regions of the Southern Ocean havedistinctive ecological systems due to the growth of microalgaein sea ice. Although sea ice microalgal production is exceededby phytoplankton production on an annual basis in most offshoreregions of the Southern Ocean, blooms of sea ice algae differconsiderably from the phytoplankton in terms of timing and distribution.Thus sea ice algae provide food resources for higher trophiclevel organisms in seasons and regions where water column biologicalproduction is low or negligible. A flux of biogenic materialfrom sea ice to the water column and benthos follows ice melt,and some of the algal species are known to occur in ensuingphytoplankton blooms. A review of algal species in pack iceand offshore plankton showed that dominance is common for threespecies: Phaeocystis antarctica, Fragilariopsis cylindrus andFragilariopsis curta. The degree to which dominance by thesespecies is a product of successional processes in sea ice communitiescould be an important in determining their biogeochemical contributionto the Southern Ocean and their ability to seed blooms in marginalice zones.     

Grazing and food size selection by crustacean zooplankton compared to production of bacteria and phytoplankton in a shallow Norwegian mountain lake

Food size selection by suspension-feeding crustaceans in a shallowmountain lake was investigated with radiolabelled algae andbacteria, and with a series of monodisperse fluorescent latexbeads with diameters ranging from 0.25 to 5.18 µm. JuvenileDaphnia longispina was a more efficient grazer on bacteria-sizedparticles than adults. Bosmina longispina and Cyclops scutiferwere both able to ingest bacteria-sized food particles. Thecrustaceans harvested {small tilde} 100% of the primary productionduring the day investigated, but only 20% of the bacterial production.To keep the bacterial biomass in or near steady state, the microzooplanktonmust be harvesting most of the bacterial production.     

A model of the predatory impact of larval marine fish on the population dynamics of their zooplankton prey

A simulation model of the population dynamics of two speciesof calanoid copepods (Calanus.r pacificus and Pseudocalanussp.) was forced with predation pressure from a genetic, hypotheticalpopulation of larval marine fish. Results of the model are sensitiveto changes in parameters describing the dynamics of both predatorand prey populations, including initial numbers, fecundity,growth, mortality, size of prey organisms and feeding selectivityof the predators; the relative importance of these parametersis tested by way of a brute-force sensitivity analysis. Usingresults from recent ichthyoplankton surveys in Dabob Bay, WA,USA, the model was also forced with predation from populationsof larval Pacific herring (Clupea harengus pallasi) and Pacificwhiting (Merluccius productus). Results of the various simulationruns lead to the conclusion that marine fish larvae can significantlyimpact the population dynamics of their calanoid copepod prey,but that the magnitude of this impact is highly dependent onspecies-specific values of various population parameters.     

Use of immobilized algae for estimating bioavailable phosphorus released by zooplankton

Cells of the green alga Selenastrum capricomuxum were immobilizedin permeable alginate beads to prevent them from being grazedby zooplankton. Algae were able to grow in these beads and wereused as a new technique to estimate bioavailable phosphorus(P) released by zooplankton. P-limited algal cells were encapsulatedin alginate beads and used to measure P-release by Daphnia pulexfeeding on P-saturated and P-limited free algal cells. Daphnidsgrazing on P-saturated cells released 20 times more P availablefor the immobilized algae than animals grazing on P-limitedcells (0.06 versus 0.003 µg P mg^–1 Daphnia-DW h^–1).     

Spatial and temporal patterns of zooplankton on baleen whale feeding grounds in the southern Gulf of Maine

This study addresses the regional variation of zooplankton inthe Great South Channel area in the southern Gulf of Maine betweenCape Cod, Massachusetts and Georges Bank. This is a region ofparticular interest because of the intense concentrations ofthe copepod Calanus finmarchicus in the spring, along with theco-occurrence of right whales that feed upon these copepod aggregations.Zooplankton in the Great South Channel were sampled with theMOCNESS plankton sampler during spring 1988 and 1989 as partof the SCOPEX (South Channel Ocean Productivity Experiment)project. Zooplankton variation was addressed through comparisonsof taxonomic composition and water column abundances among towswithin and between years, and between locations with and withoutright whales. Results showed that zooplankton community compositionwas highly similar between tows within each year average percentsimilarity index (PSI) for pairwise comparisons of tows = 82.2and 88.8% in 1988 and 1989, respectively] as well as betweenyears (mean PSI = 84.4). The copepods C.finmarchicus and Pseudocalanusspp. dominated the zooplankton in terms of total water columnabundance (>94% of all zooplankton), with C.finmarchicuscomprising an average of 84% of the copepods. Highest abundancesof these copepods (particularly the younger life stages) coincidedwith a region of low-salinity transport from the north. In addition,these copepods had higher abundances in 1989, which may be relatedto the fact that low-salinity transport was approximately twiceas large in 1989 as in 1988. Given the physical dynamics ofthe region, it is possible that developing copepod populationswere adverted into the Great South Channel from the northwesternGulf of Maine via the low-salinity plume. Each year, whaleswere located in areas of both high and low copepod abundance,and tended to concentrate near the leading edge of the low-salinityplume. In 1988, there were no significant differences in zooplanktonabundance between right whale areas and non-whale areas forany taxon. In 1989, whale areas had a greater proportion ofC.finmarchicus relative to other zooplankton in 1989, whichsuggests that whales preferred regions enriched in C.finmarchicus.     

Toxic and inhibitory effects of the blue-green alga Microcystis aeruginosa on herbivorous zooplankton

Blooms of blue-green algae are often associated with declinesin populations of large-bodied cladocerans and increased importanceof small cladocerans, copepods, and rotifers. We conducted toxicityand herbivory experiments, using a wide range of herbivore taxa,to test the hypothesis that the blue-green alga Microcystisaeruginosa most strongly inhibits large cladocerans. For a varietyof herbivore taxa, M. aeruginosa was toxic or non-nutritious,and inhibited feeding on co-occurring nutritious food. The rotiferBrachionus calyciflorus was unique in several respects: it wasunaffected by M. aeruginosa toxins, it showed some ability togrow and reproduce on a diet of M. aeruginosa, and it maintainedhigh feeding rates on co-occurring nutritious food in the presenceof bloom densities of M. aeruginosa. There was a strong relationbetween the toxicity of M. aeruginosa and its inhibitory effecton herbivore feeding rates. Copepods strongly avoided consumingM. aeruginosa, but all cladocerans and rotifers tested filteredunicellular M. aeruginosa at rates similar to or higher thannutritious Chlamydomonas reinhardi. Our results indicate thatthere are a variety of mechanisms whereby herbivorous zooplanktoncan coexist with blooms of M. aeruginosa, including resistanceto toxic chemicals (B. calyciflorus), and avoidance of consumptionof M. aeruginosa by chemosensory means (copepods), or by theinability to consume large colonies (some small cladocerans). ¹Present Address: Department of Biology, George Mason University,4400 University Drive, Fairfax, VA 22030, USA     

Feeding behaviour in Meganyctiphanes norvegica (M. Sars) (Crustacea: Euphausiacea)

Meganyctiphanes norvegica (M. Sars) will feed upon the centric diatom Thalassiosira weissflogii (Grunow) G. Fryxell & Hasle but cannot fulfil its energy requirement for metabolism on this food. Its daily metabolic requirement can be exceeded when the euphausiid feeds upon the copepods Calanus finmarchicus Gunner or Centropages typicus Krøeyer, but not when feeding upon the smaller copepods Pseudocalanus spp. or Acartia spp. When feeding upon a natural copepod assemblage Meganyctiphanes norvegica requires high concentrations of copepods to achieve its metabolic requirements, suggesting that the euphausiid may exploit vertically patchy concentrations of prey. Short-term predation rates on Pseudocalanus spp. were also used to estimate feeding rates. Feeding in Meganyctiphanes norvegica appears to be adapted to a spatially variable food supply and rapid exploitation of food sources concentrated into patches or layers. The filter area of the feeding basket of M. norvegica is proportionally smaller than the filter area of Euphausia superba Dana, but has the same allometric length exponent. The filter area probably reflects the difference between the primarily carnivorous diet of Meganyctiphanes norvegica and herbivorous diet of Euphausia superba.     

Effects of UV-B irradiated algae on zooplankton grazing

We tested the effects of UV-B stressed algae on grazing rates of zooplankton. Four algal species (Chlamydomonas reinhardtii, Cryptomonas sp., Scenedesmus obliquus and Microcystis aeruginosa) were used as food and fed to three zooplankton species (Daphnia galeata, Bosmina longirostris and Brachionus calyciflorus), representing different taxonomic groups. The phytoplankton species were cultured under PAR conditions, and under PAR supplemented with UV-B radiation at two intensities (0.3 W m^–2 and 0.7 W m^–2, 6 hours per day). Ingestion and incorporation experiments were performed at two food levels (0.1 and 1.0 mg C l^–1) using radiotracer techniques. The effect of food concentration on ingestion and incorporation rate was significant for all three zooplankton species, but the effect of UV-B radiation was more complex. The reactions of the zooplankton species to UV-B stressed algae were different. UV-B stressed algae did not affect Daphnia grazing rates. For Bosmina the rates increased when feeding on UV-B stressed Microcystis and decreased when feeding on UV-B stressed Chlamydomonas, compared with non-stressed algae. Brachionus grazing rates were increased when feeding on UV-B stressed Cryptomonas and UV-B stressed Scenedesmus, and decreased when feeding on UV-B stressed Microcystis, compared with non-stressed algae. These results suggest that on a short time scale UV-B radiation may result in increased grazing rates of zooplankton, but also in decreased grazing rates. Long term effects of UV-B radiation on phytoplankton and zooplankton communities are therefore difficult to predict.     

The influence of food resources on the development, survival and reproduction of the two cyclopoid copepods: Cyclops vicinus and Mesocyclops leuckarti

Laboratory experiments were conducted to determine whether thetwo cyclopoid copepods. Cyclops vicious and Mesocyclops leuckaru.exploit the same food resources. The food requirements of juvenilesof the two cyclopoid copepods were investigated. Moreover, theimportance of algae for the predaceous adults was studied. Naupliiof both M leuckaru and C.vicinus successfully developed intocopepodites when fed the motile algae Chlamydomonas reinhardtii.Chlamydomonas sphaeroides and Cryptomonas sp. Threshold foodconcentrations for naupliar development varied between offeredalgae and between the two cyclopoid species. The food thresholdfor successful naupliar development, when reared on C.reinhardui,was lower for M.leuckarti (0.3 mg C 1^–1) than for C.vicinus(0.5 mg C l^–1) whereas a similar food threshold was foundusing Cryptornonas sp (0.3 mg C ^–1) and C.sphaeroides(<0.2 mg C 1^–1), Naupliar development time was inverselyrelated to food concentration. Food required for copepoditedevelopment differed for the two cyclopoid species. Cyclopsvicinus was able to develop to the adult stage on a pure dietof any one of the three algal species. whereas M.leuckarti requireda prey supply of the rotifer Brachionus rubens. Food composition.i.e. algal species, algal concentration and rotifer abundance,influenced copepodite survivorship of both cyclopoids and wasalways higher in the presence of B.rubens. Under similar foodconditions, mortality was higher for M.leuckarti than for Cvicinus. Mesocyclops leuckaru females were very dependent onanimal food. The predation rate of M.leuckaru was not lowerin the presence of algae. Egg production of M.leuckarti waslow on a pure algal diet and significantly higher when B rubenswas present. The results were used to discuss the life cyclestrategy and the possibility of exploitative competition ofthe two cyclopoid copepods.