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.     

High Organic Carbon Export Precludes Eutrophication Responses in Experimental Rocky Shore Communities

We studied the effect of nutrient inputs on the carbon (C) budget of rocky shore communities using a set of eight large experimental mesocosms. The mesocosms received a range of inorganic nitrogen (N) and phosphorus (P) additions, at an N:P ratio of 16. These additions were designed to elevate the background concentration, relative to that in eutrophic Oslofjord (Norway) waters, by 1, 2, 4, 8, 16, 32 μmol dissolved inorganic nitrogen (DIN)l⁻¹ (and the corresponding P increase). Two unamended mesocosms were used as controls. The nutrients were added continuously for 27 months before gross primary production (GPP), respiration (R), net community production (NCP), and dissolved organic carbon (DOC) production were assessed for the dominant algal species (Fucus serratus) and for the whole experimental ecosystem. Inputs and outputs of DOC and particulate organic carbon (POC) from the mesocosms were also quantified. The F. serratus communities were generally autotrophic (average P/R ratio = 1.33 ± 0.12), with the GPP independent of the nutrient inputs to the mesocosms, and maintained a high net DOC production during both day (0.026 ± 0.008 g C m⁻² h⁻¹) and night (0.015 ± 0.004 g C m⁻² h⁻¹). All the experimental rocky shore ecosystems were autotrophic (P/R ratio = 2.04 ± 0.28), and neither macroalgal biomass nor production varied significantly with increasing nutrient inputs. Most of the excess production from these autotrophic ecosystems was exported from the systems as DOC, which accounted for 69% and 58% of the NCP of the dominant community and the experimental ecosystem, respectively, the rest being lost as POC. High DOC release and subsequent export from the highly energetic environments occupied by rocky shore communities may prevent the development of eutrophication symptoms and render these communities resistant to eutrophication. Received 10 October 2001; accepted 18 July 2002.     

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.     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Contrasting microplanktonic composition and food web structure in two coastal embayments (Long Island, NY, USA)

The neighboring Great South Bay (GSB) and Peconic Bay (PB) ofLong Island, NY, USA, were observed to support distinctive microplanktoniccommunities and trophic structure over most of an annual cycle(1998–99). Trophic structure analyses were based on 15months of sampling for inorganic and organic nutrients, size-fractionatedchlorophyll a (Chl a) and nanoplankton and microplankton abundances.While dissolved inorganic nitrogen (DIN) inventories were notdemonstrably different between bays, dissolved organic carbon(DOC) and nitrogen were significantly higher in GSB than inPB and covaried with Chl a concentrations. Likewise, total biomasses(µg C L^–1) and mean seasonal biomass ratios of heterotrophicnanoplankton (HNAN) to autotrophic nanoplankton (ANAN) weresubstantially higher in GSB (>0.30) than in PB (<0.15)from spring to autumn 1998. The higher nanoflagellate biomassin GSB appears to have been indirectly supported by elevatedconcentrations of dissolved organic matter (DOM). During winterand spring 1999, biomass ratios in GSB dropped to levels similarto those in PB and coincided with a clear water event in GSBthat may have been caused by increased bivalve suspension feeding.Even though these bays share similar broad-scale oceanographic/hydrogeologicsettings and a common assortment of planktonic taxa, the structureand function of their planktonic communities were fundamentallydistinct.     

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.     

The annual patterns of photosynthesis in two large, freshwater, ultra-oligotrophic Antarctic lakes

The phytoplankton productivity and biomass of two large, freshwater Antarctic lakes (Vestfold Hills, eastern Antarctica) were investigated over a 12-month period. Crooked Lake was sampled at one site, while Lake Druzhby, a complex lake with two shallow and one deep basin, was subject to a more detailed investigation. Concentrations of chlorophyll a were usually below 1 µg l^-1, indicating ultra-oligotrophic conditions. Despite periodic low nutrient levels, low temperatures (range 0.4-2.8°C) and periodic poor light climate, some degree of photosynthesis was measurable throughout the year, including the dark winter phase. Snow cover had a pronounced impact on the light climate of the water column and inhibited photosynthesis. Mean rates of carbon fixation in the 0- to 15-m water column varied between 0 and 38.47 µg C l^-1 day^-1 in Crooked Lake and 0.24 and 37.68 µg C l^-1 day^-1 in Lake Druzhby. There were significant differences in the seasonal patterns of primary production between the basins of Lake Druzhby. The shallow basins had highest productivity in August, whereas the deep basin had highest rates in summer. Chlorophyll specific rates of photosynthesis or assimilation numbers [µg C (chl. a)^-1 h^-1] varied between 0.05 and 44.9, and photosynthetic efficiency [µg C (chl. a)^-1 h^-1 µmol m^-2 s^-1] between 0.02 and 5.19. The data suggest that the phytoplankton of these lakes is adapted to low irradiance levels, low temperatures and nutrient limitation.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Size structures of microplankton biomass and production in Jiaozhou Bay, China

Seasonal investigations of size-fractionated biomass and productionwere carried out from February 1992 to May 1993 in JiaozhouBay, China. Microplankton assemblages were separated into threefractions: pico- (0.7–2 µm), nano- (2–20 µm)and netplankton (20–200 µm). The biomass was measuredas chlorophyll a (Chi a), paniculate organic carbon (POC) andparticipate organic nitrogen (PON). The production was determinedby ¹⁴C and ¹⁵N tracer techniques. The seasonal patterns in biomass,though variable, were characterized by higher values in springand lower values in autumn and summer (for Chi a only). Theseasonal patterns in production, on the other hand, were moreclear with higher values occurring in summer and spring, andlower values occurring in autumn and winter. Averaged over thewhole study period, the respective proportions of total biomassaccounted for by net-, nano- and picoplankton were 26, 45 and29% for Chi a, 32, 33 and 35% for POC, and 26, 32 and 42% forPON. The contributions to total primary production by net-,nano- and picoplankton were 31, 35 and 34%, respectively. Therespective proportions of total NH₄⁺–N uptake accountedfor by net-, nano- and picoplankton were 28, 33 and 39% in thedaytime, and 10, 29 and 61% at night. The respective contributionsto total NO₃^–-N uptake by net-, nano- and picoplanktonwere 37, 40 and 23% in the daytime, and 13, 23 and 64% at night.Some comprehensive ratios, including C/N biomass ratio, Chla/C ratio, C uptake/Chl a ratio, C:N uptake ratio and the f-ratio,were also calculated size separately, and their biological andecological meanings are discussed.     

Seasonal variations of bacterial abundance and biomass and their relation to phytoplankton in the hypertrophic tropical lagoon Cienaga Grande de Santa Marta, Colombia

The seasonal development of bacteria was studied in the hypertrophiccoastal lagoon Ciénaga Grande de Santa Marta (Caribbeancoast of Colombia). This large but only 1.5 m deep lagoon issubject to strong seasonal variations of salinity from almostfully marine (April/May) to brackish conditions in October/November.Chlorophyll ranged from 6 to 182 µg L^–1, and grossprimary production amounted to 1690 g C m^–2 per year.Total bacterial number (TBN) ranged from 6.5 to 90.5 x 10⁹ cellsL^–1 and bacterial biomass (BBM) from 77 to 1542 µgC L^–1, which are among the highest ever reported for naturalcoastal waters. Neither TBN nor BBM varied significantly withsalinity, phytoplankton or seston concentrations. Only the bacterialmean cell volume showed a significant relation to salinity,being highest (0.066 µm³) during the period of increasingand lowest (0.032 µm³) during decreasing salinity. Bacterialprotein accounted for 24% (19–26%) and phytoplankton proteinfor 57% (53–71%) of total seston protein. The ratio (annualmean) of bacterial carbon to phytoplankton carbon was 0.44 (range0.04–1.43). At low phytoplankton abundance [chlorophylla (Chl a) < 25 µg L^–1], bacterial carbon wasalmost equal to phytoplankton biomass (i.e. the mean ratio was1.04). In contrast, at Chl a > 100 µg L^–1, BBMwas low compared to phytoplankton biomass (the mean ratio was0.16). In general, BBM varied less than phytoplankton biomass.Most probably, the missing correlation between bacterial andphytoplankton variables was due to (i) organic material partlyderived from allochthonous sources serving as food resourcefor bacteria and (ii) a strong resuspension of bacteria fromthe sediment caused by frequent wind-induced mixing of the veryshallow lagoon.     

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.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

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.     

Photosynthetic organic carbon production and respiratory organic carbon consumption in the trophogenic layer of Lake Biwa

Measurement of the photosynthetic production rate in Lake Biwawas camed out from May 1985 to September 1987. In the light-saturatedlayer, the seasonal variation in the photosynthesis rate perchlorophyll a was regulated by water temperature. The depth-integratedphotosynthetic production rate was 0.21-1.48 g C m^–2 day^–1and the maximum value was observed in midsummer when the watertemperature of the mixed surface layer was highesL The criticalnutrient for photosynthesis may be dissolved reactive phosphorus,which was generally <1 µg P 1^–1 throughout theobservation period. In the trophogenic layer, respiratory organiccarbon consumption, estimated from measurement of respiratoiyelectron transport system activity, was 0.35-1.07 (mean 0.66)g C m^–1 day^–1 and corresponded, on average, to 79%of the photosynthetic carbon production rate. This implies thatthe major part of photosynthetic fixed organic matter mightbe recycled in the trophogenic layer. The estimated settlingorganic carbon flux at 20 m depth, from calculation of theseparameters and changes in the particulate organic carbon concentration,was 0.01 (-0.09 to 0.13) g C m^–1 day^–1 The meansettling organic carbon flux measured by sediment trap at 20m was 0.19 (0.09-0.31) g C m^–1 day^–1 higher thanthe estimated value. It seemed that organic matter collectedby sediment trap may contain allochthonous matter and resuspendedepilimnetic sediment matter.     

Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary productionstudies conducted in the vicinity of the Subtropical Front (STF),an adjacent warm-core eddy, and in Sub-antarctic waters duringthe third South African Antarctic Marine Ecosystem Study (SAAMESIII) in austral winter (June/July) 1993. Throughout the investigation,total chlorophyll (Chl a) biomass and production were dominatedby small nano- and picophytoplankton. No distinct patterns intotal Chl a were evident. At stations (n = 7) occupied in thevicinity of the STF, total integrated biomass values rangedfrom 31 to 53 mg Chl a m^–2. In the vicinity of the eddy,integrated biomass at the eddy edge (n = 3) ranged from 24 to54 mg Chl a m^–2 and from 32 to 43 mg Chl a m^–2 inthe eddy (n = 2). At the station occupied in the Sub-antarcticwaters, total integrated biomass was 43 mg Chl a m^–2.Total daily integrated production was highest at stations occupiedin the vicinity of the STF and at the eddy edge. Here, totalintegrated production ranged from 150 to 423 mg C m^–2day^–1 and from 244 to 326mg C m^–2 day^–1, respectively.In the eddy centre, total integrated production varied between134 and 156 mg C m^–2 day^–1. At the station occupiedin the Sub-antarctic waters, the lowest integrated production(141 mg C m^–2 day^–1) during the entire survey wasrecorded. Availability of macronutrients did not appear to limittotal production. However, the low silicate concentrations duringthe survey may account for the predominance of small nano- andpicophytoplankton. Differences in production rates between theeddy edge and eddy core were related to water column stability.In contrast, at stations occupied in the vicinity of the STF,the control of phytoplankton production appears to be relatedto several processes, including water column stability and,possibly, iron availability.     

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.     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

Autotrophic picoplankton in southern Lake Baikal: abundance, growth and grazing mortality during summer

Autotrophic picoplankton were highly abundant during the thermalstratification period in late July in the pelagic area (waterdepth 500–1300 m) of southern Lake Baikal; maximum numberswere 2 x 10⁶ cells ml^–1 in the euphotic zone ({small tilde}15m). Unicellular cyanobacteria generally dominated the picoplanktoncommunity, although unidentified picoplankton that fluorescedred under blue excitation were also abundant (maximum numbers4 x 10⁵ cells ml^–1) and contributed up to {small tilde}40%of the total autotrophic picoplankton on occasions. Carbon andnitrogen biomasses of autotrophic picoplankton estimated byconversion from biovolumes were 14–84 µg C l^–1and 3.6–21 µg N l^–1. These were comparableto or exceeded the biomass of heterotrophic bacteria. Autotropicpicoplankton and bacteria accounted for as much as 33% of paniculateorganic carbon and 81% of nitrogen in the euphotic zone. Measurementsof the photosynthetic uptake of [^l4C]bicarbonate and the growthof picoplankton in diluted or size-fractionated waters revealedthat 80% of total primary production was due to picoplankton,and that much of this production was consumed by grazers inthe <20 µ.m cell-size category. These results suggestthat picoplankton-protozoan trophic coupling is important inthe pelagic food web and biogeochemical cycling of Lake Baikalduring summer.     

Biogeographic study of the planktonic communities of the Prince Edward Islands (Southern Ocean)

Microphytoplankton and zooplankton composition and distributionin the vicinity of the Prince Edward Islands and at the Sub-antarcticFront (SAF) were investigated in late austral summer (April/May)1996. Samples were collected for analysis of chlorophyll a concentration(Chi a), microphytoplankton and zooplankton abundance. Generally,the highest Chl a concentrations (up to 2.0 µg l^–1)and zooplankton densities (up to 192 ind. m^–3) were recordedat stations within the inter-island area while the lowest values(<0.4 µg l^–1) were observed at stations upstreamof the islands. High Chl a and zooplankton biomass values werealso associated with the SAF. Microphytoplankton were dominatedby chain-forming species of the genera Chaetoceros (mainly C.neglectus),Fragilariopsis spp. and the large diatom Dactyliosolen antarcticus.The zooplankton assemblages were always dominated by mesozooplanktonwhich at times contributed up to 98% of total zooplankton abundanceand up to 95% of total biomass. Among mesozooplankton, copepods,mainly Clausocalanus brevipes and Metridia lucens numericallydominated. Among the macrozooplankton euphausiids, mainly Euphausiavallentini, E.longirostis and Stylocheiron maximum, and chaetognaths(Sagitta gazellae) accounted for the bulk of abundance and biomass.Cluster and ordination analysis did not identify any distinctbiogeographic regions among either the microphytoplankton orzooplankton.