The effect of zooplankton grazing on estuarine blooms of the toxic dinoflagellate Gonyaulax tamarensis

A series of short-term ^{in situ} experiments was conducted intwo Cape Cod embayments to estimate mortality rates of the toxicdinoflagellate, Gonyaulax tamarensis, resulting from grazingby zooplankton. Rates of grazing by the whole zooplankton communityand by specific zooplankton populations were measured at variouspoints in the G. tamarensis bloom cycle. The planktonic larvaeof the spionid polychaete Polydora ligni and the tintinnid ciliateFavella sp. were important grazers in the systems studied. Gonyaulax-specificclearance rates effected by Polydora ranged from 0.02 to 0.5ml individual^–1 h^–1; for Favella the range was aboutan order to magnitude lower. Peak population densities wereclose to 900 and 400 individuals 1^–1 for P. ligni andFavella, respectively. Whether measured directly or predictedas the product of individual clearance rates and numerical abundance,rates of grazing were often higher than estimated algal divisionrates in years when blooms failed to develop. A simulation modelcorroborated the results of the field study, demonstrating thatgrazing can be a significant source of mortality during blooms,and can suppress bloom development when grazers are abundant.     

Effects of temperature and salinity on the growth of the red tide flagellates Heterocapsa circularisquama (Dinophyceae) and Chattonella verruculosa (Raphidophyceae)

Growth responses of the red tide flagellates, Heterocapsa circularisquama(Dinophyceae) and Chattonella verruculosa (Raphidophyceae),were examined with 36 different combinations of temperature(5–30°C) and salinity (10–35 PSU). Heterocapsacircularisquama did not grow at or below a temperature of 10°C.The maximum growth rate of H.circularisquama (1.3 divisionsday^–1) was obtained with a combination of 30°C and30 PSU. In contrast, C. verruculosa did not grow at 10 PSU andat temperatures of 25°C or more. The maximum growth rateof C. verruculosa (1.74 divisions day^–1) was obtainedwith a combination of 15°C and 25 PSU. A significant temperature-salinityinteraction on growth was found by factorial analysis. Basedon the physiological characteristics obtained in the presentstudy, these novel flagellates have a potential for future outbreaksof red tides in pre viously unaffected waters.     

Effect of temperature on growth and ingestion rates of Favella sp

This study describes the effect of temperature on the growthand ingestion rates of the tintinnid, Favella sp. cultured withthe dinoflagellate Heterocapsa triquetra. In vivo fluorescencewas used to monitor the change in density of the H. triquetrapopulation over 4- to 5-day periods in control tubes containingonly algae, and in experimental tubes containing algae and tintinnids.A ‘switchover point’ occurred in the temperaturedependency of the growth rate such that below 11.4°C, H.triquetra grew more quickly than Favella sp. and above thistemperature the situation was reversed. Ingestion rates of Favellaon H. triquetra were found to be temperature dependent in anonlinear fashion. The rate doubled (from 2.5 to 5.3 cells animal^–1h^–1) between 11.4 and 16.4°C whereas there was nochange in ingestion rates between 8.0 and 11.4°C, or between16.4 and 21.1°C.     

Cyst formation: an important mechanism for the termination of Scrippsiella trochoidea (Dinophyceae) bloom

A sediment trap study was conducted at Daya Bay, South ChinaSea, to investigate the relationships between encystment andpopulation dynamics of Scrippsiella trochoidea from December1999 to January 2001. A dense bloom of S. trochoidea occurredduring the study period from August to September 2000, withthe maximum cell number of 3.18 x 10⁴ cells mL^–1.Two morphotypes of cysts, one with a thick calcareous wall (calcifiedcyst) and another without the obvious calcareous cover (non-calcifiedcyst), were observed during this investigation. The morphologicaland excystment characteristics of these two cyst types werestudied as well. Mass encystments of S. trochoidea, with themaximum of 3.05 x 10⁵ cysts m^–2 d^–1for calcified cyst, and 1.54 x 10⁷ cysts m^–2 d^–1for non-calcified cyst, coincided with the maximum abundanceof the vegetative cells. Encystment caused the transfer of atotal of 2.24–4.49 x 10⁸ cells m^–2 vegetativecells from the water column to the sea bottom during the bloomand resulted in a considerable loss of the bloom population.High assemblages of cysts of S. trochoidea were detected inthe surface sediments as well. This rich ‘seed bed’in the surface sediments caused by the high efficiency of encystmentafter blooms acting as a benthic reservoir for future vegetativepopulation, together with the short dormant period (15–26days) and high germination rate (50–90%), may explainthe repeated occurrence of S. trochoidea blooms in Daya Bay.     

Effects of prey motility and concentration on feeding in Acartia tonsa and Temora longicornis: the importance of feeding modes

Feeding experiments were conducted with the ambush-feeding copepodAcartia tonsa and the feeding-current-generating copepod Temoralongicornis. The copepods were offered a mixed diet of the dinoflagellateHeterocapsa triquetra and the ciliate Balanion comatum of similarcell size. The dinoflagellate was offered at a constant concentrationof 10–15 cells mL^–1, whereas the ciliate was offeredat a variety of concentrations, ranging from 7 to 57 cells mL^–1.Copepods with different feeding modes possess different mechanismsfor prey detection, suggesting that the two copepods would responddifferently to the two prey types. Both copepods had significantlyhigher clearance rates on the highly motile ciliate than onthe less motile dinoflagellate. In encounters between A. tonsaand its prey, we argue that this is due to the higher hydromechanicalsignal generated by the ciliate. The advection feeding copepodT. longicornis fed on the two prey according to their relativeconcentrations; in this case, we suggest that although B. comatumis capable of detecting feeding-current-generating predators,the feeding current velocity generated by T. longicornis isgreater than the escape velocity of this ciliate.     

Microbial dynamics during the decline of a spring diatom bloom in the Northeast Atlantic

The microbial dynamics during a spring diatom bloom declinewas monitored in the Northeast Atlantic during a 5-day Lagrangianstudy (8–12 April 2002). Phytoplankton abundance, compositionand health status were related to viral and bacterial abundance,zooplankton abundance and grazing rates, as well as bacterialproduction. Phytoplankton reached maximum concentration on Day3 (Chl a >5 µg L^–1) and declined on Day 5 (Chla 2 µg L^–1) and was dominated (70% of Chl a) bydiatoms. Bacterial production increased substantially to >20µg C L^–1 day^–1 on Day 3 and concomitantlylarge viruses decreased in number by half to <10 x 10³ mL^–1.This was followed by a 5-fold increase in large viruses on Day5, indicating infection and subsequent lysis on Days 3 and 5,respectively. Micro- and mesozooplankton grazing were not theprincipal cause for the decline of the bloom and pheophorbide-ashowing little variation in concentration from Days 1–4(100 ng L^–1) although doubled on Day 5. The poor physiologicalstatus of the diatoms, indicated by the high chlorophyllide-aconcentrations (50–480 ng L^–1), likely promoteda series of closely interrelated events involving bacteria andviruses leading to the demise of the diatom bloom.     

Growth and decline of a diatom spring bloom phytoplankton species composition, formation of marine snow and the role of heterotrophic dinoflagellates

During the spring of 1994, we determined the factors responsiblefor the decline of the seasonal diatom bloom in the Gullmarfjord, on the west coast of Sweden. Four species constituted>75% of the biomass—Detonula confervacea, Chaetocerosdiadema, Skeletonema costatum and Thalassiosira nordenskioeldii—reachingconcentrations of 4900, 350, 8200 and 270 cells ml^–1,respectively. Growth of phytoplankton was exponential (growthrate = 0.12 day^–1) from 3 to 21 March, after which a galewith winds >15 m s^–1 caused massive aggregation. Amaximum of 130 p.p.m. (v/v) of marine snow aggregates was observedby in situ video at the peak of the bloom. Critical concentrations(Jackson, Deep-Sea Res., 37, 1197–1211, 1990) were similarto observed showing that coagulation theory could explain thesudden decline of the bloom. The heterotrophic dinoflagellateGyrodinium cf. spirale increased exponentially after the peakof the bloom with maximum (temperature-adjusted) growth rates.After the rapid aggregation and sedimentation of the bloom,they were able to control any further growth of diatoms. Nitrateand silicate were never depleted, but phosphate may have beenlimiting by the end of the study period. We conclude that massaggregation during a gale marked the end of the bloom, and thatintense grazing by heterotrophic dinoflagellates prevented anysubsequent increase of diatoms.     

Fine scale spatial correlations between planktonic ciliates and dinoflagellates

Fine scale spatial distributions of planktonic ciliates whichprey on dinoflagellates were investigated in a small estuary.The horizontal distributions of Favella sp., Balanion sp. andStrobilidium sp. were positively correlated with the patchydistribution of dinoflagellates. The vertical distribution ofFavella was positively corrdated with the distribution of dinoflagellates.Throughout the diel cycle, the vertical distribution of Faveilawas similar to the vertical distribution of dinoflagellates.We speculate that behavioral responses are responsible for theassociation of diliates with their prey in the water column.Vertically coincident ciliate and algal populations should respondsimilarly to turbulent mixing and water displacement causedby wind stress, bathymetry, or frontal convergences. This wouldresult in horizontal patches in which the ciliates and algaeremain associated. This fine scale spatial coupling betweenciliates and their prey should result in higher ciliate growthrates and greater impact of ciliate grazing on phytoplanktonpopulations than would be predicted from average ciliate oralgal densities. Coincident patches of algae and ciliates mayalso provide higher food concentrations for larger grazers whichcan use both resources. ^{Contribution No. 5532, W.H.O.I.}     

The seasonal abundance of the phototrophic ciliate Mesodinium rubrum in Southampton Water, England

The abundance of the marine phototrophic planktonic ciliateMesodinium rubrum was monitored throughout an annual cycle attwo stations in the Southampton Water estuary. Seasonal changesin the concentration of nitrate, ammonia and phosphate weremonitored both at the inner estuary station (NW Netley) andouter estuary station (Calshot). Nutrient levels in the winterwere similar at both stations, and were diminished during sequentialdiatom blooms dominated initially by Sketetonema costatum andthen by Rhizosolenia dclicatula. Nitrate was reduced to a seasonalminimum in the outer estuary following these spring diatom blooms,but in the inner estuary was sustained >500 µg I^–1until the onset of the M.rubrum bloom. During the developmentof a visible red tide of M.rubrum in June/July at NW Netley,nutrient concentrations were considerably reduced. Cell numbersof M.rubrum varied between 2 and 3 cells ml^– during winterto >400 cells ml^–1 during the bloom at NW Netley, whereasat Calshot cell abundance did not increase above 25 cells ml^–1at any time of the year. At NW Netley, dense accumulations ofthe ciliate occurred over restricted depth intervals duringthe bloom and possible factors influencing the observed verticaldistribution of cells are considered. ¹Present address: Biology Department, Faculty of Science, AddisAbaba University, PO Box 1176, Addis Ababa, Ethiopia     

Pseudobalanion planctonicum (Ciliophora, Prostomatida): ecological significance of an algivorous nanociliate in a deep meso-eutrophic lake

Pseudobalanion plancioracum was the most abundant ciliate speciesin the pelagic zone of Lake Constance, FRG, over a 3 year period.Annual averages accounted for 30, 35 and 43% of total ciliatenumbers in the uppermost 20 m of the water column in 1987, 1988and 1989 respectively. Highest cell numbers were observed inearly spring, simultaneously with the first phytoplankton maximum.The small ciliate (mean length 15 µm, mean cell volume1300 µm³) is a raptorial feeder and predominantly consumesphytoplankton. In laboratory cultures, P planctonicum grew wellon a diet of Rhodomonas sp. (Cryptophyceae). Maximum growthrates increased from 0.46 day^–1 at 5 5°C to 1.52 day^–1at 18.5° C, while temperatures above 21°C were lethal.Depending on food concentration, 0.2–4.4 Rhodomonas cellswere ingested per ciliate and hour.     

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.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

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     

Comparative growth rates and yields of ciliates and heterotrophic dinoflagellates

Growth rates, ingestion rates and grazer yields (grazer volumeproduced/prey volume consumed) were measured for six protozoanspecies (ciliates: Favella sp., Strombidinopsis acuminatum,Uronema sp.; heterotrophic dinoflagellates: Amphidinium sp.,Gymnodinium sp., Noctiluca scintillans) in laboratory batchculture experiments. Comparative growth data indicate that theprymnesiophyte Isochrysis galbana, the prasinophyte Mantoniellasquamata, two cryptophyte species and several autotrophic dinoflagellatespecies were suitable foods for these grazers. When grown onoptimized diets at 13C, maximum ciliate growth rates (range0.77–1.01 day^–1 uniformly exceeded maximum heterotrophicdioflagellate growth rates (range 0.41–0.48 day^–1).A compilation of published data demonstrates that this growthrate difference persists across a range of ciliate and dinoflagellatetaxa and cell sizes. Comparison of volume-specific ingestionrates and yields for the six species studied here showed thatthere was no single explanation for this growth rate disparity.Heterotrophic dinoflagellates exhibited both low ingestion ratesand, in one case, low yields; ciliates were able to achievehigher growth rates via either higher ingestion rates or higheryields, depending on ciliate species. Volume yield increasedover time throughout the exponential growth phase in nearlyall experiments, suggesting variation in response to changingfood concentrations or long-term acclimation to culture conditions.Higher maximum ciliate growth rates mean that these grazershave the potential to exercise tighter control over incipientblooms of their prey than do heterotrophic dinoflagellates.     

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.     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

Fluxes of diatoms in the Dona Paula Bay, west coast of India

Sediment traps were deployed at a station in the Dona PaulaBay to collect sedimenting particles at weekly intervals fromNovember to May during 1995–1997. Sedimented particleswere analysed for total diatom flux, chlorophyll a (Chl a) andparticulate organic carbon (POC). The highest diatom flux wasrecorded in April–May for both the years. Fluxes of diatomsvaried from0.6 x 10⁴ cells m^–2 day^–1 (November 1995)to 121.47 x 10⁴ cells m^–2 day^–1 (December 1996).In all, 19 diatom genera were identified in the sedimented material.Navicula, Nitzschia, Pleurosigma, Licmophora, Coscinodiscus,Rhizosolenia and Surirella were the most abundant genera inthe sedimented material throughout the sampling period. Meanflux of POC and diatom carbon was 251 and 0.39 mg C m^–2day^–1, respectively. The diatom carbon accounted for 0.15%of the POC flux. Mass flux of diatoms showed significant negativecorrelation with the concentration of nitrate and phosphate.This suggests that the nutrient concentration played an importantrole in influencing the sedimentation of diatoms at this coastalstation.     

Phytoplankton and nutrients in the waters north-west of Spitsbergen in the autumn of 1979

Phytoplankton biomass, primary production rates and inorganicnutrients were measured in the uppermost layer of the ice-edgeregion and in open water and compared with environmental factorsduring a three-week cruise in September – October 1979.Biomass and production values were low (maximum 2.2 µgchl a l^–1, 2.5 mg C m^–3 h^–1). A post-bloomcommunity of diatoms, consisting mainly of representatives ofChaetoceros, Leptocylindrus, Nitzschia and Thalassiosira, waspredominant. Concentrations of phosphate were quite low (maximum0.55 µM I^–1). Nitrate and silicate ranged from nomeasurable quantities to 5.7 µM l^–1 and 3.8 µMl^–1, respectively. The possibility of light and nutrientlimitation on phytoplankton growth is discussed.     

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.     

Zooplankton grazing impact in the plume of dilution of the Gironde estuary (France) prior to the spring bloom

The Bay of Biscay is a coastal area intensively exploited forfishing and is submitted to important human actions (proximityof important industrial and agricultural areas). Thus, the understandingof the materials and energy transfers in such ecosystems isof great interest. However, investigations on zooplankton (animportant component of the marine food web) are very scarcein this area. Our study concerns the grazing impact of the zooplanktonin shelf waters in the plume of dilution of the Gironde estuaryprior to the spring bloom. Samples were taken through the photiclayer in April 1993. Our results have shown that Temora longicornisand Paracalanus parvus dominated the ‘herbivorous’planktonic community. The grazing impact of the zooplanktoncommunity on the total phytoplankton stock and on the totalprimary production was low (9–14 and 17–21% day^–1,respectively) during this period, which is in accordance withmost results in coastal areas. However, due to the small sizeof the algae (