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.     

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.     

Composition, spatial and temporal variations of Chaetognatha in Guanabara Bay, Brazil

This paper presents the specific composition and distributionof adult chaetognaths in Guanabara Bay, Brazil. A comparisonhas been made between chaetognath and copepod distribution patterns.Three areas were distinguished within Guanabara Bay: an externalarea, influenced by oceanic waters; an internal area, influencedby fluvial waters; and a transitional area between the two.Monthly samples were collected by horizontal surface hauls usinga conical net with 200µm mesh. Three species were identified:Sagitta friderici, S.enflata and S.hispida. Sagitta fridericiwas the dominant species and occurred throughout the year atthe three areas. Sagitta enflata and S.hispida were limitedto the more saline waters. Generally, greater densities of chaetognathswere observed following copepod peakden sities. Temperatureand salinity associated with copepod abundance were the mostimportant factors affecting the distribution pattern and theabundance of chaetognath species in Guanabara Bay.     

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.     

Gelatinous invertebrate zooplankton of the South Adriatic: species composition and vertical distribution

The species composition and vertical distribution of gelatinousinvertebrate zooplankton were investigated from April 1993 toJune 1994 in the 0–1000 m water column at a deep-sea stationin the northern part of the South Adriatic Pit. Fifty-sevenspecies were identified: 11 hydromedusae, 13 calycophores, 3ctenophores, 3 heteropods, 10 pteropods, 8 polychaetes and 9chaetognaths. The pteropod Desmopterus papilio and the heteropodProtatlanta mediterranea were recorded for the first time inthe Adriatic Sea. Data from this study differed from those ofprevious investigations in the South Adriatic as regards thenumerically dominant polychaete and pteropod species. All investigatedgroups generally were more abundant in the upper 100 m and decreasedwith depth. Different vertical distributions of life stageswere observed for those species that occupy a wide depth range:Persa incolorata, Solmissus albescens, Limacina inflata, Cymbuliaperoni, Pelagobia longicirrata, Sagitta lyra and Sagitta decipiens.     

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.     

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.     

Comparative experimental studies on the feeding of chaetognaths and ctenophores

There is very little quantitative experimental information onfeeding in chaetognaths and ctenophores because of the difficultyof obtaining undamaged specimens. As important plankton predators,a knowledge of their daily rations as a function of food concentrationis vital to ecosystem models of water column trophodynamics.At specified food concentrations, three ctenophore species alwaysingested higher specific daily rations than three species ofchaetognaths. Food consumption of ctenophores increased linearlythroughout the food concentration range (exceeding 10⁶ µgC/m³ Although food consumption of chaetognaths reached an upperlimit at very high food concentrations, it seemed unlikely thatsatiation would occur under environmental conditions. Below10,000 food items/m³ chaetognaths were unable to obtain l% oftheir specific daily ration, although the few environmentalstudies based on gut contents suggest substantial feeding evenbelow this concentration. The apparent threshold may reflectthe environmental patch density necessary for feeding to begin.At high food concentrations daily rations of newly-hatched ctenophoresand chaetognaths can exceed well over 10 times their body carbon.In ctenophores even adults can consume these extremely largerations although chaetognaths follow the more usual pattern,where ration decreases throughout life. Gut residence time offood (where single items were ingested) increased with age andsize of food item selected in Sagitta hispida from one to twohours, levelling off after animals reached 6 mm. Mnemiopsismccradyi maintained a uniform digestion time of one hour asit grew, except for a brief period when it first selected larger-sizedfood. In older animals of both species, multiple ingestion becamemore common, where gut residence time was directly proportionalto number of food items consumed, ranging up to 5 hours forfive or more food items. Chaetognaths showed more variabilityof gut residence time than did ctenophores. It was concludedthat these animals are much better at catching food in the environmentthan laboratory experiments would suggest.     

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

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

Feeding rates in the chaetognath Sagitta elegans: effects of prey size, prey swimming behaviour and small-scale turbulence

The gut contents of Sagitta elegans were sampled twice daily(noon and midnight) during 9 days in October at an anchor stationin the northern North Sea. Observations of the ambient preyfield and of turbulent dissipation rates were collected simultaneously.The average number of prey per chaetognath was among the highestever recorded, 0.57 ± 0.10. Total gut content was independentof ambient prey concentration, suggesting that feeding ratewas saturated. Clearance rates were estimated from gut contentsand ambient prey concentrations and a literature-based estimateof digestion time. The clearance rate to prey size showed adome-shaped relationship. The maximum clearance rates, about100–300 ml h^-1, were observed for prey sizes correspondingto 6–10% of Sagitta length. Clearance rates varied notonly with prey size, but also with prey type. For example, copepodmales were cleared at rates up to an order of magnitude higherthan similarly sized females, probably owing to differencesin swimming behaviour. Sagitta elegans is an ambush predatorthat perceives its prey by hydromechanical signals. Faster swimmingprey generates stronger signals and is, hence, perceived atlonger distances. We develop a simple prey encounter rate modelby describing the swimming prey as a ‘force dipole’and assuming that a critical signal strength is required toelicit an attack. By fitting the model to the observations,a critical signal strength of 10^-2 cm s^-1 is estimated; thisis very similar to estimates for copepods that also perceiveprey by mechanoreceptory setae. Gut contents were independentof turbulent dissipation rate. Because feeding rates were saturated,we did not expect to see positive effects of turbulence. However,the strong wind-generated turbulent dissipation rates observedduring the study (10^-3–10^-1 cm² s^-3 in the upper mixedlayer) could lead to negative effects by interfering with preyperception. At a dissipation rate of 10^-2 cm² s^-3 a 10-mm longS. elegans would experience fluid signals of order 0.3 cm s^-1due to turbulence, 30 times stronger than the signal strengthfrom the prey. It is, therefore, suggested that S. elegans isable to separate prey signals from turbulence signals due totheir different spatial characteristics.     

Regional, vertical and seasonal distribution of phytoplankton and photosynthetic pigments in Lake Baikal

A 3-year phytoplankton study was carried out in Lake Baikal(Siberia) as part of the CONTINENT project and in conjunctionwith a 60-year long monitoring programme by the Irkutsk StateUniversity. A combination of microscopy and high performanceliquid chromatography (HPLC) pigment analysis was used. Allover the lake, the dominant functional group (by biovolume)was the vernal diatom blooms, due to the dominance of endemicCyclotella species. Chlorophyll a (Chl a) was significantlyhighest at the Selenga and Barguzin inflows (2.39 ± 0.34and 2.49 ± 0.18 nmol L^–1, mean ± 95% CI,respectively) and higher in the South than in the North (1.43± 0.26 and 0.96 ± 0.13 nmol L^–1). This variationof Chl a reflected changes in the phytoplankton composition.Diatoms and Chrysophyceae were the major contributors to thetotal Chl a except in the South (Chlorophyceae) and SelengaDelta (cyanobacterial picoplankton). There were also indicationsof species composition changes due to enhanced P-loading fromthe Selenga River. However, canonical analyses indicated thattemperature and stratification were the major driving forcesfor regional distribution patterns and seasonal succession.It seems likely that further global warming will cause a shiftin the species and group composition towards small cells atthe expense of the large endemic diatom flora.     

Plankton composition and cycling of carbon during the rainy season in a tropical coastal ecosystem, Zanzibar, Tanzania

Biomass, species composition and production of the planktoniccommunity were investigated during the rainy season in May andJune 1999 outside Zanzibar Island, Tanzania. In general, theplankton biomass of different organisms was uniform betweendepths as well as over time. The integrated water column primaryproduction ranged from 204 to 4142 mg C m^–2 day^–1.Bacterial production varied between 10 and 72 mg C m^–2day^–1, comprising ~5% of the total bacterial standingstock. The data obtained from these experiments are summarizedin a carbon budget. At the most 77% of the total primary productionchannelled through the heterotrophic flagellates, ciliates andheterotrophic dinoflagellates to higher trophic levels. Of theestimated carbon demand for mesozooplankton, 28% could potentiallybe met by ciliates and heterotrophic dinoflagellates.     

Vertical distribution, life cycle and production of the chaetognath Sagitta crassa in Tokyo Bay, Japan

The daytime vertical distribution of Sagiita crassa in TokyoBay was examined from February 13, 1988 to February 20, 1989.High densities of larger-size chaetognaths were found near thesea bottom, whereas the smaller animals tended to inhabit theupper layers. This feature of distribution is discussed in relationto the distribution of their main food organisms, e g. Pseudodiaptomusmarinus, Acartia omoru, Centropages abdomialis and Oithona davisae.The two periods of replacement of two morphs were confirmedby the variation only in mean body length of this chaetognath,unlike the previous authors who made additional morphologicalobservations. It was hypothesized that S.crassa has at leastfive generations Two generations, including mostly the largerforms, had higher growth rates than the generation consistingmainly of the small form. Yearly respiration of S.crassa was8.2 g C m^–Abstract. Yearly production of this animal wasestimated to be 3.8 g C m^–. A feeding estimate revealedthat chaetognaths require a prey production of 13.1 g C m^–year^–1. The impact of this chaetognath on the prey populationin Tokyo Bay and the propriety of an estimated value of annualproduction of S crassa is discussed.     

Effects of Salinity and Temperature on Survival and Reproduction of Boeckella hamata (Copepoda: Calanoida) from a Periodically Brackish Lake

The freshwater calanoid copepod, Boeckella hamata Brehm, occursperiodically in Lake Waihola, New Zealand, a shallow, tidally-influencedlake that experiences daily and seasonal fluctuations in salinity,and seasonal changes in temperature. To determine whether theseenvironmental changes might contribute to the copepod's disappearancefrom the lake in summer, we tested the survival and egg productionof B. hamata at a range of salinities (20-3000 mg^–1 Cl)and two temperatures (10°C, 20°C). Adults survived for20 days or more only at salinities     

The importance of macrophyte bed size for cladoceran composition and horizontal migration in a shallow lake

Cladoceran composition and diel horizontal migration were studiedin 2, 10 and 25 m diameter macrophyte exclosures establishedin the littoral zone of shallow Lake Stigsholm, Denmark. Theexclosures were protected from waterfowl grazing, but open tofish. The macrophyte community cornprized Potwnogeton pectinatus,Potamogeton pusillus and Callitriche hemaphroditica. Cladoceranswere sampled randomly every third hour inside and outside themacrophyte exclosures during a 24 h period. In the 2m exclosure,the pelagic species Ceriodaphnia spp. and Bosmina spp. dominatedduring the day, mean density being as high as 3430 indiv. l^–1During the night, density decreased to 10–20% of the daytimedensity thus indicating diel horizontal migration. In the 10and 25 m exciosures, the daytime mean density of Ceriodaphniaspp. was 865 and 202 indiv. l^–1, respectively, and didnot decrease at night. In contrast to the pelagic species, thedensity of macrophyte-associated species tended to be higherin the 10 and 25 m exclosure than in the 2 m exclosure. In thedaytime, Eurycercus lamellatus density in the 2, 10 and 25 rnmacrophyte exclosures was 7, 28 and 16 indiv. l^–1 respectively,while that of Simocephalus vetulus was 11, 171 and 92 mdiv.l^–1, respectively. There was no thy-night difference inthe density of macrophyte-associated species. We conclude thatcladoceran community composition varies with macrophyte bedsize, and that the edge zone between the bed and open wateris an important daytime refuge for potentially migrating pelagiccladocerans.     

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

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

Trophic importance of the chaetognaths Eukrohnia hamata and Sagitta gazellae in the pelagic system of the Prince Edward Islands (Southern Ocean)

Trophodynamics and predation impact of the 2 dominant chaetognaths Eukrohnia hamata and Sagitta gazellae were investigated at 19 stations in the vicinity of the Prince Edward Islands and at a 24-h station occupied at the sub-Antarctic Front in late summer (April/May) 1996. During the entire investigation, the zooplankton assemblages were numerically dominated by copepods with densities ranging from 21 to 170 ind. m⁻³. Amongst the copepods, Clausocalanus brevipes, Metridia gerlachei and M. lucens dominated accounting for >90% of the total. Generally, chaetognaths were identified as the second most important group composing at times up to 30% (mean = 14.7%) of total zooplankton abundance. Of the two chaetognath species, E.␣hamata was generally numerically dominant. Gut content analysis showed that both chaetognath species are opportunistic predators generally feeding on the most abundant prey, copepods. No feeding patterns were evident during the 24-h station, suggesting that both species feed continuously. The feeding rates of E. hamata ranged from 0 to 0.50 prey ind. day⁻¹ and between 0 and 0.90 prey ind. day⁻¹ for S. gazellae. The maximum total predation impact of E. hamata was equivalent to 5.2% of the copepod standing stock or up to 103% of copepod production per day. For S. gazellae the predation impact was lower, reaching a level of 3.2% of the copepod standing stock or 63% of the daily copepod production. Chaetognaths can, therefore, be regarded as an important pelagic predator of the Prince Edward Islands subsystem. Received: 27 March 1997 / Accepted: 11 September 1997     

Distribution and feeding of chaetognaths in the epipelagic zone of the Lazarev Sea (Antarctica) during austral summer

This study seeks to determine the effects of local hydrography on the distribution, abundance and feeding of chaetognaths in the Lazarev Sea, an area strongly controlled by physical processes which has been held responsible for initiating the Weddell Polynya. Zooplankton samples were taken at 39 stations on four transects located between 6°W and 3°E and from 60°S to 70°S between surface and 350 m. The dominant species, Eukrohnia hamata, accounted for 86.5% of all chaetognaths, followed by Sagitta gazellae (8.1%) and Sagitta marri (5.4%). These three species showed distinct vertical and horizontal distribution patterns. While E. hamata and S. marri had maximum abundances below 250 m depth, S. gazellae showed a narrow distribution band in the upper 150 m depth. The distribution pattern was strongly modified at the Greenwich meridian with an upward transport of a high abundance of deep dwelling organisms (S. marri and E. hamata) and a displacement of S. gazellae to the surface, likely coupled with the rise of the warm, saline halo around the Maud Rise. Small copepods were the main prey of all three chaetognath species. Feeding rates (FR) varied among species and depth. Sagitta marri showed the highest FR with 0.38 prey d⁻¹, followed by S. gazellae and E. hamata (0.22 and 0.07 prey d⁻¹). Feeding rates were usually highest in the 25–80-m stratum. Size distribution and maturity of E. hamata revealed a dominance of small and immature organisms along all depths and stations, suggesting that this area might be acting as an important source of recently spawned organisms to the surface.