Phytoplankton composition and cell carbon distribution in Prydz Bay, Antarctica: relation to organic particulate matter and its {delta}13C values

In January-February 1991, in Prydz Bay, phytoplankton bloomwas evident in the inner shelf area with the dominant diatomsbeing represented mainly by pennate species of the Nitzschia-Fragilariopsisgroup. Dinoflagellates and naked flagellates were most abundantin the centre of the bay; however, larger heterotrophic speciesprevailed at the southern stations. Cell carbon values (average317 µg l^–1; range 92-1048 µg l^–1) foundin the bloom in the south were chiefly due to pennate diatomsand larger heterotrophic dinoflagellates. Much lower carbonvalues (average 51 µg l^–1; range 7-147 µgl^–1) in the outer shelf region were mainly contributedby large centric diatoms (70-110 mu;m) and small dinoflagellates(5-25 µm). Wide ranges of algal cell sizes were observedin both southern and northern communities; the overlapping ofsizes of diatoms and flagellates, the latter containing heterotrophs,suggested complex trophic relationships within the planktonand an enhanced heterotrophic activity in the south. North-to-southvariations in surface     

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.     

Sedimentation of phytoplankton populations dominated by Microcystis in a shallow lake

The sedimentary flux of phytoplankton was measured using sedimenttraps in a shallow hypertrophic lake (Lake Kasumigaura), whereMicrocystis bloomed, from June to November 1983 The sedimenttraps were set at 0.5, 1.5 and 3.0 m depth in Takahamairi Bay(3.5 m depth). Microcystis spp. (including M.aerugmosa and M.viridis)in the traps were rare until early August, but increased thereafter.Sinking rates of Microcystis were 0.0045, 0.020 and 0.24 m day^–1in June–August, September and October respectively, whichwere far lower than those of Melosira (0.2–1.7 m day^–1)and Syncdra (0.2–1.0 m day^–1). The total sedimentaryfluxes of POC and that of algal carbon during the study periodwere 283.2 and 96.7 gC m^–2 which were 59.5% and 20.3%of the gross primary production (475.8 gC m^–2) respectively.The sedimentary flux of living algae measured by algal countswas large in June but small in August and September. On theother hand, the flux of detritus obtained by subtracting totalalgal carbon from POC was small in June and July but large inAugust and September. Therefore diatoms, which appeared mostlyin June, tended to sink as live algae, while Microcystis sankas detritus after being decomposed or consumed in the waterIt was concluded from the results of carbon budget calculationsand the respiration rate of the 1- to 20-µm fraction thatthe activity of decomposers or consumers increased greatly inthe short period at the end of the bloom of Microcystis.     

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.     

Effect of u.v.-B irradiance on the response of 15N-nitrate uptake of Lauderia annulate and Synedra planctonica

The marine diatoms Lauderia annulata and Synedra planctonicaharvested during exponential growth were exposed to differentdoses of u.v.-B (286, 439 and 710 J m^–2 d^–1) for2 days. Uptake of ¹⁵N-nitrate was estimated before, during andafter u.v.-B radiation over 2 days. Exposure to high levelsof u.v.-B (710 J m^–2 d^–1) caused irreversible damageat the second daily irradiance. Lauderia cells were less affectedby u.v.-B stress than Synedra cells. ¹⁵N-nitrate uptake wasreduced under u.v.-B irradiance but could be reactivated within1 day following exposure to a low dose (286 J m^–2 d^–1).Higher levels of u.v.-B radiation (710 J m^–2 d^–1)led to irreversible damage. The pattern of ¹⁵N-incorporationinto several amino acids of Lauderia varied after 2 days ofu.v.-B radiation. ¹⁵N enrichment of glutamine increased markedlyafter u.v.-B stress (717 J m^–2 d^–1) whereas ^I5Nexcess of aspartic acid was significantly reduced. Results arediscussed with reference to the u.v.-B damage of the nitratetransport system.     

Comparison of east and west chlorophyll a standing stock and oceanic habitat along the Transition Domain of the North Pacific

Chlorophyll (Chl) a was measured every 10 m from 0 to 150 min the Transition Domain (TD), located between 37 and 45°N,and from 160°E to 160°W, in May and June (Leg 1) andin June and July (Leg 2), 1993–96. Total Chl a standingstocks integrated from 0 to 150 m were mostly within the rangeof 20 and 50 mg m^–2. High standing stocks (>50 mg m^–2)were generally observed westof 180°, with the exceptionof the sporadic high values at the easternmost station. Thetotal Chl a standing stock tended to be higher in the westernTD (160°E–172°30'E) than in the central (175°E–175°W)and eastern (170°W–160°W) TD on Leg 1, but thesame result was not observed on Leg 2. It was likely that largephytoplankton (2–10 and >10 µm fractions) contributedto the high total Chl a standing stock. We suggest that thehigh total Chl a standing stock on Leg 1, in late spring andearly summer, reflects the contribution of the spring bloomin the subarctic region of the northwestern Pacific Ocean. Thedistribution of total Chl a standing stock on Leg 2 was scarcelyaffected by the spring phytoplankton bloom, suggesting thattotal Chl a standing stock is basically nearly uniform in theTD in spring and summer. Moreover, year-to-year variation inthe total Chl a standing stock was observed in the western TDon Leg 1, suggesting that phytoplankton productivity and/orthe timing of the main period of the bloom exhibits interannualvariations.     

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.     

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.     

Infection of Coscinodiscus granii by the parasitoid nanoflagellate Pirsonia diadema: III. Effects of turbulence on the incidence of infection

The influence of turbulence on the incidence of infection ofthe diatom Coscinodiscus granii by the parasitoid nanoflagellatePirsonia diadema was investigated experimentally with two initialhost densities. Independently of the initial diatom densitiesof 7 and 44 cells ml^–1, under calm conditions both diatomsand parasitoids became extinct within 6–9 days. Turbulence,however, led to the survival of diatoms at a reduced densityof 0.1–2 cells ml^–1 for >30 days. A population-dynamicmodel is formulated that takes into account the non-homogeneousdistribution of infecting flagellates among host diatoms. Applicationof the results to parasitoid–diatom interactions in naturalwaters suggests that, under turbulent conditions, endemic infectionsmay effectively prevent the mass development of host diatoms.     

On the trophic fate of Phaeocystis pouchetii (Harlot). III. Functional responses in grazing demonstrated on juvenile stages of Calanus finmarchicus (Copepoda) fed diatoms and Phaeocystis

The objective of this study was to quantify the functional responsein feeding rate in the various developmental stages of Calanusfinmarchicus to different concentrations of the diatoms Thalassiosiranordenskioeldii and Porosira glacialis, and the haptophyseanPhaeocystis pouchetii. Grazing of copepodite stage I–VC.finmarchicus was measured using two different approaches.Feeding rates were obtained from either incubation experiments,estimating the rate of removal of particles from suspension,or by quantifying the turnover rate of the plant pigments inthe gut. Clearance as a function of algal concentration (1–30µg plant pigment 1^–1) was described in juvenilestages of C.finmarchicus fed the diatoms T.nordenskioeldii [20µm equivalent spherical diameter (ESD)], P.glacialis (40µm ESD), and two size categories (30–100 µmand >100 µm ESD) of the gelatinous alga P.pouchetii.When the copepodite stages were fed T.nordenskioeldii, the gutcontent of plant pigments was in general higher than when fedP.glacialis. Rates obtained were variable when the same copepoditestages were offered the two size categories of P.pouchetii,but within the same order of magnitude as those obtained forthe larger diatom. At unialgal diets, diatoms were more readilyconsumed than the larger size fraction among colonies of P.pouchetiiby copepodite stage I–III C.finmarchicus. But given anappropriate prey size, C.finmarchicus grazed both diatoms andcolonies of gelatinous algae at equal rates. A linear relationshipbetween gut content and food concentrations <10 µgchlorophyll 1^–1 was found. This indicates that the ingestionrate in C.finmarchicus is directly proportional to the ambientfood concentration during the most productive period in Mayand June in high latitudes irrespective of algal species present. ¹Present address: Marine Biological Laboratory, University ofCopenhagen, Strandpromenaden 5, DK-3000 Helsingør, Denmark ²Present address: Greater Copenhagen Council, Gl. KøgeLandevej 1–3, DK-2550 Valby, Denmark     

Specific growth rates of heterotrophic plankton organisms in a eutrophic lake during a spring bloom

The in situ growth of the dominating pelagic organisms at severaltrophic levels was investigated during a spring bloom characterizedby well-mixed cold water. The study includes primary productionand the carbon flow through the nano-, micro- and mesozooplanktonpopulations based on population dynamics and specific growthrates. The phytoplankton biomass and production were totallydominated by small algae <20 µm. of which {small tilde}5%were <3µm. potentially a food source for the nano-and microzooplankton. The mean carbon-specific primary productionwas 0.15 day^–1 and was regulated solely by light. Themean volume-based specific growth rate of bacterioplankton wasmodest. 0.1 day^–1. and probably controlled by the lowtemperature. The volume-based specific growth rates of heterotrophicnanoflagellates. ciliates. rotifers and copepods were 0.35.0.13. 0.16 and 0.03 day^–1, respectively. The observedgrowth of the heterotrophic plankton was generally not foodlimited, but was controlled by temperature. The stable temperatureduring the experiment therefore allows a cross-taxonomic comparisonof specific growth rates. The b exponent in the allometric relationship(G = aV^th) between volume-specific growth rate (G) and individualbody size (V) was –0.15 ± 0.03 for all filtratingzooplankton. indicating an in situ scaling not far from thephysiological principles onginally demonstrated for laboratorypopulations.     

Autotrophic and heterotrophic nanoplankton in the diet of the estuarine copepods Eurytemora affinis and Acartia bifilosa

The ingestion of autotrophic and heterotrophic nanoplanktonby two estuarine copepods, Eurytenora affinis and Acarith bifilosa,was measured in various environmental conditions using the incubationmethod and epifluorescence microscopy. Egg production of thespecies was also deter mined in order to estimate their carbonrequirements. Assuming a gross efficiency of egg productionof 0.3, nanoplanktonic carbon ingested always met the carbonrequirements suggesting that, most of the time, other carbonsources could be unnecessaly. Nanoplankton ingestion by A.bifilosa(from 128 to 1693 cells ind.^–1 h^–1) was dominatedby autotrophic forms (60–97%) and was seriously affectedby high (>100 mg l^–1 suspended particulate matter (SPM)concentrations. Nanoplankton ingestion by E.affinis (from 300to 1049 cells ind^–1 h^–1) was relatively stable incomparison, but this latter species seemed to switch its grazingpressure from autotrophic to heterotrophic forms when SPM concentrationsincreased. Thus, two copepod species, living in the same estuary,presented two different feeding behaviours, probably to maximizeenergy input per unit of energy expenditure. Such differencescould contribute to the spatial and seasonal segregation ofthese species which is usually observed.      

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.     

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.     

Effects of increasing salinity on a cyanobacteria bloom in the Potomac River estuary

From June through August, 1985, a bloom of the cyanobacteriumMicrocyslis aeruginosa was observed in the upper Potomac Riverreaching densities of 193 x 10⁶ cells 1^–1 and 83% of totalcells in the surface mixed layer. However, in regions typifiedby salinities of 1–2 p.p.t. immediately down-river, thealga disappeared from the phytoplankton assemblage, never contributing>17% of total phytoplankton numbers. In an attempt to determinethe effect of increasing salinity on limiting horizontal distributionsof cyanobacteria blooms in the Potomac River estuary, threelaboratory experiments were conducted. Bloom samples collectedfrom the river were exposed to daily salinity increases of 1–2p.p.t. through the addition of NaCl or a complement of fullsea salts. Relative to samples receiving no salt supplement,densities of Microcyslis spp. declined in salinity-stressedsamples. However, total cyanobacteria remained constant or increaseddue to rapid growth of Aphanizomenon. The addition of a mixtureof full sea salts favored aggregation of cyanobacteria, yieldingan average of 132 cells colony^–1; 36 cells colony^–1were observed in populations maintained in river water and NaCl-supplementedsamples. Chlorophyll concentrations declined slightly relativeto assemblages receiving no salt additions while carbon fixationwas reduced in salinity-stressed assemblages. These resultssuggest that salinities from 0.5 to 7 p.p.t., typical of theoligohaline region of the Potomac River, could limit the distributionof Microcystis-dominated blooms down-estuary. In addition, increasingsalinities should result in the aggregation of photosyntheticallyinhibited cells and, through prolonged exposure to increasingosmotic stress, produce large, rapidly sinking detrital particlessupporting microbial decomposition in oligohaline and mesohalineregions of the lower Potomac River estuary, perhaps perpetuatinghigh microbial oxygen demand and anoxia in this portion of theriver/estuary.     

Ecological studies on the phytoplankton of Korsfjorden, western Norway. The dynamics of a spring bloom seen in relation to hydrographical conditions and light regime

The investigation was carried out at one station in Korsfjorden,a typical deep silled fjord of western Norway. During 14 cruisesfrom 4 February to 30 June 1977 ATP, chlorophll a, phaeopigmenta, and in situ ¹⁴C-assimilation were measured in the net (>30µm), nano and ultraplankton (<5 µm). Sampleswere collected from five light depths within euphotic zone.The impact of hydrographical conditions and light regime onthe bloom dynamics was also studied. In the periods 4 February-7 March and 13 April-30 June, ultraplankton contributed >60%to the total primary production while net and nanoplankton dominatedfrom 7 March to 13 April. The diatoms Skeletonema costatum,Chaetoceros compressits and C. debilis, and Rhizosolenia hebetatavar. semispina made up the main part of the biomass on 21 March,28 March and 4 April respectively. A shade adapted diatom societywas located at the top of the nutricline in late June with S.costatum, Chaetoceros spp., and Thalassiosira spp. as the dominantspecies. The highest assimilation number of eight for the netplankton and four for the ultraplankton were found at the depthof 32% light intensity on 28 March and 24 May respectively.Linear relationships were found between chlorophyll a and ATPfor the different size fractions with regression slopes rangingfrom 4.3 to 5.8. The total primary production for the periodof investigation was calculated to 74 g C m^–2. Light regimeand water column stability were decisive factors for the outburstof the first diatom bloom in late March. Grazing on net planktondiatoms increased during late March-early April. Changes inthe longshore wind-stress component were found to be essentialfor the understanding of the bloom dynamics.     

Buoyancy properties of the giant diatom Ethmodiscus

Net-collected Ethmodiscus spp. were examined and found to becapable of positive buoyancy with ascent rates averaging 1.4m h^–1 on individually measured cells, and 0.3 m h^–1using a homogeneous settling technique. These data, togetherwith published reports of positive buoyancy in Rhizosolenia,indicate that upward movement is a widespread phenomenon inlarge oceanic diatoms. These observations also suggest thatpublished size-rate relationships for marine diatoms requirerevision to include the effects of positive buoyancy.     

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     

Spatial distribution of transparent exopolymer particles in the Bransfield Strait, Antarctica

Transparent exopolymer particles (TEP) are recognized to playan important role in the flux of exported carbon to the deepocean. However, there is little information on how TEP standingstocks are affected by different hydrographic conditions andother relevant ecological factors in situ. This lack of knowledgeis particularly serious for the Southern Ocean. During Australsummer 1999, the Strait of Bransfield presented high mesoscalevariability. Two fronts were present, the Bransfield hydrographicfront and a slope front along the South Shetland Islands andseveral mesoscale anticyclonic eddies and/or frontal meanders.The spatial distributions of biological properties were largelyaffected by this complex hydrography. Chlorophyll a (Chl a)(0.05–4.81 µg L^–1), TEP (from undetectableto 346 µg GXeq L^–1) and heterotrophic bacteria (HB)(1.7–9.4 x 10⁵ cells mL^–1) were positively correlateddespite the wide hydrographic heterogeneity of the BransfieldStrait. Higher abundances of autotrophic biomass, and correspondlyhigher TEP and heterotrophic bacteria (HB), were found in themore stratified waters. TEP spatial distribution was mostlyrelated to the abundance of autotrophic biomass although localhigh TEP concentrations were not matched by similarly high valuesof Chl a in some areas where diatoms were relatively abundant.     

Predation and respiration by the small cyclopoid copepod Oithona similisr: How important is feeding on ciliates and heterotrophic flagellates?

Feeding on natural plankton populations and respiration of thesmall cyclopoid copepod Oithona similis were measured duringthe warm season in Buzzards Bay, Massachusetts, USA. AlthoughO.similis did not significantly ingest small autotrophic andheterotrophic flagellates (2–8 µn), this copepodactively fed on >10 µm particles, including autotrophic/heterotrophic(dino)flagel-lates and ciliates, with clearance rates of 0.03–0.38ml animal^–1 h^–1. The clearance rates increased withthe prey size. O.similis also fed on copepod nauplii (mainlycomposed of the N1 stage of Acartia tonsa with a clearance rateof 0.16 ml animal^–1 h^–1. Daily carbon ration fromthe combination of these food items averaged 148 ng C animal^–1day^–1 (41% of body C day^–1), with ciliates and heterotrophicdino-flagellates being the main food source ({small tilde}69%of total carbon ration). Respiration rates were 020–0.23µl O₂ animal^–1 day^–1. Assuming a respiratoryquotient of 0.8 and digestion efficiency of 0.7, the carbonrequirement for respiration was calculated to be 125–143ng C animal^–1 day^–1, close to the daily carbon rationestimated above. We conclude that predation on ciliates andheterotrophic dinoflagellates was important for O.similis tosustain its population in our study area during the warm season.