Interannual variability in a predator-prey interaction: climate, chaetognaths and copepods in the southeastern Bering Sea

The interaction of the chaetognath Sagitta elegans with thecopepod community of the southeast Bering Sea middle shelf wasexamined in relation to environmental conditions during 1995–1999.Predation impact was estimated for 2 years, 1995 and 1997, usinggut content analysis, experimentally derived digestion time(DT) and abundances of chaetognaths and prey. Pseudocalanusconcentrations correlated with water temperature and Calanusmarshallae with sea ice extent. Sagitta elegans were less abundantbut individuals were larger in 1995, when C. marshallae predominated,compared to 1997, when Pseudocalanus and Acartia were the primaryprey. Predation by S. elegans removed <1% standing stockday^–1 of Pseudocalanus or C. marshallae in 1995 and 1.7to 2.3% of Pseudocalanus in 1997. The percent of the copepodcommunity biomass required by chaetognaths was estimated tobe <1% in 1995 compared with 8–12% in 1997. Calanusmarshallae may be more vulnerable than Pseudocalanus to cumulativepredation effects because of its reproductive strategy. Theeffect of chaetognath predation on the copepod community dependson which copepod species is predominant and its susceptibilityto cumulative predation effects, as well as on daily predationimpact, both of which varied between years with different climaticconditions.     

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.     

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.     

DENSITY FLUCTUATIONS, GROWTH AND DRY TISSUE PRODUCTION OF HYDROCOCCUS BRAZIERI (TENISON WOODS, 1876) AND ARTHRITICA SEMEN (MENKE, 1843) IN PEEL INLET, WESTERN AUSTRALIA

Hydrococcus brazieri and Arthritica semen on a sandflat at Coodanup,Peel Inlet, Western Australia, accounted for 89.5% of totalmollusc numbers. The mean density of H. brazieri was 9487 m^–2and A. semen averaged 8105 m^–2. H. brazieri grew at 0.5mm month^–1 and reached maturity in 4 months; A. semengrew at 0.3 mm month^–1 and reached maturity in 6 months.Somatic production in the Peel-Harvey estuarine system was estimatedto be 0.5 g m^–2yr^–1for H. brazieri and 4.1 g m^–2yr^–1for A. semen. The positions of these two species in the estuarinefoodweb is discussed. (Received 26 June 1981;     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

Photoadaptation in Antarctic phytopfankton: variations in growth rate, chemical composition and P versus I curves

The response of phytoplankton to variations in the light regimewas studied during the VULCAN and ACDA cruises in the Antarctic.Unenriched batch cultures of 12–19 days' duration reachedchl concentrations of 10–50 µg^–1 and exhibitedexponential growth rates, with the maximal rate being 0.41 doubl,day^–1. Ice edge algae exhibited maximum growth rates atphoton flux densities (PFD) of 30–100 µE m^–2S^–1and the growth rate was reduced by about 30% at 500–1000µE m^–2S^–1 The chl/C ratio ranged between 0.004and 0.018, with the lowest ratios at PFDs above 500 µEm^–2S^–1 chl/C ratios were also below maximum at PFDsbelow 40–50 µE m^–2S^–1 The C:N:P ratioswere close to the Redfield ratios; the Si/C ratio averaged 0.16(atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050in different culture senes. When thawed after having been frozenfor 10 days, shade-adapted cultures were in a much better conditionthan sun-adapted ones. P versus I data showed that the maximumassimilation number varied from 0.75 to 4.4 µg C (µgchl)^–1h^–1. It varied inversely with the chl/C ratio;therefore the maximum carbon turnover rate varied little betweensamples (0.024/0.035 h^–1). Low biomass communities exhibitedrelatively high values for (the initial slope of P versus Icurves), low values for 1_sat (160–330 µE m^–2S^–1),and they were susceptible to photoinhibition. In contrast, communitiesdominated by Odontella weissflogii exhibited low values for, a high value for I_sat (560 µE m^–2S^–1 andthey tolerated high PFDs. The photo-adaptational status of thephytoplankton in natural water samples is discussed relativeto the profile of water column stability and mixing processes.     

Production of Oikopleura longicauda (Tunicata: Appendicularia) in Toyama Bay, southern Japan Sea

Oikopleura longicauda occurred throughout the year in ToyamaBay, southern Japan Sea, and analysis of its size compositionand maturity revealed that reproduction was continuous overtheyear. Somatic growth production (P_g) varied with season from0.03 to 103 mg carbon (C) m^–2day^–1 (annual P_g 4.5g C m^–2), and house production (P_e) from 0.11 to 266 mgC m^–2 day^–1 (annualP_e 11.3 g C m^–2). The annualP_g/B ratio was 176. Compared with production data of some predominantzooplankton species in Toyama Bay, it is suggested that despitetheir smaller biomass, appendicularians are an important secondaryproducer.     

Seasonal and interannual variability of chlorophyll a and primary production in the Equatorial Atlantic: in situ and remote sensing observations

The seasonal variability of phytoplankton in the EquatorialAtlantic was analysed using Sea-viewing Wide Field-of-view Sensor(SeaWiFS)-derived chlorophyll a (Chl a) concentration data from1998 to 2001, together with in situ Chl a and primary productiondata obtained during seven cruises carried out between 1995and 2000. Monthly averaged SeaWiFS Chl a distributions werein agreement with previous observations in the Equatorial Atlantic,showing marked differences between 10° W in the EasternTropical Atlantic (ETRA) and 25° W in the Western TropicalAtlantic (WTRA) provinces (Longhurst et al. 1995. J. PlanktonRes., 17, 1245–1271). The seasonal cycle of SeaWiFS-derivedChl a concentration calculated for 0–10° S, 0–20°W (ETRA) is consistent with in situ Chl a measurements, withvalues ranging from 0.16 mg m^–3, from February to April,to 0.52 mg m^–3 in August. Lower variability was observedin 10° N–10° S, 20–30° W (WTRA) whereminimum and maximum concentrations occurred in April (0.15 mgm^–3) and in August (0.24 mg m^–3), respectively.A significant empirical relationship between depth-integratedprimary production and in situ measured sea surface Chl a wasfound for ETRA, allowing us to estimate the seasonal cycle ofdepth-integrated primary production from SeaWiFS-derived Chla. As for Chl a, this model was verified in a small area ofthe Eastern Equatorial Atlantic (0–10° S, 0–20°W), although in this instance it was not completely able todescribe the magnitude and temporal variability of in situ primaryproduction measurements. The annual euphotic depth-integratedprimary production rate estimated for ETRA by our empiricalmodel was 1.4 Gt C year^–1, which represents 16% of theopen ocean primary production estimated for the whole AtlanticOcean.     

WATER EXCHANGE, TEMPERATURE TOLERANCE, OXYGEN CONSUMPTION AND ACTIVITY OF THE NAMIB DESERT SNAIL, TRIGONEPHRUS SP.

Water exchange, temperature tolerance and oxygen consumptionof the snail, Trigonephrus sp., from the southern Namib desertof Namibia were examined and related to activity. At 25°Cand 15% R.H. mean water loss and food and water uptake were5.95 mg. day^–1 and 630 mg.day^–1, respectively. Bodytemperature tracked sand temperature. Snails tolerated sandtemperatures as high as 45°C. Mean ± S.D. oxygenconsumption rates were 32.0 ± 2.94 µlO₂.g totalbody mass^–1.h^–1 at 15°C, when the snails wereactive, and 11.27 µlO₂.g total body mass^–1.h^–1at 25°C, when the snails were inactive. These values are2-6 times lower than those recorded for the similarly sizedmesic snail, Helix aspersa. Activity experiments indicated thatlow ambient temperatures and high humidities were favoured bythe snails. This, together with the burying behaviour of thesesnails during high temperatures, suggests that they limit stressby restricting activity to physiologically-favourable periods,even though more-extreme conditions may be tolerated. (Received 7 June 1990; accepted 20 November 1990)     

PREY CAPTURE, PREFERENCE AND CONSUMPTION BYLINATELLA CAUDATA (GASTROPODA: TONNOIDEA: RANELLIDAE) IN HONG KONG

On sheltered rocky beaches in Hong Kong, Linatella caudata preysupon the lower-littoral and sub-littoral fringe fauna. Ascidiantests are cut with the radula, while the proboscis is insertedbetween the parted shell valves of bivalves. The salivary glandssecrete sulphuric acid that is not used in prey penetration,as in the Cassidae, but is more likely used in digestion ordefence. In preference experiments, prey were chosen in the sequence:Barbatia virescens (Bivalvia: Arcidae) > Styela plicata (Ascidiacea)/Saccostreacucullata (Bivalvia: Ostreidae) > Lunella coronata (Gastropoda:Turbinidae) > Balanus amphitrite (Crustacea: Cirripedia).52% of all attacks were upon Barbatia Lunella and Balanus wereattacked rarely. Prey preferences by individual gastropods werealso demonstrated. Calibration plots of total weight against wet and dry tissueweight of prey (Barbatia virescens) and, finally, the predatorwere obtained and used in estimations of consumption. On average,an adult Linatella caudata (10-15g) consumed three B. virescens.week^–1 The mean weight of B. virescens flesh consumed.week^–1 ranged between 0. 208–0.412g dry weight prey.gdry weight predator^–1, i.e., a mean of 28.2% of the predator'body weight.week^–1 or 4%.day^–1. Such a figure accordswell with estimates of consumption obtained for other adultpredatory gastropods from Hong Kong. (Received 4 September 1989; accepted 5 December 1989)     

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.     

Planktonic primary production in the German Wadden Sea

By combining weekly data of irradiance, attenuation and chlorophylla concentrations with photosynthesis (P) versus light intensity(E) curve characteristics, the annual cycle of planktonic primaryproduction in the estuarine part of the Northfrisian WaddenSea was computed for a 2 year period. Daily water column particulategross production ranged from 5 to 2200 mg C m^–2 day^–1and showed a seasonal pattern similar to chlorophyll a. Budgetcalculation yielded annual gross particulate primary productionsof 124 and 176 g C m^–2 year^–1 in 1995 and 1996,respectively. Annual amounts of phytoplankton respiration, calculatedaccording to a two-compartment model of Langdon [in Li,W.K.W.and Maestrini,S.Y. (eds), Measurement of Primary Productionfrom the Molecular to the Global Scale. International Councilfor the Exploration of the Sea, Copenhagen, 1993, pp. 20–36],and dissolved production in 1996, were both in the range of24–39 g C m^–2 year^–1. Annual total net productionwas thus very similar to particulate gross production (127 and177 g C m^–2 year^–1 in 1995 and 1996, respectively).Phytoplankton growth was low or even negative in winter. Inspring and summer, production/biomass (Pr/B) ratios varied from0.2 up to 1.7. Phytoplankton growth during the growth seasonalways surpassed average flushing time in the area, thus underliningthe potential of local phytoplankton bloom development in thispart of the Wadden Sea. The chlorophyll-specific maximum photosyntheticrate (P^B_max) ranged from 0.8 to 9.9 mg C mg^–1 Chl h^–1and was strongly correlated with water temperature (r² = 0.67).By contrast, there was no clear seasonal cycle in ^B, which rangedfrom 0.007 to 0.039 mg C mg^–1 Chl h^–1 (µmolphotons m^–2 s^–1)^–1. Its variability was muchless than P^B_max and independent of temperature. The magnitudeand part of the variability of P^B_max and ^B are presumably causedby changes in species composition, as evidenced from the rangeof these parameters found among 10 predominant diatom speciesisolated from the Wadden Sea. The ratio of average light conditionsin the water column (E_av) to the light saturation parameterE_k indicates that primary production in the Wadden Sea regionunder study is predominantly controlled by light limitationand that nutrient limitation was likely to occur for a few hoursper day only during 5 (dissolved inorganic nitrogen) to 10 (PO₄,Si) weeks in the 2 year period investigated.     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

A QUANTITATIVE STUDY OF A CERASTODERMA -- NEPHTHYS COMMUNITY IN THE LIMFJORD, DENMARK, WITH SPECIAL REFERENCE TO PRODUCTION OF CERASTODERMA EDULE

Cerastoderma edule is the dominant member of a non-random Cerastoderma—Nephthyscommunity, occupying a physically, homogeneous, submerged sandbankin the Limfjord, Denmark. C. edule is patchily distributed withinthis community, growth rates showing a clear density-dependence.Individual production is about 35%lower in high density patchesthan outside them; at low density annual individual flesh dryweight increment is 295 mg. Seasonal variation in water, ash,lipid, protein and glycogen contents is studied, and a sizedependent(= density-dependent) relationship is shown to exist for someparameters. Population production, estimated by a new graphicalmethod, and taking into account gametic and shell protein production,attains 150 g dry flesh weight, m^–2.yr^–1 at meanpopulation density rising to 250 g dry flesh weight. m^–2.yr^–1in the high density patches. ¹Present address: Institut für Physiologische Chemie, Abt.Zellbiochemie, Universitat Hamburg, Martinistrasse 52, 2000Hamburg 20 (Received 10 March 1980;     

Light, temperature and nitrogen as interacting factors affecting diel vertical migrations of dinoflagellates in culture

Diel vertical migrations of the marine dinoflagellates Gonyaulaxpolyedra Stein and Ceratium furca (Ehr.) Clap, et Lachm. werefollowed in a laboratory tube (2.02 m x 0.25 m) under a 12:12hlight:dark cycle. The effects of temperature stratification,two levels of surface irradiance and nitrogen depletion on patternsof vertical migrations were examined. At temperatures between22–26°C with small temperature gradients, both speciesmigrated at a rate of 0.7 –1.0 m h^–1. Steeper thermoclines(ca. 0.8°C 0.1 m^–1) with temperatures below ca. 20°Ccaused a marked decrease in swimming speed which resulted inaccumulations of cells in these thermocline regions. Under conditionsof nutrient sufficiency both algae migrated into the surfacelayers at irradiance values of over 1000 µE m^–2s^–1. Increasing nitrogen depletion caused the downwardmigration of both algae to commence progressively earlier inthe day and before the end of the light period. The earlierdownward migrations enabled a more complete descent throughthe thermocline. Nitrogen depleted cells of Gonyaulax continuedto undertake vertical migrations but avoided high irradiancesthus forming subsurface maxima at irradiance levels close to150 µE m^–2 s^–1. Ceratium cells which exhaustedboth inorganic nitrogen and phosphorus ceased to migrate accompaniedby a large change in cellular fluorescence.     

Copulation in the neritic chaetognath Sagitta crassa

The number of spermatophores found attached to Sagitta crassa,the dominant chateognath in Tokyo Bay, ranged from one to four.Individuals with one spermatophore were most abundant, accountingfor 70% of chaetognaths with spermatophores (A). Using the meanminimum body length of chaetognaths with spermatophores (A),those without spermatophores were subdivided into two categories:those >7 mm (B) were considered to be mature animals, andthose <7 mm (C) to be immature. The ratio of chaetognathswith spermatopbores (A) to all chaetognaths (A+B+C) varied withtime, with high values from 0040 through 0120 h as in a previousstudy. Variations in the ratio of chaetognaths with spermatophores(A) to all adults (A+B) showed a pronounced peak at 0100 h.Thus, it was confirmed that copulation of S. crassa is a periodicactivity with a peak soon after midnight. Most of the chaetognathpairs exchange one spermatophore. The frequency of copulationis considered to be two times per night at most because thechaetognaths have a single pair of seminal vesicles. However,the presence of chaetognaths with 3–4 spermatophores suggeststhat occasionally a reciprocal exchange of spermatophores doesnot occur. This could be called false copulation. It is suggestedthat copulation in S. crassa including false copulation occursat most four times per night.     

Summer coastal zooplankton biomass and copepod community structure near the Italian Terra Nova Base (Terra Nova Bay, Ross Sea, Antarctica)

The structure of the zooplankton biotic community and of copepodpopulation in the coastal area of Terra Nova Bay (Ross Sea,Antarctica) was investigated during the 10th Italian AntarcticExpedition (1994/1995). Zooplankton biotic community consistedmainly of pteropods (Limacina helicina and Clione antarctica),Cyclopoid (Oithona similis), Poecilostomatoid (Oncaea curvata)and Calanoid (Ctenocalanus vanus, Paraeuchaeta antarctica, Metridiagerlachei and Stephos longipes) copepods, ostracods, larvalpolychaetes and larval euphausiids. Zooplankton abundance rangedfrom 48.1 ind m^–3 to 5968.9 ind m^–3, and copepodabundance ranged from 45.2 ind m^–3 to 3965.3 ind m^–3.The highest peak of zooplankton abundance was observed between25 m and the surface and was mainly due to the contributionof O. similis, O. curvata and C. vanus. Zooplankton biomassranged from 5.28 mg m^–3 to 13.04 mg m^–3 dry weight;the maximum value was observed between 25 m and the surface.Total lipid content varied from 216.44 to 460.73 mg g^–1dry weight.