Distribution of Calanus species in Kongsfjorden, a glacial fjord in Svalbard

The distribution of Calanus species was investigated in Kongsfjordenin summer of 1996 and 1997. In both years Calanus finmarchicusand Calanus glacialis dominated, although the boreal C. finmarchicuswas more abundant than the Arctic C. glacialis in 1997. Thiscoincided with a 2°C higher water temperature at 50 m in1997, indicating stronger influence of Atlantic origin waterthat year. Advected Calanus finmarchicus occurred in deep andsubsurface layers of the outer fjord in 1996 (200 ind. m^-3,mainly CIII). A less abundant local population aggregated insurface layers of the inner fjord (100 ind. m^-3). Similarly,advected C. finmarchicus occurred in subsurface layers in 1997(446 ind. m^-3, mainly CIII and CIV) and a local population insurface layers (183 ind. m^-3, mainly CI). Calanus glacialisin 1996 aggregated as CII and CIII in the deep layers of theouter fjord (272 ind. m^-3), whereas CIII–CV were abundant(216 ind. m^-3) in cold surface waters of the inner fjord. In1997 C. glacialis (mostly CIII–CV) was more abundant inthe outer than in the inner part of the fjord (40 and 192 ind.m^-3, respectively). Within Kongsfjorden, Calanus finmarchicusneeds one year to complete its life cycle, whereas Calanus glacialisneeds two. Calanus hyperboreus seems to be an expatriate inthe fjord system.     

Seasonal variations in abundance, development and vertical distribution of Calanus finmarchicus, C. hyperboreus and C. glacialis in the East Icelandic Current

The seasonal variation in abundance and development of Calanusfinmarchicus, Calanus hyperboreus and Calanus glacialis in relationto hydrography and chlorophyll (Chl) a was studied in the Arcticwaters of the East Icelandic Current to the north-east of Icelandfrom March 1995 to February 1996. The sampling was carried outat approximately monthly intervals on a transect of five stationsextending from 67°00'N, 13°55'W to 68°00'N, 12°40'W.In April, May and June, vertical distribution was also investigated.Spring warming of the surface waters began in May, with maximumtemperatures recorded in August (~5°C, mean for uppermost50 m). Below 75 m, temperature remained at <0°C throughoutthe year. The spring bloom of phytoplankton started in earlyMay and the highest Chl a concentrations were measured duringlate May to early June (~1 mg Chl a m^-3). Calanus finmarchicusdominated in terms of numbers (~75%), while C. hyperboreus dominatedbiomass (~76%). Calanus glacialis occurred only in low numbers(~1%) and was only a small portion of biomass (~0.7%). The abundanceof all species was low during the winter and peaked once duringsummer: C. finmarchicus in July (~16 000 ind. m^-2) and C. glacialisand C. hyperboreus in June (~370 and ~7700 ind. m^-2, respectively).The biomass of C. finmarchicus had two maxima, in April (~1.9g m^-2) and July (~1.5 g m^-2), while C. hyperboreus peaked inJune (~12.3 g m^-2). Calanus finmarchicus was estimated to spawnin early May at about the start of the spring bloom, while C.hyperboreus spawned prior to the spring bloom, in late Februaryto early March. On the basis of copepod stage distribution,C. finmarchicus was considered to have a 1-year life cycle andC. hyperboreus at least a 2-year life cycle.     

Egg production and growth rates of Calanus euxinus (Copepoda) in the Black Sea

Egg production rates (EPRs) of Calanus euxinus were measuredin the Black Sea during October 2000 and May 2001. EPRs weregenerally low, on average 1.7 eggs female^–1 day^–1in October 2000 and 3.9 eggs female^–1 day^–1 in May2001. The relationships between EPRs and gonad maturity, depth-integratedchlorophyll a (Chl a) and mean surface layer temperature wereexamined. The EPRs were not related to depth-integrated Chla, but were negatively correlated with temperature. EPRs werestrongly related to the proportion of mature females. Growthrates of C. euxinus were derived from the EPRs. The mean growthrate was 0.011 day^–1 in October 2000 and 0.03 day^–1in May 2001. Growth rates were not significantly correlatedwith Chl a concentrations, but were negatively related to femaleweight and temperature.     

Sinking rates of heterogeneous, temperate phytoplankton populations

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

Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea

Simultaneous ingestion and egg production experiments were conductedwith female Calanus finmarchicus in April/May and July/August2002 in the Irminger Sea. Experimental animals were providedwith natural microplankton food assemblages and incubated underin situ conditions for 24 h. The quantity of food consumed wassignificantly related to the concentration of prey cells, withtotal daily ingestion rates ranging from 0.6 to 8.1 µgof carbon female^–1 day^–1, corresponding to carbon-specificrates of 0.6–4.7% day^–1. Egg production rates (EPRs)remained relatively low (0.3–11 eggs female^–1 day^–1)during both periods of investigation and were not influencedby food availability. The data were used to construct energeticbudgets in which the microplankton carbon ingested, includingciliates, was compared with the carbon utilized for egg productionand respiration. These budgets showed that ingestion alone couldnot provide the necessary carbon to sustain the observed demandsfor growth and metabolism. Although ciliates constituted >80%of the total material ingested at times, they were not sufficientto provide the metabolic shortfall. Indeed, the females weretypically lacking 5 µg of carbon each day, 5% of theircarbon biomass. Our study results highlight the possible importanceof internal reserves in sustaining reproduction in C. finmarchicusduring periods of food scarcity.     

Plankton community respiration in Bransfield Strait (Antarctic Ocean) during austral spring

Community respiration (R) was determined in Bransfield Straitfrom oxygen changes in water samples incubated in borosilicatebottles maintained at in situ temperature. The respiratory electrontransport system (ETS) activity of seawater communities wasalso measured from the same samples. Both data sets were relatedby the regression equation: log R (mg O₂ m^–3 day^–1)=0.462+0.730xlogETS activity mg O₂ m^–3 day^–1) (r=0.80, n=23). Fromthis equation and 37 ETS activity depth profiles, we calculatedthe integrated (0–100 m) community respiration as beingin the range 1.2–4.5 g O₂ m^–2 day^–1 (mean=2.2).These values do not differ significantly from other publishedresults for the Arctic and Antarctic Oceans. Assuming a respiratoryquotient of unity, the areal respiration ranges between 0.45and 1.69 g C m^–2 day^–1 (mean=0.8). This would representan important sink for the primary production reported for BransStrait. The spatial distribution of community respiration showedhigher values associated with the warmer and phytoplankton-richwaters outflowing from Gerlache Strait into Bransfield Strait,and with the front that separates Bellingshausen Sea watersfrom Weddell Sea waters. We suggest that this pattern of distributionmay be related to the transport of organic matter by the BransfieldCurrent along the front.     

The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensisand Calanoides carinatus, the dominant large calanoid copepodsin the southern Benguela upwelling system, as well as the fecundityof several other common copepods, were measured between Septemberand March of 1993/94 and 1994/95. Mean egg production of mostcopepods was low at >30 eggs female^-1 day^-1 {Calanoides carinatus23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rhincalanusnasutus 26.1), whereas the mean fecundity of Centropages brachiatuswas significantly greater (83.6 eggs female^–1 day^-1).This study also presents the first comprehensive field estimatesof the fecundity of Nanno-calanus minor (mean: 26.1 eggs female^–1day^–1, range: 0.0–96.2 eggs female^–1 day^–1)and of somatic growth of N6 and all copepodite stages of Calanoidescarinatus (decreasing from 0.58 day^–1 for N6 to 0.04 day^–1for C5). Somatic growth rates of Calanus agulhensis also declinedwith age: from 0.57 day1 for N6 to 0.09 day1 for C5. Data ongrowth rates were used to assess the relative importance offood [as measured by total chlorophyll (Chi) a concentration],phytoplankton cell size (proportion of cells >10 µm)and temperature to the growth of copepods. Multiple regressionresults suggested that fecundity and somatic growth rates werepositively related to both Chi a concentration and phytoplanktoncell size, but not to temperature. Although it was not possibleto separate the effects of Chi a concentration and phytoplanktoncell size, data from previous laboratory experiments suggestthat copepod growth is not limited by small cells per se, butby the low Chi a concentrations that are associated with theseparticles in the field. Despite growth not being directly relatedto temperature, a dome-shaped relationship was evident in somespecies, with slower growth rates at cool (<13°C) andwarm (>18°C) temperatures. The shape of this relationshipmirrors that of Chi a versus temperature, where poor Chi a concentrationsare associated with cool and warm temperatures. It is concludedthat the effect of food limitation on growth of copepods outweighsthat of temperature in the southern Benguela region. Sourcesof variability in relationships between growth and Chi a concentrationare discussed.     

Growth and development of Metridia pacifica (Copepoda: Calanoida) in the northern Gulf of Alaska

Juvenile growth and development rates for Metridia pacifica,one of the dominant larger copepods in the subarctic Pacific,were investigated from March through October of 2001–2004in the northern Gulf of Alaska. The relationship between prosomelength (PL, µm) and dry weight (DW, µg) was determined:log₁₀ DW = 3.29 x log₁₀ PL – 8.75. The stage durationsof copepodites ranged from 3 to 52.5 days but were 8–15days under optimal condition. Seasonally, growth rates increasedfrom March to October and typically ranged between 0.004 and0.285 day^–1, averaging 0.114 ± 0.007 day^–1(mean ± SE). After standardization to 5°C (Q₁₀ of2.7), growth rates averaged 0.083 ± 0.005 day^–1and were significantly correlated to chlorophyll a, with saturatedgrowth rates of 0.149 day^–1 for C1–C3, 0.102 day^–1for C4–C5 and 0.136 day^–1 for all stages combined.Measured juvenile growth rates were comparable with specificegg production rates in this species. The comparisons of ourrates in this study with those predicted by the global modelsof copepod growth rates suggested that further refinement ofthese models is required.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.     

Seasonal Cycles of Egg Production of Two Planktonic Copepods, Centropages typicus and Temora stylifera, in the North-western Mediterranean Sea

Reproduction of the dominant copepods Centropages typicus andTemora stylifera was studied at a permanent station in the LigurianSea (north-western Mediterranean). Seasonal patterns of eggproduction, clutch size, egg size and female prosome lengthwere followed from January 1998 to December 1999. Female carboncontent and weight-specific egg production were compared inautumn 1998 and spring 1999. Reproductive patterns of C. typicusand T. stylifera were very similar, indicating that reproductionwas affected by the same environmental factors. Reproductiveactivity was highest in autumn in both species and years. Asecond peak of egg production was observed in early summer,which was less intense in 1999 after a bloom of salps. Egg productionrates reached maximal values of 33.5 and 33.3 eggs female^–1day^–1 and annual means of 10.8 and 11.7 eggs female^–1day^–1 in Centropages and Temora, respectively. Maximalweight-specific egg production was 0.21 day^–1 in bothspecies in November 1998, when female carbon contents were 6.7(C. typicus) and 12.0 µg (T. stylifera). No statisticalrelationship between egg production and food availability ortemperature was detected. Reproductive activity did not reflectthe seasonal abundance patterns, with C. typicus dominatingin spring and T. stylifera in autumn.     

Separation of marine seston and density determination of marine diatoms by density gradient centrifugation

An attempt has been made to separate constituents of marineseston samples: inorganic material, detritus and the algal species,by density gradient centrifugation, without affecting the physiologicalstate of the algae. A relatively inert gradient material, consistingof Percoll, salt and sucrose, was composed. Since the densitiesof detritus and algae as well as those of different algal speciesoften overlapped, only 10 of the 100 samples processed in thecourse of the year showed a reasonable separation. However,an enrichment with respect to one or more species was oftenachieved. Densities of eleven species of marine diatoms andof one dinoflagellate have been determined at different timesof the year. For eight diatom species and for the dinoflagellatethe following specific density ranges were established: Bidduiphiaaurita: 1.18–1.23 g cm^–3, Biddulphia sinensis: 1.03–1.08g cm^–3, Cerataulina bergonii: 1.03–1.06 g cm^–3,Ditylum brightwellii: 1.07–1.13 g cm^–3, Rhizosoleniadelicatula: 1.04–1.09 g cm^–3, Skeletonema costatum:1.12–1.17 g cm^–3, Streptotheca thamensis: 1.04–1.10g cm^–3 , Thalassiosira rotula: 1.05–1.10 g cm^–3,Peridinium sp.: 1.08–1.12 g cm^–3. No seasonal variationin density was demonstrated. Gradients of different compositiondid not influence density measurements.     

Photosynthetic organic carbon production and respiratory organic carbon consumption in the trophogenic layer of Lake Biwa

Measurement of the photosynthetic production rate in Lake Biwawas camed out from May 1985 to September 1987. In the light-saturatedlayer, the seasonal variation in the photosynthesis rate perchlorophyll a was regulated by water temperature. The depth-integratedphotosynthetic production rate was 0.21-1.48 g C m^–2 day^–1and the maximum value was observed in midsummer when the watertemperature of the mixed surface layer was highesL The criticalnutrient for photosynthesis may be dissolved reactive phosphorus,which was generally <1 µg P 1^–1 throughout theobservation period. In the trophogenic layer, respiratory organiccarbon consumption, estimated from measurement of respiratoiyelectron transport system activity, was 0.35-1.07 (mean 0.66)g C m^–1 day^–1 and corresponded, on average, to 79%of the photosynthetic carbon production rate. This implies thatthe major part of photosynthetic fixed organic matter mightbe recycled in the trophogenic layer. The estimated settlingorganic carbon flux at 20 m depth, from calculation of theseparameters and changes in the particulate organic carbon concentration,was 0.01 (-0.09 to 0.13) g C m^–1 day^–1 The meansettling organic carbon flux measured by sediment trap at 20m was 0.19 (0.09-0.31) g C m^–1 day^–1 higher thanthe estimated value. It seemed that organic matter collectedby sediment trap may contain allochthonous matter and resuspendedepilimnetic sediment matter.     

A comparison of seasonal growth and development of the copepods Calanus marshallae and C. pacificus in the northern Gulf of Alaska

The juvenile growth rates and development times of subarcticCalanus marshallae and temperate/sub-tropical C. pacificus wereinvestigated during nine cruises (May through October, 2001–04)in the northern Gulf of Alaska. The artificial cohort methodbased on a length-weight regression was used for growth estimatesand the reciprocal of the molting rate for developmental time.The copepodite stage duration ranged from 3 to 16 days for C.marshallae (C1–C4) and 3–23 days for C. pacificus(C1–C5). Seasonally, copepodid growth rates increasedfrom May to October, ranging between 0.055 and 0.291 day^–1(mean ± SE: 0.176 ± 0.008 day^–1) for C.marshallae, while growth rates increased from August to Octoberbetween 0.018 and 0.296 day^–1 (mean ± SE: 0.142± 0.016 day^–1) for C. pacificus. After standardizationto 5°C (Q₁₀ of 2.7), growth rate averaged 0.118 ±0.007 day^–1 and 0.075 ± 0.009 day^–1 for C.marshallae and C. pacificus, respectively. Calanus marshallaegrowth rate is satisfactorily described by a Michaelis–Mentenmodel using chlorophyll-a concentration (r² = 0.33) after temperaturecorrection, but the prediction improves with a composite nonlinearmodel combining body weight into the Michaelis–Mentenfunction (r² = 0.55). Considering the limited range of dataavailable for C. pacificus, the combination of the data forboth species suggests that C. pacificus has a similar functionalresponse to growth despite the differences in the geographicand temporal distributions with C. marshallae. Measured juvenilegrowth rates of the two Calanus species in this study were comparableto other calanoid species in the same area and showed reasonableagreement to Calanus growth models but less with global copepodgrowth models.     

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.     

Microplankton and primary production in the Sea of Okhotsk in summer 1994

Phytoplankton composition, density, vertical distribution andprimary production were investigated in the Sea of Okhotsk andin the adjacent northern north Pacific in July–August1994, together with measurements of density and distributionof planktonic microheterotrophs: bacteria, nanoheterotrophsand ciliates. Different phases of phytoplankton seasonal successionwere encountered during the period of investigation in variousregions of this sea. Primary production measured at 144 stationswas found to be greatest (1.5–4 g C m^-2day^-1) in areasof spring-phase succession along the Sakhalin shelf and theKashevarov bank. Periodic relapses of the spring blooms of ‘heavy’diatoms during the whole growth season were recorded over thisbank. The summer phase of the phytoplankton minimum prevailedin the central and eastern parts of the sea, manifested by thedominance of nanoflagellates in terms of phytoplankton biomass.Primary production was 0.5–1 g C m^-2 day^-1. The earlyautumn phase of succession was typical of the Kurile straitarea and the adjacent north Pacific. Primary production therevaried from 0.7 to 2 g C m^-2 day^-1. The integrated phytoplanktonbiomass in the water column varied from 9–12 g m^-2 inzones supporting the summer minimum assemblage to 15–20g m^-2 in zones of early autumn recovery of phytoplankton growth,and up to 40–70 g m^-2 in areas of remnant or relapseddiatom blooms. The numerical density of bacterioplankton wasbetween 1 x 10⁶ and 3 x 10⁶ cells ml^-1 and its wet biomass wasbetween 100 and 370 mg m^-3. In deep waters it was 8–15mg m^-3. The integrated bacterioplankton biomass in the upperwater column varied from 6 to 29 g m^-2. The numerical densityof zooflagellates varied in the upper layer between 0.8 x 10⁶and 4 x 10⁶ l^-1 and their biomass was between 20 and 50 mg m^-3.In deep waters they were still present at a density of 0.05x 10⁶ to 0.2 x 10⁶ cells l^-1. The biomass of planktonic ciliatesvaried between stations from 20 to 100 mg m^-3. The joint biomassof planktonic protozoa in the water column was between 3 and12 g m^-3 at most of the stations.     

The contribution of ammonia excreted by zooplankton to phytoplankton production in Narragansett Bay

Ammonia excreted by mixed zooplankton populations over an annual(1972–1973) cycle in Narragansett Bay varied from 0.04to 3.21 µg at NH₃-N dry wt^–1 day^–1, exclusiveof two exceptional rates measured one year apart: 11.74 and18.39 µg at NH³-N mg dry wt^–1 day^–1. Grossphytoplankton production integrated over the year (1972–1973)averaged 151 mg C m^–3 day^–1 for an 8 m water column;peaks of 332 and 905 mg C m^–3 day^–1 occurred duringthe winter-spring and summer blooms, respectively. Excretedammonia, integrated seasonally and annually, contributed only0.2% and 4.9% of the nitrogen required for observed gross productionduring the winter-spring and summer blooms, respectively, and4.4% annually. However, excreted ammonia may be an importantsource of the nitrogen required by Skeletonema costatum, thedominant diatom in Narragansett Bay, during the post-bloom periodwhen 186% of the nitrogen required for its net production wasmet by ammonia excretion. A combination of zooplankton ammoniaexcretion and benthic ammonia flux contributed 22% of the nitrogenrequired for the annual gross production (440 g C m^–2)while 51% of the nitrogen required for the net production ofSkeletonema was accounted for by regenerated nitrogen. ¹This research was supported by NSF grant GA 31319X awardedto Dr.T.J.Smayda.     

Reproductive response of the copepods Calanoides carinatus and Calanus agulhensis to varying periods of starvation in the southern Benguela upwelling region

Egg production by the calanoid copepods Calanoides carinatusand Calanus agulhensis fed excess Thalassiosira weissflogiiwas monitored in the laboratory following starvation periodsof 1, 3, 5, 7 and 9 days. Following short (1–3 day) periodsof starvation, egg production by C.agulhensis returned to thesatiated rate (51.1 eggs {female} day^–1) more rapidly(after 0.9–2.4 days of excess food) than that of Ca. carinatus(after 2.8–3.1 days). However, following longer (5–9day) periods of starvation, Ca. carinatus regained satiatedlevels of egg production (55.8 eggs {female}^–1 day^–1)more rapidly (after 3.1–4.0 days of excess food) thanC. agulhensis (after 3.8–5.2 days of feeding following5–7 days of starvation). Moreover, many C. agulhensisfemales did not regain normal rates of egg production after9 days of starvation. For both species, the time required foregg production to recover was proportional to the starvationperiod, although only up to 7 days for C. agulhensis, and wasthe same following 4.25 days of starvation. Previously fed Ca.carinatus terminated egg production more rapidly than C. agulhensiswhen starved. The ability of Ca. carinatus to tolerate, andrecover rapidly from, prolonged periods of starvation, combinedwith a comparatively fast development time and high rate ofegg production, provides this species with a strong competitiveadvantage over C. agulhensis in the highly pulsed food environmentof the southern Benguela upwelling region.     

Noctiluca scintillans MACARTNEY in the Northern Adriatic Sea: long-term dynamics, relationships with temperature and eutrophication, and role in the food web

The first ‘bloom’ of Noctiluca scintillans in theNorthern Adriatic Sea was recorded in 1977. The organism causedseveral red tides in the whole basin during the late 1970s,a period characterized by increasing nutrient loads. Duringthe 1980s and early 1990s, there was no ‘red tide’,but the species was an almost constant summer presence, associatedwith high temperatures. Noctiluca scintillans was almost completelyabsent from 1994 until May 1997, concurrent with a general planktondecrease. From summer 1997, N. scintillans was recorded againin the whole basin, although there was no other signal of increasingeutrophication. In contrast to all previous observations, duringwinter 2002–2003, N. scintillans was continuously sampledin the Gulf of Trieste. We estimated experimentally growth andgrazing rates of the dinoflagellate at 9–10°C in cultureand consuming the natural assemblage. Noctiluca scintillanswas able to reproduce actively at low temperatures, showingsimilar growth rates in both experiments (k = 0.2 day^–1).The values found were close to those reported in the literaturefor higher temperatures. The natural diet was mainly composedof phytoplankton (ingestion = 0.008 µg C Noctiluca ^–1day^–1), microzooplankton (ingestion = 0.008 µg CNoctiluca ^–1 day^–1) and bacteria (ingestion = 0.005µg C Noctiluca ^–1 day^–1) with an average carboncontent of 0.138 ± 0.020 µg C Noctiluca cell^–1.     

Growth and ingestion rates of larval fish populations in the coastal waters of Israel

Clupeoid larvae were collected on eight cruises between February1984 and February 1985 in the coastal waters of Israel. Fromanalysis of daily growth increments of otoliths, growth ratesof the abundant clupeoids, Engraulis encrasicolus, Sardina pilchardusand Sardinella aurita were found to be 0.55 mm day^–1,0.67 mm day^–1 and 0.60 mm day^–1, respectively, duringthe first month after hatching. Ingestion rates were estimatedusing an equation from the literature relating ingestion andgrowth of larval fish. Ingestion calculated for populationsof fish larvae in pelagic waters ranged from 0 to >23 mgC m^–2 day^–1 with maximum rates observed in April.Annual ingestion by larval fish at a pelagic station near Haifawas calculated to be 2.2 g C m^–2 year^–1, 10–20%of annual primary production estimated from ¹⁴C uptake.     

The fate of discarded appendicularian houses: degradation by the copepod, Microsetella norvegica, and other agents

Despite the potential importance of zooplankton in degradationof marine snow, the association of colonising zooplankton withdiscarded appendicularian houses has not been investigated innorthern areas. We sampled the vertical distributions of appendicularians,houses and potential zooplankton colonisers at two stationsin the central North Sea during late summer. In addition, grazingexperiments were performed with the copepod Microsetella norvegica,which was assumed to be the main contributor to house degradation.The results were used in (i) inverse modelling, to estimatethe factors which were significant in shaping the vertical distributionof houses and (ii) calculations to estimate potential housedegradation rates. M. Norvegica was able to feed on appendicularianhouses, with feeding rates up to 0.42 g C (g C)^–1 day^–1(0.14 µg C ind.^–1 day^–1). The model resultssuggested that the vertical distribution of houses was shapedby sinking of houses, bacterial degradation and feeding of M.norvegica and invertebrate larvae. The estimated community degradationrate by M. norvegica was low, whereas invertebrate larvae haddegradation rates close to bacterial degradation. We concludethat at the typical concentrations of M. norvegica in the NorthSea (10⁴ ind. m^–2), its role in marine snow degradationis likely to be small. Degradation by other zooplankton groups,such as invertebrate larvae, can, however, be substantial.