Zooplankton annual cycle in a Mediterranean coastal area

The annual cycle of the zooplankton community in the SaronikosGulf (Aegean Sea, Greece) was studied over a period of 2 years.The sampling scheme included monthly hauls at three stationsdifferentiated according to depth and neritic character. Maximumvalues of total zooplankton abundance were found in summer monthsup to early autumn and partially in spring. Copepods dominatedduring most of the year (Clausocalanus furcatus, Paracalanusparvus, Temora stylifera, Ctenocalanus vanus, Oithona similis,Oithona plumifera), while cladocerans (Penilia avirostris) wereabundant in summer months and in September. No important fluctuationswere detected between the 2 years of study, while monthly oneswere more significant in the more neritic station during thewinter–spring period. Correspondence analysis showed thatthe seasonal evolution of zooplankton is related to environmentalparameters: temperature, hydrography expressed in open sea influenceand topography.     

Egg production rates and stage-specific development times of Clausocalanus furcatus (Copepoda, Calanoida) in the northern Gulf of Mexico

The calanoid copepod Clausocalanus furcatus is widely distributedin tropical and subtropical oceans, yet knowledge of its reproductionand development remains limited. We estimated egg productionrates and stage-specific development times from laboratory incubationsand field samples collected at a petroleum platform in the northernGulf of Mexico. Egg production experiments were performed in2002 and 2003 at a laboratory on the platform using freshlycollected female copepods. Median stage development durationswere also measured during the incubations. Logistic regressionmodels were used to estimate median stage durations. The mediandevelopment times of Clausocalanus furcatus from hatching toadulthood ranged from 13 to 20 days. Field samples were collectedfrom waters beneath the platform at 12-h intervals using a 30-LNiskin water bottle during March–April and May–June2003, and the Edmonson egg ratio method was used to estimatethe mean in situ egg production rate. This field-derived meanegg production rate was significantly lower than the 12.08 eggsfemale^–1 day^–1 measured from incubation experiments,and this difference may reflect high in situ egg mortality.In situ egg production rates also showed high variability andwere almost five times higher during March–April comparedto May–June.     

Mesozooplankton community structure and grazing impact in the Polar Frontal Zone of the south Indian Ocean during austral autumn 2002

Mesozooplankton community structure and grazing impact were investigated at 13 stations in the Polar Frontal Zone during the second Marion Offshore Variability Ecosystem Study (MOVES II), conducted during April 2002. Total integrated chl- a biomass ranged between 11.17 and 28.34 mg chl- a m^-2 and was always dominated by nano- and picophytoplankton (<20 µm). Throughout the study, small copepods, mainly Oithona similis and Ctenocalanus vanus, numerically dominated the mesozooplankton community, composing up to 85% (range 30-85%) of the total abundance. Grazing activity of the four most abundant copepods ( O. similis, C. vanus, Calanus simillimus and Clausocalanus spp.), constituting up to 93% of total mesozooplankton abundance, was investigated using the gut fluorescence technique. Results of gut fluorescence analyses indicated that Calanus simillimus and Clausocalanus spp. exhibited diel variability in gut pigments with maximum values recorded at night. In contrast, O. similis and Ctenocalanus vanus did not demonstrate diel variation in gut pigment contents. Ingestion rates of the four copepods ranged from 23.23 to 1462,02 ng(pigm.)ind^-1day^-1. The combined grazing impact of the four copepods ranged between 1 and 36% of the phytoplankton standing stock per day, with the highest daily impact occurring at stations occupied in the vicinity of the Antarctic Polar Front (~35.86% at station 23). Among the copepods, O. similis and Ctenocalanus vanus represented the most important consumers of phytoplankton biomass, collectively responsible for up to 89% (range 15-89%) of the total daily grazing impact. Carbon specific ingestion rates of the copepods varied between 42 and 320% body carbon per day.     

Annual Zooplankton Succession in Coastal NW Mediterranean Waters: The Importance of the Smaller Size Fractions

Zooplankton abundance, biomass (biovolume) and taxonomic compositionwere studied within an annual cycle (August 1995–October1996) in the Bay of Blanes (northwest Mediterranean). Weeklyzooplankton sampling included oblique tows made with a 200 µmJuday–Bogorov net, and vertical tows made with a 53 µmnet, to adequately sample both mesoplankton and the smallerzooplankton fractions. Total zooplankton abundance showed highvariability, lacking any clear seasonal pattern. However, thedifferent species within the zooplankton community displayeda clear succession throughout the year. In general, cyclopoidcopepods (Oithona spp.) and cladocerans (Peniliaavirostris)dominated the summer and autumn communities, whereas in winterand spring, calanoid copepods (Clausocalanus spp., Paracalanussp. and Centropages typicus) were predominant. The zooplanktonannual cycle in the Bay of Blanes does not resemble those ofother Mediterraneanlittoral areas, probably due to the inherentparticularity and variability associated with open coastal environments.On average, the abundance of organisms estimated with a traditional200 µm Juday–Bogorov net was 8.1 times lower thanthe values obtained with a 53 µm net. Even if only organisms>200 µm collected in the 53 µm tows were considered,the total abundance within the 53 µm net was still 4.4times higher than the estimates from the Juday–Bogorovnet. These results suggest the need for accurate samplings ofthe entire zooplankton assemblage when characterizing the structureand dynamics of zooplanktonic communities.     

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.     

Particulate and dissolved phytoplankton production of the Patos Lagoon estuary, southern Brazil: comparison of methods and influencing factors

Uptake rates of ¹⁴C (filtration and the acidification-bubblingmethod—ABM) were measured weekly in a shallow region ofthe Patos Lagoon estuary (3207'S, 5206'W) between March 1989and March 1990. Phytoplankton production varied seasonally,the lowest values occurring in the austral winter (June–August1989) and the highest rates during spring and summer (March1989; September 1989–March 1990). Particulate carbon productionvaried between 0.65 and 70.6 mg C m^–3 h^–1 and wasmostly associated with organisms <20 µm (mean = 73.4%).Dissolved organic carbon (DOC) released by phytoplankton variedbetween 0.1 and 89.3 mg C m^–3 h^–1 representing     

Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates wereinvestigated along a 50°S–50°Nlatitudinal transect,during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises(boreal spring–autumn 1997, boreal spring–summer1998). Copepod abundance was higher at high latitudes in spring,near northwest Africa, in the equatorial and Benguela upwellingsystems, and in the Subtropical Convergence, and lower in oligotrophicgyres. Gut contents were not related to phytoplankton biomassor production. Gut evacuation rate averaged 0.03 min^-1, andwas not related to latitude or body size. Conservative estimatesof copepod community total ingestion rates ranged between 3.4and 173 mg C m^-2 day^-1 for AMT4, 1.6–252 mg C m^-2 day^-1in AMT5 and 10–160 mg C m^-2 day^-1 in AMT6. Maximum valueswere always in the upwelling regions, the subtropical convergenceand high latitudes in the Northern Hemisphere during borealspring. Calculated ingestion rates translate into average dailyminimal consumption values of 2.07%, 1.89% and 2.6% of totalchlorophyll stock, or 8.02%, 14.5% and 12.9% of total primaryproduction ingested daily on AMT4, 5 and 6 respectively. Grazingimpact increases considerably if we consider ingestion of phytoplanktonlarger than 2 µm, especially under the influence of theEquatorial and North African upwellings, where copepod ingestionrepresents up to 30% of the biomass and >100% of productionby large cells.     

Seasonal and interannual variability of size-fractionated phytoplankton biomass and community structure at station Kerfix, off the Kerguelen Islands, Antarctica

Time series of phytoplankton biomass and taxonomic compositionhave been obtained for the 3 years 1992, 1993 and 1994 in thenorthern part of the Southern Ocean (station Kerfix, 5040'S,6825;E) Autotrophic biomass was low throughout the year (<0.2mg m^–3 except during a short period in summer when a maximumof 1.2 mg chlorophyll (Chl) a m– was reached. During winter,the integrated biomass was low (<10 mg m^–2) and associatedwith deeply mixed water, whereas the high summer biomass (>20mg m^–2) was associated with increased water column stability.During summer blooms, the >10 µ;m size fraction contributed60% to total integrated biomass. Large autotrophic dinoflagellates,mainly Prorocentrum spp., were associated with the summer phytoplankton maxima and accounted for >80% of the total autotrophcarbon biomass. In November and December, the presence of thelarge heterotrophic dinoflagellates Protoperidinium spp. andGyro dinium spp. contributed a high proportion of total carbonbiomass. During winter, the <10 µm size fraction contributed80% of total Chi a biomass with domination of the picoplanktonsize fraction. The natural assemblage included mainly nakedflagellates such as species of the Prasinophyceae, Cryptophyceaeand Prymnesiophyceae. During spring, picocyanobacteria occurredin sub-surface water with a maximum abundance in September of106 cells 1^–1     

Diel vertical migration of the planktonic copepods at an upwelling station north of Taiwan, western North Pacific

A total of 178 copepod species were identified in an upwellingarea of the Mienhua Canyon off northern Taiwan, western NorthPacific during a spring cruise in 1995. Paracalanus aculeatus,Oncaea venusta and Clausocalanus furcatus were the three dominantspecies, comprising 43% of the total copepod numbers. Most copepodspecies performed normal diel vertical migration, descendingduring daytime and ascending at night to different depth zonesand with different rates. Some dominant copepod species, suchas P. aculeatus, C. furcatus, Temora discaudata and Canthocalanuspauper, apparently congregated in the surface water (between0 and     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Determining the mass density of marine copepods and their eggs with a critical focus on some of the previously used methods

We collected Calanus finmarchicus copepodites CIV, CV and CVIfemales in a deep fjord on the west coast of Norway during April1996, May 1997 and November 1998. Eggs of C. finmarchicus andCalanus glacialis were collected during May 1989 in the BarentsSea. The sinking speeds of animals and eggs were measured ina homogeneous column with seawater of known density, and Stokeslaw was applied to estimate their mass density. Also the densitycontrast between the organisms and seawater was calculated.The mean mass density of C. finmarchicus ranged from 1.0274to 1.0452 g cm^–3. During spring copepodite stage CV hada significantly lower mean mass density (1.0345 g cm^–3)compared to CIV (1.0381 g cm^–3) and CVI females (1.0408g cm^–3). Copepods collected during winter had a distinctlylower mass density. The sinking speed of C. glacialis eggs followeda unimodal distribution, with a mean of 25.9 m day^–1,while sinking speeds of C. finmarchicus eggs were bimodal, thetwo groups of eggs having a mean sinking speed of 23.3 m day^–1and 35.4 m day^–1 respectively. Correspondingly the meanmass density was 1.0556 g cm^–3 for C. glacialis eggs andfor the two groups of C. finmarchicus eggs 1.0639 g cm^–3and 1.0812 g cm^–3. Results of earlier work, particularlyusing density gradient methods to determine mass density ofzooplankton, are critically reviewed, and it is suggested thatthis method should not be used to determine the mass densityof small organisms the size of C. finmarchicus.     

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     

The dynamics of heterotrophic nanoflagellates and bacterioplankton in a large ultra-oligotrophic Antarctic lake

The abundance of both heterotrophic nanoflagellates (HNAN) andbacterioplankton in a large (9km²) ultraoligotrophic Antarcticlake (Crooked Lake) were investigated from December 1992 untilNovember 1993. HNAN abundance peaked in spring, summer and autumn,falling to lowest numbers during the winter. Numbers rangedbetween 0 and 50.9x10⁴ l^–1. Bacterioplankton abundancewas highest during the late summer and then fell progressivelytowards winter and autumn (range 1.19–4.46x10⁶ l^–1)In contrast to numbers, mean cell volumes (MCV) of the bacteriareached their highest in spring, and consequently highest bacterialbiomass occurred at this time. MCV ranged between 0.052 and0.224µm³. Bacterial production measurements followingthe incorporation of [³H] thymidine into DNA and [¹⁴C] leucineinto protein using a doubling-labelling procedure were undertakenin January, June, August, October and November. Rates variedbetween 2.8 and 52 ng C l¹ h¹. On occasions, a significant differencein production rates based on the uptake of leucine and thymidinewas observed, suggesting unbalanced growth. Highest rates ofproduction coincided with times of high dissolved organic carbonlevels in the water column and lowest production with low levelsof DOC. HNAN grazing rates were measured by following the uptakeof fluorescently labelled bacteria and averaged 4.8 bacterialcells individual¹ day¹ at 2 and 4°C. Specific growth rates(h¹) ranged around 0.00070–0.00077 in both the field andlaboratory, giving doubling times of 37.3 and 41.0 days, respectively.These low rates of grazing and growth indicate that there isno adaptation to low temperatures in these freshwater protists.Based on these data, the gross production efficiency is 24%.HNAN removed between 0.1 and 9.7% of bacterial production perday.     

Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer 1999: the distribution of taxonomic groups in relation to physical/chemical conditions in the photic zone

A study of the phytoplankton community in the Faroe-ShetlandChannel was conducted in July 1999. Samples were collected atvarious depths in the photic zone along three transects (thenorthern entrance, the center and the southern entrance). Exceptfor a few easterly stations where nitrate and silicate werebelow 1 µM, all nutrients (phosphate, silicate, ammonium,nitrite and nitrate) were non-limiting for phytoplankton growth.HPLC pigment analysis revealed a pronounced (>50%) dominanceof Prymnesiophyceae at all stations. Their pigment ratio ofdiatoxanthin + diadinoxanthin/Chl a (DDX/Chl a) indicated thatthe phytoplankton community was controlled by light. Primaryproduction in the delayed spring bloom varied from 1.2 to 1.8g C m^–2 day^–1 along the northern transect. Alongthe other two transects, primary production ranged from 1.6to 3.8 g C m^–2 day^–1. Associated with the characteristicsindicating the establishment of a bloom, the relative contributionof diatoms and Prymnesiophyceae increased, whereas that of Prasinophyceae,Cryptophyceae, Chrysophyceae and Cyanobacteriaceae decreased.With respect to their vertical distribution, Cyanobacteriaceae,Chrysophyceae and Dinophyceae tended to have a higher abundance,relative to other taxonomic groups, in the surface layers. Therelative abundance of diatoms and Chlorophyceae increased withdepth. The DDX/Chl a ratio of the Prymnesiophyceae decreasedwith depth, indicating that vertical mixing in the upper 30m of the photic zone occurred less frequently than the timespan of physiological acclimation of cellular pigment composition.     

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.     

Seasonal reproductive biology of Centropages tenuiremis (copepoda) in Xiamen waters, People's Republic of China

The egg production rate (EPR) and the effects of environmentalvariations on diapause induction in the copepod Centropagestenuiremis in Xiamen waters were studied in 2002 and 2003. TheEPR ranged from 7.33 ± 8.14 (on 18 February 2003) to91.08 ± 20.61 (on 6 June 2003) eggs female^–1day^–1(mean ± 95% confidence limits), and displayed a significantseasonal pattern. Before the period of maximum egg production,EPR increased with seawater temperature and chlorophyll a concentration;but after the maximum, no significant relationship was foundbetween EPR and chlorophyll a concentration, and there was anegative linear relationship between EPR and temperature. Seawatertemperature and photoperiod, respectively, played a primaryrole in the switch to diapause egg production by the femalesas shown by single-factor probit analysis. However, the differencein effect between the two factors was demonstrated by binarylogistic multivariate analysis. These results suggest that thiscopepod rapidly increases subitaneous EPR by taking full advantageof conditions in favorable seasons (winter and spring). SubitaneousEPR decreases and there is a switch to the production of diapauseeggs that survive in the bottom sediments during unfavorableseasons (summer and autumn) in Xiamen waters.     

Seasonal Cycles of Egg Production of Two Planktonic Copepods, Centropages typicus and Temora stylifera, in the North-western Mediterranean Sea

Reproduction of the dominant copepods Centropages typicus andTemora stylifera was studied at a permanent station in the LigurianSea (north-western Mediterranean). Seasonal patterns of eggproduction, clutch size, egg size and female prosome lengthwere followed from January 1998 to December 1999. Female carboncontent and weight-specific egg production were compared inautumn 1998 and spring 1999. Reproductive patterns of C. typicusand T. stylifera were very similar, indicating that reproductionwas affected by the same environmental factors. Reproductiveactivity was highest in autumn in both species and years. Asecond peak of egg production was observed in early summer,which was less intense in 1999 after a bloom of salps. Egg productionrates reached maximal values of 33.5 and 33.3 eggs female^–1day^–1 and annual means of 10.8 and 11.7 eggs female^–1day^–1 in Centropages and Temora, respectively. Maximalweight-specific egg production was 0.21 day^–1 in bothspecies in November 1998, when female carbon contents were 6.7(C. typicus) and 12.0 µg (T. stylifera). No statisticalrelationship between egg production and food availability ortemperature was detected. Reproductive activity did not reflectthe seasonal abundance patterns, with C. typicus dominatingin spring and T. stylifera in autumn.     

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.     

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.     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.