Potential contributions of vertically migrating Rhizosolenia to nutrient cycling and new production in the open ocean

We present a numerical model of nutrient uptake and photosynthesisduring migrations of the marine diatom Rhizosolenia that wasdeveloped to estimate fluxes of carbon and nitrogen due to thesemigrations in the open ocean. The predicted specific rate ofincrease of Rhizosolenia was 0.11–0.15 day¹, whereas thetotal time for one migration cycle ranged between 3 and 5 days.Using published estimates of Rhizosolenia abundance, we estimatethat new primary production due to Rhizosolenia migrations rangesbetween 0.018 and 0.033 mmol N m^–2 day^–1. Thesevalues represent up to 17% of new production due to turbulentdiffusive fluxes of nitrate into the euphotic zone and are ofthe same order of magnitude as new production due to nitrogenfixation in tropical oceans. Large-scale contributions of Rhizosoleniato oceanic new production are limited by their relatively lowstanding crop. Variations in the formulation of losses withdepth greatly affected gross and net fluxes of carbon and nitrogen.Better characterization of losses of Rhizosolenia and improvedestimates of its abundance will help determine more accuratelythe contributions of Rhizosolenia to global biogeochemical cycles. ¹Present address: Department of Agricultural and EnvironmentalScience, The Queen's University of Belfast New forge Lane, BelfastBT95PX, UK     

Nitrogen inputs into the euphotic zone by vertically migrating Rhizosolenia mats

Rhizosolenia mats conduct extensive vertical migrations in theoligotrophic central North Pacific (cNP) gyre that permit thesediatoms to acquire nitrate at depth and return to the surfacefor photosynthesis. The ultimate fate of this N within the ecosystemis unknown, but may include remineralization by grazing, lossto depth by sinking biomass, or N excretion by Rhizosoleniamats. Direct release of N by mats into the mixed layer wouldrepresent an upward biological pump that circumvents the diffusionbarriers and nutrient sinks at the base of the oceanic euphoticzone. We examined Rhizosolenia mat N release along a transect(28–31° N) in the summer of 2002 (Hawaii to California)and 2003 (Hawaii to west of Midway Island) using sensitive fluorometricand chemiluminescence methods. Nitrate, NO₂^– and NH₄⁺release was determined. Nitrate and NH₄⁺ release by the matsoccurred in both 2002 (22.84 ± 6.04 and 3.69 ±1.74 nmol N µg^–1 Chl a h^–1, respectively)and 2003 (23.74 ± 3.54 and 3.60 ± 0.74 nmol Nµg^–1 Chl a h^–1, respectively). Nitrite releaseonly occurred in the 2003 summer period but occurred in bothyears when Fe chelators were added. Fv/Fm values decreased westwardin 2003 suggesting a gradient of increasing physiological stresstowards the west. The various physiological measures are consistentwith concurrent Fe stress; however, other possibilities exist.Nitrate excretion was the dominant form of N release in bothyears and provided a substantial addition to the ambient nitratepool in the mixed layer. Rhizosolenia mat nitrate release suppliesat least 4–7% of the nitrate pool on daily basis, andpossibly as much as 27%. Rhizosolenia mats are part of a largephytoplankton community that appears to migrate, and rates couldbe significantly higher. Literature reports suggest little orno nitrification in the upper euphotic zone, and thus biologicaltransport and release of nitrate may be a major source to thisregion. This N release is uncoupled from upward CO₂ transportand, like N₂ fixation, provides a component of the N pool availablefor net carbon removal.     

A 91-year record of seasonal and interannual variability of diatoms from laminated sediments in Saanich Inlet, British Columbia

Diatom seasonal succession and interannual variability werestudied using laminated sediments from Saanich Inlet, BritishColumbia, for the years 1900–1991. Frozen sediment coresallowed fine-scale sampling of laminae for each year. Thus,three ‘seasons’ for each year were identified basedon species composition. Thalassiosira species were indicatorsof spring deposition. Skeletonema costatum was abundant in samplesfollowing Thalassiosira, probably deposited in late spring andsummer. Rhizosolenia sp. was most abundant in fall/winter samples.Diatom stratigraphies were related to sea surface temperature,salinity, sea level and the Pacific North American Index (PNA)using canonical correspondence analysis (CCA). CCA showed thatspecies of a particular season generally had optima for temperatureand salinity characteristic of that time. Interannual changesin diatom species composition and abundance were most prevalentin the decades 1920–1940, with the exception of S.costatumwhich showed cyclic changes in abundance. Skeletonema was moreabundant during periods of cool temperatures, while littoraldiatoms were more abundant during times of heavy winter rains.Sea level was an important variable in CCA and while its relationshipto diatoms is not clear, it may be related to variations innutrient supply to diatoms in surface waters.     

Zooplankton community grazing in a hypertrophic lake (Hartbeespoort Dam, South Africa)

Seasonal abundance as total biomass and specific densities ofthe main herbivorous zooplankton (>60 µm) in hypertrophicHartbeespoort Dam from 1981 to 1983 are described. After springzooplankton biomass maxima each year there followed a mid-summerdecline in the Daphnia population and a shift to a smaller bodiedcladoceran community dominated by Ceriodaphnia concomitant witha change from largely edible phytoplankton species to abundantcolonial Microcystis. In situ community grazing rates were measuredthroughout 1983 using a ¹⁴C-labelled unicellular alga. Integratedcommunity grazing rates measured in the aerobic water columnwere highest in December (260.2%/day) when Daphnia was abundantand the edible component of the phytoplankton was diminishing.Lowest integrated community grazing rates occurred in January–February(19.8–35.3%/day) and July (28.3%/day) when the phytoplanktonwas composed almost entirely of Microcysris, and Ceriodaphniadominated the zooplankton community whilst food availabilitywas low. Feeding in Ceriodaphnia was not hindered by abundantlarge Microcysris colonies; total biomass specific grazing ratewas high when Ceriodaphnia dominated and low when Daphnia dominatedthe zoo-plankton community. Results indicate that in hypertrophicconditions it is unlikely that large filter-feeders such asDaphnia are able to retard or limit the development of Cyanophyceaeblooms by high grazing pressure.     

Feeding of Sagitta enflata and vertical distribution of chaetognaths in relation to low oxygen concentrations

Zooplankton samples were collected in Mejillones Bay, northernChile (23°00'15'S, 70°26'43'W ). Sampling was conductedat 4 h intervals, for 24 h during three seasons, austral spring(October 2000), summer ( January 2001) and winter (August 2001)at three different strata (0–25, 25–50 and 50–100m). Five species of chaetognaths were collected. Sagitta enflatawas the most abundant species, representing up to 65% of allchaetognaths in total numbers, followed by Sagitta bierii, makingup 34% of the total abundance of chaetognaths. S. enflata wasdistributed mainly above the Oxygen Minimum Zone, while S. bieriiremained below this zone. Feeding rates were relatively constantwithin the upper layer (0–25 m depth), for each samplingdate, averaging 1.2 prey S. enflata day^–1, and decreasingwith depth. Gut content analyses demonstrated that predationwas principally focused on small copepods (<1500 µm),with greatest feeding activity occurring at night. The dailypredation impact on the total standing stock of small copepodsvaried seasonally between 6% in spring and 0.4% in winter. Thispercentage may represent a negligible impact on the entire copepodcommunity, but it is relevant at the species or genus level,since S. enflata removed more than 20% of the standing stockof Centropages brachiatus and Corycaeus sp. Thus, during someperiods of the year, chaetognaths may strongly influence theabundance and size distribution of copepods in coastal upwellingecosystems.     

Zooplankton composition and distribution off the coast of Galicia, Spain

During June and September 1984, zooplankton samples were collectedwith other hydrographic and biological data along the Galiciancoast (NW of Spain). In June copepods contributed {small tilde}60%to the total zooplankton community, with larvaceans, siphonophoresand cladocerans also abundant. In September >90% of the zooplanktonsampled were copepods. The dominant species of copepods in bothJune and September were Acartia clausi, Paracalanus parvus andTemora longicornis. The meroplankton was dominated by echinoderms,bryozoans, barnacle larvae and bivalve larvae. In June the averagezooplankton biomass was 31.08 mg C m^–3; the Septemberaverage was 41.69 mg C m^–3. The relationship between theslopes of the regression equations (biomass versus abundance)suggests that the zooplankton assemblage in June was composedby larger animals than in September. The major concentrationof zooplankton was between 0 and 50 m, with both June and Septemberdaytime surface samples having 6–7 times the amount oforganisms than the lower water column (50–100 m). Therewere no distinct differences in total zooplankton abundancesat the inshore and offshore stations; however, the inshore stationsoften had a higher percentage of meroplankton than the offshorestations. In June zooplankton abundance at the northern transectsand the western transects was similar. In September there weregreater concentrations of zooplankton in the western Galicianshelf as compared with the northern shelf. These differencesin the horizontal distribution of the zooplankton were relatedto upwelling events.     

Biogeographic study of the planktonic communities of the Prince Edward Islands (Southern Ocean)

Microphytoplankton and zooplankton composition and distributionin the vicinity of the Prince Edward Islands and at the Sub-antarcticFront (SAF) were investigated in late austral summer (April/May)1996. Samples were collected for analysis of chlorophyll a concentration(Chi a), microphytoplankton and zooplankton abundance. Generally,the highest Chl a concentrations (up to 2.0 µg l^–1)and zooplankton densities (up to 192 ind. m^–3) were recordedat stations within the inter-island area while the lowest values(<0.4 µg l^–1) were observed at stations upstreamof the islands. High Chl a and zooplankton biomass values werealso associated with the SAF. Microphytoplankton were dominatedby chain-forming species of the genera Chaetoceros (mainly C.neglectus),Fragilariopsis spp. and the large diatom Dactyliosolen antarcticus.The zooplankton assemblages were always dominated by mesozooplanktonwhich at times contributed up to 98% of total zooplankton abundanceand up to 95% of total biomass. Among mesozooplankton, copepods,mainly Clausocalanus brevipes and Metridia lucens numericallydominated. Among the macrozooplankton euphausiids, mainly Euphausiavallentini, E.longirostis and Stylocheiron maximum, and chaetognaths(Sagitta gazellae) accounted for the bulk of abundance and biomass.Cluster and ordination analysis did not identify any distinctbiogeographic regions among either the microphytoplankton orzooplankton.     

Distribution and species-specific egg production of Pseudocalanus in the Gulf of Alaska

Pseudocalanus species are important contributors to the secondaryproduction of the northern hemisphere mid- to high-latitudeoceans. In the coastal Gulf of Alaska, Pseudocalanus are presentyear round and are represented by three species. In 2001, Pseudocalanusmimus was the dominant Pseudocalanus species on the shelf duringspring and summer, comprising 30–100% of the total, whilePseudocalanus newmani dominated in Prince William Sound (10–90%).Pseudocalanus minutus were only abundant in Prince William Soundduring early spring. Egg production (by number and volume) wasa function of female prosome length and decreased from springto summer; however, significant variability was attributableto regional influences that were independent of size. For thesame sized female, P. newmani produced more eggs per clutchthan P. mimus. Pseudocalanus mimus, however, tended to havea larger mean egg size than P. newmani. Consequently, clutchvolumes of the two species were indistinguishable. Pseudocalanusegg production rates (EPRs) (eggs female^–1 day^–1)were lower in July and August (ca. 2–4) than April andMay (ca. 1–9), but total egg production by the population(eggs day^–1) was nearly equivalent for the two time periodsdue to higher female concentrations in summer.     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Vertical structure of small metazoan plankton, especially noncalanoid copepods. I. Deep Arabian Sea

The abundance and vertical distribution of micro-metazoans sampledwith fine nets of 0.05 mm mesh size were studied at three stationsin the Arabian Sea during the intermonsoon period (April/May1987) and down to 1850 m depth. In the epipelagic zone (0–100m). values of biomass and metazoan abundance tended to be higherthan those reported for other tropical oceanic areas. In themesopelagic zone, which is characterized by an extreme oxygendeficiency between 100 and 1000 m depth, the abundance of metazoantaxa and species numbers of non-calanoid copepods were largelyreduced. However, intermediate abundance maxima occurred withinthis zone, which were dominated by specific metazoan taxa (copepods.appendicularians) and species of non-calanoids (Oncaea sp. C).In the bathypelagic zone below 1050 m, the species diversityof the dominant copepod family Oncaeidae increased substantially.Two-thirds of a total of 69 oncaeid species recovered were confinedto this layer. As most of them were small in size and occurredin low abundance only, the increase in total oncaeid densityand/or plankton biomass was less conspicuous. Dominant Oncaeaspecies in the bathypelagic zone were O.longipes and O.brodskii.The results are compared with published data from the ArabianSea and other tropical oceanic areas with and without an extrememesopelagic oxygen minimum zone Possible causes of the intermediateabundance maxima within the oxygen deficiency zone are discussed.     

Periodicity of composition, abundance, and vertical distribution of summer zooplankton (1977/1978) in Ezcurra Inlet, Admiralty Bay (King George Island, South Shetland)

Seasonal succession and variation in species composition, density,biomass, age distribution and frequency of zooplankton (mainlythe Copepoda) were analysed during the austral summer of of1977/1978 in Ezcurra Inlet, a part of the Antarctic coastalecosystem. Small zooplankters (i.e., cyclopoids of the generaOncaea and Oithona, and calanoids Drepanopus pectinatus andScolocithricella glacialis) were found to predominate in termsof abundance and percentage contribution. The zooplankton biomasswas dominated by larger organisms of the Metrididae and Calanidae(Calanoida). The maximum abundance and maximum biomass of copepodswere recorded in February; two small peaks in copepod biomassbeing observed in late December and late January, and a lesserbiomass peak in late December. The vertical distribution ofcopepods in terms of their diel and seasonal (December, January,February, March) changes showed a day-time maximum to have occurredin the near-bottom layer, the nocturnal distribution being bimodalwith peaks within 0–10 m and 25–50 m. The summerzooplankton community in Ezcurra Inlet is controlled by trophic(phytoplankton composition and density) and hydrological (waterexchange with Bransfield Strait) conditions prevailing in thearea.     

Vertical distribution of pelagic chaetognaths and feeding of Sagitta enflata in the Central Equatorial Pacific

The vertical distribution and feeding of pelagic chaetognathsat 5°S, 160°W in the Central Equatorial Pacific wereinvestigated using a series of 0–500 m vertical haulswith a VMPS net over a 24 h period between 6 and 7 October 1990.The total number of individuals per haul was between 370 and688. Fourteen species in four genera were found at this station.The most abundant species was Sagitta enflata which comprised32.4–61.1% of the individuals collected from the 0–500m layer. Mesopelagic species made up 9.3–15.1% of thetotal number of individuals. Sagitta enflata and Pterosagittadraco were found in the upper part of the thermocline both byday and at night. The fraction of the population containingfood items (FCF) of S.enflata in the 0–50 m layer variedbetween 4.8 and 12.5% (mean 10.8%) and feeding activity washighest between sunrise and noon. The percentages of Copepoda,Foraminifera, crustacean larvae, Chaetognatha, Pteropoda, Ostracoda,fish and unidentified material in the gut of S.enflata were51.9,6.7,3.8,2.9,1.9,1.9 and 30.9%, respectively. Sagitta enflataconsumed food organisms which were mainly between 0.5 and 1.0mm in length. The daily feeding rate of S.enflata was 1.81 preyper individual, which was equivalent to 8.06 mg C m^–2day^–1. This corresponded to     

Fluxes of diatoms in the Dona Paula Bay, west coast of India

Sediment traps were deployed at a station in the Dona PaulaBay to collect sedimenting particles at weekly intervals fromNovember to May during 1995–1997. Sedimented particleswere analysed for total diatom flux, chlorophyll a (Chl a) andparticulate organic carbon (POC). The highest diatom flux wasrecorded in April–May for both the years. Fluxes of diatomsvaried from0.6 x 10⁴ cells m^–2 day^–1 (November 1995)to 121.47 x 10⁴ cells m^–2 day^–1 (December 1996).In all, 19 diatom genera were identified in the sedimented material.Navicula, Nitzschia, Pleurosigma, Licmophora, Coscinodiscus,Rhizosolenia and Surirella were the most abundant genera inthe sedimented material throughout the sampling period. Meanflux of POC and diatom carbon was 251 and 0.39 mg C m^–2day^–1, respectively. The diatom carbon accounted for 0.15%of the POC flux. Mass flux of diatoms showed significant negativecorrelation with the concentration of nitrate and phosphate.This suggests that the nutrient concentration played an importantrole in influencing the sedimentation of diatoms at this coastalstation.     

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

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

The spatial distribution of temperature, oxygen, plankton and fish determined simultaneously in Lake Kinneret, Israel

Temperature and concentrations of oxygen, phytoplankton, zooplanktonand fish were measured simultaneously on the night of 28 February-1March 1991, in Lake Kinneret, Israel. High concentrations ofoxygen, up to 19.9 mg l^–1 (207% saturation), were recorded.The phytoplankton was dominated by the dinoflagellate Peridiniwngatunense, which was horizontally and vertically unevenly distributed.We hypothesize that the oxygen supersaturation was the resultof a high standing stock of Peridinium, combined with high irradianceinput and mild wind-generated turbulence. The relationship betweenthe concentration of oxygen and Peridinium density was highlysignificant, both above the thermocline (negative correlation)and below the thermocline (positive correlation). Zooplanktonbiomass was dominated by cladocerans and cyclopoid copepods.The spatial distribution of these groups was not correlatedwith other measured parameters. Rotifers and calanoid copepodswere highly abundant in the northern part of the lake, and significantlyrelated to oxygen concentrations. It is suggested that the distributionsof rotifers and calanoid copepods are affected by the JordanRiver inflow. fish abundance was not correlated with limnologicalparameters or other biological components. We conclude thatthe distribution of the dominant fish species was governed byfactors other than those measured in this study.     

Dinoflagellate cyst production at a coastal Mediterranean site

To assess the diversity and seasonality of dinoflagellate cystproduction, surface sediment and trap samples were studied inthe Gulf of Naples (Mediterranean Sea). A total of 59 differentcyst morphotypes were recorded. At the stations within the 70m isobath, sediment assemblages were dominated by calcareousPeridiniales (66–79%), while at the deepest stations non-calcareousPeri-diniales attained the highest percentages (40–49%).The sediment trap sampling, carried out fortnightly over twoannual cycles, revealed high production rates (up to 1.7 x 10⁶cysts m^–2 day^–1) from spring to late autumn of bothyears, with a distinct seasonal production pattern. Althoughrather similar in species composition, the total cyst flux differedmarkedly between the 2 years (1.26 and 0.55 x 108 cysts m^–2year^–1, respectively). Species-specific production patternswere observed: some species formed cysts over several months,others in restricted periods of the year. Cyst-forming speciesconstituted a small part of the planktonic dinoflagellate populationsrecorded in the area. A coupling between the trap material andsurface water plankton was observed for calcareous Peridiniales.This sampling approach allowed the detection of some speciesnever recorded before in the gulf, including two potentiallytoxic species: Alexandrium andersoni and Gymnodinium catenatum-likespecies.     

DISTRIBUTION AND ABUNDANCE OF GYMNOSOMATA (GASTROPODA: OPISTHOBRANCHIA) IN THE SOUTHWEST ATLANTIC

The distribution and abundance of gymnosome gastropods in theArgentine Sea and Brazil—Malvinas Confluence during 1978–1979and 1988 were studied. The collections analyzed included 768quantitative samples obtained between 48°W and the coast,and from 35°S to 55°S. Two species were found. Spongiobranchaeaaustralis was the most frequent and abundant (up to 730 per1000 m3); its presence in the area was associated with the coreof the Malvinas Current. Clione antarctica was less abundant(maximum abundance: 230 per 1000 m³) and was also associatedwith the Malvinas Current. The geographic ranges of both speciesin the area are wider than previously described. Since the rangeof S. australis in the area extends far from the range of itsprey Clio, it is not clear whether S. australis can feed onthe thecosomatous pteropod Limacina (and not only on Clio, asdescribed in the bibliography) or it starves in that area. Duringthe 1978–1979 annual cycle, the abundance of both speciesfollowed neither the abundance patterns of their prey nor ofthe total zooplankton, and differed from each other. The residencetime of swarms of both gymnosomes were shorter than one month.As a general pattern, the aggregates are rapidly transportednorthward by the Malvinas Current and also penetrate the outershelf water, but they remain there only during a short periodand cannot preclude the final expatriation. So, the abundanceof gymnosomes in the area depends on passive migration morethan intrinsic population factors. (Received 9 July 1997; accepted 15 December 1997)     

Temporal variability in the night-time distribution of epipelagic siphonophores in the North Atlantic Ocean at Bermuda

The temporal variability of siphonophores was compared on night-to-night,month-to-month and year-to-year scales in open-net collectionsmade at or near the Bermuda Atlantic Time Series (BATS) site.These collections were made as oblique tows to 100 m on (i)one summer night in each of 1986, 1989 and 1991, (ii) one nightper month on 13 cruises between April 1991 and August 1992,and (iii) three consecutive summer nights of 1992. The meanwet displacement volume (WDV) of sipho nophores in the thesecollections was 1.4 ± 0.6 ml per 100 m³ or 20 ±10% of the total catch biovolume. The most common species belongedto the Calycophorae families Diphyidae and Abylidae: the combinednumerical abundance of Chelophyes appendiculala, Eudoxoidesmitra, Lensia subtilis. Eudox-oides spiralis, Bassia bassensisand Abytopsis eschscholtzi averaged 300–400 per 100 m³.Combined numbers of these common species varied little whensampled hour by hour at the same location during consecutivesummer nights or at different night-time sampling locations10¹–10² km apart when these were sampled within 3–5h. In contrast, most of these six species exhibited 2- to 3-folddifferences in night-time abundance between summers of differentyears and between seasons. Three seasonal groups were detected,with a winter group being the most distinct from the others.Changes in the abundance of several species were correlatedwith seasonal changes in the depth of the mixed layer.     

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

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

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.