Feeding of nauplius stages of Eudiaptomus gracilis on mixed plastic beads

Eudiaptomus gracilis makes up 30–40 and 80–90% ofthe zooplankton in Lake Balaton during the summer and the winterrespectively. More than half of the species population consistsof nauplii We studied feeding and size selectivity of naupliiin suspensions which contained polystyrol latex beads in a concentrationdose to the natural seston. Guts of NI nauplii were free ofboth beads and remnants of natural food Of NII–NVI nauplii,67–87% took in beads. Older animals consumed more andlarger particles. The maximum diameter of ingested beads reached29 µm On an average, NII nauplii took in 128 µm³of beads in 10 min, whereas older animals consumed 615–900µm³. The clearance rate remained below 0 01 µl h^–1NII nauplii strongly preferred 1 2 µm particles Oldernauplii did not show any preference or selected only slightlyfor the smallest particles. Nauplii rejected 4–11 µmbeads. In some cases a weak positive selection could be observedtoward 12 µm or larger beads.     

Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillans

The heterotrophic dinoflagellate Noctiluca scintillans has anegligible swimming ability and feeds predominantly on immobileprey. How, then, does it encounter prey? Noctiluca scintillansis positively buoyant and, therefore, we hypothesized that itintercepts prey particles during ascent and/or that microscaleshear brings it into contact with prey. Noctiluca scintillanshas a specific carbon content 1–2 orders of magnitudeless than that typical for protists and, thus, an inflated volume.It also has a density slightly less than that of the ambientwater and therefore ascends at high velocities (-1 m h^–1).In stagnant water, clearance rates of latex spheres (5–80µm) increased approximately with prey particle size squared.This scaling is consistent with N.scintillans being an interceptionfeeder. However, absolute clearance rates were substantiallylower than those predicted by modeling N.scintillans both asa spherical and as a cylindrical collector. The latter modelassumes that prey particles are collected on the string of mucusthat may form at the tip of the tentacle. Feeding, growth andprey selection experiments all demonstrated that diatoms arecleared at substantially higher rates than latex beads and otherphytoplankters, particularly dinoflagellates. We propose thatdiatoms stick more efficiently than latex beads to the mucusof N.scintillans and that dinoflagellates reduce fatal contactbehaviorally. We conclude that N.scintillans is an interceptionfeeder and that the high ascent velocity accounts for encounterswith prey. However, the flow field around the cell-mucus complexis too complicated to be described accurately by simple geometricmodels. Fluid shear (0.7–1.8 s^–1 had a negativeimpact on feeding rates, which were much less than predictedby models. Noctiluca scintillans can survive starvation forlong periods (>3 weeks), it can grow at low concentrationsof prey (-15 µg C l^–1), but growth saturates onlyat very high prey concentrations of 500–1000 µgC l^–1 or more. We demonstrate how the functional biologyof N.scintillans is consistent with its spatial and seasonaldistribution, which is characterized by persistence in the plankton,blooms in association with high concentrations of diatoms, andsurface accumulation during quiescent periods or exponentialdecline in abundance with depth during periods of turbulentmixing.     

Effects of concentration on the feeding of a marine copepod in algal monocultures and mixtures

High-speed microcinematography was used to examine the feedingbehavior of the marine copepod Eucalanus elongatus in a rangeof concentrations of algal monocultures and mixtures. Two celltypes were offered, the 13-µm diatom Thalassiosira weissflogii,which is primarily accumulated passively by low amplitude flappingof the second maxillae, and the 20 450-µm diatom Rhizosoleniaalata, which is actively captured by detection of and orientedresponse to individual cells. E. elongatus rapidly switchedback and forth between these two capture modes in mixtures ofboth diatoms, and flapped the second maxillae at low amplitudesregardless of the absolute or relative abundance of small andlarge cells. However, copepods in both monocultures and mixturesaltered the duration and/or rate of flapping of the feedingappendages with changes in algal concentration, with maximumactivity levels occurring at intermediate concentrations. Themarked reduction in feeding motions observed at the lowest algalconcentrations supports results from traditional grazing studiesand optimal foraging models, and may conserve energy duringprolonged perods of low food availability in continental slopewaters.     

Food size selectivity of Artemia franciscana at three developmental stages

Food size selectivity was examined in Artemia franciscana metanaupliiat three different developmental stages. Clearance rates weredetermined in short-term experiments either by measuring thedecrease in concentration of live particles and plastic beads,or by measuring the radioactivity accumulated in animals thatgrazed ¹⁴C-labelled live particles. The maximum clearance rateofA.franciscana metanauplii increased during development andwas measured at 50–63 µl ind.^–1 h^–1,254 µl ind.^–1 h^–1 and 1.48–2.10 ml ind.^–1h^–1 in 2-, 4- and 7-day-old metanauplii, respectively.A preference for particles with a diameter of 4–8 µmwas observed at all three developmental stages. The abilityof A.franciscana metanauplii to graze bacterial particles wasalso demonstrated, although the efficiency in grazing such smallparticles was low compared to microalgae (28, 20 and 9% of themaximum clearance rate in 2-, 4- and 7-day-old metanauplii,respectively). Electron microscopy showed that the inter-setulardistance in antennae and thoracopods was 0.20 ± 0.07,0.16 ± 0.05 and 0.18 ± 0.04 µm in 2-, 4-and 7-day-old metanauplii, respectively, and accordingly independentof stage.     

Feeding selectivities and food niche separation of Acartia clausi, Penilia avirostris (Crustacea) and Doliolum denticulatum (Thaliacea) in Blanes Bay (Catalan Sea, NW Mediterranean)

Selectivity-size spectra, clearance and ingestion rates andassimilation efficiencies of Acartia clausi (Copepoda), Peniliaavirostris (Cladocera) and Doliolum denticulatum (Doliolida)from Blanes Bay (Catalan Sea, NW Mediterranean) were evaluatedin grazing experiments over a wide range of food concentrations(0.02–8.8 mm³ L^–1 plankton assemblages from BlanesBay, grown in mesocosms at different nutrient levels). Acartiaclausi reached the highest grazing coefficients for large algae>70 µm (longest linear extension), P. avirostris forintermediate food sizes between 15 and 70 µm, and D. denticulatumfor small sizes from 2.5 to 15 µm. Penilia avirostrisand D. denticulatum acted as passive filter-feeders. Acartiaclausi gave some evidence for a supplementary raptorial feedingmode. Effective food concentration (EFC) decreased linearlywith increasing nutrient enrichment for D. denticulatum andfollowed domed curves for A. clausi and for P. avirostris withmaximum values at intermediate and high enrichment levels, respectively.Clearance rates of crustacean species showed curvilinear responseswith narrow modal ranges to increasing food concentration. Clearancerates of D. denticulatum increased abruptly and levelled intoa plateau at low food concentrations. Mean clearance rates were13.9, 25.5 and 64.1 mL ind.^–1 day^–1, respectively.No clearance could be detected for A. clausi at food concentrations<0.1 mm³ L^–1 and for P. avirostris at food concentrations     

Remote detection of algae by copepods: responses to algal size, odors and motility

Selective feeding on large algae by copepods involves remotedetection of individual particles and subsequent active captureresponses In this study we use radiotracer experiments to quantifythe clearance rates of five coexisting freshwater copepods andto investigate the relative merits of the chemoreception andmechanoreception hypotheses of remote detection Tropocyclopsand three diaptomid copepods exhibited relatively high clearancerates when feeding on low concentrations of large algae, suggestingthat most previous studies with freshwater copepods have underestimatedmaximal clearance rates and the degree of size selectivity.All five species of copepods exihibited strong selection foran intermediate-sized flagellate (25 µm Cartena) or alarge-sized nonmotile alga (80 µ.m Pediastrum) over asmall-sized flagellate (6 µ.m Chlamydomonas). The weight-specificclearance rate for Tropocylops prasmus feeding on motile Cartena(271 ml mg^–1 h^–1) was about twice that of threediaptomid copepods and more than an order-of-magnitude higherthan the estimate for Epischura lacustris feeding on its preferredalga, Pediastrum Assuming that distance chemoreception (‘smell’)is important in remote detection, we predicted that the additionof high concentrations of ‘algal odors’ would obscureany chemical gradients emanating from individual algal cellsand would thereby hinder the remote detection and active captureof large algae Contrary to this hypothesis, the addition ofamino acids, sucrose, and algal extracts had no effect on theclearance rates and selectivity of Diaptomus birgei. These results,together with recent cinematographic studies (Vanderploeg etal.,1990), suggest that mechanoreception is the primary mechanismfor the remote detection of large particles by diaptomid copepods.A raptorial cyclopoid, Tropocyclops prasinus, exhibited strongpreferences for motile algae, whereas a suspension-feeding calanoid,D birgei, did not select between motile and nonmotile cells.Motility appears to be an important factor in algal detectionfor small cyclopoid copepods but not for suspension-feedingdiaptomids     

Grazing Rates in Euplotes mutabilis: Relationship between Particle Size and Concentration

Abstract Grazing behavior of both individual cells and populations of the marine hypotrich Euplotes mutabilis, a largely benthic ciliate that feeds on suspended particles, was studied using fluorescent latex microspheres. Microspheres of sizes 0.57-, 1.90-, 3.06-, 5.66-, and 10.0-μm diam were offered at concentrations from 10² to 10⁶ ml⁻¹. Their uptake in a ten-min period was determined. Food particles within such ranges of size and concentration are found under natural conditions. The ciliates ingested particles of all sizes offered. Uptake rates at all concentrations were dependent upon particle size, with 1.90- and 3.06-μm diam microspheres having the highest uptake rate in all cases. For all sizes, there was an increase in the percentage of feeding cells and in the uptake rate (the number of particles ingested cell⁻¹ h⁻¹), with increasing particle concentration. When uptake was expressed as the volume ingested, maximum values were obtained for 5.85-μm diam microspheres at a concentration of 10⁶ ml⁻¹. By taking a small number of large particles, present at a low concentration in the medium, a ciliate can ingest a greater biovolume than by taking a high number of small particles which are abundant in the medium. These results demonstrate that some ciliates can graze particles of a wide range of sizes. Also, its nutrition can be affected by changing ambient concentrations of different prey, both through effects on the proportion of feeding cells and through the relative food content of the particles. The data can also add to the understanding of food niche partitioning between species. At low particle concentrations, particularly, it is important to consider the behavior of individual ciliates as well as of the whole population. Received: 11 February 1997; Accepted: 21 October 1997     

The importance of phytoflagellate, heterotrophic flagellate and ciliate grazing on bacteria and picophytoplankton sized prey in a coastal marine environment

The uptake of bacteria and picoplankton sized fluorescentlylabelled beads was measured off the west coast of the SouthIsland of New Zealand in winter. Phytoflagellates and heterotrophicflagellates showed similar grazing rates on 0.49 µm beads,with mean clearance rates of 1.1 and 1.8 nl ind.^–1 h^–1,respectively. Clearance rates for 1.09 µm beads were 0.9nl ind.^–1 h^–1 for heterotrophic flagellates and0.5 nl ind.^–1 h^–1 for phytoflagellates. Non-loricateciliates had clearance rates of 1.5 µl ind.^–1 h^–1for the picoplankton sized particles. The heterotrophic flagellatesshowed no significant difference between clearance rates of0.49 and 1.09 µm particles. Phytoflagellates, however,showed an apparent preference for the smaller particles. Themeasurement of significant grazing by phytoflagellate populationsin the marine environment is important and indicates that weneed to reassess our concepts of food web structure.     

Are bacteria an important food source for rotifers in eutrophic lakes?

In situ grazing measurements using fluorescent particles of0.5, 2.4 and 6.3 µm diameter in eutrophic Lake Loosdrecht(The Netherlands) showed that Anuraeopsis fissa, a small rotifer,filtered the smallest, bacteria-sized particles as efficientlyor more efficiently than the larger particles. In contrast,three other rotifer species (Brachionus angularis, Filinia longisetaand Pompholyx sulcata) filtered the bacteria-sized particlesless efficiently than the larger particles. Both Keratella cochlearisand Conochilus unicornis only ingested the bacteria-sized particles.Anuraeopsis fissa had a higher uptake of fluorescent bacteria-sizedparticles than K.cochlearis, both in 1 µm filtrate oflake water and in lake water. Within both species, uptake didnot differ between juveniles and adults. When cultured on threedifferent size fractions of lake water (1, 3 and 15 µmfiltrate) in July, all rotifer species declined in numbers onthe 1 and 3 µm filtrates, while A.fissa and B.angularisincreased in numbers on the 15 µm filtrate. The high abundanceof small bacteria in the lake water could not support rotiferpopulations. It is concluded that bacteria are not a suitablefood source of high quality for A.fissa because its populationdoes not grow even though the bacterial concentration was higherthan its estimated threshold food concentration. In August,when individually cultured, the mortality was high for all species,but especially for F.longiseta. The lifespan of K.cochleariswas reduced in the 1 and 3 µm filtrates of lake water,compared with in the 15 µm filtrate. The lifespan of A.fissawas similar in all filtrates, but reproduction was reduced inthe 1 and 3 µm filtrates, as in Keratella. On the 15 µmfiltrate, their ages at first reproduction and growth ratesdid not differ. Individuals of A.fissa older than 4 days showeda higher survival in the 15 µm filtrate than in the othertwo filtrates, as did K.cochlearis throughout its life. Hence,bacteria seem to be a more important food source for youngerindividuals of A.fissa than of K.cochlearis.     

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.     

Grazing by the calanoid copepod Neocalanus cristatus on the microbial food web in the coastal Gulf of Alaska

Neocalanus cristatus feeding on phytoplankton and microzooplanktonwas measured in the coastal Gulf of Alaska during spring andearly summer of 2001 and 2003. Neocalanus cristatus CV fed primarilyon particles >20 µm. Particles in the 5- to 20-µmsize range were ingested in some experiments under nonbloomconditions but not under bloom conditions. Particles <5 µmwere not ingested but increased during incubations because N.cristatus consumed their microzooplanktonic predators. Neocalanuscristatus are sufficiently abundant in nature to induce sucha cascade effect in situ. Microzooplankton provided >70%of the carbon ingested by N. cristatus under nonbloom conditionsbut only     

Feeding of freshwater filter-feeders at very low food concentrations: Poor evidence for "threshold feeding" and "optimal foraging" in Daphnia longispina and Eudiaptomus gracilis

The feeding behaviour of two freshwater zooplankton species,the calanoid copepod Eudiaptomus gracilis and the cladoceranDaphnia longispina, at extremely low food concentrations hasbeen studied in order to test whether the animals react as predictedby the models of optimal foraging. Three different sized algae were offered in concentrations downto 1µg C/litre at 7°C during winter and 19°C duringsummer. Ingestion rates were measured by use of a flow-through¹⁴C-technique In contrast to the findings of several authors working withmarine copepods a threshold concentration where the animalsceased filtering could be detected only in one experiment byregression analysis (Daphnia, 19°C, food: Stichococcus).A depression of the filtering rate at low concentrations wasobserved only in Daphnia at 19°C with Stichococcus and Scenedesmusas food. It is supposed that in this case the reduction of thefiltering rate is not a direct response of the filtrator, butis caused by exhaustion due to the experimental conditions. As freshwater zooplankton is very rarely exposed to food abundancesas low as in the marine environment, there seems to be no selectionpressure favouring the evolution of an "optimal forager" behaviour. ⁽¹⁾Supported by Deutsche Forschungsgemeinschaft     

Effect of mechanical deformation on structure and function of polymorphonuclear leukocytes

Kitagawa, Yuko, Stephan F. Van Eeden, Darlene M. Redenbach,Maleki Daya, Blair A. M. Walker, Maria E. Klut, Barry R. Wiggs, andJames C. Hogg. Effect of mechanical deformation on structure andfunction of polymorphonuclear leukocytes. J. Appl.Physiol. 82(5): 1397-1405, 1997.The presentstudies were designed to test the hypothesis that mechanicaldeformation of polymorphonuclear leukocytes (PMN) leads to functionalchanges that might influence their transit in the pulmonarycapillaries. Human leukocytes were passed through 5- or 3-µm-porepolycarbonate filters under controlled conditions. Morphometricanalysis showed that the majority of PMN were deformed and that thisdeformation persisted longer after filtration through 3-µm filtersthan through 5-µm filters (P < 0.05) but did not result in the cytoskeletal polarizationcharacteristic of migrating cells. Flow cytometric studies of thefiltered PMN showed that there was a transient increase in thecytosolic free Ca²⁺ concentrationafter both 3- and 5-µm filtration (P < 0.01) with an increase in F-actin content after 3-µm filtration(P < 0.05). AlthoughL-selectin expression on PMN wasnot changed by either 5- or 3-µm filtration, CD18 and CD11b wereincreased by 3-µm filtration (P < 0.05). Priming of the PMN withN-formyl-methionyl-leucyl-phenylalanine (0.5 nM) before filtration resulted in an increase of CD11b by both 5 (P < 0.05)- and 3-µm(P < 0.01) filtration. Neither 5- nor 3-µm filtration induced hydrogen peroxide production. We conclude that mechanical deformation of PMN, similar to what occurs in thepulmonary microvessels, induces both structural and functional changesin the cells, which might influence their passage through the pulmonarycapillary bed.

     

Cladoceran filtration rate-body length relations: model improvements developed for a Microcystis-dominated hypertrophic reservoir

Filtering rates were measured for zooplankton species in Situon single-celled Chlorella and on four Microcystis colony sizefractions (5–20, 20–40, 40–60 and 60–100µm) in a hypertrophic reservoir. Natural-log-transformedfiltration rates of five cladoceran species, one copepod andone rotifer were included in an all-food-particle, all-speciesmultiple regression model which explained 43% of the variancein filtration rate as a function of animal body length. An additional14% and 7.6% of the variance was attributable to food type andzooplankton species respectively, with temperature accountingfor <4% of the variance. Restricting the filtration ratemodel to cladocerans alone explained 51% of the variance asa function of animal length, 16% as a function of food type,7.5% as a function of species and only 0.2% as a function oftemperature. In linear filtration rate models for each foodtype, cladoceran body length explained 70% of the variance whenfeeding on Chlorella and between 57 and 67% of the varianceon the four Microcystis colony fractions. Models describingcladoceran filtration rates on Chlorella and the 5–20µm Microcystis colony fraction were significantly differentfrom the three models on larger colonies due to cladoceran responsesto increasing food particle size. Accordingly, a combined modelfor Microcystis colonies >20 µm was developed. Inclusionof food quality factors such as cyanophyte colony size seemsjustified in models aimed at estimating clearance rates, resourceutilization and phytoplankton grazing losses in plankton orecosystem studies when applied to eutrophic or hypertrophiclakes where large cyanophyte particles are abundant.     

Impacts of nutrients and zooplankton on the microbial food web of an ultra-oligotrophic lake

In ultra-oligotrophic lakes and the sea, calanoid copepods arethe dominant mesozoo-plankton and cladocerans are generallysparse or absent. To determine the effects of predation andnutrient enrichment on the pelagic microbial food web of anultra-oligotrophic lake, we added copepods and cladocerans atlow biomasses (<60 µg l^–1 to in situ enclosuresin Lake Wakatipu, New Zealand, in the presence and absence ofadded nutrients (nitrogen and phosphorus). In response to nutrientfertilization, the concentrations of phototrophs >3 µmand heterotrophic bacteria increased by 50 and 15%, respectively,over 4 days, but those of cyanobacterial picoplankton decreasedby 68%. The presence of calanoid copepods (Boeckella dilatata)at ambient densities (1 and 4 l^–1) rapidly and severelysuppressed ciliate population growth over 4 days and also loweredthat of flagellates >3 µm, even when microbial growthwas enhanced by added nutrients. The presence of a small cladoceran,Ceriodaphnia dubia, at double the densities, but similar biomasses,to those of copepods, depressed the net growth rates of ciliatesand flagellates to a lesser degree. The net growth rate of heterotrophicbacteria after 4 days declined with flagellate abundance, consistentwith the possibility of regulation by flagellates. Althoughbacteria and algae increased in response to nutrient fertilization(bottom-up control), predation (top-down control) appeared toplay an important role in structuring the microbial food webof this ultra-oligotrophic lake in summer.     

Changes in the optical properties of a particle suspension caused by protist grazing

The effect of grazing within microbial food webs on the opticalproperties of seawater was studied in the laboratory by inoculatinga culture of cyanobacteria Synechocystis with an oligotnchousciliate, Strombidium. During the 4 week experiment sequentialmeasurements of spectral absorption and scattering coefficientswere supplemented by determinations of the particle size distribution,prey and predator numbers and pigment concentration. All ofthese parameters changed dramatically during the course of theexperiment. Protist grazing caused a large decrease in the concentrationof particles that ranged from 0.75 to 2.5 µm in size;Synechocystis is within this range. On the other hand, the concentrationof particles <0.75 µm increased. These fine particlesconsist of small heterotrophic bacteria and disintegrated detritalmaterial. In addition, the concentration of particles in therange of 2.5–16 µm also increased; this size rangeincluded a growing population of pigmented flagellates and detritus.Such particle dynamics were associated with large decreasesin both the scattering and the absorption coefficient. The spectralshape of the scattering coefficient flattened, and the spectralabsorption coefficient shifted to enhanced absorption at shorterwavelengths. These changes suggest that detrital particles andpigmented microflagellates were the dominant contributors tolight scattering and absorption at the end of the experiment.     

Feeding and growth rates of the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

The goal of this research is to enhance our knowledge of thecontributions of doliolids to the planktonic community as consumersand secondary producers. The objectives are to quantify feedingand growth rates of Dolioletta gegenbauri gonozooids at fourfood concentrations and four temperatures in order to determinetheir impact as grazers throughout the water column. Althoughdoliolids are abundant in numerous regions of the coastal ocean,and are considered to be major planktonic grazers, data on ratesof feeding and growth are scarce. Laboratory experiments wereconducted at 16.5, 20, 23.5 and 26.5°C to quantify removalof a 50:50 volumetric concentration of Thalassiosira weissflogiiand Rhodomonas sp. at four different food concentrations of20, 60, 160 and 390 µg C l^–1. Results from theseexperiments suggest that clearance rates are similar at concentrationsfrom 20 to 60 µg C l ^–1, and decrease as the foodconcentrations increase to 160 and 390 µg C l ^–1.The ingestion rates increase over a range of phytoplankton concentrationsfrom 20 to 160 µg C l ^–1, then decrease when abnormallyhigh concentrations of 390 µg C l ^–1 are offered.Clearance and ingestion rates increase as temperature increasesfrom 16.5 to 26.5°C. The exponential growth rates rangefrom k = 0.2–0.7, with the lowest rates occurring at thehighest food concentration. Growth rates increase with increasingtemperature from K = 0.1–0.3 at 16.5°C to 0.45–0.7at 26.5°C. In each case, the small- and medium-sized zooidshad higher growth rates than the larger gonozooids. These resultssuggest that doliolid feeding and growth rates are a functionof environmental food concentrations and temperatures, and implythat they can be important consumers in a changing neritic environment.     

Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Size, composition and distribution of particles related to wind induced resuspension in a shallow tropical lagoon

The size, composition and distribution of particles in the watercolumn were surveyed in a shallow area (1 m depth) of a tropicallagoon (Cte d'Ivoire) during a sequence of wind-induced resuspension.Water samples were collected hourly near the surface duringone tidal cycle. Three characteristic periods were distinguished:a calm period with low wind speed (average 1.2 m s^–1 awindy period with wind speed >3 m^–1 s (range between4 and 6 m s^–1) inducing sediment resuspension and a relaxationperiod during the decrease of wind velocity. From the analysisof several parameters (particle size and volume, bacteria. pico-and nanophytoplankton, ciliates and detritus), sediment resuspensioncaused a regular injection of particles from the bed. The finestparticles (1.5–6 µm: chlorophytes such as Chiorellaspp., picocyanobacteria such as Synechococcus) were the firstto be affected by wind-induced turbulence, whereas large particles(6–12 µm: diatoms. cyanobacteria such as Lyngbiaspp.) were dispersed into the water column at the highest windspeed. The fate of the different seston components differedaccording to their size. Therefore, wind-induced resuspensioncould greatly influence the food web organization through thequantity, quality and size of edible particles available ata given time.     

Diel variations in gut pigment content, diel vertical migration and estimates of grazing impact for copepods in the southern Benguela upwelling region in October 1987

The gut fluorescence technique was used to estimate ingestionand filtration rates of the adult female copepods Paracalanusparvus, Cenlropages brachiatus and Calanus austrails, and copepoditestages 3, 4 and 5 of C.australis in the southern Benguela upwellingregion. During the study period chlorophyll concentrations withinthe upper 20 m of the water column were high, 5 µg I^–1in mid-shelf waters and 15–30 µg I^–1 in innershelf waters. Copepod gut pigment content was low and constantduring the day then increased sharply during the first 2 h aftersunset. Gut pigment content was 2–6 times higher duringthe night compared with daytime values. Small non-migratingcopepods (Paracalanus parvus) showed the smallest diel differencein gut pigment content and large migrating copepods (Centropagesbrachiatus and Calanus australis) the largest difference. Eggproduction rates were 20 and 50% of maximum at the mid-shelfand inner shelf stations respectively, suggesting food-limitation.Comparison of ingestion rates calculated from egg productiondata with ingestion rates calculated from gut pigment data suggestedthat the copepods were feeding omnivorously at the inner shelfstations but herbivorously at the mid-shelf stations. Assumingthat all of the phytoplankton was available as food, the nearshorecopepod assemblage grazed {small tilde}1% of the standing cropeach day, and the mid-shelf assemblage grazed 5% day^–1.Because of errors and uncertainties associated with the gutfluorescence technique, the feeding impact could be underestimatedby 2–4-fold. We discuss several approaches which couldlead to more precise estimates of feeding rates. ³Present address: Marine Sciences, SUNY, Stony Brook, NY, 11794-5000,USA