Copepod grazing impact on the trophic structure of the microbial assemblage of the San Pedro Channel, California

In August 2002 and March 2003 the trophic structure of the microbialassemblage from the San Pedro Channel, California was studiedfollowing the experimental alteration of the number of copepods.Changes in the abundance/biomass of microorganisms <80 µmduring 3-day incubations were monitored in (i) the absence ofmetazoa >80 µm, (ii) the presence of natural abundancesof metazoa and (iii) the presence of an elevated number of copepods.Prokaryotes and small-sized eukaryotes (<4 µm) dominatedplankton biomass during both experimental months. Diatoms numericallydominated the 10–80 µm plankton in August 2002,but ciliate and heterotrophic dinoflagellate biomass generallyexceeded diatom biomass on both dates. Ingestion of protozooplankton(predominantly ciliates) contributed substantially to copepoddaily carbon rations. The adult copepod assemblage removed 4.6and 36% per day of the microzooplankton standing stocks (10–80µm size fraction) in August and March, respectively. Elevatedcopepod grazing pressure on protozooplankton resulted in increasedbiomass of nanoplankton (<5 µm) presumably via a trophiccascade. Accordingly, the copepod–protozoan trophic linkappears to be a key factor structuring the planktonic microbialassemblage in the San Pedro Channel. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

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     

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.     

Photosynthetic production and exoenzymatic degradation of organic matter in the euphotic zone of a eutrophic lake

Photosynthetic production of organic matter, and its exoenzymaticdecomposition were studied in the euphotic zone of a naturallyeutrophic lake during early spring phytoplankton bloom, andafter its breakdown. Phytoplankton were the major biomass producerswhen algae were actively growing, and the algal fraction (>3.0µm) contributed on average 75–80% to the total biomassof microplankton. When the phytoplankton bloom began to declinebacterial biomass increased rapidly and, at the end of the bloom,bacteria contributed 48.7–69.98% to the total biomassof microplankton. The high bacterial abundance during phytoplanktonbloom breakdown followed the highest rates of glucose uptake,and the highest rates of alkaline phosphatase, leucine-amino-peptidase,ß-galactosidase and ß-glucosidase activities.The majority of enzyme activity was associated with the bacterialsize fraction of seston. The activities of free (dissolved inwater) exoenzymes were negligible. The synthesis of bacterialexoenzymes was under control of an induction/derepression mechanism,and depended on the amount of easily assimilable substrates,and/or the presence of polymeric organic compounds in the water,which served as substrates for exoenzymatic hydrolysis. Thetight metabolic coupling between bacterial exoenzymatic hydrolysisand uptake of low molecular weight substrates, and its ecologicalsignificance is discussed.     

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

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

The importance of small-sized copepods in a frontal area of the Aegean Sea

Distribution, production and grazing of the copepod communitywere investigated in the northern Aegean Sea, which is characterizedby a permanent thermohaline front. Cruises were conducted alonga transect crossing the frontal area during spring and latesummer. Biomass and production of autotrophs were measured bysize fractionation and heterotrophic nanoflagellates and ciliateswere also studied. Copepod biomass, production and grazing impacton the phytoplankton and ciliate populations were estimated.The copepod community was sampled with a 45 µm net toinclude the smallest species and their developmental stages.The size, structure and distribution of the phytoplankton implythat most carbon was fixed by picoplankton during both seasonsand throughout the study area. The partitioning of carbon amongthe different plankton compartments was not a broad-based pyramidand the biomass of heterotrophs was higher than that of autotrophs,except in the non-frontal region during spring. Copepod biomasswas substantially higher in the frontal area. Our results showedthat the small-sized copepods (calanoids and cyclopoids) dominatedin terms of biomass and production, but also had a greater influenceon the efficiency of the trophic coupling between the primaryproducers and the protozooplankton than the larger species,stressing their importance in the northern Aegean Sea and theEastern Mediterranean in general.     

Plankton composition and cycling of carbon during the rainy season in a tropical coastal ecosystem, Zanzibar, Tanzania

Biomass, species composition and production of the planktoniccommunity were investigated during the rainy season in May andJune 1999 outside Zanzibar Island, Tanzania. In general, theplankton biomass of different organisms was uniform betweendepths as well as over time. The integrated water column primaryproduction ranged from 204 to 4142 mg C m^–2 day^–1.Bacterial production varied between 10 and 72 mg C m^–2day^–1, comprising ~5% of the total bacterial standingstock. The data obtained from these experiments are summarizedin a carbon budget. At the most 77% of the total primary productionchannelled through the heterotrophic flagellates, ciliates andheterotrophic dinoflagellates to higher trophic levels. Of theestimated carbon demand for mesozooplankton, 28% could potentiallybe met by ciliates and heterotrophic dinoflagellates.     

Microplankton and its function in a zone of shallow hydrothermal activity:the Craternaya Bay, Kurile Islands

Biomass and productivity of microplankton were measured in theCraternaya Bay (Kurile Islands), which is influenced by hydrothermalactivity and volcanic heating. The hydrothermal fields are situatedaround its shores and underwater within the 0–20 m depth.A dense ‘bloom’ of photoautotrophic microplanktonwas observed there, dominated by diatoms, phytoflagellates andthe symbiont-containing ciliate Mesodinium rubrum. The biomassof these ciliates attained 3–11 g m^-3 in the upper waterlayer. The total biomass of the phototrophic microplankton reached30–46 g m^-3. The primary production in the water columnwas, correspondingly, enormously high: 6–10 g C m^-2 day^-1.The depth of the euphotic zone was 7 m. Pelagic photosynthesiswas inhibited in the upper 0–1 m by the spreading of alayer of low-salinity hydrothermal water. The numerical densityof bacterioplankton in the upper zone of the water column variedfrom 1 x 10⁶ to 2.9 x 10⁶ cells ml^-1, and its wet biomass from250 to 750 mg m^-3. Its production varied at stations from 70to 390 mg m^-3 day^-1. Chemosynthesis contributed up to 30% ofthis production in the sites neighbouring the hydrothermal vents.Outside their direct impact however, its share was negligible.The biomass of heterotrophic planktonic ciliates varied from30 to 270 mg m^-3. The mechanisms of possible influence of shallowvolcanic activity on development and function of microplanktonin the Craternaya Bay is discussed.     

Contribution of nanoprotists to metazooplankton diet in a mesocosm experiment in the coastal northern Baltic

Grazing by a metazooplankton community on nanoprotists <10µm was studied four times during a 21 day enclosure experimentcarried out off the SW coast of Finland in late summer. Duringthe study, the pelagic community was manipulated through nutrientenrichment (N + P) and through predation by stickleback fry.Grazing experiments were conducted in the laboratory using 5µm prefiltered, ³H-labelled nanoplankton as tracer food. Grazing by mesozooplankton (Meso) and metazaan microplankton(Micro), screened through 140 and 100 µm mesh, was studied.Owing to enrichment effects and weak predation control by fish,the biomass of Meso and Micro increased during the study peridMeso biomass consisted mostly of the copepod Eurytemora affinisand the cladoceran Bosmina longispims maritima, and Micro biomassof copepod nauplii NIII–NVI. The community clearance rateof Meso usually exceeded that of Micro when feeding on nanoprotistsThe opposite was found for the biomass-smc clearance rate, revealingnanoprotists to be a more important f d source for Micro thanfor Meso. Metazmplankton were not able to control nanoprotists,because Meso and Micro were estimated to remove on average 4and 2% of nanoprotowan biomass daily. When integrated throughthe study period, grazing on nanoprotists could meet 5 and 17%of the carbon need for Meso and Micro, 3 and 12% of their productionbeing estimated to consist of bacterial carbon transferred bynanoprotists Micro were estimated to be more closely Linkedto the microbial food web than Meso, suggesting that the trophicpusition of copepods changes slightly during their maturationfrom nauphi to larger copepodites. ¹ Present address: Department of Ecology and Systematics, Divkionof Hydrobiology, University of Helsinki, FIN-00014, Helsinki,Finland     

Community structure and bottom-up regulation of heterotrophic microplankton in arctic LTER lakes

Microplankton community structures and abundance was assessed in lakes at the Toolik Lake LTER site in northern Alaska during the summers of 1989 and 1990. The microplankton community included oligotrich ciliates, but rotifers and zooplankton nauplii comprised greater than 90% of total estimated heterotrophic microplankton biomass. Dominant rotifer taxa included Keratella cochlearis, Kellicottia longispina, Polyarthra vulgaris, Conochilus unicornis and a Synchaeta sp. Microplankton biomass was lowest in highly oligotrophic Toolik Lake (< 5 μgCl⁻¹ at the surface) and highest (up to 55 μCl⁻¹) in the most eutrophic lakes, experimentally fertilized lakes, and fertilized limnocorrals, consistent with bottom-up regulation of microplankton abundance.     

Contribution of heterotrophic plankton to nitrogen regeneration in the upwelling ecosystem of A Coruna (NW Spain)

The contribution of heterotrophic plankton to nitrogen (N) regenerationin the water column, and its significance for the requirementsof phytoplankton, were studied at the seasonal scale in thecoastal upwelling ecosystem of A Coruña (Galicia, NWSpain). During 1995–1997, monthly measurements were takenof hydrographic conditions, dissolved nutrients, and abundanceand biomass of microplanktonic heterotrophs (bacteria, flagellatesand ciliates), phytoplankton and mesozooplankton (>200 µm).Additionally, series of experiments were conducted to quantifyN fluxes, including primary production (¹⁴C method), phytoplanktonuptake of nitrate, ammonium and urea (¹⁵N-labelling techniques),microheterotrophic regeneration of ammonium, mesozooplanktongrazing (chlorophyll gut-content method) and excretion of ammoniumby mesozooplankton. Two N budgets were built for the averagesituations of high (>100 mg C m^-2 h^-1) and low (<100 mgC m^-2 h^-1) primary production. The results revealed that phytoplanktonrelied strongly on regenerated ammonium all year round (33 and43% of total N uptake in high and low production situations,respectively). This demand for ammonium was closely matchedby regeneration rates of microplankton (0.14–0.25 mmolN m^-2 h^-1), whereas zooplankton contributed on average <10%to N regeneration. Likewise, zooplankton grazing had littledirect control on phytoplanktonic biomass. The results obtainedindicate that in the A Coruña upwelling system, N biomassof heterotrophic plankton is generally higher than phytoplanktonN biomass. The high rates of N regeneration measured also suggestthat a large proportion of the organic matter produced afteran upwelling pulse is recycled in the water column through themicrobial food web.     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.     

Diurnal variations in the contributions of autotrophic picoalgae and heterotrophic bacteria to planktonic production in an upland lake

An investigation of the diurnal variation in contributions toproduction of the autotrophic and heterotrophic components ofthe picoplankton community was carried out during August andSeptember in Llyn Padarn, a mesotrophic upland lake in NorthWales. The picoplankton was separated using 1 µm pore-sizedfilters into the smaller cell sized fraction (<1 µm),the majority of the bacteria and the larger cell sized picoalgae(<3>1 µm), together with some bacteria. The distributionof bacterial heterotrophic activity between these two fractionsof picoplankton was assessed by uptake of [¹⁴C]glucose and differentialfiltration. Thus, the absolute autotrophic production by picoalgaeand the heterotrophic contribution by bacteria to picoplanktoncommunity production via uptake of extracellular organic carbon(EOC) were determined. Rates of picoplankton community productionexhibited diurnal variation with maximum rates of 19.1 mg Cm^–3 h^–1 recorded at 18.00 h at 4 m depth in September.The bacterial contribution to picoplankton community productionincreased markedly between 15.00 and 18.00 h. Rates of absoluteautotrophic production varied less over 24 h than rates of accumulationin bacteria of ¹⁴C-labelled EOC released from the entire phytoplanktoncommunity. Bacteria contributed up to 86–98% of the neworganic carbon within the picoplankton community at the endof the day. The maximum rate of absolute autotrophic productionin the picoplankton was 1.6 mg C m^–3 h^–1 at 18.00h at 1 m in August, and the maximum rate of bacterial accumulationof new organic carbon was 18.5 mg C m^–3 h^–1 at 18.00h in September at 4 m depth. The diurnal pattern of picoplanktoncommunity production involved increasing rates during the daywith a maximum at 18.00 h. Autotrophic processes were dominantin the first 3–6 h of the light cycle and heterotrophicuptake of ¹⁴C-labelled EOC was the major component from 15.00h onwards. Bacterial uptake of newly released EOC by phytoplanktonwas rapid, comprised the majority of picoplankton production,particularly later in the day, and contributed a maximum of60% of the total pariculate primary production in plankton between15.00 and 18.00 h at 4 m in September with a mean contributionof between 6 and 26% over 24 h in these investigations. Theimportance of autotrophic processes in picoplankton communityproduction has been overestimated in previous investigations.Bacteria play a major role in transferring newly produced EOCrapidly from phytoplankton to the picoplankton community. Atthe end of the day, the majority of newly produced organic carbonis in bacterial cells and this production is significant inthe dynamics of carbon production within the entire planktoniccommunity.     

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.     

Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria,and zooplankton (including heterotrophic flagellates, ciliates,rotifers and crustaceans) were studied in 55 lakes in NorthernGermany with different trophic status, ranging from mesotrophicto hypertrophic. Mean abundance and biomass of all groups increasedsignificantly with trophic level of the lake, but bacteria andmetazooplankton showed only a weak correlation and a slightincrease with chlorophyll concentration. Composition of phytoplanktonshowed a dominance of cyanobacteria in hypertrophic lakes, whereasthe importance of chrysophytes and dinophytes decreased withan increase in trophic status. Protozoans (heterotrophic flagellatesand ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophiclakes) of total zooplankton biomass on average, and were dominatedby ciliates (62–80% of protozoan biomass). Seasonally,protozoans can build up to 60% of zooplankton biomass in spring,when heterotrophic flagellates can contribute     

The effect of viral concentrate addition on the respiration rate of Chaetoceros gracilis cultures and microplankton from a shallow bay (Coliumo, Chile)

Experiments were conducted to test the possible effects of viralconcentrate additions on the respiration rates of both Chaetocerosgracilis and a natural microplankton community (<200 µm)from a shallow bay located in central-south Chile (Coliumo Bay,36°32'S, 72°57'W). Each experiment was started by adding2 ml of viral concentrate to a C. gracilis culture, microplanktoncommunity or bacterioplankton community (nominal size 0.2–1.0µm) contained in a 125 ml borosilicate bottle. The incubationslasted 24 h at in situ temperatures and included a minimum offive replicates and five controls. Respiration was measuredas oxygen consumption using a semi-automatic photometric Winklermethod. Samples were taken from the experimental bottles toassess chlorophyll a, ATP concentration and bacterioplanktonabundance throughout the incubation period. Our results showthat the addition of viral concentrates can affect the respirationrates of both natural microplankton communities and C. graciliscultures. When subjected to a viral concentrate addition, therespiration rates of the natural microplankton community decreasedby 12–50% or increased by 29%, depending on the experiment,whereas bacterioplankton respiration rates increased by 92%and decreased by 78%, and C. gracilis rates increased by ~4%and decreased by ~7%.     

Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea

Simultaneous ingestion and egg production experiments were conductedwith female Calanus finmarchicus in April/May and July/August2002 in the Irminger Sea. Experimental animals were providedwith natural microplankton food assemblages and incubated underin situ conditions for 24 h. The quantity of food consumed wassignificantly related to the concentration of prey cells, withtotal daily ingestion rates ranging from 0.6 to 8.1 µgof carbon female^–1 day^–1, corresponding to carbon-specificrates of 0.6–4.7% day^–1. Egg production rates (EPRs)remained relatively low (0.3–11 eggs female^–1 day^–1)during both periods of investigation and were not influencedby food availability. The data were used to construct energeticbudgets in which the microplankton carbon ingested, includingciliates, was compared with the carbon utilized for egg productionand respiration. These budgets showed that ingestion alone couldnot provide the necessary carbon to sustain the observed demandsfor growth and metabolism. Although ciliates constituted >80%of the total material ingested at times, they were not sufficientto provide the metabolic shortfall. Indeed, the females weretypically lacking 5 µg of carbon each day, 5% of theircarbon biomass. Our study results highlight the possible importanceof internal reserves in sustaining reproduction in C. finmarchicusduring periods of food scarcity.     

Nanoflagellate predation on auto- and heterotrophic picoplankton in the oligotrophic Mediterranean Sea

Dynamics of autotrophic and heterotrophic prokaryotes and theirconsumption by nanoflagellates were studied in the euphoticzone at nine stations located from the Levantine Basin (34°E)to the Balearic sea (5°E) in June 1999. Bacterial biomassconstituted the largest proportion of living biomass at allstations. Integrated bacterial production at the furthest eaststation, was sixfold lower than integrated bacterial productionat the furthest west (13 and 75 mg C m^-2 d^-1 respectively).Estimated heterotrophic nanoflagellate bacterivory accountedfor 45–87% of bacterial production. Small protists (<3µm) dominated the bacterivore assemblage and accountedfor more than 90% of the heterotrophic bacterial consumption.Our results indicated that there was no negative selection againstSynechococcus and that both picoplankton groups were grazedaccording to their standing stocks. An estimated consumptionof Synechococcus derived from food vacuole content analysisof nanoflagellates revealed that they consumed from 0.5 to 45%(mean 13%) of Synechococcus stock per day. These data are amongthe first documenting the relative grazing impact of heterotrophicnanoflagellates on bacteria and Synechococcus in situ. Assumingthat there was no selection for or against Prochlorococcus,heterotrophic nanoflagellates could ingest from 1.4 to 21% (mean6%) of Prochlorococcus stock per day. The amount of organiccarbon obtained by heterotrophic nanoflagellates from photosyntheticprokaryotes represented 27% of the total amount of carbon obtainedfrom total prokaryotes     

Nitrogen uptake by heterotrophic bacteria and phytoplankton in Arctic surface waters

We estimated rates of heterotrophic bacterial and phytoplanktonuptake of nitrate, ammonium, and urea using ¹⁵N-labelled nitrogenand specific metabolic inhibitors of prokaryote and eukaryotenitrogen metabolism in the surface waters of the North Water(northern Baffin Bay) during autumn that were characterizedby the absence of cyanobacteria (comprising prochlorophytes).The percentage of nitrate + ammonium uptake by heterotrophicbacteria ranged between 44 and 78% of the measured total uptakeand was the highest when the phytoplankton biomass was relativelylow (<2 µg Chlorophyll a L^–1). Phytoplanktonaccounted for a larger fraction (e.g., 58–95%) of ureauptake than heterotrophic bacteria. When our results are combinedwith those from previous studies carried out in diverse temperateand polar areas, it appears that heterotrophic bacteria accountfor 25% (14–40%; median and interquartile range) of thetotal nitrate uptake in surface waters with chlorophyll biomass<2 µg L^–1. Estimates of new production computedfrom phytoplankton carbon uptake and f-ratios may be stronglyoverestimated in regions where nitrate uptake by heterotrophicbacteria is high and the biomass of phytoplankton is low.