Potential contribution that the copepod Neocalanus tonsus makes to downward carbon flux in the Southern Ocean

We estimate that Neocalanus tonsus makes a contribution to downwardscarbon flux of 1.7–9.3 g C m^–2 year^–1, insubantarctic waters, the Subtropical Front and waters immediatelyto the north, based on its ontogenetic vertical migration minusthe biomass of eggs, the products of which are returned to thesurface the following season. This flux is an order of magnitudegreater than that estimated (0.27 g C m^–2 year^–1)for vertical migration of large copepods in the North Atlantic.Over the total 55.6 x 10⁶ km² where N. tonsus is distributed,0.17 Gt C year^–1 are estimated to be lost annually tothe ocean interior. In subantarctic water, this loss represents1.4% of primary production and is 14% greater than the measuredsedimented particulate organic carbon (POC) at 300 m. Similarly,in subtropical water, carbon loss to the ocean interior fromN. tonsus seasonal migration is estimated to be 13% lower thanmeasured POC flux. Nevertheless, N. tonsus was never found intime-incremental sediment trap samples. We hypothesize thatthe apparently proportionally different role of downwards seasonalmigration of large copepods relative to sedimented POC in theNorth Atlantic compared with the subarctic North Pacific andSouthern Ocean arises because of a combination of differencesin the nutrient status of these oceans, differences in the rateof development of grazer populations in spring, and differencesin life history characteristics of large copepods. The fluxdue to the behaviour of N. tonsus in different parts of itsrange, put into the context of the estimated global-mean netflux of 1.7–3.7 g C m^–2 year^–1 taken up bythe ocean, may be a regionally significant amount. The summerdownward migration of N. tonsus, however, does not entirelyexplain the observed seasonal variation in regional measurementsof pCO₂ off New Zealand.     

Simulated annual plankton production in the northeastern Pacific Coastal Upwelling Domain

A microcomputer simulation model is presented that describesthe generalized plankton production dynamics, in the surfacemixed layer, of the Juan de Fuca Eddy located on the southwesternBritish Columbia continental shelf. The Juan de Fuca Eddy simulationmodel evaluates how the annual biomass production of diatoms,copepods and euphausiids is forced by plankton feeding interactions,seasonal variability in upwelling, water temperature and solarradiation, and generalized fish predation. The model estimatesannual primary production of 345 g C m^–2 year^–1and secondary production of 19.4 g C m^–2 year^–1for copepods and 6 g C m^–2 year^–1 for euphausiids,during 1985–89; -90% of the annual plankton productionwas generated during the April-October upwelling season. Perturbationsof 22 abiotic and biotic parameters, one at a time by ±10%of nominal values, indicated that oceanic variability (e.g.upwelling rate) most strongly affected primary production. Conversely,zooplankton production was most sensitive to variability inbiological parameters describing zooplankton grazing potentialand growth (e.g. gross growth efficiency). Simulated seasonalbiomass patterns of diatoms, copepods and euphausiids were foundto closely match empirical data. However, euphausiid biomassproduction in the Juan de Fuca Eddy alone was unable to meetthe demands of estimated pelagic fish consumption. Local Eddyeuphausiid populations had to be supplemented, from regionaleuphausiids. by a mechanism that is proposed to be linked tothe seasonal pattern and intensity of positive Ekman transport(upwelling).     

Estimates of 'new production' in the Mid-North Atlantic

The principal aim of this paper is to demonstrate how the majorfeatures of primary production are influenced by climatologicalchange. The roles played by seasonal change in mixed layer depthand the vertical distribution of a key limiting nutrient areemphasized. The model traces the sequences of primary productionas the sun moves from the winter to summer solstice. Seasonalchange in primary production is regulated by light at high latitudesduring winter months and nutrients during summer months, whereat mid and low latitudes nutrients limit production. The annualpattern of production down the central meridian reflects thevertical distribution of nitrate-nitrogen in cross-sectionsof the mid-ocean. In turn, this pattern of nutrient distributionreflects the density structure due to the currents and gyresof the North Atlantic. The model produces estimates of ‘newprimary production’ which are consistent when comparedwith measured values. It should be useful for global estimatesof primary production. ¹‘Production’ refers to primary carbon fixationwhere nitrate is the only nitrogen source     

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.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Carbon flux by seasonal vertical migrant copepods is a small number

The abundant species of Calanus that dominate the mesozooplanktonof high North Atlantic latitudes overwinter at depths >500m, when the population loses 70–80% of its biomass bypredation and physiological stress. This represents an annualflux of carbon, obtained in the photic zone, into the interiorof the ocean of 274.5 mg C m^–2 year^–1, or 0.0018Gt C year^–1 for the North Atlantic. This is a small valuecompared with the flux of respiratory carbon by diel migrantsin warmer oceans and, when extrapolated to a global flux (0.012–0.018Gt C year^–1 over areas where winter migrations are importantis also small compared with computations of the global sinkingflux of particles through 200 m (1.6–3.8 Gt C year^–1or other relevant global carbon fluxes in the oceans.     

Production of tropical larvaceans in Kingston Harbour, Jamaica: are we ignoring an important secondary producer?

The larvacean community was observed during an 18 month periodat the mouth of eutrophic Kingston Harbour, Jamaica. Duringthis period, larvaceans averaged 3607 m^–3 with a biomassof 2.2mg ash-free dry weight m^–3 (32.6mg AFDW m^–2in a community dominated by Oiko pleura longicauda There wereno relationships between larvacean biornass and any size fractionof chlorophyll, suggesting that other factors must normallyregulate larvacean communities. The evidence indicates thatthis regulation is by predation. Annual production by larvaceanswas 586 kJ m^–3 year ^–1 (29.3 g AFDW m^–2 year^–1);production of houses could represent an added 300–600kJ m^–2 year. While copepod biomass was 10 times higherthan that of the larvaceans during the same period, copepodgrowth rates were only one.third those of larvaceans. Thus,larvacean annual production is at least 30% that of the copepods,due to their rapid growth rates, and at least 50% that of thecopepods when house production is considered. The contributionof larvaceans to plankton production has been underappreciatedhistorically when only their biomass is considered. ¹Present address :Monterey Bay Aquarium Research Institute 7700Sandholdt Road, Moss Landing, CA 95039-0628, USA     

Estimating nutrients recycling by tropical oceanic macroplankton-micronekton from biomass data

Simultaneous measurements of biomass and excretion of macroplankton-micronektonlead to similar results in tropical Atlantic and Pacific Oceans.Ratios are thus proposed to roughly estimate regenerated productionin oligotrophic tropical waters from biomass data: biomass figures(mg dry wt m^–2) should be multiplied by 0.32 to estimatethe amount of total nitrogen excreted (µg at m^–2day^–1) and by 0.027 to estimate the amount of total phosphorusexcreted (µg at m^–2 day^–1).     

Shape, size and density of daytime surface swarms of the euphausiid Meganyctiphanes norvegica in the Bay of Fundy

Daytime surface swarms of ^{Meganyctiphanes norvegica} in the Bayof Fundy were examined using a variety of techniques to providemeasurements of their shapes, sizes and densities. Shapes andsizes were determined from two aerial photographs: swarms werespherical, ribbon-like or amorphous. were up to 28 6 m longand ranged in area from 0.4–111 7 m². Densities were measuredby a bag-sampling device which gave figures of up to 41 000animals m^–3 by photographic methods which gave figuresof up to 770 000 animals m^–3 and by a plankton net whichgave maximum values of six animals m^–3 Using the photographicmethod the maximum euphausud biomass was estimated to be 154kg m^–3 within swarms and the largest swarm measured wasestimated to contain up to 2 1 tonnes of M. norvegica. Meanpatch biomass estimates for the two aerial photographs rangedfrom 77.8–778 g m^–3 and 15 6–155.6 g m^–3which are similar to figures obtained by other authors usingintegrating sampling techniques at depth.     

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.     

Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50{degrees}N-30{degrees}S transect of the Atlantic Ocean

Size-fractionated mesozooplankton grazing and metabolism wereinvestigated along the wide latitudinal range (50°N–30°S)covered during the Atlantic Meridional Transect (AMT) 11 cruise.Five different oceanic provinces were traversed in this cruise:North Atlantic Drift (NADR), North Atlantic Subtropical Gyral(NAST), Canary Coastal (CNRY), Eastern Tropical Atlantic (ETRA),and South Atlantic Gyral (SATL). CNRY and ETRA were affectedby the upwelling Mauritanian and equatorial respectively andprimary production in these provinces was higher than in theoligotrophic subtropical gyres (NAST and SATL). Both mesozooplanktonand phytoplankton biomass were highest around the equator. Theamount of chlorophyll a ingested daily by copepods was noticeablyhigher in mesotrophic than in oligotrophic provinces as shownby the spatial distribution of gut content values and the highabundances of copepods recorded at the equator. Grazing impactalong the transect ranged from 0.2 to 5.6% of the phytoplanktonstanding stock and from 1.6 to 14.5% of primary production.If only phytoplankton >2 µm are considered, the rangesare 1.0–19.4% (stock) and 3.4–44.7% (primary production).Grazing impact upon both phytoplankton biomass and primary productionfollowed a spatial distribution similar to that of chlorophylla ingestion, with higher values in upwelling zones than in thegyres. Weight-specific rates of respiration and NH₄⁺ and PO₄^3–excretion showed large variability both along the transect andwithin provinces, but did not differ between provinces. Therefore,zooplankton assemblages inhabiting the different provinces visitedin the AMT 11 seem to be adapted to the prevailing thermal conditions.Given the substantial proportion of nitrogen and phosphorusthat are supplied to primary producers through the excretoryactivity of mesozooplankton (the percentage of nitrogen andphosphorus requirements of phytoplankton accounted for by mesozooplanktonexcretion was >30% in all the provinces) it follows thatthey may play a crucial role as nutrient regenerators, especiallyin the oligotrophic gyres where regenerated production dominates.     

Regional differences in stratification and its effect on phytoplankton production and biomass in the northwestern Irish Sea

Differences in tidal mixing result in the formation of offshoremixed and stratified regions in the NW Irish Sea during springand summer. Stratification resulted from vertical gradientsin temperature, although vertical gradients in salinity wereimportant during the early stages of stratification. The northerncoastal and offshore mixed regions were characterized and distinguishedfrom the southern coastal and summer stratified regions by thepresence of more saline, cool near-surface water and incompletedepletion of dissolved inorganic nutrients. Distinct regionaldifferences in the production season of phytoplankton were observed.This lasted 6 months with a seasonal production of 155 g C m^–2in the southern coastal region. A shorter season, 4 and 2 months,and lower production of 101 and 96 g C m^–2 occurred inthe summer stratified and northern mixed regions, respectively.The southern mixed region supported the shortest season (<2months) and lowest production (66 g C m^–2). It is estimatedthat a daily light exposure of {small tilde}200 Wh m^–2is required for the onset of the production season and it isconduded that the subsurface light climate as a function ofsolar radiation and surface mixed layer depth, rather than nutrientavailability, controls its duration. The existence of a shortlate production season in the offshore regions is confirmedand this contrast with the season in the North Sea is attributedto differences in the subsurface light climate. Sustained productionin the southern coastal region may play an important role infish recruitment and offset any effect of the short late offshoreproduction season.     

The estimation of a mackerel (Scomber scombrus L.) spawning stock size by plankton survey

The International Council for the Exploration of the Sea (ICES)Mackerel Working Group recognises two mackerel stocks in Europeanwaters, the North Sea stock and the Western stock. From Marchto July 1977 an Anglo-French plankton survey was carried outin the shelf waters of the Bay of Biscay, Celtic Sea and westof Ireland to estimate the size of the Western stock. Sampleswere collected with a 76 cm Lowestoft pattern high-speed planktonsampler fished to a maximum depth of 100 m. The number of eggsm^–2 at each of five development stages were raised todaily production estimates with development rate coefficientsmeasured with live material collected in March. The mean numberof stage I eggs m^–2 d^–1 was raised to a total dailyproduction estimate for each month by the area within the minimumcontour level, and a production curve was drawn for the spawningseason. The total egg production was estimated at 1.98 x 10¹⁵eggs, + 30%, – 20%. With a mean fecundity of 361 000 anda sex ratio of 0.64 males per female this is equivalent to 8995 million fish.     

Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied,focusing on fish larvae and their copepod prey. The major objectiveswere to describe distributional overlap and evaluate the predatoryimpact by larval fish. The study was carried out across DoggerBank in the North Sea, August-September 1991. Sampling transectscrossed tidal fronts off the Bank and plankton at all trophiclevels showed peak abundance within frontal zones. Also Verticallythere was a significant overlap in distributional patterns ofthe plankton. Seven species of fish larvae were abundant, ofthese sprat (Sprattus sprattus) dominated. The abundance ofone group of fish larvae peaked in the shallow water close tothe Bank, whereas other species, including sprat, were foundin deeper water. Prey preference and predation pressure of fishlarvae were assessed using information on prey sizes and growthrates of larvae and the copepod prey. We estimated larval removalof preferred prey sizes to 3–4% day^–1, counterbalancedby a 3–7% day^–1' replenishment from copepod productionand growth. Additional predation pressure on copepods by aninvertebrate predator was estimated to 1–3%day^–1.In conclusion, the dynamics of fish larvae and other zooplankterswere closely linked. At peak abundances of fish larvae (>35mg dry weight m^–2), the accumulated predation on specificsize ranges of copepods, exerted by larvae and other predators,could exceed the ability of copepod replenishment and intra-/interspecificcompetition among predators might take place.     

Fine-scale spatial patterns in the coastal epiplankton off southern California

Fine-scale vertical (5 – 40 m) and horizontal (50 –500 m) patterns of temperature, chlorophyll and abundance ofzooplankton species were sampled with a pump filtration systemin the surface waters offshore of San Diego in May and October,1978. Intense and consistent patterns were most apparent invertical profiles. Herbivorous zooplankton were more consistentlyassociated with the estimated primary productivity maximum thanwith the deeper chlorophyll maximum layer, which representeda phytoplankton biomass maximum. Predators were positively correlatedwith abundant potential prey species. Variations in body lengthwith depth suggest that these fine-scale patterns were sufficientlystable to influence zooplankton growth. Consequences for grazingand predator – prey interactions in pelagic ecosystemsare discussed. ¹Present address: NOAA/NMFS Southwest Fisheries Center, PO Box271, La Jolla, CA 92038, USA     

Bacterial growth and grazing loss in contrasting areas of North and South Atlantic

Samples were collected from the top 200 m of the water columnat 50 stations during two cruises in different, near equinoctialseasons on an Atlantic transect near the 20°W meridian between50°N and 50°S. These samples were analysed to determinecharacteristics of the heterotrophic bacterial populations.Flow cytometry was used to enumerate these bacteria and determinetheir average size so as to calculate their biomass. Heterotrophicbacterial production, and the rate of grazing of these bacteriaby heterotrophic nanoplankton in the main depth layers, weredetermined using ³H thymidine and ¹⁴C leucine techniques. Thebiomass of heterotrophic nanoplankton in these layers was determinedusing a glucosaminidase assay. Five provinces were distinguishedalong the transect and characterized by average values of allmeasured parameters. The relative composition and activity ofthe microbial community in the water columns within each provincechanged little between the two cruises. Lowest heterotrophicbacterial biomass of 1–2 mg C m^–3 and productionof 0.1–0.2 mg C m^–3 day^–1 were found in thenorthern and southern Atlantic gyres, and were relatively similarin both seasons. Biomass and production were 2–4 timeshigher in the northern and southern temperate waters, and inequatorial waters, than in the gyres and tended to show moreseasonal variation. Production and biomass in the layer belowthe pycnocline were lower by 10–30% and about 50%, respectively,than values determined in the surface mixed layer, and variedless with latitude. Depth-integrated values of these two parameterswere generally of similar size in the mixed water layer andthe layer of the chlorophyll maximum and pycnocline, and tendedto vary with season. The specific growth rate of heterotrophicbacteria was in the range 0.05 to 0.12 day^–1 in the topmixed layer at all latitudes. In spite of the elevated temperatures,bacterial growth appears to be restricted by a shortage of nutrientsso that the microbial community cycles very slowly, with a turnovertime of the order of 1 week or more. The depth-integrated biomassof heterotrophic nanoplankton was generally about 100% of theheterotrophic bacterial biomass in the same water. Grazing bythese nanoplankton at the rate measured could consume all ofthe new production of heterotrophic bacteria in all waters,and they probably control the populations of both heterotrophicand phototrophic bacteria.     

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.     

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.     

Temporal and spatial variation of plankton abundance in the Gulf of Carpentaria, Australia 1975-1977

The Gulf of Carpentaria is a large (ca. 3.7 x 10⁵ km²) shallow(<70 m) embayment in tropical northern Australia lying between11 and 17.5°S latitude. Although it contains a multi-speciespenaeid prawn fishery which is Australia's largest and mostvaluable fishery its hydrology and planktology are largely unknown.As a background to a study of the larval ecology of penaeidstocks, ten Gulf-wide survey cruises, sampling the planktonand hydrography, were undertaken over a twenty month periodfrom August 1975 to May 1977. Though comparisons with otherstudies are difficult because of variations in sampling techniquesand biomass estimation methods, the plankton biomass in theGulf of Carpentaria appears to be high by comparison with otherareas around Australia. The mean estimate over all stationsand all cruises of 77 mg/m³ dry weight (1880 mg/m²) compareswith the very high abundances found only in seasonal upwellingareas south of Java and off the northwest shelf of Australia.Further, the Gulf of Carpentaria standing stocks of planktoncompare with other coastal areas supporting important fisheriesoff the west coast of North America, the eastern North AtlanticOcean and some European waters. Because of its depth, relativelyhigh temperature and primary production rates, secondary productionrates are assumed to be high as well but as yet are unmeasured. ^*Microfiche of station list available upon request. CSIRO MarineLaboratories Reprint No. 1280     

Acantharian abundance and symbiont productivity at the VERTEX seasonal station

The abundances of acantharians and the carbon fixation ratesof their symbiotic algae were measured over an 18 month periodat the VERTEX seasonal station, 1400 km west of Monterey, CA.Abundances varied up to 4 1 acantharians l^–1, with thehighest values in the upper euphotic zone. Integrated abundancesvaried seasonally by a factor of two and were highest (<170000 acantharians m^–2) in the summer and fall The biomassof acantharians (estimated from cell volume) ranged from 6.8to 56.7 mg C m^–2 and did not exhibit a seasonal pattern.The mean number of symbiotic algae per acantharian averaged14 7 and varied between 11 and 23 on different cruises. Approximatelyhalf of the acantharians at this station had symbiotic algae.Carbon fixation rates of the acantharian symbionts were highestnear the surface (maximum rate of 26.7 ng C acantharian^–1day^–1) and declined exponentially with depth. In the upper20 m, symbiont carbon fixation in acantharians was >4% ofthe total primary production and between 6 and 35% of the primaryproduction by plankton larger than 100 µm Exports of acanthariansfrom the euphotic zone by the sinking of intact cells were atleast 2–6% of the standing stock per day and would representup to 9% of the total sinking organic carbon flux. These ratesof carbon exports are comparable to the rate of carbon fixationby the symbionts in the acantharian population