Oithona Similis Research Articles

Zonal structure of zooplankton communities in the Southern Ocean South of Australia: results from a 2150 km continuous plankton recorder transect

The Southern Ocean south of Australia is oceanographically complex, being characterized by double branches of the Sub-Antarctic Front (SAF), Polar Front (PF) and Southern Antarctic Circumpolar Current (SACCF), in addition to the Southern Boundary (SB) of the ACC. From 25 February to 3 March 2002 a 2150-km Continuous Plankton Recorder (CPR) transect was conducted along 140 °E, between 47.02 °S and 66.36 °S, crossing each of these frontal zones. Surface temperature, salinity, and fluorescence were measured at 1-min intervals in conjunction with CPR samples. Additional physical data for the region south of 61 oS was provided by nine CTD stations. Multivariate and Indicator Species analysis of the high resolution (∼9.2 km) zooplankton samples identified six distinct assemblages which were strongly correlated with frontal/oceanographic zones. These assemblages appeared to be structured by a combination of zonal differences in water mass structure, phytoplankton regimes, and small scale intra-zonal features (e.g. eddies). The northern branch of the SAF was the strongest biogeographic boundary, separating a high proportion of sub-tropical and temperate species from the waters to its south. The study area differed from other sectors of the Southern Ocean in that the northern PF, equivalent to the PF in other sectors, was not a zone of distinct ecological transition. Two of the identified assemblages were located with the seasonal ice zone, south of the northern SACCF. Although Euphausia superba larvae were a component of both of these assemblages, this species, together with appendicularia, was most abundant south of the SB. The seasonal ice zone north of the SB was dominated by small copepods ( Oithona similis and Ctenocalanus citer), appendicularia and foraminifera. Although the physical characteristics of the frontal zones can be subtle, the demarcation between zooplankton assemblages was clear. Cross-frontal changes in zooplankton assemblages highlight their role in long-term monitoring programs as indicators of environmental change.

Multivariate analysis of the copepod community of near-shore waters in the western Gulf of Maine

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 292:233-249 (2005) - doi:10.3354/meps292233 Multivariate analysis of the copepod community of near-shore waters in the western Gulf of Maine C. A. Manning*, A. Bucklin Ocean Process Analysis Laboratory and Department of Zoology, University of New Hampshire, 142 Morse Hall, Durham, New Hampshire 03824, USA *Email: chris.manning@unh.edu ABSTRACT: Abundance patterns were described for 15 species of planktonic copepods in coastal waters of the western Gulf of Maine, NW Atlantic, between April 2002 and March 2003. Vertically stratified sampling was carried out at 4 stations along a 12 km cross-shelf transect ranging from 60 m to 105 m depth offshore of Portsmouth, NH, USA. Temporal and spatial patterns of distribution and abundance were described for adult copepods, and multivariate analyses were used to assess trends in community composition. Three species, Oithona similis, Temora longicornis, and Centropages typicus, dominated the community numerically, comprising 72.1% of all copepods identified. The abundances of 7 species exhibited strong seasonal variation, in contrast to 5 species which were abundant throughout the year. As shown by non-metric multi-dimensional scaling (MDS), the composition of the community changed in a cyclical pattern over the 1 yr period, suggesting a clear seasonal cycle. There was significant vertical partitioning of the copepod community, with significant differences among depth strata at all but the shallowest station. In contrast, significant horizontal spatial variation was evident only in the deepest stratum; surface and mid-depth samples did not differ among the 4 stations. Although many of the same species found in this study are also known from Georges Bank and Massachusetts Bay, their seasonal patterns of abundance differed notably among the regions. Such dissimilarities emphasize the importance of sampling at appropriate time and space scales in order to accurately assess population and community dynamics of coastal zooplankton assemblages. KEY WORDS: Zooplankton · Copepods · Coastal processes · Multi-dimensional scaling · Gulf of Maine Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 292. Online publication date: May 12, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Seasonal cross-shelf distribution of major zooplankton taxa on the northern Gulf of Alaska shelf relative to water mass properties, species depth preferences and vertical migration behavior

The cross-shelf distribution of major zooplankton species was examined on the northern Gulf of Alaska (GOA) shelf during the production season for four years, between October 1997 and October 2001. The zooplankton community on the northern GOA shelf consisted of oceanic and neritic species of the North Pacific subarctic species complex. Cross-shelf distribution of the major zooplankton species was influenced by their depth preferences, vertical migration behavior, salinity-temperature preferences, and by cross-shelf water-mass distribution and movement. The neritic community, dominated by Pseudocalanus spp., Metridia pacifica and Calanus marshallae, had highest abundances on the inner shelf, in the Alaska Coastal Current, and in the adjacent fjords in late spring and early summer. The oceanic community, which contained primarily Neocalanus cristatus and Eucalanus bungii, was observed in the Alaskan Stream and adjacent waters near the shelf break. A mid-shelf transition zone contained a mixture of oceanic and neritic species. Prince William Sound (PWS) contained a unique species complex of large mesopelagic copepods, amphipods and shrimp. Neocalanus flemingeri and Oithona similis were abundant in all four regions during spring and early summer. The transition zone commonly crossed much of the shelf between the shelf break and the ACC, but satellite images and CTD data indicate that occasionally a narrow shelf-break front can form, in which case distinct zooplankton species groups are observed on either side of the front. Satellite data also revealed numerous large and small eddies, which probably contribute to cross-shelf mixing in the transition zone.

The problem of invaders in the Caspian Sea in the context of the findings of new zoo- and phytoplankton species from the Black Sea

In April 2004, an expedition to the Caspian Sea discovered the species in the Middle Caspian not previously described in this sea. These species preliminary identified as Oithona similis Claus 1963, Sagitta setosa Muller 1847, and Calanus euxinus (Hulsemann) are the main representatives of edible plankton of the Black Sea; most likely they were introduced into the Caspian Sea with ballast waters of ships passing through the Volga-Don Canal. At present, it is difficult to say if these species will be established in the Caspian Sea; if so, they will clearly become a valuable food resource for planktophagous fish in the Caspian Sea. In addition, we discovered two diatom species typical for the Black Sea phytoplankton, Pseudo-nitzschia seriata (Cleve) H. Peragallo and M. Peragallo and Cerataulina pelagica (Cleve) Hendey which invaded the Caspian Sea in recent years. Pseudo-nitzschia seriata synthesizes a toxin (domoic acid) and can become harmful for the basin in the case of mass bloom. We discuss the problem of spontaneous invasion of alien species from the Black Sea and their effect on the Caspian ecosystem in the context of these findings.

Living conditions, abundance and biomass of under-ice fauna in the Storfjord area (western Barents Sea, Arctic) in late winter (March 2003)

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7–10/10) prevailed and ice thickness ranged over <0.1–1.6 m covered by <0.1–0.6 m of snow. Air temperatures ranged between −1.8 and −27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0–5 m depth) were not stratified (T=−1.9 to −2.0°C and S=34.2–34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 μg chlorophyll a l−1), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2–241.3 and 5.3–16.4 μg l−1, respectively, the C/N ratio over 11.2–15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0–1.8 ind. m−2 for Apherusa glacialis, 0–0.7 ind. m−2 for Onisimus spp., and 0–0.8 ind. m−2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181–2,487 ind. m−3 (biomass: 70–2,439 μg C m−3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34–1,485 ind. m−3), contributing 19–65% to total abundances, followed by copepod nauplii (85–548 ind. m−3) and the cyclopoid copepod Oithona similis (44–262 ind. m−3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.

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 copepod population in the coastal area of Terra Nova Bay (Ross Sea, Antarctica) was investigated during the 10th Italian Antarctic Expedition (1994/1995). Zooplankton biotic community consisted mainly of pteropods (Limacina helicina and Clione antarctica), Cyclopoid (Oithona similis), Poecilostomatoid (Oncaea curvata) and Calanoid (Ctenocalanus vanus, Paraeuchaeta antarctica, Metridia gerlachei and Stephos longipes) copepods, ostracods, larval polychaetes and larval euphausiids. Zooplankton abundance ranged from 48.1 ind m � 3 to 5968.9 ind m � 3 , and copepod abundance ranged from 45.2 ind m � 3 to 3965.3 ind m � 3 .T he highest peak of zooplankton abundance was observed between 25 m and the surface and was mainly due to the contribution of O. similis, O. curvata and C. vanus. Zooplankton biomass ranged 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 � 1 dry weight.

Spatio-temporal distribution of Oithona similis in the Bornholm Basin (Central Baltic Sea)

Oithona similis is an abundant but poorly studied cyclopoid copepod in the brackish Central Baltic Sea. We describe the spatio-temporal distribution of O. similis in a Central Baltic deep basin (Bornholm Basin) during spring and summer 1999. Using vertically resolving sampling in parallel with hydrographic measurements, we found the copepod to dwell in the permanent halocline characteristic of a Central Baltic deep basin. The habitat of O. similis is thus limited from above by low salinity and from below by low oxygen conditions, both characteristic for the area. Horizontally resolving sampling yielded abundance surfaces which were compared by analysis of variance showing similar patterns among sampling dates. Comparison with flow fields from a three-dimensional hydrodynamic model suggests that the horizontal distribution is primarily the result of circulation in the dwelling depth. The study shows how the physical environment in the area determines the spatial distribution which might affect abundance and production of this copepod.

Toxicity of the aquaculture pesticide cypermethrin to planktonic marine copepods

The acute and sublethal toxicity of cypermethrin, the active ingredient in the sea lice treatment formulation Excis®, to non-target planktonic marine copepods was determined. The comparative sensitivities of three life stages (nauplii, copepodites, adults) of four common marine copepods (Acartia clausi, Pseudocalanus elongatus, Temora longicornis and Oithona similis) were assessed in 48-h exposures, followed by a recovery period in toxicant-free sea water. The cyclopoid copepod, O. similis, was most affected by cypermethrin, with EC50 values ranging from 0.14 to 0.24 μg L−1 for nauplii and adults respectively. With the exception of T. longicornis nauplii, the calanoid copepods (A. clausi, P. elongatus and T. longicornis) responded similarly to cypermethrin. Overall, 48-h EC50 values ranged from 0.12 μg L−1 (T. longicornis nauplii) to >5 μg L−1 (P. elongatus adults). For all species, nauplii and copepodite EC50 values were lower than those of the adults. The primary toxic effect, immobilization, was generally irreversible. A sublethal test with adult A. clausi females, involving pulse exposures over 4 days measured a significant increase in egg production at the higher concentrations (1.58 and 5 μg L−1). Concentrations causing acute toxicity to planktonic copepods were lower than the recommended sea lice treatment concentration of 5 μg L−1 cypermethrin, indicating the potential for toxic effects in the field. However, acute toxicity values were higher than the Environmental Quality Standard of 0.016 μg L−1 for dispersing treatment plumes, suggesting that cypermethrin released to the marine environment following sea lice treatments is unlikely to affect adversely planktonic copepods.

Plankton community structure and carbon cycling in a coastal upwelling system. I. Bacteria, microprotozoans and phytoplankton in the diet of copepods and appendicularians

Copepod and appendicularian grazing experiments using naturally occurring plank- tonic assemblages from a coastal embayment (Mejillones Bay, northern Chile upwelling system at 23o S) were conducted between October 2000 and October 2001. Total carbon ingestion rates based on size- fractioned chlorophyll data showed that dominant copepods (Acartia tonsa, Centropages brachiatus, Oithona similis and Paracalanus parvus) ingested between 2 and 8 µgC ind. -1 d -1 , while appendicu- larians (Oikopleura dioica and O. longicauda) ingested ~3 to 4 µgC ind. -1 d -1 . Even when most cope- pods were feeding on larger cells (> 23 µm) at high rates, the smaller copepods also grazed at similar rates on nanoplankton (5 to 23 µm) and picoplankton (<5 µm). In contrast, chain-forming diatoms were cleared at very low rates by copepods. Bacteria were cleared only by appendicularians (~170 to ~400 ml ind. -1 d -1 ) but not by any copepod, while heterotrophic protists constituted a substantial pro- portion in the diet of both copepods and appendicularians (~10 to 100% body carbon d -1 ), particularly during austral spring. Occasionally, copepod C-specific ingestion on heterotrophs was similar to that on autotrophic cells. Large ciliates and dinoflagellates were cleared but not ingested by the appen- dicularian O. dioica, suggesting a mechanism of trapping large cells in their houses and implying a rapid export of fresh material. Since heterotrophs are a common component in the diet of these 2 groups (omnivory by copepods and bacterophagy by appendicularians), they can potentially affect microbial food webs in this upwelling system and thus carbon export.

Stage-specific mortality of Calanus finmarchicus, Pseudocalanus elongatus and Oithona similis on Fladen Ground, North Sea, during a spring bloom

We used Woods Population Surface Method to estimate stage-specific mortality rates for populations of Calanus finmarchicus, Pseudocalanus elongatus, and Oithona similis from abundance data obtained during the Fladen Ground Experiment (FLEX 1976) in the northern North Sea. Temperatures from the day and night depth distributions of each developmental stage were used to force temperature functions for the duration of developmental stages. In addition, we considered the influence of potential food-limitation of development, which had only minor effects on the mortality estimates. We show that the risk of mortality changes substantially over the life span of the animals, with notable differences among species. O. similis, in particular, shows negligible losses after naupliar stages 1 and 2, which may account for its numerical dominance in many ocean regions. Differences among taxa are poorly explained by allometry alone and are instead related to species- and stage-specific differences in behavior. The vertical migration behavior of late developmental stages of C. finmarchicus (i.e. their choice of habitat) might help to reduce mortality risk.

The Continuous Plankton Recorder in the Southern Ocean: a comparative analysis of zooplankton communities sampled by the CPR and vertical net hauls along 140 E

A repeat transect was run south of Tasmania, along ∼140°E, during November and December 2001. NORPAC nets were deployed during a CTD transect on the southern leg, sampling four depth zones at each of 19 stations: 0–20, 20–50, 50–100 and 100–150 m. A Continuous Plankton Recorder (CPR) was deployed on the northern leg (average sampling depth = 10.5 m). Both net systems were harnessed with 270 μm mesh and all sampling was conducted between 47°S and the Southern Polar Front (S-PF) at ∼61°S. Zooplankton in the top 150 m of the water column demonstrated strong, small-scale, vertical distribution patterns. Species richness and diversity increased with depth, and were lowest for CPR samples. Conversely, dominance decreased with depth and was highest for CPR samples. Evenness was similar for all sample groups, indicating that all communities had a similar distribution of abundance amongst species. There was little variation in abundance between NORPAC depth zones (average = 82 ± 47 individuals m−3), while abundance was substantially higher in the CPR samples (average = 144 ± 103 individuals m−3), despite it under-sampling fast-moving and delicate components of the plankton community. The higher CPR abundance was due to significantly higher abundance levels of Appendicularia, Oithona similis and Rhincalanus gigas nauplii. The NORPAC samples showed that these three taxa were most abundant in the surface waters. The significant increase in abundance in the CPR samples was attributed to the growth in size during the period between the NORPAC and CPR surveys (minimum 15 days) increasing their catchability. Both the NORPAC nets and CPR surveys identified distinct communities to the north and south of the Southern Sub-Antarctic Front. Owing to its shallow towing depth, the CPR focuses on species with surface distributions. Despite under-sampling some components of the zooplankton, the CPR provided sufficient taxonomic resolution to identify biogeographic zones in the Southern Ocean. The utility of the CPR as a long-term monitoring tool in the Southern Ocean is discussed.

Grazing impact of copepods on phytoplankton in the Bohai Sea

During spring (April/May 1999) and autumn (September/October 1998) cruises in the Bohai Sea, China, copepods were the dominant components of mesozooplankton, the most abundant species being Calanus sinicus, Centropages mcmurrichi, Paracalanus parvus, Acartia bifilosa and Oithona similis. Pigment ingestion rates by three size classes of copepods (200–500, 500–1000 and >1000 μm) were measured. In the south of the investigation area, gut pigment content (GPC), individual pigment-specific ingestion rates and grazing impacts on phytoplankton were lower in spring than in autumn. In the central area, GPC and individual pigment-specific ingestion rates were higher in spring than in autumn. The grazing impact on phytoplankton by the copepod assemblages was lower in spring than in autumn, however, because of the relatively smaller biomass in spring. In the western area where the Bohai Sea joins the Yellow Sea, GPC, individual pigment-specific ingestion rates and grazing impacts on phytoplankton were higher in spring than in autumn. Among the three size groups, the small-sized animals (200–500 μm) contributed more than 50% (range 38–98%) of the total copepod grazing during both cruises. The grazing impact on phytoplankton by copepods was equivalent to 11.9% (range 3.0–37.1%) of the chlorophyll- a standing stock and 53.3% (range 21.4–91.4%) of the primary production during the spring cruise. Grazing impact was equivalent to 6.3% (range 2.0–11.6%) of the chlorophyll- a standing stock and >100% (range 25.7–141.6%) of the primary production during the autumn cruise. The copepod community apparently consumed only a modest proportion of the standing stock of phytoplankton during spring and autumn blooms. They did, however, sometimes graze a significant proportion of daily primary production and hence were presumably able to limit the rate of further accumulation of phytoplankton, or even to prevent it.

Annual cycle in abundance, distribution, and size in relation to hydrography of important copepod species in the western Arctic Ocean

A yearlong study of the zooplankton biomass and the abundance, vertical distribution, life stage proportions, and body size and condition for five target copepod species ( Calanus glacialis, C. hyperboreus, Metridia longa, Microcalanus pygmaeus, Oithona similis) was conducted from October 1997 to October 1998 in the Western Arctic Ocean. The research was staged from Ice Station SHEBA that drifted from Canadian Basin over the Northwind Ridge and Chukchi Plateau and back over the Basin during this period. Four hydrographic regimes were surveyed during the period of the study. Zooplankton biomass was least over the basin during the fall and winter and greatest over the Chukchi Plateau during summer, with most biomass in the 200–1500 m depth interval except during summer when greatest biomass was present in the upper 200 m. The five copepod species followed two general life history strategies: (1) sustained reproduction with all life stages present throughout the year and constant depth distribution ( M. longa, M. pygmaeus, O. similis) and (2) pulsed reproduction with overlapping cohorts present and ontogenetic redistribution of preferred depths through the year ( C. glacialis, C. hyperboreus). Body size and condition did not demonstrate consistent temporal or regional patterns. Based on population age structure, both C. hyperboreus and C. glacialis were reproducing in the Arctic Ocean. However, extremely low abundances of C. glacialis suggested that this species may not be self-sustaining in the Arctic Ocean. Plankton biomass was consistent with that observed in recent studies and supported an emerging paradigm of a more productive Arctic Ocean than traditionally believed.

Zooplankton distribution in a temperate estuary (Mondego estuary southern arm: Western Portugal)

The composition and spatio-temporal distribution of zooplankton abundance and diversity were studied monthly from July 1999 to June 2000 in the southern arm of the Mondego estuarine system (Western Portugal). Two sampling stations with different salinity conditions were selected. Zooplankton samples were obtained using 63 and 125 μm mesh nets. In both taxocenosis densities were higher in autumn and late spring-summer with copepodits and adult copepods among the most abundant zooplankters, representing more than 90.0% of the total densities. Abundance increased with increasing temperature, salinity and chlorophyll a values. The most abundant species were Oithona nana, Acartia tonsa, Acartia clausi, Euterpina acutifrons, Oithona similis, Temora longicornis, Clausocalanus arcuicornis, Paracalanus parvus, and Acartia bifilosa var. inermis. In the 63 μm taxocenosis, diversity was higher closest to the mouth of the estuary, during late spring and summer, and in the inner estuary, during autumn and winter. Cluster analysis showed that spatial distribution dominated over seasonal patterns, i.e. the similarities between the clusters grouping the samples of different months is high, which was also confirmed by ANOVA analysis.

Mesozooplankton community structure across the Antarctic Circumpolar Current to the north of South Georgia: Southern Ocean

During four summer seasons mesozooplankton community composition and structure in relation to water mass distribution were investigated along a 735-km transect running across the Antarctic Circumpolar Current (ACC) to the north-west of the island of South Georgia, Southern Ocean. Samples were obtained each year during December–January from the top 200 m of the water column at 22 stations spaced 35 km apart. Cluster analysis revealed four station groupings that were geographically consistent with the different water masses identified on the basis of temperature and salinity properties along the transect. A Sub-Antarctic Zone (SAZ) community characterised by low overall plankton abundance was present at the northernmost end of the transect on three of the four cruises, separated from a Polar Frontal Zone (PFZ) community by the Sub-Antarctic Front (SAF). The PFZ community lay between the SAF and the Polar Front (PF) and was characterised by highest overall abundance and little interannual variability. Two Antarctic Zone (AAZ) communities were found south of the PF that, although taxonomically similar, differed in overall abundance. Although there were significant differences in nutrients (e.g. silicate) and phytoplankton (Chl a) between the different water masses, these factors only weakly correlated with plankton community structure. Copepods were the largest contributors to total abundance within all station groupings (median percentage 83–90% of total) and pteropods were also proportionately abundant in the PFZ (11%). With the exception of pteropods (≤11%) and ostracods (≤3%) all other non-copepod taxa contributed <1% to total abundance. All station groups were characterised by varying proportions of a relatively small subset of species, many of which were present throughout the transect, for example, Oithona similis, Ctenocalanus spp., Euchaeta antarctica, and Rhincalanus gigas. Others were particularly characteristic of different station groups, thus Neocalanus tonsus, Clausocalanus ingens, and Calocalanus spp. were characteristic of the SAZ with few in the PFZ and none in the AAZ. Microcalanus pygmaeus was particularly abundant in the AAZ compared to other regions, as was the polychaete, Pelagobia longicirrata. Other taxa, although widespread, tended to be typical of particular water masses, for example, Calanus simillimus and Limacina helicina in the PFZ, Calanoides acutus, appendicularians and P. longicirrata in the AAZ, and ostracods and chaetognaths in the SAZ. The close physical and biological coupling observed across the ACC confirms the frontal zones and particularly the PF as features across which community properties change in the Atlantic sector of the Southern Ocean.

The toxicity of emamectin benzoate, an aquaculture pesticide, to planktonic marine copepods

The acute and sublethal toxicity of emamectin benzoate to non-target planktonic marine copepods was determined. Emamectin benzoate is the active ingredient in Slice®, an in-feed pesticide formulation used to control parasitic sea lice in salmon aquaculture. The comparative sensitivity of three life stages (nauplii, copepodites, adults) of four common marine copepods (Acartia clausi, Pseudocalanus elongatus, Temora longicornis and Oithona similis) was assessed in 48-h exposures followed by a recovery period in toxicant-free sea water. The calanoid copepods responded similarly to emamectin benzoate and EC50 values were significantly lower than those for the cyclopoid O. similis. Nauplii and copepodite 48-h EC50 values were generally lower than those for the adults. EC50 values ranged from 0.12 μg/l (P. elongatus nauplii) to 232 μg/l (O. similis adults). The primary toxic effect, immobilisation, was generally irreversible. A 7-day sublethal test with adult A. clausi females measured a significant reduction in egg production at higher concentrations. The no-observed-effect-concentration (NOEC) and lowest-observed-effect-concentration (LOEC) values were 0.05 and 0.158 μg/l, respectively. Concentrations causing toxicity to planktonic copepods were considerably higher than Predicted Environmental Concentrations (PEC) in the vicinity of treated salmon farms and suggest that the use of emamectin benzoate for lice control is unlikely to adversely affect planktonic copepods.

Mesozooplankton community structure and grazing impact in the Polar Frontal Zone of the south Indian Ocean during austral autumn 2002

Mesozooplankton community structure and grazing impact were investigated at 13 stations in the Polar Frontal Zone during the second Marion Offshore Variability Ecosystem Study (MOVES II), conducted during April 2002. Total integrated chl- a biomass ranged between 11.17 and 28.34 mg chl- a m-2 and was always dominated by nano- and picophytoplankton (<20 µm). Throughout the study, small copepods, mainly Oithona similis and Ctenocalanus vanus, numerically dominated the mesozooplankton community, composing up to 85% (range 30–85%) of the total abundance. Grazing activity of the four most abundant copepods ( O. similis, C. vanus, Calanus simillimus and Clausocalanus spp.), constituting up to 93% of total mesozooplankton abundance, was investigated using the gut fluorescence technique. Results of gut fluorescence analyses indicated that Calanus simillimus and Clausocalanus spp. exhibited diel variability in gut pigments with maximum values recorded at night. In contrast, O. similis and Ctenocalanus vanus did not demonstrate diel variation in gut pigment contents. Ingestion rates of the four copepods ranged from 23.23 to 1462,02 ng(pigm.)ind-1day-1. The combined grazing impact of the four copepods ranged between 1 and 36% of the phytoplankton standing stock per day, with the highest daily impact occurring at stations occupied in the vicinity of the Antarctic Polar Front (~35.86% at station 23). Among the copepods, O. similis and Ctenocalanus vanus represented the most important consumers of phytoplankton biomass, collectively responsible for up to 89% (range 15–89%) of the total daily grazing impact. Carbon specific ingestion rates of the copepods varied between 42 and 320% body carbon per day.

Mesozooplankton community structure in the Southern Ocean upstream of the Prince Edward Islands

A meso-scale oceanographic grid survey was conducted during the first cruise of the Marion Offshore Ecosystem Variability Study in the upstream region of the Prince Edward Islands in austral autumn (April/May) 2001. Mesozooplankton samples, collected using a Bongo net (fitted with 200-µm and 300-µm mesh nets), were separated into three size fractions, 200–500 µm, 500–1,000 µm, 1,000–2,000 µm, by reverse filtration. Total surface (depth<5 m) chlorophyll-a concentration during the study ranged between 0.11 and 0.34 µg l–1 and was always dominated by picophytoplankton (0.45–2.0 µm). Total mesozooplankton abundance and biomass during the survey ranged between 49 and 1,512 ind. m–3 and between 0.7 and 25 mg Dwt. m–3, respectively. Throughout the survey, the 200 to 500 µm class numerically dominated the mesozooplankton community, with an average of ~69% (SD=±12.3%). The dominant species in the 200- to 500-µm size fraction were the copepods, Oithona similis, Calanus simillimus and Metridia lucens, and the pteropod, Limacinaretroversa. However, in terms of biomass, the 1,000- to 2,000-µm group was predominant, with dry weight values constituting an average of ~66% (SD=±10.2%). The most well-represented species in this group were the carnivorous Euphausia vallentini, Thysanoessa vicina, Sagitta gazellae and Eukrohnia hamata. Three distinct groupings of stations were identified by numerical analysis. The different station groupings identified reflect changes in the relative contributions of the dominant species, as opposed to the presence/absence of species.

Zooplankton assemblages and hydrography in the spawning area of anchovy (Engraulis anchoita) off Río de la Plata estuary (Argentina - Uruguay)

The massive spawning of anchovy (Engraulis anchoita) in Argentine waters takes place in spring in the coastal sector off the Buenos Aires Province, with the highest concentrations of spawners occurring in front of the Río de la Plata mouth. In October 1995, the size structure, composition and abundance of the zooplankton community was analyzed and related to the hydrography and the abundance of anchovy eggs. Oceanographic conditions were different from the recorded means for the season. A higher proportion of the colder and more saline shelf waters were present in the coastal area and strong haloclines were not observed. High values of plankton biomass were observed in the estuarine frontal area and in the weakly stratified waters of the median shelf. The cluster analysis of the stations gave rise to the formation of coastal (CA), transitional (TA) and shelf (SA) faunistic areas. Mesozooplankton smaller than 1 mm total length (TL) dominated the whole area. The copepods, Paracalanus spp. and Oithona similis, and the cladocerans, Evadne nordmanni and Podon spp., constituted the bulk of this mesozooplankton fraction. Oithona similis was the dominant species in the SA. The copepod, Acartia tonsa (1–2 mm TL), was extremely abundant in the estuarine front (more than 17 600 individuals m–3) followed by appendicularians, members of Pseudocalanidae, anchovy eggs and decapod larvae. Species of Calanidae (Calanoides carinatus and Calanus simillimus), the amphipod hyperiid, Themisto gaudichaudii, and the euphausiid, Euphausia lucens, largely dominated the macrozooplankton (&gt;2 mm TL) in the TA and SA. In the CA, significant positive correlations between the abundance of small copepods (size ranges &lt;1 and 1–2 mm) and anchovy eggs were found. Trophic implications of these findings for first-feeding anchovy larvae were discussed. The adequacy of the TA and SA as feeding ground for adults of anchovy was evaluated.

High summer abundances of small pelagic copepods at the Antarctic Polar Front—implications for ecosystem dynamics

AbstractCopepods represent a major component of the Antarctic food web in terms of biomass turnover. Although important in abundance, little is known about the life cycle strategies of the small species. Within the framework of the SO-JGOFS, we investigated the distribution of small copepods (< 2.0 mm) in relation to the hydrography of the Antarctic Polar Front during summer 1995/96. The community of small copepods was dominated by Oithona similis, followed by Oithona frigida and Ctenocalanus citer. The total abundance of these copepods was extremely high throughout the study area with peaks of up to 49,000 ind. m-3, to which naupliar and early copepodid stages (CI-CIII) contributed a high percentage. The accumulation of such a high standing-stock of small copepods is probably related to retention mechanisms provided by the meandering structure of the frontal system and to the biology of the dominant species Oithona similis, Oithona frigida and Ctenocalanus citer.Stage distribution and metabolic demand of the dominant species indicate a very active and productive zooplankton community with high grazing pressure on smaller plankton particles and faecal material, leading to high recycling efficiencies and low export rates due to sinking material. This study gives further support to recent findings that small copepod species and early developmental stages of all species are key components of the plankton food web of the Southern Ocean.