Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates wereinvestigated along a 50°S–50°Nlatitudinal transect,during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises(boreal spring–autumn 1997, boreal spring–summer1998). Copepod abundance was higher at high latitudes in spring,near northwest Africa, in the equatorial and Benguela upwellingsystems, and in the Subtropical Convergence, and lower in oligotrophicgyres. Gut contents were not related to phytoplankton biomassor production. Gut evacuation rate averaged 0.03 min^-1, andwas not related to latitude or body size. Conservative estimatesof copepod community total ingestion rates ranged between 3.4and 173 mg C m^-2 day^-1 for AMT4, 1.6–252 mg C m^-2 day^-1in AMT5 and 10–160 mg C m^-2 day^-1 in AMT6. Maximum valueswere always in the upwelling regions, the subtropical convergenceand high latitudes in the Northern Hemisphere during borealspring. Calculated ingestion rates translate into average dailyminimal consumption values of 2.07%, 1.89% and 2.6% of totalchlorophyll stock, or 8.02%, 14.5% and 12.9% of total primaryproduction ingested daily on AMT4, 5 and 6 respectively. Grazingimpact increases considerably if we consider ingestion of phytoplanktonlarger than 2 µm, especially under the influence of theEquatorial and North African upwellings, where copepod ingestionrepresents up to 30% of the biomass and >100% of productionby large cells.     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

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.     

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

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

Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a shallow hypereutrophic lake

The impact of a cyclopoid copepod population on the protozoacommunity (two ciliate categories and Cryptomonas) was assessedweekly during the spring cohort of Cyclops vicinus (one monthduration) in hypereutrophic Lake Søbygård by insitu gradient experiments with manipulation of ambient zooplanktonabundance. As C.vicinus always made up >92% of the zooplanktonbiomass, the response of protozoa is assumed to be a resultof predation by the copepod. Significant effects of copepodbiomass on protozoa net population growth rates were obtainedin the four experiments. Copepod clearance rates were significantlyhigher on oligotrichs than on prostomatids and Cryptomonas butdeclined for all three protozoa categories during the firstthree weeks of the copepod cohort, probably because of the changein developmental instar composition of the copepod population.Grazing impact on protozoa at ambient copepod abundance wasconsiderable (range, 0.05–0.87 day^–1) and could,together with the estimated reproductive potential of protozoans(range, –0.20–0.87 day^–1), account for thedecline in abundance and biomass of protozoa during the cohortdevelopment. Carbon flow from the protozoa to C.vicinus (range,2.8–23.5 µg C l^–1 day^–1) documents thepresence of a trophic link between protozoa and the spring cohortof C.vicinus in Lake Søbygård.     

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.     

Production of Penilia avirostris in Kingston Harbour, Jamaica

The cladoceran Penilia avirostris is one of the more abundantand widespread members of the crustacean zooplankton in nearshoretropical and subtropical waters. Its abundance, biomass, fecundity,development rate and production were estimated in Kingston Harbour,Jamaica, during an 18 month period. Mean annual abundance ofPenilia was 1821 m^–3, while biomass (excluding eggs/embryos)was 2.87 mg ash-free dry-weight (AFDW) m^–3 (43.1 mg AFDWm^–2), accounting for 13% of the copepod community biomass.Fecundity increased with body size. There was no clear seasonalpattern of abundance, size or fecundity, nor were physical orbiological variables correlated to these variations. Developmenttime averaged 20.5 h for juveniles and 41.4 h for adult femalesduring incubations; there was no clear evidence of a diel patternto molting. Growth rate appeared to be exponential, with correspondingsomatic growth rates, averaging 0.27 day^–1 for juveniles,and 0.34 day^–1 for somatic plus reproductive growth inadult females. Annual production was estimated as 173 kJ m^–2year^–1,     

Geographic distribution, seasonal life cycle, biomass and production of a planktonic copepod Calarms sinicus in the Inland Sea of Japan and its neighboring Pacific Ocean

The geographical distribution, seasonal life cycle, biomassand production of a copepod Calanus sinicus were investigatedin and around Kii Channel of the Inland Sea of Japan. The distributioncenter of the population was located in Kii Channel. The patternof the seasonal variation in abundance of copepodites and adultsdiffered geographically within the study area. In Kii Channel,for example, they were most abundant in June-July and leastabundant in October. Over the study area, the reproduction ofC.sinicus took place throughout the year, indicating the absenceof diapause phase. In adults, females usually outnumbered males.The prosome length of late copepodites and adults was inverselycorrelated with water temperature. The annual mean biomass washighest (4.87 mg C m^–3 or 231 mg C m^–2) in Kii Channel.The potential production rate of the population exhibited aseasonal variation more or less similar to that of the biomassand the annual potential production rate was 358 mg C m^–3year^–1 (14.1 g C m^–2 year^–1) in Kii Channel.Daily production and biomass (P/B) ratios in Kii Channel increasedfrom 0.11 at 11.8°C to 0.26 at 20°C.     

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.     

Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary productionstudies conducted in the vicinity of the Subtropical Front (STF),an adjacent warm-core eddy, and in Sub-antarctic waters duringthe third South African Antarctic Marine Ecosystem Study (SAAMESIII) in austral winter (June/July) 1993. Throughout the investigation,total chlorophyll (Chl a) biomass and production were dominatedby small nano- and picophytoplankton. No distinct patterns intotal Chl a were evident. At stations (n = 7) occupied in thevicinity of the STF, total integrated biomass values rangedfrom 31 to 53 mg Chl a m^–2. In the vicinity of the eddy,integrated biomass at the eddy edge (n = 3) ranged from 24 to54 mg Chl a m^–2 and from 32 to 43 mg Chl a m^–2 inthe eddy (n = 2). At the station occupied in the Sub-antarcticwaters, total integrated biomass was 43 mg Chl a m^–2.Total daily integrated production was highest at stations occupiedin the vicinity of the STF and at the eddy edge. Here, totalintegrated production ranged from 150 to 423 mg C m^–2day^–1 and from 244 to 326mg C m^–2 day^–1, respectively.In the eddy centre, total integrated production varied between134 and 156 mg C m^–2 day^–1. At the station occupiedin the Sub-antarctic waters, the lowest integrated production(141 mg C m^–2 day^–1) during the entire survey wasrecorded. Availability of macronutrients did not appear to limittotal production. However, the low silicate concentrations duringthe survey may account for the predominance of small nano- andpicophytoplankton. Differences in production rates between theeddy edge and eddy core were related to water column stability.In contrast, at stations occupied in the vicinity of the STF,the control of phytoplankton production appears to be relatedto several processes, including water column stability and,possibly, iron availability.     

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.     

Pelagic carbon metabolism in a eutrophic lake during a clear-water phase

Dissolved and paniculate organic carbon (DOC and POC, respectively),primary production, bacterial production, bacterial carbon demandand community grazing were measured for 9 weeks in eutrophicFrederiksborg Slotssø. The period covered the declineof the spring bloom, a clear-water phase and a summer phasewith increasing phytoplankton biomass. The process rates andchanges in pools of organic carbon were combined in a carbonbudget for the epilimnion. The POC budget showed a close balancefor both the post-spring bloom and the clear-water phase, whilea surplus was found in the summer phase. Production of POC wasdominated by phytoplankton (2/3) compared to bacteria (1/3)during all phases, and there was a significant correlation betweenphytoplankton and bacterial production rates (r² = 0.48, P <0.039). Bacterial demand for DOC was balanced by productionand changes in the pool of DOC during the decline of the springbloom, but the calculated demand exceeded the supply by 81 and167%, respectively, during the other two periods. The discrepancywas most probably due to an underestimation of bacterial growthefficiency and an overestimation of in situ bacterial productionin carbon units. Production of bacterial substrate by zooplanktonactivity was estimated to be higher than the direct excretionof organic carbon from phytoplankton. The biological successionwas regulated by the balance between area primary productionand community grazing. The clear-water phase was initiated bya combination of low primary production due to low surface irradianceand high community grazing (100 mmol C m^–2 day^–1),which caused a decrease in phytoplankton biomass. However, dueto the high initial phytoplankton biomass, community grazingwas not high enough to cause a significant decrease in areaprimary production. The summer phase was initiated by a decreasein community grazing followed by an increase in phytoplanktonbiomass. Based on these observations and calculations of areaprimary production as a function of chlorophyll concentrations,we suggest that the possibility for zooplankton to regulatephytoplankton biomass in temperate lakes decreases with increasingnutrient level.     

'St Martin's Summer': the case of an autumn phytoplankton bloom in the Gulf of Naples (Mediterranean Sea)

Phytopiankton abundance. species composition and primary productionof the Gulf of Naples were investigated during an autumn bloomin November 1985. Hydrographic data and surface phytoplanktonsamples were collected during a 3 day cruise, whereas investigationson in situ primary production and phytoplankton vertical distributionwere conducted from a second boat on three different dates.A coast-offshore gradient was recorded for most of the chemicaland biological parameters analysed. At stations closer to thecoast, which were affected by land run-off, phytoplankton populationsattained concentrations of 2.5 10⁶ cells l^–1 with amarked dominance of diatoms belonging to the genera Thalassiosiraand Chaetoceros. The most striking character of the system wasa remarkably high carbon assimilation rate (up to 1.04 g C m^–2day^–2) at stations closer to the coast. The causativemechanism for this bloom appeared to be land-derived nutrientenrichment, possibly enhanced by autumn rains, followed by aperiod of favourable meteorological conditions, which occursrecurrently in the region for a brief period around November,known locally as ‘St Martin's Summer’. We hypothesizethat a similar mechanism can stimulate phytoplankton growthmore than once every year. since high-stability penods followingmeteoro logical perturbations can occur several times in temperateregions of the northern hemisphere in autumn.     

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.     

The annual cycle of Mesodinium rubrum in the waters surrounding the Isles of Shoals, Gulf of Maine

Mesodinium rubrum was collected in Kemmerer bottles, fixed inBouin's solution and protargol stained. Cell volume showed aseasonal change of over an order of magnitude, being largest(3.25x10⁴ µm³) in the early spring and smallest in thesummer. Cell abundance was highest in the spring and lowestin the summer. Biomass followed a similar trend ranging from{small tilde}1 to 147 J m^–3 Production, estimated by amultiple regression that incorporated ambient temperature andcell volume, was 2.5 kJ m^–3 day^–1 This is {smalltilde}0.3% of the primary production.     

Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer 1999: the distribution of taxonomic groups in relation to physical/chemical conditions in the photic zone

A study of the phytoplankton community in the Faroe-ShetlandChannel was conducted in July 1999. Samples were collected atvarious depths in the photic zone along three transects (thenorthern entrance, the center and the southern entrance). Exceptfor a few easterly stations where nitrate and silicate werebelow 1 µM, all nutrients (phosphate, silicate, ammonium,nitrite and nitrate) were non-limiting for phytoplankton growth.HPLC pigment analysis revealed a pronounced (>50%) dominanceof Prymnesiophyceae at all stations. Their pigment ratio ofdiatoxanthin + diadinoxanthin/Chl a (DDX/Chl a) indicated thatthe phytoplankton community was controlled by light. Primaryproduction in the delayed spring bloom varied from 1.2 to 1.8g C m^–2 day^–1 along the northern transect. Alongthe other two transects, primary production ranged from 1.6to 3.8 g C m^–2 day^–1. Associated with the characteristicsindicating the establishment of a bloom, the relative contributionof diatoms and Prymnesiophyceae increased, whereas that of Prasinophyceae,Cryptophyceae, Chrysophyceae and Cyanobacteriaceae decreased.With respect to their vertical distribution, Cyanobacteriaceae,Chrysophyceae and Dinophyceae tended to have a higher abundance,relative to other taxonomic groups, in the surface layers. Therelative abundance of diatoms and Chlorophyceae increased withdepth. The DDX/Chl a ratio of the Prymnesiophyceae decreasedwith depth, indicating that vertical mixing in the upper 30m of the photic zone occurred less frequently than the timespan of physiological acclimation of cellular pigment composition.     

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.     

Ecological studies on the phytoplankton of Boknafjorden, western Norway. II. Environmental control of photosynthesis

Both predicted (incubator) and measured (in situ) ¹⁴C-assimilationrates were studied from February to November 1981 at three stationsin Boknafjorden, a deep silled fjord of western Norway. Sampleswere taken from different light depths within the euphotic zone.A high degree of conformity was found between the two approaches.Daily values of carbon assimilation integrated over the euphoticzone varied between 0.05 and 1.4 g C m^–2. Yearly primaryproduction varied between stations from 82 to 112 g C m^–2(120–148 g C m^–2 when based on average light conditions).The light-saturation curve parameters ^B and P^B_max ranged from0.0056 to 0.0537 mg C mg Chla^–1 h^–1 µE^–1m² and from 0.7 to 8.5 mg C mg Chla^–1 h^–2 (in situassimilation numbers ranged from 0.9 to 9.3 mg C mg Chla^–1h^–1) respectively, which compare well with those publishedfrom the northwestern side of the Atlantic. The overall importanceof light in controlling photosynthesis throughout the year wasrevealed by the light utilization index , estimated to be 0.43mg C mg Chla^–1 E^–1 m². The maximum quantum yieldwas encountered on August 17, with 0.089 mol CE^–1. Chla/Cratios above and below 0.010 were found to be typical for shade-and light-adapted cells respectively. Assimilation numbers andgrowth rates were linearly related only when considering light-adaptedcells. Consistent with the findings of this study, the applicabilityof I_K, ^B and P^B_max as indicators of light-shade adaptation propertiesshould be questioned. Maximum growth rates were encounteredduring an autumn bloom of the dinoflagellate Gyrodinium aureolum(1.0 doublings day^–1), while 0.7–0.8 doublings day^–1were found for a winter bloom (water temperature of 2°C)of the diatom Skeletonema costatum. No unambiguous temperatureeffect on assimilation number and growth of phytoplankton couldbe recognized in Boknafjorden. A tendency towards increasedassimilation numbers coinciding with increased water columnstability was revealed. The highest P^B_max values were oftenencountered at almost undetectable nutrient concentrations.At least during summer this could be attributed to recyclingof nutrients by macro- and/or microzooplankton, responsiblefor a greater part of the primary production now being grazeddown. This study supports the convention that the depth of theeuphotic zone may extend considerably below the 1% light depth.     

Growth and development of Neocalanus flemingeri/plumchrus in the northern Gulf of Alaska: validation of the artificial-cohort method in cold waters

In situ growth and development of Neocalanus flemingeri/plumchrusstage C1–C4 copepodites were estimated by both the artificial-cohortand the single-stage incubation methods in March, April andMay of 2001–2005 at 5–6°C. Results from thesetwo methods were comparable and consistent. In the field, C1–C4stage durations ranged from 7 to >100 days, dependent ontemperature and chlorophyll a (Chl a) concentration. Averagestage durations were 12.4–14.1 days, yielding an averageof 56 days to reach C5, but under optimal conditions stage durationswere closer to 10 days, shortening the time to reach C5 (fromC1) to 46 days. Generally, growth rates decreased with increasingstage, ranging from 0.28 day^–1 to close to zero but weretypically between 0.20 and 0.05 day^–1, averaging 0.110± 0.006 day^–1 (mean ± SE) for single-stageand 0.107 ± 0.005 day^–1 (mean ± SE) forartificial-cohort methods. Growth was well described by equationsof Michaelis–Menten form, with maximum growth rates (G_max)of 0.17–0.18 day^–1 and half saturation Chl a concentrations(K_chl) of 0.45–0.46 mg m^–3 for combined C1–3,while G_max dropped to 0.08–0.09 day^–1 but K_chl remainedat 0.38–0.93 mg m^–3 for C4. In this study, in situgrowth of N. flemingeri/plumchrus was frequently food limitedto some degree, particularly during March. A comparison withglobal models of copepod growth rates suggests that these modelsstill require considerable refinement. We suggest that the artificial-cohortmethod is the most practical approach to generating the multispeciesdata required to address these deficiencies.     

The role of faecal material in the particulate organic carbon flux in the northern Humboldt Current, Chile (23{degrees}S), before and during the 1997-1998 El Nino

Sedimentation rates of faecal material, phytoplankton and microzooplanktonand production rates of faecal material from crustaceans andpelagic tunicates were estimated during the austral summer andwinter 1997, and summer 1998, in the northern Humboldt Current(23°S, off Antofagasta, Chile). Sampling periods coveredpre-El Niño (January 1997) and El Niño 1997–98(July 1997 and January 1998). Samples were collected using floatingsediment traps deployed at 65, 100, 200 and 300 m depth in oceanicand coastal areas. Sedimentation rates during January 1997 were,on average, 152 ± 23 and 85 ± 57 mg C m^–2day^–1 at 65 and 300 m depth, respectively. During July,these rates averaged 93 ± 56 mg C m^–2 day^–1at 65 m depth and 35 ± 12 mg C m^–2 day^–1at 300 m depth, while in January 1998 they were 98 and 109 ±37 mg C m^–2 day^–1 at 65 and 200 m depth, respectively.Recognizable faecal material made up the bulk of the sedimentingmatter, accounting for 8 ± 5% (n = 14), 31 ± 26%(n = 16) and 8 ± 5% (n = 5) of the average total organiccarbon recorded from all sediment trap samples collected duringJanuary and July 1997 and January 1998, respectively. However,at300 m depth, the contribution of recognizable faecal materialto total sedimented organic carbon increased to 43 ±33% (n = 4) during July 1997. The remaining sedimenting particlesconsisted mainly of tintinnids, crustacean exuviae, heterotrophicdinoflagellates (both thecated and athecated) and diatom cells.During this study, we estimated that only a minor fraction (average± SD = 5 ± 8%) of the copepod faecal materialproduced within the photic zone sedimented down to 300 m depth,suggesting an efficient recycling within the overlaying watercolumn. On the other hand, an important fraction (47 ±30%) of the euphausiid faecal strings was collected in the 300m depth trap, suggesting that this material would enhance thedownward flux of particulate organic matter (POC). POC fluxesto 65 and 300 m depth traps were in the range of 4–20%and 3–8% of the estimated primary production during thewhole study period. It is postulated that the overall verticalflux of particulates and, in particular, faecal pellets wasdetermined by a combination of three factors. The first wasthe composition of the zooplankton assemblages in the studyarea. When the dominant group was calanoid copepods, their faecesseemed to contribute poorly to the vertical flux of particulates.On the other hand, when the dominant group was euphausiids,a significant proportion of their faecal material was collectedin the sediment trap located at 300 m depth. The second wasthe relatively high abundance of cyclopoid copepods from thegenera Oncaea, Corycaeus and Oithona, which are reported tofeed on aggregates of phytodetritus and faecal pellets producedby calanoid copepods, suggesting that they may act as a naturalfilter to sedimenting particulates. The third was the compositionand size spectrum of the phyto- and microzooplankton assemblageswhich are potential food sources for the meso- and macrozooplankton.These factors were partially modulated by both the 1997–1998El Niño and seasonality.