Abundance, breeding and temporal size distribution of the chaetognath Sagitta setosa in the Kattegat

During a hydrographical survey in the northern Kattegat, 1975to 1977, vertical plankton hauls were taken monthly at fourstations between Göteborg and Frederikshavn. Hydrographicalparameters were measured at 10 stations along the transect.Sagitta setosa J. Mailer was by far the most dominant chaetognath(99% of individuals). Sagitta elegans Verrill and Eukrohniahamata Möbius occurred simultaneously with inflowing highsaline water during autumn and winter. The transport of S. setosaby currents was an important factor influencing the abundancesand the breeding period in the Kattegat. The abundance peakin 1977 was only one fifth of that found in 1975. The startand the duration of the breeding period (defined as presenceof stage 1 individuals <3 mm) varied between the three years.Breeding occurred between the middle of July and December witha peak in August and September. Changes in size distributionwere small during winter and spring, possibly due to low temperatures.S. setosa has probably a life span of one year in the Kattegat.Some earlier results regarding the number of breeding periodsof S. setosa per year are questioned. It is proposed that thenumber of generations per year of S. setosa and the start andthe duration of the breeding period(s) in western European watersshould be regarded as an open question until further informationis available.     

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

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

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.     

Summer vertical distribution of paralarval gonatid squids in the northeast Pacific

The vertical distribution patterns of paralarvae from severalabundant cephalopod taxa were examined from depth-stratifiedtows in the northeast Pacific (44–56°N, 145–165°W)during three summer surveys in 1999–2001. A total of 309cephalopods representing 10 taxa in three families were collected.Gonatid squids composed 97% of the total catch, and the mostnumerous taxa were Berryteuthis anonychus (59% of the totalcatch), Gonatus spp. (21%) and Gonatopsis borealis (17%). B.anonychus and Gonatus spp. were both most abundant in the upper20 m; catches of both taxa varied significantly with depth andwere significantly higher above the thermocline than in andbelow the thermocline. Gonatopsis borealis was collected mostlybetween 20 and 50 m, and catches were significantly higher inthe thermocline than above and below the thermocline. Paralarvaeof the three major taxa showed no evidence of diel verticalmigration. Mantle lengths of Gonatus spp. and G. borealis eachvaried significantly with depth, and Gonatus spp. showed a strongpositive correlation between mantle length and depth.     

Population structure, vertical distribution and diel migration of Sagitta setosa (Chaetognatha) in the south-western part of the Black Sea

The population structure of Sagitta setosa from the south-westernpart of the Black Sea was examined between May 1994 and December1996. In addition, the vertical distribution and diel verticalmigration of S.setosa were studied for juveniles (     

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.     

Influence of the pycnocline on the vertical migration of chaetognaths in the northern Benguela

The vertical distribution and migration patterns of chaetognathswere studied at a sampling station off northern Namibia (18°00'S,10°30'E during a 48 h sampling cycle. The sampling areawas characterized by mixing of the Angola current with the northernmostwaters of the Benguela current in the surface region. The continuousflow of the Angola current during the study period gave riseto a thermocline and a halocline at a depth of 40 m. The samplingintervals used to study the behaviour of epipelagic chaetognathspecies under these conditions were 200–100, 100–60,60–40, 40–20 and 20–0 m. A total of 10 differentchaetognath species were captured. Sagitia enflata was the predominantspecies, with a mean density of 4400 individuals per 1000 m³in daytime hauls, accounting for 54.1% of all individuals collectedin the samples. Sagitia minima was the next most predominantspecies, with a mean density of 2400 individuals per 1000 m³in daytime hauls, accounting for 16.6% of the chaetognath communitysampled. Three maturity stages were considered in analysingpossible ontogenetic migrations. Nearly all the species wereaggregated above the pycnocline, and stages I and II of manyspecies carried out short migrations in the surface layers.The limited migration pattern, together with a staggered distributionof the different species and stages above the pycnocline, hasbeen interpreted as a space partitioning mechanism to preventintra- and interspecific competition.     

Seasonal variation and life history of the pelagic chaetognatha, Sagitta elegans Verrill, in Toyama Bay, southern Japan Sea

The seasonal variation and life history of the pelagic chaetognatha,Sagitta elegans Verrill, in Toyama Bay, southern Japan Sea,were investigated using a time series of 0–500 m verticalhauls with a Norpac net from 1 February 1990 to 30 January 1991.Nine species of one genus occurred in Toyama Bay throughoutthe year, including the cold-water species S.elegans. Therewas no remarkable variation in abundance throughout the year,although many individuals were collected in August. Juvenilesoccurred mostly in spring and summer, from late March to August.Adults (Stage 3) occurred in all seasons, except summer. Thebody length of adults ranged from 26 to 30 mm. There were twoprincipal spawning periods. One was in March-May and the otherwas in August. Life spans of both cohorts were 10–12 months.Copepods were major prey of S.elegans inhabiting Toyama Bay.The annual mean food-containing ratio (FRC) was 6.1%, but values>10% occurred in February, April, May, July and September.     

The diel migrations and vertical distributions of zooplankton and micronekton in the Northwestern Mediterranean Sea. 1. Euphausiids, mysids, decapods and fishes

Vertical distributions and diel migrations of the main speciesof micronekton, four euphausiids, one mysid, one decapod andthree fishes, were described in detail in the 0–1000 mwater column on a fixed station in the Northwestern MediterraneanSea. The euphausiids Euphausia krohni and Thysanopoda aequalis,the decapod Gennadas elegans and, to a lesser extent, the fishArgyropelecus hemigymnus were shown to perform clear diel verticalmigrations. Results of horizontal hauls at a given depth aroundsunrise and sunset showed a marked diurnal symmetry of the migratorycycles, particularly for E.krohni, T.aequalis and G.elegans.The behaviour of the euphausiid Nematoscelis megalops was morecomplex: it presented a repetitive bimodal day distributionand only part of its population migrated. As very weak or non-migratorswe found the euphausiid Stylocheiron longicorne and the bathypelagicmysid Eucopia unguiculata, for which migration concerned onlysome of the older individuals. The fishes Cyclothone braueriand Cyclothone pygmaea appeared to be non-migrants. As depthincreased, C.braueri was replaced by C.pygmaea, with maximumconcentrations at 350–550 and 550–700 m depth, respectively.     

The life history and vertical distribution of the chaetognath, Sagitta elegans, in Dabob Bay, Washington

Seasonal changes in the population structure and patterns ofvertical distribution and migration of the dominant chaetognath,Sagitta elegans, in a fjord off Puget Sound were determinedwith samples collected at approximately semi-monthly intervalsfrom November 1972 to November 1973. S. elegans bred continuouslyfrom March through August, with major recruitment in June andJuly. Seasonal changes in the vertical distribution of the populationand in the proportion of the population that vertically migratedreflected the stage of maturity and size of the individuals.Small chaetognaths were non-migratory and distributed in thetop 100 m. Stage II individuals were dispersed throughout thewater column; some did migration was evident. Breeding stageswere restricted to a layer between SO and 100 m during the dayand migrated to the surface layer at night, enhancing the probabilityof successful reproduction.     

Thermal Tolerance of the Bivalve Mollusc, Modiolus modiolus L.

SYNOPSIS. A critically heat-stressed population of Modiolusmodiolus has been studied in a tidal pool. Over a period of5.5 hours the temperature in the shallows rose from 19°Cto 32.5°C. Some of the animals were killed outright; otherssuccumbed over the next few days. Most of the animals, althoughdistressed at the time of exposure, seeined to have recoveredby the following week. The animals most adversely affected werethose whose shells protruded above the surface of the pool;these individuals gaped and became desiccated. The reactionsof the species are consistent with previous hysiological experiments.High temperature plays a role in limiting the vertical distributionof Modiolus modiolus.     

The combined effects of temperature and food supply on Calanus sinicus in the southern Yellow Sea in summer

The effects of temperature and food availability on the lifehistory strategy of the planktonic copepod Calanus sinicus inthe southern Yellow Sea in summer were studied in this paper.The fifth copepodite stage (CV) dominates the population inthe central part of the southern Yellow Sea, where the YellowSea Cold Water Mass (YSCWM) occurs below the thermocline. Incubationexperiments were conducted on CV C. sinicus caught from theYSCWM to examine the effects of temperature and food availability.Temperature at the surface (27°C) is lethal to CVs regardlessof food availability. At the temperature in the middle of thethermocline (18°C), survival time of the specimens dependson food availability, being     

Water-column chlorophyll in an oligotrophic environment: correction for the sampling depths and variations of the vertical structure of density, and observation of a growth period

The chlorophyll content of a water column (WCC), which is commonlyused as an index of the phytoplankton abundance, is affectedby the choice of the sampling depths and by the variations ofthe vertical structure of density. For instance, the thicknessof the water layer, between two sigma-t values, which containsthe deep chlorophyll maximum, can vary with internal waves.The resulting noise often dominates the mesoscale variationsof the observed water-column chlorophyll (OWCC). Sigma-t dependentstatistics (mean, standard deviation) of the chlorophyll concentrationare computed using the observations at 29 casts from a 22-day-longfixed station in an oligotrophic environment at 15•S, 173°E.For each cast, these statistics, the sampling depths, and thewater density at these sampling depths, allow the estimationof a station-dependent ‘expected water-column chlorophyll’(EWCC). The ratio of EWCC to the overall likelihood of WCC duringthe fixed station (i.e. the mean of all OWCC) is a measure ofthe effect of sampling and variable density structure at eachcast. When this effect is removed, the noise in WCC estimatesdecreases significantly. The time variations of WCC during thefixed station then show a trend with relatively high valuesduring the first days, followed by a 12-day-long period withlow values. A regular increase occurred from 1 October, whichwas accompanied by high carbon fixation rates and was mainlydue to an increase of the chlorophyll concentration betweenthe surface and the deep chlorophyll maximum. New productionduring this active phase was estimated to be 535 mgC m^–2day^–1, corresponding to 62% of the total production. Breakingof internal waves which were recorded at the beginning of thegrowth phase and vertical mixing of nutrients can explain theobservation.     

Environmental factors influencing diurnal distribution of zooplankton and ichthyoplankton

The diurnal vertical distribution of a large number of speciesof zooplankton, icbthyoplankton and micronekton were determinedin the top 150 m in three locations in the Shelf Water, on theNova Scotia Shelf, and Slope and on Georges Bank during springand fall periods. Species were categorized as to their trophiclevel and their type of diurnal migration behaviour. The influenceof temperature, salinity, and water density on the diurnal verticaldistribution of the species was examined. Temperature was foundto have the greatest influence on the distribution of the largestnumber of species. Diurnal migration behavior of the same speciesin Shelf and Slope water and at different times of the yearwas examined. Results showed that species changed their behaviorin the two water masses, while some species changed their migrationbehavior at different times of the year. During the night inApril the most abundant copepod species, Calanus finmarchicus,making up about 80% of the biomass, was found concentrated abovethe thermocline and the main chlorophyll layer. The majorityof the less abundant species of copepods were found below thethermocline and the chlorophyll layer. At night in August thetwo most abundant copepod species, Centropoger typicus and Paracalanusparvus, making up at least 80% of the zooplankton biomass, werealso concentrated above the thermocline and the main chlorophyllLayer. Three species of copepods were concentrated at the depthof the main chlorophyll layer and two species were concentratedbelow the chlorophyll layer and thermocline. The vertical distributionof other zooplankton and ichthyoplankton species was examinedin relation to the thermocline and chlorophyll layer. Relationshipsbetween concentrations of six species of fish larvae and allspecies of copepods in the same samples showed a general increasein the numbers of larvae m^–3 as the numbers of copepodsm^–3 increased in a range of 500–4000 m^–3.However, the concentration of Merluccius bilinearis decreasedas the concentration of copepods exceeded 4000 m^–3 suggestingthat high concentrations of copepods may not be a favourableenvironment for the larvae.     

Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m^–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m^–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.     

Predator-limited population growth of the copepod Pseudocalanus sp.

The impact of predators on population growth of Pseudocalanussp. was investigated in Dabob Bay, Washington. Mortality ofPseudocalanus sp. was determined from stage-specific survivorship,from seasonal changes in mortality rates of adult males andfemales and from incidence of injuries to adult copepods. Theprincipal predators of adult Pseudocalanus were identified asthe predatory copepod Euchaeta elongata, the omnivorous euphausiidEuphausia pacifica and the chaetognath Sagirta elegans. Predatorattack rates - and prey mortality rates - are highly density-dependentand thus sensitive to prey dispersion in the water column, particularlyto layering in the vertical plane. Predation rates by the threeprincipal predators exceeded 100% of the recruitment rate toadult Pseudocalanus sp. beginning in early summer, thus restrictingpopulation growth. Planktivorous fish predation (by adult three-spinestickleback, Gasterosteus aculeatus, and juvenile chum salmon,Oncorhynchus keta) on Pseudocalanus sp. adults was estimatedto be two orders of magnitude lower than consumption rates bypredatory zooplankton, at a deep water station in July. Analysisof seasonal changes in prey ingested by Sagitta elegans revealedthat Pseudocalanus sp. was the major prey item of S. elegansin April (61.0% of prey) and in June (67.0% of prey), thereafterdeclining seasonally in importance. Predation by S. elegansvaried seasonally with changes in chaetognath stage structure,vertical distribution and diapause, not size structure alone.Although chaetognath recruitment and population growth appearto be directly coupled to the abundance of Pseudocalanus sp.,predation by S. elegans has little reciprocal impact on Pseudocalanussp. population growth; hence asymmetries may occur in the interaction of planktonic prey and predators.     

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.     

Diel periodicity of Subantarctic copepods: relationships between vertical migration, gut fullness and gut evacuation rate

A zooplankton community in the Polar Frontal Zone north of SouthGeorgia was sampled for 5 days in February 1994. Feeding ofvarious copepodite stages of six copepod species was assessedby a series of gut fluorescence/gut evacuation experiments.Feeding periodicity was compared to vertical distribution andmigration patterns, as revealed by Longhurst Hardy PlanktonRecorder (LHPR) and ring net catches. Despite chlorophyll alevels reaching only 0.8 mg m^–3 and daily carbon rationsbased on phytoplankton intake being low, feeding of all thecopepods was restricted mainly to the 8 h night-time perid.During the daytime, the epipelagic community was verticallydispersed within the top 100 m. At night, upward migration bymost species led to a convergence of almost all zooplanktersin the upper half of the surface mixed layer. However, large-amplitudevertical migrations which crossed the thermocline were performedonly by Metridia lucens and Pleuromamma robusta. Although feedingby both migrants and non-migrants was mainly at night, therewas no diel signal in gut evacuation rate. The gut evacuationrates of the nine copepod species and stages differed significantly(5-fold) and were negatively related to the extent of theirdiel vertical migration. The long gut-passage times of the migratoryspecies, M.lucens and P.robusta, would have allowed them timeto defaecate some of the food eaten near the surface at depth,contributing to an active carbon transfer out of the mixed layer.However, their scarcity at this site meant that their grazingcomprised only -<1.4% of total copepod ingestion, so theircombined gut flux is likely to have been negligible (-0.4 mgcarbon day^–1).     

Appendicularian assemblages in a shelf area and their relationship with temperature

The structure of the community of appendicularians was describedby multivariate analyses throughout a seasonal cycle on thecentral Cantabrian coast. It is shown by correlation and principalcomponents analysis that the appendicularian species may bearranged in a successional sequence in relative abundance thatis closely coupled to a temperature gradient. This sequencestarts with Fritillaria borealis, which exhibits highest relativeabundance during January, being sequentially followed by Oikopleuradioica, Fritillaria pellucida, Oikopleura fusiformis and Oikopleuralongicauda. This species numerically dominated the communityfrom September to December. Sea surface temperature and thetemperature at the depth of the chlorophyll maximum were bothreliable predictors of the species composition of the community.However, only the latter provided an adequate explanation forthe persistence of cryophilic communities in stratified oceanicenvironments and the dominance of thermophilic communities afterthe autumn mixing period. Under stratified conditions, surfacetemperatures are high (up to 21°C), but temperatures atthe depth of the chlorophyll maximum are low (<15°C).These differences disappear after the autumn mixing, when thewater column exhibits a uniform temperature profile (16°C).Critically, however, although there is a sharp decline in surfacetemperature, water at the depth of the chlorophyll maximum iswarmer than during stratification. A simple conceptual modelis proposed to account for these features and predictions aremade regarding the vertical distribution of appendiculariansduring stratification. The relevance of non-anthropomorphictemperature measures, such as the temperature at the depth ofthe chlorophyll maximum, for other zooplankton groups is alsodiscussed.