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.     

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.     

Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillans

The heterotrophic dinoflagellate Noctiluca scintillans has anegligible swimming ability and feeds predominantly on immobileprey. How, then, does it encounter prey? Noctiluca scintillansis positively buoyant and, therefore, we hypothesized that itintercepts prey particles during ascent and/or that microscaleshear brings it into contact with prey. Noctiluca scintillanshas a specific carbon content 1–2 orders of magnitudeless than that typical for protists and, thus, an inflated volume.It also has a density slightly less than that of the ambientwater and therefore ascends at high velocities (-1 m h^–1).In stagnant water, clearance rates of latex spheres (5–80µm) increased approximately with prey particle size squared.This scaling is consistent with N.scintillans being an interceptionfeeder. However, absolute clearance rates were substantiallylower than those predicted by modeling N.scintillans both asa spherical and as a cylindrical collector. The latter modelassumes that prey particles are collected on the string of mucusthat may form at the tip of the tentacle. Feeding, growth andprey selection experiments all demonstrated that diatoms arecleared at substantially higher rates than latex beads and otherphytoplankters, particularly dinoflagellates. We propose thatdiatoms stick more efficiently than latex beads to the mucusof N.scintillans and that dinoflagellates reduce fatal contactbehaviorally. We conclude that N.scintillans is an interceptionfeeder and that the high ascent velocity accounts for encounterswith prey. However, the flow field around the cell-mucus complexis too complicated to be described accurately by simple geometricmodels. Fluid shear (0.7–1.8 s^–1 had a negativeimpact on feeding rates, which were much less than predictedby models. Noctiluca scintillans can survive starvation forlong periods (>3 weeks), it can grow at low concentrationsof prey (-15 µg C l^–1), but growth saturates onlyat very high prey concentrations of 500–1000 µgC l^–1 or more. We demonstrate how the functional biologyof N.scintillans is consistent with its spatial and seasonaldistribution, which is characterized by persistence in the plankton,blooms in association with high concentrations of diatoms, andsurface accumulation during quiescent periods or exponentialdecline in abundance with depth during periods of turbulentmixing.     

The vitamin B requirement of Phaeocystis globosa (Prymnesiophyceae)

In batch cultures of flagellates and non-flagellate cells ofPhaeocystis globosa, the biomass yield was significantly enhancedby the addition of a mixture of the vitamins thiamine (B1),cyanocobalamin (B12) and biotin (H). A bioassay with B1 andB12 using the non-flagellate cells of P.globosa showed thatthis prymnesiophyte is a B1 auxotroph. The bioassay also indicateda significant difference in growth rate between culture mediumwith 10 nmol l^–1 B1 (µ = 0.80 day^–1) and culturemedium with 10 nmol l^–1 B12 (µ = 0.52 day^–1).These findings are discussed in relation to the hypothesis thatcentric diatoms, through vitamin B1 excretion or B12 depletion,initiate Phaeocystis blooms. It is concluded, however, thatan alternative hypothesis, that diatoms provide a solid substratefor colony initiation, has more experimental support.     

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.     

Impacts of nutrients and zooplankton on the microbial food web of an ultra-oligotrophic lake

In ultra-oligotrophic lakes and the sea, calanoid copepods arethe dominant mesozoo-plankton and cladocerans are generallysparse or absent. To determine the effects of predation andnutrient enrichment on the pelagic microbial food web of anultra-oligotrophic lake, we added copepods and cladocerans atlow biomasses (<60 µg l^–1 to in situ enclosuresin Lake Wakatipu, New Zealand, in the presence and absence ofadded nutrients (nitrogen and phosphorus). In response to nutrientfertilization, the concentrations of phototrophs >3 µmand heterotrophic bacteria increased by 50 and 15%, respectively,over 4 days, but those of cyanobacterial picoplankton decreasedby 68%. The presence of calanoid copepods (Boeckella dilatata)at ambient densities (1 and 4 l^–1) rapidly and severelysuppressed ciliate population growth over 4 days and also loweredthat of flagellates >3 µm, even when microbial growthwas enhanced by added nutrients. The presence of a small cladoceran,Ceriodaphnia dubia, at double the densities, but similar biomasses,to those of copepods, depressed the net growth rates of ciliatesand flagellates to a lesser degree. The net growth rate of heterotrophicbacteria after 4 days declined with flagellate abundance, consistentwith the possibility of regulation by flagellates. Althoughbacteria and algae increased in response to nutrient fertilization(bottom-up control), predation (top-down control) appeared toplay an important role in structuring the microbial food webof this ultra-oligotrophic lake in summer.     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

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

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

Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonla diadema: I. Behavioural studies on the infection process

The investigation of successive steps involved in the infectionprocess of the marine diatoms Coscinodiscus granii and Coscuwdiscuswailesii by the host-specific parasitoid nanoflagellate (PNF)Pirsonia diadenw showed that flagellates reacted chemokJinokineticallywith changes of swimming pattern to the presence of a host diatom.Chemosensory stimulation appeared to induce readiness for infection,whereas attachment and penetration of the diatom cell wall wasinduced by a mechanosensory response to morphological featureson the diatom frustules. The mean swimming speed of P.diademaflagellates decreased during their infective lifetime of 3 daysfrom an average of 78 µm s^–1 to 51 µm s^–1while the frequency of small loops in the swimming pattern increasedfrom 0.8 to 6.3 loops min^–1. At high Cgranii densities,an epidemic was delayed. It is suggested that this could becaused by overlapping gradients of extracellular material releasedby the diatoms which impaired the sensing of spatial gradientsby PNF and, therefore, the location of hosts.     

Phytoplankton and nutrients in the waters north-west of Spitsbergen in the autumn of 1979

Phytoplankton biomass, primary production rates and inorganicnutrients were measured in the uppermost layer of the ice-edgeregion and in open water and compared with environmental factorsduring a three-week cruise in September – October 1979.Biomass and production values were low (maximum 2.2 µgchl a l^–1, 2.5 mg C m^–3 h^–1). A post-bloomcommunity of diatoms, consisting mainly of representatives ofChaetoceros, Leptocylindrus, Nitzschia and Thalassiosira, waspredominant. Concentrations of phosphate were quite low (maximum0.55 µM I^–1). Nitrate and silicate ranged from nomeasurable quantities to 5.7 µM l^–1 and 3.8 µMl^–1, respectively. The possibility of light and nutrientlimitation on phytoplankton growth is discussed.     

A comparative study of the size-dependent organic composition of marine diatoms and dinoflagellates

Cellular chlorophyll a, protein, carbohydrate and lipid contentwas determined for eleven clones of centric marine diatoms (volume89–1.47 x 10⁷ µ³) and eight species of marine dinoflagellates(597–4.45 x 10⁴ µ³) cultured under continuous illuminationat 18°C and 20°C, respectively. In both groups the logof cellular concentrations of each constituent was directlyrelated to the log of cell volume; diatoms generally had lowercellular concentrations than dinoflagellates of an equivalentvolume. Diatom chlorophyll a, protein and lipid concentrationsnormalized to a unit cell volume (pg µ^–3) decreasedexponentially with increasing cell size; this decrease is aconsequence of the diatoms' unique morphology restricting cellcytoplasm to a thin parietal layer within the frustule. Althoughdinoflagellates yield a wide range of cytoplasm concentrations,small dinoflagellates contained up to 3-fold higher cytoplasmconcentrations of all constituents than diatoms of equal volume.The log of cellular caloric values, summed from the caloricequivalents of cellular protein, carbohydrate and lipid, wasa linear function of log volume. Diatoms contained ca. halfthe caloric value of dinoflagellates of an equivalent volume.Although the evaluation of caloric content provides a basisfor comparing the "nutritional value" of phytoplankton groups,evidence from the literature suggests subjective factors suchas taste and digestibility are equally important in determiningnutritional values of individual species.     

Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon: the Mar Menor

The seasonal distribution of plankton in a Mediterranean hypersalinecoastal lagoon has been studied through a dataset, comprisingthe taxonomic composition and the size–abundance distributionof both phyto- and zooplankton, measured by image analysis techniquesduring a one-year time series of weekly samplings. The studiedorganisms ranged from small nanoplanktonic heterotrophic flagellates(2 µm diameter) to fish larvae (>2 µm). The phytoplanktonannual succession was characterized by a winter period dominatedby Rhodomonas spp. and Cryptomonas spp. with Cyclotella spp.as the main diatom represented, a spring phase where diatoms(mainly Cyclotella) were the dominant group with some monospecificblooms of other diatoms (mainly of Chaetoceros sp.), a summerphase characterized by diatoms with blooms of Niztschia closterium,and a post-summer phase where dinoflagellates increased withpeaks of Ceratium furca. High densities of the microbial foodweb elements, flagellates and ciliates, indicate the importanceof the microbial loop in the ecosystem. Meroplankton contributedwidely to the seasonal character of the zooplankton distribution.Copepods, represented by Oithona nana, Centropages ponticusand Acartia spp. (mainly latisetosa), remained relatively constantthroughout the year, exhibiting a lower density in the warmerwater period (July–September). At the end of the samplingperiod, a massive proliferation of copepods (>1000 ind l^–1), mainly due to O. nana, took place. The autotrophsto heterotrophs biovolume ratio (A:H) remained lower than 1throughout the year except when, occasionally, large phytoplanktoncells bloomed. Persistent very low values of A:H suggest thatadditional sources of energy, such as the microbial loop ordetrital pathways, would be needed to sustain the high heterotrophicbiovolume found in the lagoon.     

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.     

Growth and decline of a diatom spring bloom phytoplankton species composition, formation of marine snow and the role of heterotrophic dinoflagellates

During the spring of 1994, we determined the factors responsiblefor the decline of the seasonal diatom bloom in the Gullmarfjord, on the west coast of Sweden. Four species constituted>75% of the biomass—Detonula confervacea, Chaetocerosdiadema, Skeletonema costatum and Thalassiosira nordenskioeldii—reachingconcentrations of 4900, 350, 8200 and 270 cells ml^–1,respectively. Growth of phytoplankton was exponential (growthrate = 0.12 day^–1) from 3 to 21 March, after which a galewith winds >15 m s^–1 caused massive aggregation. Amaximum of 130 p.p.m. (v/v) of marine snow aggregates was observedby in situ video at the peak of the bloom. Critical concentrations(Jackson, Deep-Sea Res., 37, 1197–1211, 1990) were similarto observed showing that coagulation theory could explain thesudden decline of the bloom. The heterotrophic dinoflagellateGyrodinium cf. spirale increased exponentially after the peakof the bloom with maximum (temperature-adjusted) growth rates.After the rapid aggregation and sedimentation of the bloom,they were able to control any further growth of diatoms. Nitrateand silicate were never depleted, but phosphate may have beenlimiting by the end of the study period. We conclude that massaggregation during a gale marked the end of the bloom, and thatintense grazing by heterotrophic dinoflagellates prevented anysubsequent increase of diatoms.     

Effects of diffusion and upwelling on the formation of red tides

In this paper, records on the timing and location of specificred tides monitored once or twice a week in Mikawa Bay, Japan,are related to horizontal and vertical mixing rates determinedfrom a numerical model. Horizontal (K_h) and vertical (K_z) diffusioncoefficients, and upwelling velocities, were estimated usinga box model analysis. In the wind-mixed period and in the upperlayer during the stratified period, K_h was estimated to be ofthe order of 10² m² s^–1. During the stratified period,K_z was estimated to be of the order of 10^–5 m² s^–1.The upwelling velocity was calculated to be in the range 0.35–5.1m day^–1 with an average of 1.5 m day^–1. Comparisonbetween the literature values of the specific growth rate (µ)of the red tide-forming diatoms and calculated K_h values duringthe red tides show that diatoms which have a low µ cannotform red tides in a strongly diffusive environment, while specieshaving a high µ can form red tides even in a strong diffusiveenvironment. On the other hand, no clear relationship was foundbetween µ of the flagellate group and K_h, although theflagellate group formed red tides even in severe diffusive conditions.From the comparison between the literature values of sinkingrate and swimming speed and the physical parameters associatedwith vertical processes, it was concluded that flagellates willform red tides, even in severe diffusive conditions, by usingtheir swimming ability, while diatoms form red tides by theirhigh growth rates with the aid of vertical diffusion and theupwelling movement of water.     

Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonia diadema: II. Selective infection behaviour for host species and individual host cells

The parasitoid nanoflagellate (PNF) Pirsonia diadema is hostspecific for the marine centric diatom Coscinodiscus spp. Experimentsshowed that flagellates significantly prefer C. wailesii overC.granii as host species (interspecific selectivity). This preferencewas independent of light conditions (dark, irradiance of 10and 70 µmol m^–2 s^–1) and temperature (10 and15C). Among unicellular host diatoms, the infection behaviourwas selective for individual cells: already infected C.graniicells were more attractive for further flagellate attachmentthan non-infected cells (intraspecific selectivity). Individualcells (     

Growth and grazing rates of Protoperidinium hirobis Abe, a thecate heterotrophic dinoflagellate

Growth and feeding rates of a laboratory-reared small thecateheterotrophic dinoflagellate, Protoperidinium hirobis Abè,grown on the diatom Leptocylindrus danicus, were measured inbatch cultures. Ingestion rates were determined directly bythe enumeration of empty diatom frustules produced by dinoflagellatefeeding. Both growth and feeding rates saturated at diatom concentrationsof {small tilde} 10⁴ cells ml^–1, and reached maximum valuesof 1.7 divisions day^–1 and 23 diatoms grazer^–1 day^–1,respectively. This rate of cell division is notably high comparedto photosynthetic dinoflagellates, which seldom grow fasterthan 1 division day^–1. A maximal clearance rate of 0.5µl h^–1 was measured. Mean cell size varied proportionallywith food abundance, with food-saturated cells having doublethe mean volume of food-depleted cells. Tuning of cell divisionand grazing rate patterns were also examined; while mitosisoccurred chiefly during the dark period, no diel variationsin feeding rate were detected. These rates represent the firstdirect growth and ingestion measurements to be made for a thecateheterotrophic dinoflagellate. They serve to underscore one functionthese dinoflagellates perform within the microzooplanktonicfood web: that of transforming large diatoms into particlesmore easily ingested by microzooplankters.     

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     

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

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

Factors controlling production of phytoplankton and bacteria under ice in a humic, boreal lake

The factors controlling pelagic primary and bacterial productionof a humic, boreal lake in winter were investigated, combininglaboratory and field experiments where some of the predictedconsequences of the climate change, i.e.the increased load ofphosphate-phosphorus and dissolved organic matter (DOM), weresimulated. In situ incubations were performed in eight acrylictubes lowered underneath the ice cover for 1–4 months.In the lake, production of phytoplankton (0.03–0.33 µgC l^–1 day^–1) was lower than that of bacteria (0.2–2.3µg C l^–1 day^–1) from the end of January tomid-April. Later in spring, the light conditions improved dueto the disappearance of snow and finally ice itself, and primaryproduction was revived. The importance of light as the factorcontrolling primary production in winter was confirmed in laboratoryexperiments where additions of phosphorus and DOM did not enhancethe primary production. These same enrichments resulted in highergrowth rates and production of bacteria. However, bacterioplanktonwas simultaneously controlled by heterotrophic flagellates,as in the laboratory experiments the exclusion of flagellatesalways resulted in higher yields of bacteria of increased cellsize. There was also some evidence that the quality of substratesis of importance for microbial activities. The importance oftemperature was highlighted with the significant correlation(r² = 0.59) between bacterial production and temperature withinthe range 0.6–2.1C. Thus, on the boreal zone the possiblewarming of climate may lead to enhanced activities in winter,but under unchanged light conditions no effects on primary productionwill be expected.