Microzooplankton grazing in a coastal embayment off Concepcion, Chile, (~36{degrees} S) during non-upwelling conditions

The impact of grazing by natural assemblages of microzooplanktonwas estimated in an upwelling area (Concepción, Chile)during the non-upwelling season in 2003 and 2004. Seawater dilutionexperiments using chlorophyll a (Chl a) as a tracer were usedto estimate daily rates of phytoplankton growth and microzooplanktongrazing. Initial Chl a concentrations ranged from 0.4 to 1.4mg Chl a m^–3 and phytoplankton prey biomass and abundancewere numerically dominated by components <20 µm. Phytoplanktongrowth and microzooplankton grazing rates were 0.19–0.25day^–1 and 0.26–0.52 day ^–1, respectively.These results suggest that microzooplankton exert a significantremoval of primary production (>100%) during the non-upwellingperiod.     

Nanoheterotroph grazing on bacteria and cyanobacteria in oxic and suboxic waters in coastal upwelling areas off northern Chile

The vertical distribution and abundance of microbial assemblagesand the grazing of nanoheterotrophs upon prokaryotes in oxicand suboxic waters were examined in two coastal upwelling areasoff northern Chile where a shallow Oxygen Minimum Zone (OMZ)is characteristic. Prokaryotic prey included bacterioplanktonand cyanobacteria (Synechococcus); both displayed a bimodaldistribution, with abundance maxima above and within the upperOMZ. Flagellates numerically dominated the nanoplankton andwere mostly concentrated in the oxic layer. Mean ingestion ratesof cyanobacteria by nanoflagellates (vacuole content method)ranged from 0.2 to 1.1 cells flagellate^–1 h^–1 andmean consumption rates (34–160 cells mL^–1 h^–1)were four times higher in the oxic layer. With the selectiveinhibitors technique, specific grazing rates on bacteria werelow (<0.1 h^–1) and consumption did not control bacterialproduction in the surface layer but did so in the suboxic layer(accounting for >100% of bacterial production). With thesame method, the specific grazing rate on cyanobacteria rangedbetween zero and 0.23 h^–1 with no clear differences betweenoxygen conditions; prey growth and production were always higherthan the grazing pressure (accounting for <17% of cyanobacterialproduction). The impact of grazing by nanoheterotrophs in regulatingthe production of prokaryotes in oxic and suboxic waters inthis region is discussed.     

Estimation of ammonium regeneration efficiencies associated with bacterivory in pelagic food webs via a 15N tracer method

Phagotrophic protists are major components of pelagic food webs,both as consumers of bacterial and phytoplankton cells, andas regenerators of inorganic nutrients. In this study, we estimatedthe efficiency of ammonium regeneration by protists feedingon bacteria within natural plank-tonic assemblages, using a¹⁵N tracer method, in which the excretion of ¹⁵N-labeled ammoniumdue to grazing on ¹⁵N pre-labeled bacteria was followed overtime. We tested this approach in experiments based on the additionof heat-killed ¹⁵N-labeled bacteria to laboratory cultures andto samples of coastal seawater. During two experiments, variationin abundance of bacterivores and bacterioplankton resulted innon-constant grazing rates. Deterministic computer models thatused abundance of bacteria and protists as variables were developedto estimate best-fit values of grazing mortality (g, h^–1)and of ammonium regeneration efficiency (R_E, fraction of theinitial ¹⁵N label in added bacteria which is released as ammonium).Estimated ammonium R_E were 0.30–0.35 for one trophic linksystems with both a monospecific culture and a mixed speciesassemblage of bacterivorous flagellates. R_E was higher for multi-trophicstep food webs: 0.60 for 5 µm pre-screened coastal seawaterand 0.90 for whole coastal seawater.     

Grazing on bacteria by flagellates and cladocerans in lakes of contrasting food-web structure

We tested the hypothesis that grazing on bacteria would varybetween lakes with differing plankton community structures.Paul and Tuesday lakes (Gogebic County, MI) are respectivelydominated by piscivorous and planktivorous fish. Consequently,zooplankton in Paul are primarily large daphnids, while zooplanktonin Tuesday are primarily small cladocerans and copepods. Wemeasured flagellate grazing on bacteria using a fluorescentminicell method, while cladoceran grazing was estimated fromthe relationship between body length and filtering rate. Wepredicted that cladoceran grazing on bacteria would be higherin Paul, and flagellate grazing would be higher in Tuesday.Cladoceran grazing on bacteria was important in both lakes contraryto our initial expectation. Large populations of the small cladoceran,Bosmina longirostris, in Tuesday exerted a grazing pressure(0.18–35x10⁶ bacteria 1^–1 h^–1) approximatelyequal to that of the large cladoceran, Daphnia pulex, in Paul(0.34–30x10⁶ bacteria 1^–1 h^–1). Flagellategrazing was higher in Tuesday as predicted (range: Paul, 0.1–6x10⁶bacteria 1^–1 h^–1; Tuesday, 0.2–20x10⁶ bacteria1^–1 h^–1). However, there was not a simple relationshipbetween total abundance of flagellates and total grazing rates.High community grazing by flagellates occurred when attachedchoanoflagellates were present. These flagellates had higheringestion rates than free forms. We find no clear evidence thatdifferences in food-web structure between the two lakes influencethe process of grazing on bacteria. Instead, our results emphasizethe significance of cladocerans and attached flagellates asconsumers of bacteria in freshwater ecosystems.     

Development and fate of Eutreptiella gymnastica bloom in nutrient-enriched enclosures in the coastal Baltic Sea

Response of the phytoplankton community to bottom-up (nutrients,organic carbon source) and top-down (fish) manipulations, bothsingly and together, were studied daily during a 3 week periodin July 1993 by using eight 50 m³ mesocosms in the coastal northernBaltic Sea. Nutrient additions (once per week) invoked a seriesof blooms of Eutreptiella gymnastica Throndsen (Euglenophyceae)(up to 13 x 10³ cells ml^–1) which formed the major part(60–90%) of the total autotrophic biomass. After rapiddepletion of nutrients (2–3 days) from the surface layer(0–6 m) downwards migration and a subsequent peak of E.gymnasticain the lower part of the water columns (6–12 m) followed.Settled material collected from the bottom of the enclosurescontained a considerable amount of E.gymnastica cells and restingcysts. Nevertheless, sinking loss rates of E.gymnastica wereestimated to be less than 1% day^–1 of the suspended cellnumbers. The fate of E.gymnastica blooms was estimated to begrazing through mesozooplankton. However, provided the nutrientsare plentiful in the water column, the growth potential of E.gymnasticaappears to exceed the ambient grazing pressure. If the nutrientsbecome depleted, it seems to be effectively controlled by mesozooplanktongrazing, which is probably limiting the likelihood of massiveE.gymnastica blooms in the coastal Baltic Sea. Our study suggeststhat E.gymnastica appears to be a fast-growing fugitive (bloom)species with flexible behavioural (vertical migration) and lifehistory (cyst formation) adaptations which is able to exertdominant role and direct trophic relations similar to otherbloom species adapted for decaying turbulence and high nutrientenvironments.     

Dynamics of microplankton communities at the ice-edge zone of the Lazarev Sea during a summer drogue study

Microzooplankton grazing and community structure were investigatedin the austral summer of 1995 during a Southern Ocean Drogueand Ocean Flux Study (SODOFS) at the ice-edge zone of the LazarevSea. Grazing was estimated at the surface chlorophyll maximum(5–10 m) by employing the sequential dilution technique.Chlorophyll a concentrations were dominated by chainformingmicrophytoplankton (>20 µm) of the genera Chaetocerosand Nitzschia. Microzooplankton were numerically dominated byaloricate ciliates and dinoflagellates (Protoperidinium sp.,Amphisoleta sp. and Gymnodinium sp.). Instantaneous growth ratesof nanophytoplankton (<20 µm) varied between 0.019and 0.080 day^–1, equivalent to between 0.03 and 0.12 chlorophylldoublings day^–1. Instantaneous grazing rates of microzooplanktonon nanophytoplankton varied from 0.012 to 0.052 day^–1.This corresponds to a nanophytoplankton daily loss of between1.3 and 7.0% (mean = 3.76%) of the initial standing stock, andbetween 45 and 97% (mean = 70.37%) of the daily potential production.Growth rates of microphytoplankton (>20 µm) were lower,varying between 0.011 and 0.070 day^–1, equivalent to 0.015–0.097chlorophyll doublings day^–1. At only three of the 10 stationsdid grazing by microzooplankton result in a decrease in microphytoplanktonconcentration. At these stations instantaneous grazing ratesof microzooplankton on microphytoplankton ranged between 0.009and 0.015 day^–1, equivalent to a daily loss of <1.56%(mean = 1.11%) of initial standing stock and <40% (mean =28.55%) of the potential production. Time series grazing experimentsconducted at 6 h intervals did not show any diel patterns ofgrazing by microzooplankton. Our data show that microzooplanktongrazing at the ice edge were not sufficient to prevent chlorophylla accumulation in regions dominated by rnicrophytoplankton.Here, the major biological routes for the uptake of carbon thereforeappear to be grazing by metazoans or the sedimentation of phytoplanktoncells. Under these conditions, the biological pump will be relativelyefficient in the drawdown of atmospheric CO₂.     

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

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

Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gutcontents, evacuation and egestion rates, as well as digestiveefficiency, were carried out during February-March 1994 in thevicin ity of South Georgia to estimate in situ daily ration.These were combined with acoustically derived biomass data tocalculate the grazing impact of Antarctic krill and its contributionto the carbon flux in the region. Individual levels of gut pigmentconcentrations and evacuation rates ranged from 27 to 1831 ngchlorophyll a-eq. ind.^–1 and from 0.133 to 0.424 h^–1,respectively. Losses of pigment fluor escence during digestionwere very high, ranging from 58 to 98% of the total pigmentdigested. Daily carbon consumption estimated using the gut fluorescencemethod varied from 0.234 to 0.931 mg C ind.^–1 day^–1(or 0.4–1.7% of body carbon), compared to {small tilde}2.73mg C ind.^–1 day^–1 (or {small tilde}5% of body carbon)using the faecal pellet production data. The 3-fold higher dailyration estimated using egestion rate data may be explained bypredation on micro-and mesozooplankton. Maximum krill grazingimpact ranged from 0.4 to 1.9% of the total phytoplankton stockor from 10 to 59% of the total daily primary production. However,grazing impact on the microphytoplankton (>20 µm) wassubstantially higher, at times exceeding 100% of the daily microphytoplanktonproduction. It is suggested that to meet its energetic demands,kriil must consume a substantial proportion of heterotrophiccarbon. ³Present address Zoology Department, University of Fort Hare,P/Bag X1314, Alice, 5700, South Africa     

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

The dynamics of the phytoplankton community were investigatedin a marine coastal lagoon (Thau, NW Mediterranean) from February1999 to January 2000. Dilution experiments, chlorophyll a (Chla) size-fractionation and primary production measurements wereconducted monthly. Maximum growth and microzooplankton grazingrates were estimated from Chl a biomass fractions to separatepico- from nano- and microphytoplankton and by flow cytometryto distinguish between picoeukaryotes and picocyanobacteria.In spring, the phytoplankton community was dominated by Chaetocerossp. and Skeletonema costatum, which represented most of biomass(B) and primary production (P). Nano- and microphytoplanktongrowth was controlled by nutrient availability and exceededlosses due to microzooplankton grazing (g). Picoeukaryote andcyanobacteria growth was positively correlated with water temperatureand/or irradiance, reaching maximum values in the summer (2.38and 1.44 day^–1 for picoeukaryotes and cyanobacteria, respectively).Picophytoplankton accounted for 57% of the biomass-specificprimary productivity (P/B). Picophytoplankton was strongly controlledby protist grazers (g = 0.09–1.66 day^–1 for picoeukaryotes,g = 0.25–1.17 day^–1 for cyanobacteria), and microzooplanktonconsumption removed 71% of the daily picoplanktonic growth.Picoeukaryotes, which numerically dominate the picoplanktoncommunity, are an important source of organic carbon for theprotistan community and contribute to the carbon flow to highertrophic levels.     

Discontinuite hydrobiologique dans le detroit du Pas-de-Calais

Hydrobiological studies in the Straits of Dover show a differencebetween the characteristics of the French coastal water andthe open sea water. A cruise between Boulogne-sur-mer and Dover(October 15, 1985) confirms the existence of a frontal area(4 miles off the French coast). This area is characterized byan increase of the salinity (1%) and decrease of the suspendedmatter content (from 27 to 20 mg l^–1), of primary production(from 15 to 3 mg C m^–3h^–1) and of chlorophyll aconcentration (from 4.5 to 1.5 mg m^–3). The presence ofdifferent zooplanktonic species in the two water masses alsocharacterizes this discontinuity. Oncaea sp., Corycaeus anglicus,Centropages typicus and Calanus helgolandicus are indicatorspecies of open sea water, while Cyciopina littoralis and ‘Saphirella’are only present in coastal water.     

Mesozooplankton grazing under conditions of extreme eutrophication in Guanabara Bay, Brazil

The objective of the present study was to quantify mesozooplanktongrazing in the eutrophic waters of Guanabara Bay. Mesozooplankton(>200 µm) was dominated by the copepods Acartia lilljeborgi,Acartia tonsa, Parvocalanus crassirostris and Paracalanus furcatus.Dinoflagellates, specifically the species Prorocentrum triestinum,were an important group for mesozooplankton nutrition, beingingested in significant amounts during all experiments. On average,12.3 ± 2.9 P. triestinum cells were ingested copepod^–1min^–1 (other dinoflagellates: 11 ± 8 cells copepod^–1min^–1). Filamentous cyanophyceae and nanoplankton wereingested in one experiment each, but the mesozooplankton communitygenerally preferred dinoflagellates to these groups, which werealways abundant in the water column. Euglenophyceae were notingested, although they dominated in one experiment. Mesozooplanktoningested, on average, only 0.2% of the nano- and microplanktonbiomass per day. The results suggest that grazing was not acontrolling process for the nano- and microplankton communityin the study area. Addition of zoeae larvae of Chasmagnatusgranulata (Decapoda: Brachyura: Grapsidae) in one experimenthad a significant effect on the mortality of adult copepods,probably due to a predator–prey relationship.     

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)     

The contribution of picophytoplankton to community structure in a Mediterranean brackish environment

The seasonal composition of phytoplankton communities was investigatedin a Mediterranean brackish area (Varano lagoon). Twelve stationswere sampled monthly from March 1997 to February 1998. Numbersof prokaryotic and eukaryotic picophytoplankton cells were estimatedby epifluorescence microscopy, while larger phytoplankton (nanoand micro fractions) were enumerated by the Utermöhl settlingtechnique. Picophytoplankton densities ranged from 0.7 to 448.6cells x 10⁶ l^–1. Nano- and microphytoplankton abundancesvaried between 0.2 and 7.9 cells x 10⁶ l^–1. The picoplanktonfraction was represented mainly by cyanobacteria and the Utermöhlfraction by nano-sized phytoflagellates (56.2%) and diatoms(20.1%). The phytoflagellates had a greater abundance over timewhile diatoms reached the highest densities in summer and fall.In Varano lagoon, phytoplankton development is related to ‘nitrogen-poor'waters and to phosphorus availability. Suspension-feeding bivalves(Mytilus galloprovincialis) are sufficiently abundant to filtera volume equivalent to the volume of Varano lagoon at leastonce daily. These observations suggest that grazing exerts animportant influence on phytoplankton dynamics, mainly on themicro fraction, and that diatoms seem to play an important rolein the food web dynamics of this coastal fishery.     

Temporal changes in the ciliate assemblage and consecutive estimates of their grazing effect during the course of a Heterocapsa circularisquama bloom

Temporal changes in ciliate assemblages during the course ofa bloom of the harmful microalga Heterocapsa circularisquama(Dinophyceae) were investigated and consecutive estimates ofspecies-specific maximum grazing losses were analyzed from Augustto September 1998 at a site in western Hiroshima Bay, the SetoInland Sea of Japan. Temporal increases of the H. circularisquamamean concentration in the water column were observed twice (25–29August and 7–10 September) with the maximum concentration(ca. 4000 cells mL^–1) being recorded on 25 August. Themain ciliate genera during the bloom were Favella, Tontonia,Eutintinnus, Tintinnopsis and Amphorellopsis. Increases of Favellaand Tontonia were observed when the concentration of H. circularisquamaranged from 260 to 1170 cells mL^–1. Total maximum grazingloss estimated from the abundance and ingestion rate of eachciliate species on H. circularisquama ranged from 1 to 75% standingstock removed d^–1 of the H. circularisquama concentration.High grazing losses mainly due to the genera Favella and Tontoniaoccurred during the period when the H. circularisquama concentrationwas decreasing. These results suggest that grazing by ciliateassemblages can influence the population dynamics of H. circularisquamadespite the potentially toxic nature of the phytoplankter.     

Dynamics of herbivorous grazing by the heterotrophic dinoflagellate oxyrrhis marina

In a series of batch experiments in the dark the heterotrophicdinoflagellate Oxyrrhis marina grazed three phytoplankton prey(Phaeodactylun tricornutum, Isochrysis galbana and Dunaliellateriolecta) with equal efficiency. Growth rates of the dinoflagellateranged between 0.8 and 1.3 day–1 Maximum observed ingestionrates on a cell basis varied according to the size of the preyfrom about 50 cells flagellate^–1 day^–1 when D.tertiolectawas the prey to 250–350 cells fiagellate^–1 day^–1when the other species were eaten. However, when compared ona nitrogen basis, ingestion rates were independent of prey type.Both ingestion and growth ceased when prey cell concentrationsfell below a threshold concentration of about 10⁵ cells ml^–1.Maximum specific clearance rates were 0.8x10⁴0ndash;5.7x10⁴it day which is considerably lower than that found for heterotrophicdinoflagellates in oceanic waters and may explain why O.marinagenerally thrives only in productive waters. The timing of NHregeneration was linked to the C:N ratio of the prey at thestart of grazing. Regeneration efficiencies for NH₄. never exceeded7%; during the exponential phase and were 45% well into thestationary phase. These results are comparable to those obtainedwith heterotrophic flagellates and demonstrate that the bioenergeticpatterns of grazing and nutrient cycling by different protozoaare very similar. Moreover, they support the notion that toachieve 90+% nutrient regeneration in the open ocean, as iscurrently believed, the microbial food loop must consist ofmultiple feeding steps. Alternatively, nutrient regenerationefficiencies may be considerably lower than 90%.     

Summer coastal zooplankton biomass and copepod community structure near the Italian Terra Nova Base (Terra Nova Bay, Ross Sea, Antarctica)

The structure of the zooplankton biotic community and of copepodpopulation in the coastal area of Terra Nova Bay (Ross Sea,Antarctica) was investigated during the 10th Italian AntarcticExpedition (1994/1995). Zooplankton biotic community consistedmainly of pteropods (Limacina helicina and Clione antarctica),Cyclopoid (Oithona similis), Poecilostomatoid (Oncaea curvata)and Calanoid (Ctenocalanus vanus, Paraeuchaeta antarctica, Metridiagerlachei and Stephos longipes) copepods, ostracods, larvalpolychaetes and larval euphausiids. Zooplankton abundance rangedfrom 48.1 ind m^–3 to 5968.9 ind m^–3, and copepodabundance ranged from 45.2 ind m^–3 to 3965.3 ind m^–3.The highest peak of zooplankton abundance was observed between25 m and the surface and was mainly due to the contributionof O. similis, O. curvata and C. vanus. Zooplankton biomassranged from 5.28 mg m^–3 to 13.04 mg m^–3 dry weight;the maximum value was observed between 25 m and the surface.Total lipid content varied from 216.44 to 460.73 mg g^–1dry weight.     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

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     

Sublethal effects of the toxic dinoflagellate Karenia brevis on marine copepod behavior

Apart from grazing interactions, little is known regarding thesublethal effects of Karenia brevis cells on copepod behavior.We conducted grazing and mortality experiments with K. breviscells and brevetoxins (PbTx-2), establishing routes of toxicityfor the copepods Acartia tonsa, Temora turbinata and Centropagestypicus. Subsequent behavioral experiments determined whethercopepod swimming and photobehavior, both behaviors involvedin predator avoidance, were impaired at sublethal K. brevisand PbTx-2 levels. Copepods variably grazed toxic K. brevisand non-toxic Prorocentrum minimum at bloom concentrations.Although copepods accumulated brevetoxins, significant mortalitywas only observed in T. turbinata at the highest test concentration(1 x 10⁷ K. brevis cells L^–1). Acartia tonsa exhibitedminimal sublethal behavioral effects. However, there were significanteffects on the swimming and photobehavior of T. turbinata andC. typicus at the lowest sublethal concentrations tested (0.15µg PbTx-2 L^–1, 1 x 10⁵ K. brevis cells L^–1).Although physiological incapacitation may have altered copepodbehavior, starvation likely played a major role as well. Thesedata suggest that sublethal effects of K. brevis and brevetoxinon copepod behavior occur and predicting the role of zooplanktongrazers in trophic transfer of algal toxins requires knowledgeof species-specific sublethal effects.     

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

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