Productivity of picoplankton compared with that of larger phytoplankton in the subarctic region

We determined the productivity (µg C µg^–1Chi a h^–1) of size-fractionated phytoplankton in the northernNorth Pacific and the Bering Sea in summer and winter. Picoplankton(<2 µm) were more productive than larger sized phytoplankton(2–10 and 10–200 µm) in the subtropical region,where the in situ temperature was >10°C; whereas picoplanktonin the subarctic region were similar in productivity or lessproductive than larger sized plankton, where the in situ temperaturewas <10°C. The result from the subtropical region inthis study agrees with previous results from tropical and subtropical waters, which indicate that phytoplankton productivitytends to decrease with increasing cell size. The result fromthe subarctic region, however, differs from previous results.We observed a positive linear regression for in situ temperatureand picoplankton productivity, but this trend was not seen inthe larger sized phytoplankton. The results show that the productivityof picoplankton is markedly influenced by in situ temperaturecompared with that of larger sized plankton. Low tem peratureappears to account largely for the observation that the productivityof picoplankton is not significantly higher than that of largersized phytoplankton in the subarctic region.     

Size-fractionated biomass, photosynthesis and dark CO2 fixation in a tropical oceanic environment

This study examines the spatial distribution and size structureof phytoplankton biomass and productivity in relation to thevertical structrure of the Andaman Sea (northeastern IndianOcean). This region was characterized by low concentrationsof nutrients and high levels of insolation. Nitrogen availabilityappeared to control overall productivity with nitrate-based‘new’ production accounting for 8–24% of thetotal primary production. Euphotic column chlorophyll (chl a)averaged 52.5 mg m^–2 of which a major portion was locatedas a subsurface chl a maximum (SCM) at     

Primary and bacterial production in lakes: are they coupled over depth?

The coupling of primary and bacterial production over depthwas examined in three lakes which differed greatly in verticalpatterns of primary productivity. We measured bacterial production,chlorophyll and light, and estimated primary production in PaulLake (Gogebic County, Michigan) and Crystal and Trout lakes(Vilas County, Wisconsin) during the summer stratification period(May–September 1991). Bacterial productivity was measuredusing the [³H]leucine incorporation method and primary productivityestimated from measured photosynthesis–irradiance relationships.Three distinct vertical patterns were observed. In Paul Lake,bacterial production was highest at the interface between theaerobic and anaerobic layers, well below the depth of maximumprimary production. In Crystal Lake, bacterial production wasuniform with depth, although primary productivity was highestin the hypolimnion. In the largest lake. Trout Lake, primaryand bacterial production tended to co-vary with maximum ratesof both processes occurring in the metalimnion. Overall, bacterialproductivity was poorly related to contemporaneous primary productionin the three lakes, suggesting that other factors, such as nutrientrecycling, phytoplankton loss rates and allocthonous loading,determine patterns in the depth distribution of bacterial productivity.     

Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Sobygaard

The impact of fish-mediated changes on the structure and grazingof zooplankton on phytoplankton and bacterioplankton was studiedin Lake Søbygaard during the period 1984–92 bymeans of in vitro grazing experiments (¹⁴C-labelled phytoplankton,³H-labelled bacterioplankton) and model predictions. Measuredzooplankton clearance rates ranged from 0–25 ml l^–1h^–1 on phytoplankton to 0–33 ml l^–1 h^–1on bacterioplankton.The highest rates were found during thesummer when Daphnia spp. were dominant. As the phytoplanktonbiomass was substantially greater than that of bacterioplanktonthroughout the study period, ingestion of phytoplankton was26-fold greater than that of bacterioplankton. Multiple regressionanalysis of the experimental data revealed that Daphnia spp.,Bosmina longirostris and Cyclops vicinus, which were the dominantzooplankton, all contributed significantly to the variationin ingestion of phytoplankton, while only Daphnia spp. contributedsignificantly to that of bacterioplankton. Using estimated meanvalues for clearance and ingestion rates for different zooplankters,we calculated zooplankton grazing on phytoplankton and bacterioplanktonon the basis of monitoring data of lake plankton obtained duringa 9 year study period. Summer mean grazing ranged from 2 to4% of phytoplankton production and 2% of bacterioplankton productionto maxima of 53 and 88%, respectively. The grazing percentagedecreased with increasing density of planktivorous fish caughtin August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundanceseemed highest for bacterioplankton. Accordingly, the percentagecontribution of bacterioplankton to the total ingestion of thetwo carbon sources decreased from a summer mean value of 8%in Daphnia-dominated communities at lower fish density to 0.7–1.1%at high fish density, when cyclopoid copepods or Bosmina androtifers dominated. Likewise, the percentage of phytoplanktonproduction channelled through the bacteria varied, it beinghighest (5–8%) at high fish densities. It is argued thatthe negative impact of zooplankton grazing on bacterioplanktonin shallow lakes is highest at intermediate phosphorus levels,under which conditions Daphnia dominate the zooplankton community.     

Microzooplankton grazing in a coastal embayment off Concepcion, Chile, (~36{degrees} S) during non-upwelling conditions

The impact of grazing by natural assemblages of microzooplanktonwas estimated in an upwelling area (Concepción, Chile)during the non-upwelling season in 2003 and 2004. Seawater dilutionexperiments using chlorophyll a (Chl a) as a tracer were usedto estimate daily rates of phytoplankton growth and microzooplanktongrazing. Initial Chl a concentrations ranged from 0.4 to 1.4mg Chl a m^–3 and phytoplankton prey biomass and abundancewere numerically dominated by components <20 µm. Phytoplanktongrowth and microzooplankton grazing rates were 0.19–0.25day^–1 and 0.26–0.52 day ^–1, respectively.These results suggest that microzooplankton exert a significantremoval of primary production (>100%) during the non-upwellingperiod.     

Modelling of autumn plankton bloom dynamics

A simple system of parametrically forced ordinary differentialequations is used to model autumn phytoplankton blooms in temperateoceans by a mechanism involving deepening of the upper mixedlayer. Blooms are triggered provided the increase in nutrientsin the mixed layer is rapid within the first few days of deepeningand provided light-limited phytoplankton growth rate is relativelyhigh. Blooms exist as transient trajectories between quasi-equilibriumstates, rather than as bifurcations of steady states; thereforevery gradual deepening cannot trigger blooms. Very rapid deepeningalso prevents blooms due to the deleterious effect on phytoplanktongrowth rate. The mechanisms identified by this simple modelare vindicated by considering alternative grazing and deepeningregimes and by comparison with a more ecologically complex model(Fasham, 1993, in The Global Carbon Cycle, Springer-Verlag).Modelled estimates of primary productivity from both the simplemodel and the complex model parameterized for Ocean WeatherStation ‘India’ are around 0.5 g C m^–2 day^–1during the autumn bloom, therefore comprising a significantcomponent of annual production in temperate areas.     

Light rather than iron controls photosynthate production and allocation in Southern Ocean phytoplankton populations during austral autumn

The role of iron and light in controlling photosynthate productionand allocation in phytoplankton populations of the Atlanticsector of the Southern Ocean was investigated in April–May1999. The ¹⁴C incorporation into five biochemical pools (glucan,amino acids, proteins, lipids and polysaccharides) was measuredduring iron/light perturbation experiments. The diurnal Chla-specific rates of carbon incorporation into these pools didnot change in response to iron addition, yet were decreasedat 20 µmol photons m^–2 s^–1, an irradiancecomparable with the one at 20–45 m in situ depth. Thissuggests that the low phytoplankton biomass encountered (0.1–0.6µg Chl a L^–1) was mainly caused by light limitationin the deep wind mixed layer (>40 m). Regional differencesin Chl a-specific carbon incorporation rates were not foundin spite of differences in phytoplankton species composition:at the Antarctic Polar Front, biomass was dominated by a diatompopulation of Fragilariopsis kerguelensis, whereas smaller cells,including chrysophytes, were relatively more abundant in theAntarctic Circumpolar Current beyond the influence of frontalsystems. Because mixing was often in excess of 100 m in thelatter region, diatom cells may have been unable to fulfil theircharacteristically high Fe demand at low average light conditions,and thus became co-limited by both resources. Using a modelthat describes the ¹⁴C incorporation, the consistency was shownbetween the dynamics in the glucan pool in the field experimentsand in laboratory experiments with an Antarctic diatom, Chaetocerosbrevis. The glucan respiration rate was almost twice as highduring the dark phase as during the light phase, which is consistentwith the role of glucan as a reserve supplying energy and carbonskeletons for continued protein synthesis during the night.     

Photosynthetic electron transport in Dunaliella tertiolecta (Chlorophyceae) measured by fast repetition rate fluorometry: relation to carbon assimilation

A comparison of photosynthesis-irradiance response curves (P–Eresponse curves) obtained through fast repetition rate (FRR)fluorometry and radiocarbon (¹⁴C) tracer method was made inthe chlorophyte, Dunaliella tertiolecta, grown under differentirradiance conditions. In FRR-based P–E response curveexperiments, actinic light provided by white light-emittingdiodes (LEDs) was increased gradually from 0 to 1500 µmolquanta m^–2 s^–1 and the rate of photosyntheticelectron transport was determined at each light level. Short-termexperiments (20 min) of ¹⁴C-based P–E response curvewere carried out with an improved photosynthetron, which containswhite LEDs as the light source. Irrespective of growth irradiance,the ratios of FRR to ¹⁴C-based initial slopes were almost uniform.The ratios of FRR- to ¹⁴C-based maximum rates were 25–36%higher than those of FRR- to ¹⁴C-based initial slopes. The relationshipbetween electron transport and carbon assimilation was non-linearwith increasing discrepancy towards high actinic light. Thisnon-linear relationship between FRR- and ¹⁴C-based estimatesis primarily due to the effect of physiological processes stimulatedat high levels of light, such as cyclic electron flow and theMehler reaction. The results of this study indicate that theFRR fluorometry can be used as a good indicator of photosyntheticrates from low to middle light levels, but becomes increasinglyquestionable as the maximum photosynthetic rate is approached.The degree to which this relationship is further affected bynutrient-status warrants investigation.     

The seasonal productivity of phytoplankton in a hypertrophic, gravel-pit lake

The seasonal time course of phytoplankton primary productivitywas studied weekly in a hypertrophic, gravel-pit lake closeto Madrid, Spain. Chlorophyll a ranged 22–445 mg m^–2.Gross primary productivity attained 0.28±0.14 g C m^–2h^–1 (range: 0.06–0.60), its yearly value being 900g C m^–2, but the shallow euphotic depths and the highplankton respiration ensured that net productivity was generallylow. Respiration losses amounted to 0.31±0.24 g O₂ m^–2h^–1, with phytoplankton respiration roughly attainingone-half of overall plankton respiration. Areal phytoplanktonproductivity and plankton respiration followed a seasonal trendbut this was not the case for photosynthetic capacity. Surfacephotoinhibition was evenly distributed throughout the study.Quantum yields showed an increasing depth trend, but no seasonaltrend. Both P_max and I_k were both temperature- and irradiance-dependent.As compared with lakes of lesser trophic degree, phytoplanktonprimary production in hypertrophic lakes might be increasednot only by higher nutrient contents but also by low chlorophyll-specificattenuation coefficients and low background, non-algal attenuation,thereby allowing for higher areal chlorophyll contents and hencehigher areal productivity. Our study suggests that physical(irradiance and water column stability) as well as chemicalfeatures (dissolved inorganic carbon and soluble reactive phosphorus)may control seasonality of phytoplankton primary productionin this lake despite recent claims that only physical factorsare of significance in hypertrophic lakes. However, this doesnot explain all the variability observed and so a food web controlis also likely to be operating.     

Size and species-specific primary productivity and community structure of phytoplankton in Tokyo Bay

Combined methods of size fractionation and single-cell isolationwere used to investigate the seasonal variation of phytoplanktondynamics in Tokyo Bay with an emphasis on primary productivity.Red tides occurred in Tokyo Bay from spring to autumn; a diatom,Skeletonema costatum, and a raphidophycean, Heterosigma akashiwo,were the most important primary producers. Small diatoms andflagellates, including these species, were dominant and showedrapid changes of phytoplankton community structure within severaldays in summer. The nanoplankton (3–20 µm) fractioncontributed most to chlorophyll a concentration and primaryproductivity during spring to autumn, whereas the microplankton(>20 µm) contribution was remarkable in winter. Picoplankton(<3 µm phytoplankton) remained relatively constantthroughout the year. A significant reverse relationship wasobtained between assimilation rate and chlorophyll a contentfor the total and nanoplankton population; the assimilationrate was high at the initial phase of the bloom, then decreasedto a minimum level at the peak of the bloom. Factors controllingthe reduction of assimilation rates at the peak, and changesin phytoplankton community structure, are discussed.     

Sinking rates of heterogeneous, temperate phytoplankton populations

Throughout the summer of 1978, the sinking rates of phytoplanktonwithin the Controlled Experimental Ecosystems (CEE's) were monitoredusing a technique based upon measurement of the transit timeof radioactively (¹⁴C) labeled cells. The experimental frameworkof FOODWEB 1 offered an unprecedented opportunity to documentthe sinking rates of heterogeneous phytoplankton of diversetaxonomic composition, growing under a variety of nutrient regimes;the absence of advective exchange in the CEE's provided knowledgeof the preconditioning history of the phytoplankton sampledat any given time. Sinking rates of whole phytoplankton assemblages (not size-fractioned)ranged from 0.32 – 1.69 m·day^–1; the averagerate (± s.d.) observed was 0.64 ± 0.31 m·day^–1.The most notable deviations from the mean value occurred whenthe population size distribution and taxonomic composition shifteddue to blooms. The relationship between phytoplankton sinkingand ambient nutrient levels was studied by following the ratesof a given size fraction (8–53 µm) for ten daysfollowing nutrient enrichment of a CEE. Over this time sinkingrates ranged from 1.08– 1.53 m·day^–1; decreasedrates occurred after nutrification, yet over the course of theentire trial sinking rates were not significantly (p >0.05)correlated to the ambient levels of any single nutrient species. The sinking rate implications of spore formation were also studied,and showed that sinking rates of Chaetoceros constrictus andC. socialis spores (2.75 ± 0.61 m·day^–1)were ca 5-fold greater than rates measured when the vegetativestages of these species dominated the population, reflectingthe influence of physiological mechanisms in controlling phytoplanktonbuoyancy. An example of the potential influence of colony formation uponbuoyancy was noted in observations of C. socialis which occasionallywas found to exist in large spherical configurations made ofcoiled cell chains and having low (0.40 m·day^–1)sinking rates. A hydrodynamic rationale is presented to showhow such a colony together with enveloped water may behave asa unit particle having lower excess density, and therefore lowobserved sinking rate, despite its conspicuously large size. Overall, sinking rates were not significantly correlated withany of the measured nutrient or photic parameters. There were,however, trials and comparisons which showed evidence for: (1)higher sinking rates in populations dominated by large cells,(2) decreased sinking rates after nutrient enrichment, and (3)buoyancy response to light levels. It is suggested that correlationof sinking rates with synoptic environmental measurements atany given time is not explicit because the associations mayinvoke lag times of physiological response. The interpretationmade from these findings is that the preconditioning historyof the population, rather than the prevailing conditions atthe time of a given measurement, is most closely associatedwith population buoyancy modifications.     

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.     

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

The dynamics of the phytoplankton community were investigatedin a marine coastal lagoon (Thau, NW Mediterranean) from February1999 to January 2000. Dilution experiments, chlorophyll a (Chla) size-fractionation and primary production measurements wereconducted monthly. Maximum growth and microzooplankton grazingrates were estimated from Chl a biomass fractions to separatepico- from nano- and microphytoplankton and by flow cytometryto distinguish between picoeukaryotes and picocyanobacteria.In spring, the phytoplankton community was dominated by Chaetocerossp. and Skeletonema costatum, which represented most of biomass(B) and primary production (P). Nano- and microphytoplanktongrowth was controlled by nutrient availability and exceededlosses due to microzooplankton grazing (g). Picoeukaryote andcyanobacteria growth was positively correlated with water temperatureand/or irradiance, reaching maximum values in the summer (2.38and 1.44 day^–1 for picoeukaryotes and cyanobacteria, respectively).Picophytoplankton accounted for 57% of the biomass-specificprimary productivity (P/B). Picophytoplankton was strongly controlledby protist grazers (g = 0.09–1.66 day^–1 for picoeukaryotes,g = 0.25–1.17 day^–1 for cyanobacteria), and microzooplanktonconsumption removed 71% of the daily picoplanktonic growth.Picoeukaryotes, which numerically dominate the picoplanktoncommunity, are an important source of organic carbon for theprotistan community and contribute to the carbon flow to highertrophic levels.     

Pathways of carbon cycling in marine surface waters: the fate of small-sized phytoplankton in the Northeast Water Polynya

The fate of small-sized phytoplankton (<5 µm) and pathwaysof carbon cycling in surface waters, i.e. recycling within orexport out of the mixed layer, were investigated in the NortheastWater (NEW) Polynya (77–81°N) from 23 May to 22 July1993. The sampling covered a wide range of ice, hydrographicand nutrient conditions. Chlorophyll a concentrations, phytoplanktonproduction rates and zooplankton abundances were determinedin the field, and potential rates of grazing by protozoa, copepodsand appendicularians were calculated from abundances, usingassumptions from the literature. To our knowledge, this is thefirst published attempt to assess concurrently the grazing ofthese three plankton groups in the Arctic. The production rateof small-sized phytoplankton was significantly higher in ice-freecompared with ice-covered areas, but the biomasses of small-sizedphytoplankton and zooplankton were not. Potential recycling,downward export and horizontal advection of phytoplankton werecalculated by resolving carbon budgets for the mixed layer.A large fraction of the small-sized phytoplankton produced insidethe polynya was advected horizontally to the ice-covered partof the NEW, where these algae were necessary to sustain theheterotrophic community. We conclude that the fate of small-sizedphytoplankton production was mostly recycling (>70%). Downwardexport would have occurred infrequently, as a result of intensegrazing by appendicularians. Size-differential pathways of carboncycling in planktonic food webs are discussed.     

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.     

Experimental evaluation of herbivory in the ctenophore Mnemiopsis leidyi relevant to ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA

Herbivory of Mnemiopsis leidyi and its interactions with phytoplanktonand non-gelatinous zooplankton were examined in small-scalemicrocosm experiments. Clearance rates for M. leidyi incubatedwith phytoplankton were generally negative, but ranged up to4.5 1 ctenophore^–1 day^–1 when the large (80 µmø) diatom Ditylum brightwelli was offered as a food source.These highest ingestion rates would provide Mnemiopsis withonly 21 % of its daily carbon requirements for respiration.Mean shrinkage of M. leidyi was 8.2–51% when incubatedwith phytoplankton. Although M. leidyi neither fed activelyon phytoplankton, nor satisfied its nutritional needs on sucha diet, the chain-forming diatom Skeletonema costatum becameentangled in mucus strands and balls produced by M. leidyi inthe absence of zooplankton. Attachment onto mucus occurred atphytoplankton concentrations commonly observed in NarragansettBay and may be important in the formation of "marine snow" duringsummer M. leidyi pulses; phytoplankton sinking rate and the"package size" available to herbivores would also be affected.The experiments support our previous hypothesis based on fieldobservations in Narragansett Bay that M. leidyi indirectly regulatesphytoplankton abundance there during the summer bloom as a consequenceof predation on zooplankton. The extent to which M. leidyi influencedphytoplankton dynamics in the microcosms was dependent on therelative abundance and physiological state of the three trophiclevels. A food web diagram for M. leidyi is presented.     

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

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

Copepod egg production, moulting and growth rates, and secondary production, in the Skagerrak in August 1988

Measurements of hydrography, chlorophyll, moulting rates ofjuvenile copepods and egg production rates of adult female copepodswere made at eight stations along a transect across the Skagerrak.The goals of the study were to determine (i) if there were correlationsbetween spatial variations in hydrography, phytoplankton andcopepod production rates, (ii) if copepod egg production rateswere correlated with juvenile growth rates, and (iii) if therewas evidence of food-niche separation among co-occumng femalecopepods The 200 km wide Skagerrak had a stratified water columnin the center and a mixed water column along the margins. Suchspatial variations should lead to a dominance of small phytoplanktoncells in the center and large cells along the margins; however,during our study blooms of Gyrodinium aureolum and Ceratium(three species) masked any locally driven differences in cellsize: 50% of chla was >11 µm, 5% in the 11–50µm fraction and 45% <50 µm. averaged for allstations. Chlorophyll ranged from 0.2 to 2.5 µg l^–1at most depths and stations. Specific growth rates of copepodsaveraged 0.10 day^–1 for adult females and 0.27 day^–1for juveniles The latter is similar to maximum rates known fromlaboratory studies, thus were probably not food-limited. Eggproduction rates were food-limited with the degree of limitationvarying among species: 75% of maximum for Centropages typicus, 50% for Calanus finmarchicus, 30% for Paracalanus parvus and 15% for Acartia longiremis and Temora longicornis. Thedegree of limitation was unrelated to female body size suggestingfood-niche separation among adults. Copepod production, summedover all species, ranged from 3 to 8 mg carbon m^–3day^–1and averaged 4.6 mg carbon m^–1 day^–1. Egg productionaccounted for 25% of the total.     

An empirical model of primary productivity (14C) using mesocosm data along a nutrient gradient

The two parameters of the hyperbolic tangent equation, P_m and, were estimated from in situ vertical profiles of primary productionusing mesocosm data along a nutrient gradient. The parameters,derived from 4-h (around noon) ¹⁴C incubations, were used togetherwith the photosynthesis-light curve and hourly solar radiationdata to calculate daily primary production rates (P_d). Approximately40% of the daily production occurred in the 4 h around noon.Considering parameter uncertainty, there was no indication ofan increase in variation in production with increased nutrientloading, nor did biomass-specific P-I parameters increase. Annualproduction ranged from 82 to 901 g C m^–2 year^–1and was highest in the highest nutrient treatment tank. Dailyproductivity ranged from 0.02 to 9.1 g C m^–2 day^–1and was significantly correlated, in all treatments, with acomposite parameter BI₀/k (where B is phytoplankton biomass;I₀ is daily radiation and k is the extinction coefficient).Linear regressions of P_d against BI₀/k indicated that much ofthe variability (86%) in productivity was explained by lightavailability and phytoplankton biomass. Two approaches for predictingproductivity were compared: (i) predicting production directlyfrom environmental variables (i.e. BI₀/k) and (ii) predictingthe parameters of the P-I curve from environmental variablesand using these to calculate daily production.     

Feeding by larval and post-larval ctenophores on microzooplankton

Feeding by the coastal ctenophorc, Mnemiopsis leidyi, on microplanktonwas investigated. Larval ctenophores (tentaculate stage) grewbest and had the highest survival rates when offered a mixtureof ciliates and copepod nauplii. Larvae did not survive whenoffered phytoplankton alone. Clearing of planktonjc ciliatesby post-larval ctenophores was a function of the ciliate speciesand the size of the predator. Removal of small ciliates (<20µm in size) and phytoplankton was negligible. Small post-larvalctenophores (volume <4 cm³) had higher biovolume-specificclearing rates (0.5–1.5 1 cm^–3 day^–1) thandid larger ctenophores fed the same ciliate species. Duringin situ incubations, adult M. leidyi removed ciliates, rotifersand copepod nauplii from natural microplankton assemblages.The data indicate that non-crustacean microzooplanlctoo arean important component of the diet of larval and post-larvallocate cteoophores, particularly when copepod standing stocksare low.