Mixotrophic nanoplankton in oligotrophic surface waters of the Sargasso Sea may employ phagotrophy to obtain major nutrients

The vertical distribution and abundance of mixotrophic nanoplanktonwas examined during two cruises to the Sargasso Sea south ofBermuda. Fluorescently labeled bacteria and cyanobacteria wereused as tracers of ingestion in experiments designed to determineabundances of mixotrophic nanoplankton. Phagotrophic nanoplanktonic(2–20 µm) algae ranged from undetectable to >100ml^–1, and were more abundant near the surface (up to 140ml^–1) than in the deeper euphotic zone. On two occasions,50% of the phototrophic nanoplankton in surface waters wereobserved with ingested fluorescent tracers. The contributionof mixotrophic algae to the total phototrophic nanoplanktonassemblage in the deep chlorophyll maximum, however, did notexceed 0.5%. It is possible that mixotrophic algae were moreabundant in the deep chlorophyll maximum, but were not phagotrophicallyactive. Two 4 day experimental incubations were subsequentlycarried out to examine the adaptive significance of phagotrophicbehavior for algae in surface waters of the Sargasso Sea. Greatermixotrophic nanoplankton abundances were observed in treatmentsthat received no nutrient inputs and were limited by the availabilityof inorganic nutrients during the experiments. A decrease inthe abundance of mixotrophic algae or a decrease in their phagotrophicactivity occurred with nutrient enrichment. Based on the experimentalresults, we suggest that phagotrophy was a mechanism by whichthese algae supplemented nutrient acquisition during periodsof low dissolved nutrient concentrations. Higher abundancesof mixotrophic nanoplankton observed in the upper 50 m of theSargasso Sea may have been due to the generally low nutrientconcentrations in these waters.     

Organisms associated with stimulated epipelagic bioluminescence in the Sargasso Sea and the Gulf Stream

On three cruises, vertical profiles of stimulated bioluminescencewere measured during the late evening in the upper 200 m ofthe Sargasso Sea using a submarine photometer. On one cruise,organisms were collected in a 25 µm porosity net afterpassing through the photometer where the intensity and lightcontent of their bioluminescence were recorded. Correlationsof bioluminescence and organisms suggested that the majorityof the stimulated bioluminescence produced in the Sargasso Seawas from zooplankton: crustaceans (ostracods, copepods, copepodlarvae, euphausid larvae), larvaceans and colonial radiolarians.In addition, the photosynthetic dinoflagellate Pyrocystis noctilucaappeared to produce 5–30% of the measured bioluminescenceat some stations. Other dinoflagellates, although numerous,were dim and thus produced less than a few percent of the stimulatedbioluminescent light. The subsurface peaks in the Gulf Streamand northern Sargasso Sea were due primarily to ostracods andlarvaceans. In the Anegada Passage in October, and in the northernSargasso Sea and the Gulf Stream in August, there were pronouncedsubsurface peaks in bioluminescence associated with the thermocline.In Anegada Passage and the Sargasso Sea just north of PuertoRico in October, and in the Gulf Stream in August, the subsurfacebioluminescence peak was in or slightly above the chlorophyllmaximum. However, at the Sargasso Sea stations in August, itwas 10–40 m above the depth of the chlorophyll maximum.     

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.     

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.     

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

An investigation of the diurnal variation in productivity andcontribution to production of populations of autotrophic picoplankton(0.2–2.0 µm), nanoplankton (>2 <20 µm)and microplankton (>20 µm) was carried out at monthlyintervals, from May to October 1989, in Llyn Padarn a mesotrophicupland lake in North Wales. Maximum rates and contributionsto production of the lake by autotrophic picoplankton occurredduring mid-late summer, with the highest average daily contributionfrom picoplankton (64%) recorded in September at 4 m depth.Diurnal variation in contributions from picoplankton was pronounced,with greatest input, recorded at the end of the day, duringthe period of picoplankton dominance in mid-late summer. Maximumcontribution from picoplantkon (86% of total, 9.2 mg C m^–3h^–1) was recorded in September. Nanoplankton primary productionwas of greatest significance in June and July, although levelswere lower than for picoplankton in subsequent months. Contributionsvia nanoplankton increased with depth in the lake at this time,reaching a maximum of 78% of the total at the end of the dayat 9 m depth in early July. At this time, diurnal variationin contributions via nanoplankton was considerable, with maximumphotosynthesis generally at the end of the photoperiod at depthsof 4 and 9 m. Microplankton made the greatest impact on primaryproduction during the mixed water conditions of spring and autumn,and at these times did variation in production was less thanthose of both pico and nanoplankton during summer thermal stratification.Photosynthetic capacity was lower for picoplankton than fornanoplankton and microplankton; the highest values were 5, 33and 51 mg C (mg chl a)^–1) h^–1) for pico-, nano-and microplankton, respectively. The photosynthetic efficiencyof all three size categories of phytoplankton increased withdepth. Maximum values were similar for all phytoplankton groups,between 75 and 131 mg C (mg chl a)^–1) E^–1) m² butmean levels of photosynthetic efficiency for the 6 months werelower for picoplankton than for nano- or microplankton. Ratesof carbon fixation per cell for picoplankton spanned three ordersof magnitude, varied considerably diurnally and reached maximumvalues of 484 fg C(cell)^–1) h^–1) in the afternoonin near-surface waters in the early stages of exponential populationgrowth in July. During the population maximum of picoplanktonin August and September, maximum daily values of carbon fixationper cell, assimilation number and photosynthetic efficiencywere all recorded at the end of the day. The seasonal and diurnalpatterns of production of the three size categories of planktonicalgae in Llyn Padarn were distinct. During spring, microplankton(mainly diatoms) were the dominant primary producers. As thermalstratification developed, nanoplankton were the major contributorsto phytoplanktonic production, particularly in the deeper regionsof the euphotic zone. Picoplankton made the greatest contributionto production in August and September, exhibiting maximum inputtowards the end of the light cycle. Diatoms became the majorphotosynthetic plankton in the mixed water conditions prevalentin Uyn Padarn in October.     

Bioerosion along a bathymetric gradient in a cold-temperate setting (Kosterfjord, SW Sweden): an experimental study

In the cold-temperate setting of the Swedish Kosterfjord, a 2-year experiment was launched in order to assess bioerosion rates and to investigate the endolithic borer communities in relation to light availability (relative bathymetry), hydrography and exposure time. The inventory of microendolithic traces, studied by SEM analysis of epoxy resin casts of planted bivalve shells, yields diverse ichnocoenoses comprising a total of 21 traces produced by boring cyanobacteria (7), chlorophytes (4), fungi (6) and traces of uncertain affinity (4). The link between the endoliths (biotaxa) and the traces they leave (ichnotaxa) is evaluated by the study of the boring organisms in situ by transmission light microscopy of planted Iceland spar and bivalve shells. Additionally, the activity of various macroborers (foraminiferans, polychaetes, echinoids, gastropods and sponges) is documented, adding to a distinct diversity maximum at 7 m water depth. A highly condensed photic zonation, due to the high latitude (59°) and eutrophic conditions, is recorded by the measurement of the Photosynthetically Active Radiation (PAR) and is confirmed by the bathymetric range of the photic related ichnocoenoses. At 1 m water depth, a mature shallow euphotic ichnocoenosis dominated by cyanobacteria and at 7 m, a deep euphotic ichnocoenosis dominated by chlorophytes, respectively, is developed after as little as 12 months exposure. With the vanishing light availability from 15 m downwards, the ichnocoenoses development is significantly slowed and only immature dysphotic and aphotic borer communities (dominated by fungi) are encountered. Strong fluctuations of salinity (down to 8%) and temperature (0–20°C) in the euphotic zone indicate most phototrophs present to be considerably euryhaline and eurytherm, while most endolithic fungi appear preferentially in the deeper, more stable marine waters.     

Vertical migration of Rhizosolenia mats and their significance to NO3- fluxes in the central North Pacific gyre

Rhizosolenia mat abundance, distribution and chemical compositionwere studied on two cruises in the central North Pacific gyrein order to determine large-scale distribution patterns andcontribution to upward nitrogen (N) flux. These macroscopicdiatom mats are composed of multiple species of Rhizosoleniathat exploit subsurface nitrate pools by vertically migratingbelow the euphotic zone. Although numerically dominated by thesmall-diameter species, R.fallax (73–95% of total numbers),mat biovolume was dominated by large-diameter (>50 µmdiameter) Rhizosolenia spp. (85–99% of total volume).Integrated mat abundance was substantially higher when matsaccumulated at the surface during calm weather (     

Size-fractionated Carboxylase Activities During a 32 h Cycle at 30 m Depth in the North-western Mediterranean Sea After an Episodic Wind Event

Using size-fractionation filtration (1 µm), we associatedcarboxylase activities (Rubisco, ß-carboxylases) andchlorophyll measurements with cell enumeration by flow cytometryat a permanent site of the central Ligurian Sea in the north-westernMediterranean Sea (73°25'N–7°51' E). The analyseswere carried out over a day/night cycle (at 30 m depth) followinga strong wind event, during the transition period from springmesotrophic to summer oligotrophic conditions. The highest valuesof Rubisco activity and ß-carboxylase activity perchlorophyll a (Chl a) for >1 µm cells were observedduring the light period of the cycle, reaching 18.9 and 4.3nmol CO₂ (µg Chl a)^–1 h^–1, respectively. Thishigher activity is assumed to be correlated with a dominanceof nanoflagellates in the phytoplankton community. Such phytoplanktonspecies generally had higher ß-carboxylase activity,expressed as a percentage of Rubisco activity (the ßC/Rratio), than diatoms. Using flow cytometry analysis to enumeratethose cells <1 µm in size, we followed the values ofRubisco activity and pigment content expressed per cell, forpicophytoplankton cells. The photoautotrophic activity, measuredas the in vitro Rubisco activity for small picoeukaryote cells,was higher than for cyanobacteria cells with lower apparentcell size. These results suggested an optimum of CO₂ assimilationreached by the pico- and nano-phytoplankton in accordance withthe cell size and growth rates from previous observations inthe literature.     

Chroococcoid cyanobacteria Synechococcus spp. in the Black Sea: pigments, size, distribution, growth and diurnal variability

Phycoerythrin-containing unicellular cyanobacteria Synechococcusspp. were studied for the first time during April–May,1994 and September–October, 1996, in the western and southernBlack Sea for pigments, size and abundance distribution viaspectrometry, epifluorescence microscopy and flow cytometry.Abundance distribution in the surface mixed layer in April–May,1994, revealed that cells were more concentrated in offshorewaters than in coastal regions under the direct influence ofthe river Danube. However, in the south, higher surface cellconcentrations were characteristic of the nearshore areas duringSeptember–October, 1996. A highly significant correlationwas observed between cell abundance and ambient physico-chemicalparameters with depth. Visual inspection of the individual cellsunder the epifluorescence microscope revealed that cells atthe subsurface, chlorophyll a maximum layer (SCML, based onin situ fluorometer readings) fluoresce more brightly and forlonger than those at the surface and at lower depths. Spectralproperties of a total of 64 Synechococcus spp. clonal isolatesfrom different depths within the euphotic layer (about the top60 m) in the southern Black Sea coast showed that all have type2 phycoerythrobilin in common, lacking phycourobilin. In vivofluorescence emission maxima for phycoerythrobilin were aboutthe same (~578 nm) for all isolates. All isolates had in vivoabsorption maxima at between 435 and 442 nm, and at about 681nm due to chlorophyll a. It was shown from the flow cytometermean forward light scatter data for size distribution that cellsat the surface mixed layer (0–10 m) were larger than cellsat lower depths (20–60 m). Based on in vivo fluorescencemeasurements, significant differences in the acclimated growthrates of clones from different depths were observed. Time versuscell count plots showed that cells of the cyanobacteria Synechococcusspp. are under grazing pressure, from midnight until noon, andslowly begin to rebuild their population in the afternoon bydividing throughout the evening.     

The composition of particulate organic matter and biomass in the Peruvian upwelling region during ICANE 1977 (Nov. 14 - Dec. 2)

This paper presents the results of plankton studies in the Peruvianupwelling region off Chimbote from November - December, 1977,during the Investigación Cooperativa de la Anchovetay su Ecosistema (ICANE). Primary productivity, respiratory ETSactivity, composition of particulate organic matter, and microplanktoncell numbers were determined. Phytoplankton growth, and bacterialand ciliate carbon-uptake rates were computed from cell counts. Inshore waters were dominated by diatoms and were more productivethan offshore waters which were dominated by dinoflagellatesand ciliates. Particulate primary production averaged 5.26 ±5.24 g C m^–2d^–1, and the POC standing stock was7.75 ± 2.74 g C m^–2 for the euphotic layer of 7shelf stations. On the shelf, microplankton respiration rateswere higher in plankton populations dominated by dinoflagellatesand ciliates (47% of particulate primary production per 24 h)than those in diatom dominated populations (11%, respectively).The diatom populations, which were dominated by Chaetocerosspecies, varied in their ecophysiological properties (assimilationnumbers, proportion of water soluble carbohydrate, and protein/nitrogenratios). The relationships between these variations and growthconditions were investigated. A 40 h time-series station revealedpatchiness which was superimposed on physiological changes ofthe plankters. Bacterial numbers of 2 x 10⁶ cells/ml were foundin the euphotic layer corresponding to approximately 17 µgC/l bacterial biomass (or 6% of the POC standing stock). Ciliatebiomass (Lohmanniella oviformis was the dominating species)ranging from 2 to 9% of the POC standing stock were found evenin diatom dominated populations. From a rough carbon balancefor the euphotic layer it was deduced that in diatom dominatedpopulations, 36–77% of particulate primary productionwas potentially available to adult anchoveta grazing.     

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

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

Phytoplankton and zooplankton seasonal dynamics in a subtropical estuary: importance of cyanobacteria

A seasonal study of phytoplankton and zooplankton was conductedfrom 1999 to 2001 in Pensacola Bay, Florida, USA, to furtherthe understanding of pelagic food webs in sub-tropical estuaries.Monthly measurements included size-fractionated chlorophyll(whole water, <5 µm, <20 µm), net- and picophytoplanktoncomposition analyzed using microscopy, flow cytometry, and HPLCpigment analysis. Additionally, zooplankton abundance and dryweight were determined from net tows. The results show a phytoplanktoncommunity dominated by the small size fraction (<5 µm),especially during the warm periods. The <5 µm chlorophyllfraction was strongly correlated with cyanobacterial abundanceand zeaxanthin. Cyanobacteria (cf. Synechococcus) abundancepeaked during summer in the upper estuary, typically exceeding3 x 10⁹ L^-1, and was strongly correlated with temperature. Cyanobacteriaabundance at the freshwater end of the Bay (in the EscambiaRiver) was very low, suggesting that cyanobacteria were notdelivered via freshwater. Two pigmentation types of cyanobacteriawere observed. Phycoerythrin-containing cells (PE-rich) weremore abundant at the marine end, while phycocyanin-containingcells (PC-rich) were more abundant in the upper estuary. Thelarger algae (>5–10 µm) were predominantly composedof diatoms, followed by chlorophytes, cryptophytes and dinoflagellates.The three most abundant genera of diatoms were Thalassiosira,Pennales and Cyclotella. Zooplankton biomass averaged 12.2 µgC L^-1, with peak biomass occurring during May (     

Vertical distribution of protozoan and microcopepod communities in the South Adriatic Pit

The qualitative composition, numerical abundance and verticaldistribution of radiolarians, tintinnines, nauplii and smallcopepod assemblages were studied in the central area of theSouthern Adriatic Pit at three stations during 10 cruises fromOctober 1985 to May 1990. The samples were collected with aplankton net of 53 µm mesh size equipped with a closingsystem in eight vertical layers. Data are presented for 53 radiolarians,61tintinnines and for the first time for 22 poecilostomatoidspecies in the Adriatic Sea. According to the numerical abundanceof assemblages and the frequency of occurrence of the bulk ofthe individual species population in the water column, fourcommunities could be defined: the surface (0–50 m), thesubsurface (50–100m), the midwater (100–600 m) andthe deep-sea community (below 600 m). The euphotic layer wascharacterized by tintinnines, copepod nauplii, cyclopoids andjuvenile calanoid copepods, while radiolarians and poecilostomatoidcopepods dominated in the deeper layers. The highest numericaldifferences between stations and seasons were noted only onthe surface. Towards the deeper layers, the differences wereconstantly smaller, and below400 m there was a uniform distributionof all assemblages. This research, on the basis of the numericalabundance of protozoans and micrometazoans, shows that the centralpart of the South Adriatic Pit is considerably richer than mentionedby earlier authors.     

Annual pattern of micro- and mesozooplankton abundance and biomass in a subtropical estuary

The pattern of biomass and abundance of microzooplankton andmesozooplankton were studied over an annual cycle in the NuecesEstuary, Texas. Zooplankton samples and associated hydrographicdata were collected at four locations at biweekly intervalsfrom September 1987 through October 1988. This is a broad, shallowbay system with an average depth of 2.4 m. The concentrationof chlorophyll a in the surface waters averaged 7.4 µgl^–1with 85% passing through a 20 µ mesh. Microzooplankton(20–200 µ in length) were extremely abundant throughoutthis study. Abundances of ciliates (including both aloricateciliates and tintinnids) ranged from 5000 to 400 000 l^–,with a mean of 38 000 l^–1 of seawater over the entirecourse of the study. Mesozooplankton (200–2000 µmin length) abundance averaged 6100 m^–3 for samples collectedduring the day and 10 100 m^–3 for samples collected atnight. Mesozooplankton were dominated by Acartia tonsa whichmade up {small tilde}50% of the total. Biomass estimates formicrozooplankton (based on volume estimates) were often higherthan measured biomass of mesozooplankton. Given the shortergeneration times and higher metabolic rate of microzooplanktoncompared to mesozooplankton, microzooplankton should have agreater effect on the trophic dynamics of the Nueces Estuarythan mesozooplankton.     

Phytoplankton size fractions in a tropical neritic ecosystem near Kingston, Jamaica

Temporal patterns in phytoplankton size fractions were studiedfor >1 year on the shelf south of Kingston, Jamaica. On average,in the East Channel, total chlorophyll was partitioned as 42%netplankton (>20 µm), 30% nanoplankton (2–20µm) and 28% picoplankton (0.2–2 µm); correspondingaverages for size-fractionated primary production were 27, 30and 43%. Netplankton was more variable than the other size fractions,and it predominated at times of high total chlorophyll concentration.When total chlorophyll was low, picoplankton predominated. Regressionmodels using wind and rainfall parameters could explain up to46% of the variation in chlorophyll concentrations. In theseregression models, the variation accounted for was always greatestfor the net- and least for the picoplankton. A suggested mechanismfor meteorological-phytoplankton coupling is that during periodsof easterly winds (90–150°), offshore water (in whichpicoplankton dominates) is advected onto the shelf, either displacingor diluting the nearshore water. At times of weaker westerlywinds (230–290°) there is a persistence of the phytoplanktoncommunity in nutrient-rich shelf waters and net- and nanoplanktonbiomass increases.     

Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

The contribution of nanoplankton (< 10 µm fraction)to winter – spring (1977 – 78) and summer (1978,1979) phytoplankton nitrogen dynamics in lower NarragansettBay was estimated from ammonium, nitrate and urea uptake ratesmeasured by ¹⁵N tracer methods. During the winter – spring,an average of 80% of chlorophyll a and nitrogen uptake was associatedwith phytoplankton retained by a 10 µm screen. In contrast,means of 51 – 58% of the summer chlorophyll a standingcrops and 64 – 70% of nitrogen uptake were associatedwith cells passing a 10 µm screen. Specific uptake ratesof winter – spring nanoplankton populations were consistentlylower than those of the total population. Specific uptake ratesof fractionated and unfractionated summer populations were notsignificantly different. Ammonium uptake averaged between 50and 67% of the total nitrogen uptake for both the total populationand the < 10µm fraction. The total population and the10 µm fraction displayed similar preferences for individualnitrogen species. Though composed of smaller cells, flagellatedominated nanoplankton assemblages may not necessarily takeup nitrogen at faster rates than diatom dominated assemblagesof larger phytoplankters in natural populations. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia     

Seasonal patterns of phytoplankton productivity and abundance in nearshore oligotrophic waters of the Levant Basin (Mediterranean)

A biweekly sampling program from two stations at the easternLevant Basin was carried out during a 1-year period (1983).The first station (neritic) was located 2 km offshore over theIsraeli continental shelf, while the second (pelagic) was located10 km offshore slightly beyond the continental shelf. It wasfound that during summer the relatively close pelagic watershad chlorophyll a concentrations comparable with the most oligotrophicdeep sea regions of the world's oceans. During winter and spring,profound fluctuations were observed in both phytoplankton standingcrop and primary productivity at the neritic station. This wasin response to weather phenomena, such as heavy rains or storms,which did not affect the pelagic Station to such an extent.The picoplanklon size fraction (<3 µm) dominated atthe neritic station during summer and fall, while the nanoplanktonfraction (3–20 µm) dominated during spring. At thepelagic station the picoplankton fraction dominated almost allyear round, but it is suspected that some portion of it wasphotosynthetically inactive.     

Nitrogen budget in the euphotic zone of Lake Biwa from spring to summer, 1986

The production rate of participate nitrogen (PN) in Lake Biwafrom March to June 1986 was calculated by monthly measurementsof the proteinaceous nitrogen production rates. The ‘new’production rate was estimated from the decrease in nitrate inthe euphotic zone; during this period (91 days) it was estimatedas 93 mg atoms m^–2, and accounted for 20% of the ‘total’PN production (460 mg atoms m^–2). This implies that {smalltilde}80% of the PN produced might be recycled in the euphoticzone. The increase in PN in the euphotic zone during this period(27 mg atoms m^–2) accounted for 5.9% of the ‘total’PN production. This indicates that the remaining 14% of PN producedin the euphotic zone was vertically transported. Sediment trapexperiments at 30 m depth indicate that 8.1% of the PN producedin the euphotic zone was measured as downward flux at 30 m depthduring the stagnation period. Decompositional loss of PN between12.5 (bottom of the euphotic zone) and 30 m depth was estimatedas 17 mg atoms m^–2. This was calculated on the basis ofaccumulation of ammonium, which accounted for 3.7% of the ‘total’PN production. The flux from the euphotic zone thus accountedfor 12% of the ‘total’ PN production, suggestingthe validity of the production model.     

Vertical distribution of picoeukaryotic diversity in the Sargasso Sea

Eukaryotic molecular diversity within the picoplanktonic size-fraction has primarily been studied in marine surface waters. Here, the vertical distribution of picoeukaryotic diversity was investigated in the Sargasso Sea from euphotic to abyssal waters, using size-fractionated samples (< 2 microm). 18S rRNA gene clone libraries were used to generate sequences from euphotic zone samples (deep chlorophyll maximum to the surface); the permanent thermocline (500 m); and the pelagic deep-sea (3000 m). Euphotic zone and deep-sea data contrasted strongly, the former displaying greater diversity at the first-rank taxon level, based on 232 nearly full-length sequences. Deep-sea sequences belonged almost exclusively to the Alveolata and Radiolaria, while surface samples also contained known and putative photosynthetic groups, such as unique Chlorarachniophyta and Chrysophyceae sequences. Phylogenetic analyses placed most Alveolata and Stramenopile sequences within previously reported 'environmental' clades, i.e. clades within the Novel Alveolate groups I and II (NAI and NAII), or the novel Marine Stramenopiles (MAST). However, some deep-sea NAII formed distinct, bootstrap supported clades. Stramenopiles were recovered from the euphotic zone only, although many MAST are reportedly heterotrophic, making the observed distribution a point for further investigation. An unexpectedly high proportion of radiolarian sequences were recovered. From these, five environmental radiolarian clades, RAD-I to RAD-V, were identified. RAD-IV and RAD-V were composed of Taxopodida-like sequences, with the former solely containing Sargasso Sea sequences, although from all depth zones sampled. Our findings highlight the vast diversity of these protists, most of which remain uncultured and of unknown ecological function.     

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.