DISTRIBUTION AND ABUNDANCE OF GYMNOSOMATA (GASTROPODA: OPISTHOBRANCHIA) IN THE SOUTHWEST ATLANTIC

The distribution and abundance of gymnosome gastropods in theArgentine Sea and Brazil—Malvinas Confluence during 1978–1979and 1988 were studied. The collections analyzed included 768quantitative samples obtained between 48°W and the coast,and from 35°S to 55°S. Two species were found. Spongiobranchaeaaustralis was the most frequent and abundant (up to 730 per1000 m3); its presence in the area was associated with the coreof the Malvinas Current. Clione antarctica was less abundant(maximum abundance: 230 per 1000 m³) and was also associatedwith the Malvinas Current. The geographic ranges of both speciesin the area are wider than previously described. Since the rangeof S. australis in the area extends far from the range of itsprey Clio, it is not clear whether S. australis can feed onthe thecosomatous pteropod Limacina (and not only on Clio, asdescribed in the bibliography) or it starves in that area. Duringthe 1978–1979 annual cycle, the abundance of both speciesfollowed neither the abundance patterns of their prey nor ofthe total zooplankton, and differed from each other. The residencetime of swarms of both gymnosomes were shorter than one month.As a general pattern, the aggregates are rapidly transportednorthward by the Malvinas Current and also penetrate the outershelf water, but they remain there only during a short periodand cannot preclude the final expatriation. So, the abundanceof gymnosomes in the area depends on passive migration morethan intrinsic population factors. (Received 9 July 1997; accepted 15 December 1997)     

Periodicity of composition, abundance, and vertical distribution of summer zooplankton (1977/1978) in Ezcurra Inlet, Admiralty Bay (King George Island, South Shetland)

Seasonal succession and variation in species composition, density,biomass, age distribution and frequency of zooplankton (mainlythe Copepoda) were analysed during the austral summer of of1977/1978 in Ezcurra Inlet, a part of the Antarctic coastalecosystem. Small zooplankters (i.e., cyclopoids of the generaOncaea and Oithona, and calanoids Drepanopus pectinatus andScolocithricella glacialis) were found to predominate in termsof abundance and percentage contribution. The zooplankton biomasswas dominated by larger organisms of the Metrididae and Calanidae(Calanoida). The maximum abundance and maximum biomass of copepodswere recorded in February; two small peaks in copepod biomassbeing observed in late December and late January, and a lesserbiomass peak in late December. The vertical distribution ofcopepods in terms of their diel and seasonal (December, January,February, March) changes showed a day-time maximum to have occurredin the near-bottom layer, the nocturnal distribution being bimodalwith peaks within 0–10 m and 25–50 m. The summerzooplankton community in Ezcurra Inlet is controlled by trophic(phytoplankton composition and density) and hydrological (waterexchange with Bransfield Strait) conditions prevailing in thearea.     

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in BorgeBay, Signy Island, were determined at monthly intervals betweenApril 1990 and June 1991. At least 24 different ciliate taxawere recorded from samples preserved in Lugol's iodine, includingthe tintinnids Codonellopsis balechi, Cymalocylis convallaria,Laackmaniella naviculaefera and Salpingella sp., and the aloricatetaxa Didinium sp. and Mesodinium rubrum. Ciliate abundance andbiomass exhibited a clear seasonal cycle with high values duringthe austral summer and low values in the austral winter. Abundanceranged from 0.3 10³l^–1 in September to 2.3 10³l^–1in January, while biomass ranged from 0.5 µg C l^–1in October to 12.6 µg C l^–1 in December. Small ciliatesdominated abundance throughout the year, and biomass duringwinter. Larger ciliates contributed most to biomass during summer.Aloricate ciliates were common throughout the year, while tintinnidscontributed substantially to abundance and biomass only duringsummer. Salpingella sp. was the commonest tintinnid, but C.convallariacontributed most to tintinnid biomass. The seasonal patternof ciliate abundance and biomass matched that of chlorophylla concentration and bacterial biomass, suggesting tight trophiccoupling between ciliates and other components of the pelagicmicrobial community. ¹Present address: Scott Polar Research Institute, Universityof Cambridge, Lensfield Road, Cambridge CB2 1ER, UK     

Temporal variability in the night-time distribution of epipelagic siphonophores in the North Atlantic Ocean at Bermuda

The temporal variability of siphonophores was compared on night-to-night,month-to-month and year-to-year scales in open-net collectionsmade at or near the Bermuda Atlantic Time Series (BATS) site.These collections were made as oblique tows to 100 m on (i)one summer night in each of 1986, 1989 and 1991, (ii) one nightper month on 13 cruises between April 1991 and August 1992,and (iii) three consecutive summer nights of 1992. The meanwet displacement volume (WDV) of sipho nophores in the thesecollections was 1.4 ± 0.6 ml per 100 m³ or 20 ±10% of the total catch biovolume. The most common species belongedto the Calycophorae families Diphyidae and Abylidae: the combinednumerical abundance of Chelophyes appendiculala, Eudoxoidesmitra, Lensia subtilis. Eudox-oides spiralis, Bassia bassensisand Abytopsis eschscholtzi averaged 300–400 per 100 m³.Combined numbers of these common species varied little whensampled hour by hour at the same location during consecutivesummer nights or at different night-time sampling locations10¹–10² km apart when these were sampled within 3–5h. In contrast, most of these six species exhibited 2- to 3-folddifferences in night-time abundance between summers of differentyears and between seasons. Three seasonal groups were detected,with a winter group being the most distinct from the others.Changes in the abundance of several species were correlatedwith seasonal changes in the depth of the mixed layer.     

DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

Seasonal abundance and distribution of Caridea larvae in Isafjord-deep, north-west Iceland

The abundance and distribution of Candea larvae was studiedin Ísafjord-deep, north-west Iceland, at approximatelymonthly intervals from February 1987 to February 1988 Zooplanktonsampling was made at nine stations along the length of the fjord,while temperature and chlorophyll a measurements from one ofthe stations are also presented Larvae of six species occurredin the samples, Eualus pusiolus and Pandalus borealis were mostnumerous, constituting 62 8 and 25 9% of the larvae respectively.The other species were, in declining order of abundance, Pandalusmontagui, Spirontocaris spp. (S spinus and s lilljeborgii) andSabinea septemcarinata. Eualus pusiolus was of highest abundancein the outer and middle parts of the fjord, while P.borealiswas most common in the middle and inner parts The onset of hatchingof all species in April–May appeared closely linked tothe phytoplankton spring bloom, while the temperature in thefjord was by then near the annual low (2–3°C). Exceptfor E pusiolus, of which a small part of the population produceda second brood during the summer, most of the larvae had disappearedfrom the plankton by the middle of August The monthly carapacegrowth of P.borealis larvae during the summer months was estimatedto be 1.0 mm.     

Seasonal dynamics in the abundance of Micromonas pusilla (Prasinophyceae) and its viruses in the Gulf of Naples (Mediterranean Sea)

To assess the role of viruses in the bloom dynamics of Micromonaspusilla in the Gulf of Naples (Mediterranean Sea), variationsof host and virus abundance were followed over one annual cycleand in late winter–spring of three consecutive years.Micromonas pusilla was recorded from autumn to spring, withpeak values up to 6.6 x 10³ cells ml^–1, but was undetectablein summer. Free M.pusilla viruses were detectable in all seasons,with concentrations from 0.02 viruses ml^–1 to 1.9 x 10³viruses ml^–1, exceeding host abundances only in one case.We found a great intraspecific variability in host susceptibilityto viruses present in natural samples, with viral titres rangingover one or two orders of magnitude for the same samples incubatedon different M.pusilla strains. Over the winter–springperiods, a highly dynamic situation was evident, with wide fluctuationsfor both host and virus abundances from one week to another.In some cases, peak host concentrations were accompanied byan increase in viral numbers, whereas in other cases the respectivefluctuations were uncoupled. Although fluctuations of M.pusillaabundance could be influenced by viral infection, there wasno evidence that viruses were able to terminate host blooms.The summer decline of M.pusilla populations did not appear tobe related to the impact of viral infection.     

Distribution of holoplanktonic typhloscolecids (Annelida-Polychaeta) in the eastern tropical Pacific Ocean

The distribution and composition of the holoplanktonic speciesof the family Typhloscolecidae are reported from 579 samplesobtained over an extensive area in the eastern tropical PacificOcean. The EASTROPAC Oceanographic Program collected the samplesin twelve cruises during five periods made between January–September1967 and January–April 1968. The 2105 specimens identifiedbelonged to three genera and four species. Typhloscolex muelleriand Sagitella kowalewski were dominant together, accountingfor 57–59% of the relative abundance and 36–49%of the occurrence. Travisiopsis dubia is recorded for the firsttime from the eastern tropical Pacific, and Travisiopsis lanceolatais reported in almost the same locality as its first recordin the surveyed region.     

Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m^–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m^–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.     

Seasonal variation in the species composition of tintinnid cilates in Hiroshima Bay, the Seto Inland Sea of Japan

Seasonal changes in the species composition of tintinnid ciliateswere examined based on time-series samples taken at 2 week intervalsover a 3 year period in Hiroshima Bay, the Seto Inland Sea ofJapan. The maximum abundance of total tintinnids over the entireperiod was 5.7 x 10³ indi viduals l^– Among 32 speciesidentified, a consistent seasonal occurrence was recognizedin 22 species. The relationships between various environmentalfactors and the abundance of each species of tintinnids wereanalyzed using principal component analysis From this analysis,the abundance of many tintinnids was revealed to be associatedwith temperature, the <20 µm size fraction of chlorophylla and water column stability, but not with the <20 µmsize fraction of chlorophyll a, nor with salinity. From theseresults, tintinnid species were divided into five associationtypes: species whose abundance increased with increasing temperature,decreasing temperature, nanophytoplankton abundance, increasingwater column mixing, or increasing water stratification coupledwith low temperature.     

A 91-year record of seasonal and interannual variability of diatoms from laminated sediments in Saanich Inlet, British Columbia

Diatom seasonal succession and interannual variability werestudied using laminated sediments from Saanich Inlet, BritishColumbia, for the years 1900–1991. Frozen sediment coresallowed fine-scale sampling of laminae for each year. Thus,three ‘seasons’ for each year were identified basedon species composition. Thalassiosira species were indicatorsof spring deposition. Skeletonema costatum was abundant in samplesfollowing Thalassiosira, probably deposited in late spring andsummer. Rhizosolenia sp. was most abundant in fall/winter samples.Diatom stratigraphies were related to sea surface temperature,salinity, sea level and the Pacific North American Index (PNA)using canonical correspondence analysis (CCA). CCA showed thatspecies of a particular season generally had optima for temperatureand salinity characteristic of that time. Interannual changesin diatom species composition and abundance were most prevalentin the decades 1920–1940, with the exception of S.costatumwhich showed cyclic changes in abundance. Skeletonema was moreabundant during periods of cool temperatures, while littoraldiatoms were more abundant during times of heavy winter rains.Sea level was an important variable in CCA and while its relationshipto diatoms is not clear, it may be related to variations innutrient supply to diatoms in surface waters.     

Separation of marine seston and density determination of marine diatoms by density gradient centrifugation

An attempt has been made to separate constituents of marineseston samples: inorganic material, detritus and the algal species,by density gradient centrifugation, without affecting the physiologicalstate of the algae. A relatively inert gradient material, consistingof Percoll, salt and sucrose, was composed. Since the densitiesof detritus and algae as well as those of different algal speciesoften overlapped, only 10 of the 100 samples processed in thecourse of the year showed a reasonable separation. However,an enrichment with respect to one or more species was oftenachieved. Densities of eleven species of marine diatoms andof one dinoflagellate have been determined at different timesof the year. For eight diatom species and for the dinoflagellatethe following specific density ranges were established: Bidduiphiaaurita: 1.18–1.23 g cm^–3, Biddulphia sinensis: 1.03–1.08g cm^–3, Cerataulina bergonii: 1.03–1.06 g cm^–3,Ditylum brightwellii: 1.07–1.13 g cm^–3, Rhizosoleniadelicatula: 1.04–1.09 g cm^–3, Skeletonema costatum:1.12–1.17 g cm^–3, Streptotheca thamensis: 1.04–1.10g cm^–3 , Thalassiosira rotula: 1.05–1.10 g cm^–3,Peridinium sp.: 1.08–1.12 g cm^–3. No seasonal variationin density was demonstrated. Gradients of different compositiondid not influence density measurements.     

Vertical migration of Rhizosolenia mats and their significance to NO3- fluxes in the central North Pacific gyre

Rhizosolenia mat abundance, distribution and chemical compositionwere studied on two cruises in the central North Pacific gyrein order to determine large-scale distribution patterns andcontribution to upward nitrogen (N) flux. These macroscopicdiatom mats are composed of multiple species of Rhizosoleniathat exploit subsurface nitrate pools by vertically migratingbelow the euphotic zone. Although numerically dominated by thesmall-diameter species, R.fallax (73–95% of total numbers),mat biovolume was dominated by large-diameter (>50 µmdiameter) Rhizosolenia spp. (85–99% of total volume).Integrated mat abundance was substantially higher when matsaccumulated at the surface during calm weather (     

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.     

Feeding of Sagitta enflata and vertical distribution of chaetognaths in relation to low oxygen concentrations

Zooplankton samples were collected in Mejillones Bay, northernChile (23°00'15'S, 70°26'43'W ). Sampling was conductedat 4 h intervals, for 24 h during three seasons, austral spring(October 2000), summer ( January 2001) and winter (August 2001)at three different strata (0–25, 25–50 and 50–100m). Five species of chaetognaths were collected. Sagitta enflatawas the most abundant species, representing up to 65% of allchaetognaths in total numbers, followed by Sagitta bierii, makingup 34% of the total abundance of chaetognaths. S. enflata wasdistributed mainly above the Oxygen Minimum Zone, while S. bieriiremained below this zone. Feeding rates were relatively constantwithin the upper layer (0–25 m depth), for each samplingdate, averaging 1.2 prey S. enflata day^–1, and decreasingwith depth. Gut content analyses demonstrated that predationwas principally focused on small copepods (<1500 µm),with greatest feeding activity occurring at night. The dailypredation impact on the total standing stock of small copepodsvaried seasonally between 6% in spring and 0.4% in winter. Thispercentage may represent a negligible impact on the entire copepodcommunity, but it is relevant at the species or genus level,since S. enflata removed more than 20% of the standing stockof Centropages brachiatus and Corycaeus sp. Thus, during someperiods of the year, chaetognaths may strongly influence theabundance and size distribution of copepods in coastal upwellingecosystems.     

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.     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

Distribution pattern of decapod larvae off the north-western Iberian Peninsula coast (NE Atlantic)

In October 1977 the model of general circulation of the watermasses off the coast of Galicia, and the presence of a coastalupwelling, led to a high primary productivity. This high productivityin turn favoured the development of a rich population of decapodlarvae. The abundance and distribution pattern of these organismswere closely linked (i) to the abundant presence of the correspondingadult species in the area, (ii) with the spatial distributionof phytoplanktonic populations concurrently studied by Estrada[J. Plankton Res ., 6, 417–434 (1984)] and (iii) withthe hydrodynamic pattern in the area. Fifty-two decapod larvaetaxa were identified and Solenocera membranacea, Pisidia longicornis,Pilumnushirtellus and Goneplax rhomboides were the most representativespecies It was observed that the greatest concentrations oflarvae (3387 larvae 10^–2 m^–3) were to be found nearthe mouth of the Rfas Baixas (situated in the south-west ofthe coastal area) and in some zones further out to sea (863larvae 10^–2 m^–3) (due to a process of hydrodynamictransport)     

Production of Penilia avirostris in Kingston Harbour, Jamaica

The cladoceran Penilia avirostris is one of the more abundantand widespread members of the crustacean zooplankton in nearshoretropical and subtropical waters. Its abundance, biomass, fecundity,development rate and production were estimated in Kingston Harbour,Jamaica, during an 18 month period. Mean annual abundance ofPenilia was 1821 m^–3, while biomass (excluding eggs/embryos)was 2.87 mg ash-free dry-weight (AFDW) m^–3 (43.1 mg AFDWm^–2), accounting for 13% of the copepod community biomass.Fecundity increased with body size. There was no clear seasonalpattern of abundance, size or fecundity, nor were physical orbiological variables correlated to these variations. Developmenttime averaged 20.5 h for juveniles and 41.4 h for adult femalesduring incubations; there was no clear evidence of a diel patternto molting. Growth rate appeared to be exponential, with correspondingsomatic growth rates, averaging 0.27 day^–1 for juveniles,and 0.34 day^–1 for somatic plus reproductive growth inadult females. Annual production was estimated as 173 kJ m^–2year^–1,     

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.