Article

Comparison of zooplankton distribution patterns between four seasons in the Indian Ocean sector of the Southern Ocean

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Abstract

We investigated the composition, distribution and abundance of zooplankton in the Indian Ocean sector of the Southern Ocean during the austral summer (December/January) of 2004/05, 2005/06, 2007/08 and 2008/09 using a Continuous Plankton Recorder (CPR). CPR tows were conducted along two transects during voyages south of Cape Town to north of Syowa station and from north of the Mawson station area to south of Fremantle. High zooplankton abundance was recorded on each transect in the Polar Frontal Zone (PFZ) and the northern area of the Antarctic Zone (AZ). Community structure in these zones was dominated by common taxa including the ubiquitous Oithona similis and calanoid copepodites, accounting for >50% of total abundance, and Calanus simillimus, Ctenocalanus citer, Clausocalanus laticeps and Metridia lucens also occurred in high densities. Appendicularians of the genus Fritillaria were the most important component in the Cape Town to Syowa station area in 2008, with 36.9% of total abundance. The average chlorophyll a level at this time of year was the lowest (0.32 mg m−3) among all transects. Appendicularians are suited to oceanic oligotrophic waters; therefore, they are suited to low phytoplankton density. Foraminiferans were numerically dominant throughout the Mawson station area to Fremantle transect in 2005. Unlike Fritillaria spp., foraminiferans prefer high phytoplankton density. The elevated average chlorophyll a biomass in 2005 (0.64 mg m−3) provided favorable conditions for Foraminifera, which were dominant and widespread. CPR surveys provide information on the fine scale structure of the inter-annual distribution changes in micro- and meso- zooplankton assemblages, and the CPR is one of ideal method to monitor organisms that are indicators of environmental change.

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... In relation to species composition, the Southern Ocean near surface zooplankton typically comprises, the ubiquitous Oithona similis, small calanoid copepods Calanus simillimus, Ctenocalanus citer, Clausocalanus brevipes, Clausocalanus laticeps, the larger copepod species Rhincalanus gigas, foraminifera, larvaceans, the pteropod Limacina spp. and at times chaetognaths Hosie, 2005, 2006a, b;Mcleod et al., 2010;Takahashi et al., 2010aTakahashi et al., , b, 2011bTakahashi et al., , 2017bHosie et al., 2014). Hosie (2005, 2006b, c) referred to this collection as the main summer taxa, with notably O. similis, foraminifera and larvaceans representing the consistent "core taxa", occurring in >97% of samples and usually contributing an average of 75% to total sample abundance. ...
... This allows more confidence in properly identifying and assessing temporal variation. In relation to inter-annual variation along the same transects, the TV Umitaka-maru traversed the same transects southeast from Cape Town and north-east to Fremantle over consecutive austral summers;, 2008(Takahashi et al., 2010a. This provided an opportunity to conduct CPR tows for four seasons along almost the same cruise track at the same time of year. ...
... This coincided with the lowest chlorophyll a levels recorded on all transects. Takahashi et al. (2010a) noted that larvaceans are suited to oceanic oligotrophic waters and therefore suited to low phytoplankton density. The second different assemblage, more an episodic event, was dominated by very abundances of foraminifera in 2005 (Fig. 10). ...
Technical Report
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Zooplankton are a crucial link in the Antarctic marine ecosystem and changes in the zooplankton are likely to have substantial flow on effects through the rest of the food web. This report has collated the current knowledge of the status of zooplankton, e.g. what species are known, community structure, biogeography, based on the analysis of the SO-CPR dataset, and 69 publications in peer-reviewed research papers, chapters, atlases, and reviews. The report brings all that information together and highlights achievements over nearly 30 years, and includes some new analyses identifying trends (seasonal or long term) in relation to changes in abundance, shifts in distribution, and community composition. This report is a product of the Expert Group on CPR Research. It is hoped that this will be a living report that will include updated information as the SO-CPR datasets continues to grow and be analysed.
... Due to its remoteness, the zooplankton in the Indian sector of the Southern Ocean (SO) is still poorly characterized. (Labat et al. 2002;Hunt and Hosie 2003, 2005, 2006Takahashi et al. 2010), with emphasis on the horizontal distribution of copepods. There is almost no information available on the vertical distribution of zooplankton in the permanent open ocean zone of the Indian sector of the SO. ...
... It seems, the Indian Ocean sector of the Southern Ocean was numerically dominated by small calanoids (e.g., Ctenocalanus sp. and Oithona spp.). This same trend has been also noted by other Antarctic researchers (Atkinson 1998;Dubischar et al. 2002;Hunt and Hosie 2003;Takahashi et al. 2010). Oithona frigida showed the least abundance 45 ind m −3 in the STF and 63 ind m −3 in the SAF, respectively. ...
... Thus, the copepod-dominant community structure observed in the study area were typical of the circumpolar distribution pattern in the permanent open ocean zone of the SO. This concurs with previous CPR studies undertaken in various regions of the SO (Hunt and Hosie 2003, 2005, 2006Takahashi et al. 2010). Furthermore, Knox (2007) stated that copepods usually dominate the SO mesozooplankton assemblages both numerically in terms of biomass and abundance distribution. ...
Article
The abundance and vertical distribution of zooplankton community structure in the mesopelagic zone are important to better understand their role in the food web dynamics in the Southern Ocean ecosystem. During the austral summer of 2015, in the Indian sector of the Southern Ocean, the vertical profiles of zooplankton community structures between 0 and 1000 m were investigated using a Hydro-Bios, Multi Plankton Sampler (200-µm mesh, 0.25 m² mouth area). A strong contrast in terms of population structure and biovolume was observed between the Subtropical Front and the Polar Front 2. High zooplankton abundance was recorded on each transect in the Polar Front. Zooplankton abundance was numerically dominated by calanoids and cyclopoids, constituting approximately 86% of the total zooplankton count. Abundance of copepods were the highest within the mixed layer depth. Under warm, stratified conditions, surface waters were dominated by picophytoplankton. These conditions result in the zooplankton being dominated by small crustaceans. This long, inefficient food web is of poor nutritional quality, supporting a smaller biomass of higher trophic levels. In contrast, under cold and well mixed conditions, surface waters were dominated by microphytoplankton. These conditions result in the zooplankton being dominated by crustaceans, such as large copepods. This short, nutritionally rich and efficient food web supports higher trophic levels.
... Major interannual changes in zooplankton abundances were reported from south of Cape Town to south of Tasmania (Takahashi et al., 2010), on the basis of a comparison of zooplankton distribution patterns observed in four consecutive summer seasons. In this near-surface zooplankton community, largely dominated by copepods, Foraminifera presented very high abundances and were numerically dominant, up to 80%, along some transects located in the AAZ, southeast of Kerguelen Island in 2005. ...
... In this near-surface zooplankton community, largely dominated by copepods, Foraminifera presented very high abundances and were numerically dominant, up to 80%, along some transects located in the AAZ, southeast of Kerguelen Island in 2005. The elevated average Chl-a biomass observed in summer 2005 (0.64 mg m À 3 ) might have provided favorable conditions for Foraminifera (Takahashi et al., 2010). South of Tasmania, a CPR survey was conducted during the austral summer (Hunt and Hosie, 2006b. ...
... In the Indian Ocean sector of the Southern Ocean, the close vicinity of several oceanic fronts, and the occurrence of numerous instable meanders and eddies generated by the Antarctic Circumpolar Circulation (Phillips and Rintoul, 2000;Sokolov and Rintoul 2009a), may produce contrasting surface environments over short distances that favor different zooplankton communities (Takahashi et al., 2010). However, it appears also that not only the surface environment at the time of towing, but also environmental conditions well below the sampling depth of the CPR, may influence the zooplankton abundance and community structure at the surface (Takahashi et al., 2010). ...
... O. antarctica has been observed feeding on small copepods (Hopkins et al. 1993). O. similis are able to adapt to the low phytoplankton densities of the permanent open ocean zone (Takahashi et al. 2010). In the present study, Oncaea spp. ...
... The polar species N. pachyderma is known to be omnivorous but to exhibit a strong preference for phytoplankton (Hembleben et al. 1989;Lee and Anderson 1991). Bergami et al. (2009) andTakahashi et al. (2010) found correlations between foraminiferans abundance and Chl a concentration. In the present study, the Chl a concentration generally increased from fast ice to pack ice to open ocean stations. ...
Article
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In the Southern Ocean, zooplankton research has focused on krill and macro-zooplankton despite the high densities of micro- and meso-zooplankton. We investigated their community structure in relation to different sea ice conditions around Japan’s Syowa Station in Lützow-Holm Bay, in the summers of 2011 and 2012. Zooplankton samples were collected using vertical hauls (0–150 m), with a closing net of 100-μm mesh size. The results of cluster analysis showed that the communities in this region were separated into fast ice, pack ice, and open ocean fauna. The fast ice fauna had lower zooplankton abundance (393.8–958.9 inds. m−3) and was dominated by cyclopoid copepods of Oncaea spp. (54.9–74.8 %) and Oithona similis (6.6–19.9 %). Deep-water calanoid copepods were also found at the fast ice stations. Pack ice and open ocean fauna had higher zooplankton abundance (943.6–2,639.8 inds. m−3) and were characterized by a high density of foraminiferans in both years (6.6–61.9 %). Their test size distribution indicated that these organisms were possibly released from melting sea ice. The pteropod Limacina spp. was a major contributor to total abundance of zooplankton in the open ocean zone in 2012 (26.4 %). The physical and/or biological changes between 2 years may affect the abundance and distribution of the dominant zooplankton taxa such as cyclopoid copepods, foraminiferans, and pteropods. Information on the relationships between the different species associated with sea ice will help to infer the possible future impacts of climate change on the sea ice regions.
... In four transects from south of Cape Town to south of Tasmania, Takahashi et al. (2010) reported major interannual changes in zooplankton abundances on the basis of a comparison of zooplankton distribution patterns observed in four summer seasons. In this zooplankton community largely dominated by copepods, Foraminifera presented very high abundances and were numerically dominant, up to 80%, along some transects located in the AAZ, southeast of Kerguelen Island in 2005. ...
... These values are in the same order of magnitude than the mean abundances of Foraminifera recorded in February in the SO-CPR data base ). In the Indian Ocean sector of the Southern Ocean, the close vicinity of several oceanic fronts, and the occurrence of numerous meanders and eddies generated by the Antarctic Circumpolar Circulation (Philips and Rintoul, 2000;Sokolov and Rintoul 2009a), may produce contrasting surface environments over short distances that favour different zooplankton communities ( Takahashi et al., 2010). However, it appears also that not only the surface environment at the time of towing, but also environmental conditions well below the sampling depth of the CPR, may influence the zooplankton abundance and community structure at the surface (Takahashi et al., 2010). ...
Thesis
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Les foraminifères planctoniques vivants (LPF) contribuent à la pompe biologique du carbone océanique en générant des flux de Corg (cytoplasme) et de Cinorg (test calcaire). Dans cette étude, la morphométrie des tests, les abondances et les compositions spécifiques des assemblages de LPF dans l'océan Indien Sud (30°S-60°S, 50°E-80°E), ont été caractérisées à partir de la collecte par filet à plancton stratifié (Multinet) sur 19 stations échantillonnées pendant trois étés consécutifs (2012- 2014). En démontrant l'efficacité d'échantillonnage du Continuous Plankton Recorder pour spatialiser les données observées en 19 stations, l’étude de la dynamique de population des LPF montre l'effet de la position des fronts sur la production des LPF. Pour mieux contraindre l'impact des LPF dans la pompe biologique du carbone des hautes latitudes, la biomasse protéique et la masse calcique de plus de 2000 foraminifères ont été mesurées. Les différences de biomasse protéique et de poids normalisé par la taille entre années, espèces et masses d'eau suggèrent que les paramètres environnementaux affectent la production de Corg et de Cinorg des LPF. Le rôle des LPF sur la pompe biologique de carbone marin dépend des conditions hydrologiques et trophiques du milieu. Le rapport Corg/Cinorg est très différent selon les espèces considérées. L'applicabilité des tests de foraminifères planctoniques comme proxy de paléopompe du carbone dans les hautes latitudes dépendrait donc de l'effet exercé par les variations des conditions écologiques, et de la composition de l’assemblage. Cette étude propose une première estimation des budgets Corg et Cinorg produits par les LPF dans l’Océan Indien Austral.
... Krill is widely recognized as the key species in the Antarctic food web. However, small herbivorous zooplankton such as copepods have been estimated to have greater biomass than that of the Antarctic krill and this signifies the importance of other plankton communities in these waters (Takahashi et al. 2010); Although different aspects of the planktonic food web have been addressed from the Indian sector of the SO (Mayzaud et al. 2002;Fielding et al. 2007;Jasmine et al. 2009;Takahashi et al. 2010;Takahashi et al. 2011), studies of the temporal variations in the planktonic food-web structure at the major fronts in relation to the prevailing hydrographic and biological conditions, based on continuous field observations are lacking. Therefore, considering the significance of STF and PF as the two prominent (in terms of production) frontal regions of the SO, this study was designed to describe the planktonic food-web structure at these two fronts with respect to chl a and prevailing environmental conditions during the 2011 austral summer. ...
... Krill is widely recognized as the key species in the Antarctic food web. However, small herbivorous zooplankton such as copepods have been estimated to have greater biomass than that of the Antarctic krill and this signifies the importance of other plankton communities in these waters (Takahashi et al. 2010); Although different aspects of the planktonic food web have been addressed from the Indian sector of the SO (Mayzaud et al. 2002;Fielding et al. 2007;Jasmine et al. 2009;Takahashi et al. 2010;Takahashi et al. 2011), studies of the temporal variations in the planktonic food-web structure at the major fronts in relation to the prevailing hydrographic and biological conditions, based on continuous field observations are lacking. Therefore, considering the significance of STF and PF as the two prominent (in terms of production) frontal regions of the SO, this study was designed to describe the planktonic food-web structure at these two fronts with respect to chl a and prevailing environmental conditions during the 2011 austral summer. ...
Article
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This study aims to describe the planktonic food web structure with respect to phytoplankton biomass (chlorophyll a) and prevailing environmental conditions at the South Subtropical Front (SSTF) and the Polar Front (PF) in the Indian sector of the Southern Ocean. Sampling was carried out at each front for 72 hrs, at 6-hr intervals, during the austral summer 2011. Considerable variations were observed in the hydrography between these two fronts. A strong temperature minimum layer was observed at the PF. Although the surface primary production and chlorophyll a values showed similar trends at both the fronts, the water column values of these parameters showed major disparities. The phytoplankton composition also revealed marked difference between the fronts. A deep chlorophyll maximum concordant with the upper limit of the temperature minimum layer was prominent at the PF. The microzooplankton abundance at the SSTF was twice as high as at the PF. The mesozooplankton biovolume and population density also showed considerable variations between these fronts. Noticeable diel variations were observed in the surface mesozooplankton biovolumes at both the fronts and the copepod Pleuromamma gracilis showed active diel vertical migration at SSTF. Both the grazing and senescence indices showed significant variations between these fronts, suggesting a disparity in the ecological efficiency of the two regions. The variability observed in the plankton community structure with respect to the hydrography and the biological components measured suggests that a multivorous food web at the SSTF and a conventional food web at the PF prevailed during the period of study.
... Daponte and Esnal [21] validated the Antarctic species of the "Oikopleura gaussica group" in the regions near South Georgia and in the Bellingshausen Sea, and Kalarus and Panasiuk [16] researched the zonal distribution of these animals in the Drake Passage. Most other studies conducted in recent decades have examined entire zooplankton communities and have examined appendicularians only as a single group or by focusing on the dominant species [22][23][24][25]. In Antarctic or sub-Antarctic bays or fjords, studies on larvaceans are even rarer. ...
Article
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Appendicularians are important but remain poorly studied groups of zooplankton in polar regions. The present research is based on samples collected in Admiralty Bay (King George Island) during a year-long period. Six larvacean species were noted, among which Fritillaria borealis and Oikopleura gaussica were found to be the most numerous, while the other species were relatively rare. Fritillaria borealis was a dominant part of the late summer (warm water) community, while O. gaussica had the highest presence in the winter (cold water) community. The abundance of appendicularians recorded in the bay was less numerous than that described by other authors. The most important factors influencing annual changes in the larvaceans in the bay was season, but only in the case of the two species. These facts were probably linked to the very dynamic changes in the abiotic conditions in the fjord, and the influx of specific masses of water.
... University, Sapporo 2009 (Fukuchi and Conlan 2010; Stoddart 2010). Of the 22 contribution, 13 are on marine themes: Coppola et al. (2010); de Pascale et al. (2010); Jadwiszczak (2010); Lautredou et al. (2010) ; Massom and Stammerjohn (2010); McLeod et al. (2010); Naito et al. (2010); Russo et al. (2010); Sato-Okoshi et al. (2010); Takahashi et al. (2010); Toda et al. (2010); Vallesi et al. (2010). The international project ''Cooperative East Antarctic Marine Census project (CEAMARC)'', was conducted by a consortium of 3 ships from Australia, France and Japan, and scientists/students from several nations. ...
Chapter
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The Census of Antarctic Marine Life (CAML, www.caml.aq) was a 5-year long international project that focused the attention on the ice-bound oceans of Antarctica during the International Polar Year (IPY) in 2007–08, bringing together researchers from 30 different countries and more than 50 institutions. It was one of the fifteen IPY-endorsed biological projects devoted to Antarctica (Project #83) and coordinated field operations of 18 research voyages in Antarctica during IPY and/or within the CAML life-span. CAML’s main objectives were to study the evolution of life in Antarctic waters to determine how this had influenced the diversity of the present biota and to use these observations to predict how it might respond to future change. CAML was also one of the fourteen projects of the international Census of Marine Life (CoML, www.coml.org) (Gutt et al. 2010), each focusing on specific geographic environments or subject areas, with the aim to understand marine biodiversity and set reference baselines to allow measuring change. CAML’s sister project was the Arctic Ocean Diversity (ArcOD), devoted to the census of Arctic marine biodiversity. Thanks to strong interaction with this project, it is now possible to draw comparisons between differences in ecological structure and dynamics of the Arctic and Southern Oceans (Gradinger et al. 2010; Gutt et al. 2010).
... The biomass of adult E. crystallorophias, when it occurred in high densities close to the ice shelf of the Ross Sea (Sala et al. 2002), was higher (0.1-474 mg m -3 ) than our values. Mesozooplankton abundance was generally low, but comparable to abundance reported in previous studies of different parts of the Southern Ocean (Table 1; Fig. 5; Hosie et al. 2000;Ward et al. 2004;Hunt and Hosie 2006;Swadling et al. 2010;Takahashi et al. 2010). Pakhomov et al. (1998) reported that swarms of E. crystallorophias were not observed in the Lazarev Sea, but the presence of high numbers in their Bongo net samples suggests E. crystallorophias was abundant during the austral spring. ...
Article
Full-text available
The rapid melting of glaciers as well as the loss of sea ice in the Amundsen Sea makes it an ideal environmental setting for the investigation of the impacts of climate change in the Antarctic on the distribution and production of mesozooplankton. We examined the latitudinal distribution of mesozooplankton and their grazing impacts on phytoplankton in the Amundsen Sea during the early austral summer from December 27, 2010 to January 13, 2011. Mesozooplankton followed a latitudinal distribution in relation to hydrographic and environmental features, with copepods dominating in the oceanic area and euphausiids dominating in the polynya. Greater Euphausia crystallorophias biomass in the polynya was associated with lower salinity and higher food concentration (chlorophyll a, choanoflagellates, and heterotrophic dinoflagellates). The grazing impact of three copepods (Rhincalanus gigas, Calanoides acutus, and Metridia gerlachei) on phytoplankton was low, with the consumption of 3 % of phytoplankton standing stock and about 4 % of daily primary production. Estimated daily carbon rations for each of the three copepods were also relatively low (<10 %), barely enough to cover metabolic demands. This suggests that copepods may rely on food other than phytoplankton and that much of the primary production is channeled through microzooplankton. Daily carbon rations for E. crystallorophias were high (up to 49 %) with the grazing impact accounting for 17 % of the phytoplankton biomass and 84 % of primary production. The presence of E. crystallorophias appears to be a critical factor regulating phytoplankton blooms and determining the fate of fixed carbon in the coastal polynyas of the Amundsen Sea.
... The first was in the sea-ice zone (SIZ) around year 2000 when smaller zooplankton became more dominant instead of Antarctic krill (Hosie and Raymond, unpublished data). The second change occurred in 2004/05 north of the SIZ when pelagic foraminiferans exceeded 50%, and at times 80%, of the numerical abundance instead of the 8% long-term average, replacing Oithona as the dominant species ( Takahashi et al., 2010a). Such changes in food size and type could have a major impact on the survival of higher predators. ...
Article
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There is an enormous amount of data on Southern Ocean (SO) zooplankton, mostly on their distribution with a minority addressing rate processes. This review aims to summarise these data and show where it resides, to assist SO food-web modellers or those with limited specialist knowledge of SO zooplankton. First, a brief overview is provided of the diversity and basic biology of SO zooplankton, with an emphasis on abundance, distribution and feeding. Second, advice is provided on the uses, strengths and limitations of zooplankton data as inputs to SO data compilations or food-web models. Copepods overall comprise >75% of the SO zooplankton biomass (excluding Euphausia superba). Total mesozooplankton biomass density differs little between the Antarctic sectors, but latitudinally it is maximal in the Polar Frontal Zone and declines to the north and south. Those compiling data on numerical density (no. m-2 or no. m-3) need to allow for differences in the extent of identification of early larval stages. Likewise, the time of year, depth of sampling and mesh size of sampler greatly influence the recorded abundance, since the populations can make seasonal vertical migrations and their pulsed reproduction causes great seasonal changes in size structure and abundance. Other issues are specific to polar environments, for example, lipid storage which leads to significantly different length-mass and mass-rate relationships than are reported in global literature compilations. Likewise, stenothermy (narrow temperature tolerance) means that fixed (Q10-type) temperature relationships based on global literature compilations must be applied with great caution in SO-specific studies. Protozoa/micrometazoa (<200 μm) are the main grazers in the SO, since mesozooplankton typically remove <30% of primary production. This emphasises the dominant role of microbial food chains involving small metazoans, relative to the classic short diatom-krill-whale type food chains. Even within regions of abundant krill, copepod production in summer roughly triples that of postlarval E. superba. This fact reflects a large flow of energy through multiple trophic levels, via copepods and their major invertebrate predators such as other predatory copepods, chaetognaths, small omnivorous euphausiids, amphipods up to myctophid fish and birds.
... and Oncaea spp.). This same trend has been noted by other Antarctic researchers (Fransz 1988;Errhif et al. 1997;Atkinson 1998;Schnack-Schiel et al. 1998;Dubischar et al. 2002;Hunt and Hosie 2003;Pakhomov and Froneman 2004;Pane et al. 2004;Takahashi et al. 2010). ...
... and Oncaea spp.). This same trend has been noted by other Antarctic researchers (Fransz 1988;Errhif et al. 1997;Atkinson 1998;Schnack-Schiel et al. 1998;Dubischar et al. 2002;Hunt and Hosie 2003;Pakhomov and Froneman 2004;Pane et al. 2004;Takahashi et al. 2010). ...
Article
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Due in part to its remote location, the zooplankton of the Ross Sea and adjacent waters is poorly characterized. Very little depth-integrated information exists for this region, including measurements of mesozooplankton biomass, abundance and community structure throughout the water column. Furthermore, because large-mesh nets are often used, small copepods have been undersampled. Mesozooplankton research during the New Zealand International Polar Year—Census of Antarctic Marine Life voyage attempted to address these knowledge gaps. Depth-stratified net sampling was conducted from the surface to the seafloor at 11 stations in the Ross Sea and the adjacent Antarctic Circumpolar Current region of the Southern Ocean. Apart from high, localized contributions from pteropods and salps, mesozooplankton was numerically dominated by small calanoid and cyclopoid copepods, and densities were highest near the surface. Maximum mesozooplankton densities reached 640 ind m−3, and those of Oncaea spp. and Oithona spp. reached 111 and 256 ind m−3, respectively. On the Ross Sea shelf, Ctenocalanus sp. made significant contributions to total mesozooplankton numbers, while Oithona similis was highly abundant on and around Admiralty Seamount. On the Ross Sea slope, mixtures of Oithona spp. and Oncaea spp. dominated, and at the Scott Seamounts, mixtures of Oithona spp. and Ctenocalanus sp. occurred. Total water column, depth-integrated mesozooplankton biomass ranged between 0.6 and 7.1 g C m−2. Mesozooplankton biomass in the Ross Sea was at the higher end of previously recorded levels in the region and may rival that of productive sub-Antarctic regions.
... At times, occasional salp swarms, often represented by Salpa thompsoni, can be efficient grazers able to remove an amount of organic carbon almost equivalent to all of the primary production ( Pakhomov et al., 2002;Atkinson et al., 2004). The dominant copepods are commonly small species such as Oithona similis (Takahashi et al., 2010), and these contribute to the conservation of mass and the recycling efficiency of biogenic elements in the surface layer (Pond and Ward, 2011). ...
Article
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In this review article, plankton community structure observations are analyzed both for artificial iron fertilization experiments and also for experiments dedicated to the study of naturally iron-fertilized systems in the Atlantic, Indian and Pacific sectors of the Southern Ocean in the POOZ (Permanently Open Ocean Zone) and the PFZ (Polar Frontal Zone). Observations made in natural systems are combined with those from artificially perturbed systems, in order to evaluate the seasonal evolution of pelagic communities, taking into account controlling factors related to the life cycles and the ecophysiology of dominant organisms. The analysis considers several types of planktonic communities, including both autotrophs and heterotrophs. These communities are spatially segregated owing to different life strategies. A conceptual general scheme is proposed to account for these observations and their variability, regardless of experiment type. Diatoms can be separated into 2 groups: Group 1 has slightly silicified fast growing cells that are homogeneously distributed in the surface mixed layer, and Group 2 has strongly silicified slowly growing cells within discrete layers. During the growth season, Group 1 diatoms show a typical seasonal succession of dominant species, within time windows of development that are conditioned by physical factors (light and temperature) as well as endogenous specific rhythms (internal clock), and biomass accumulation is controlled by the availability of nutrients. Group 1 diatoms are not directly grazed by mesozooplankton which is fed by protozooplankton, linking the microbial food web to higher trophic levels. Instead, successive dominant species of Group 1 are degraded via bacterial activity at the end of their growth season. Organic detritus fragments feed protozooplankton and mesozooplankton. The effective silicon pump leads to the progressive disappearance of silicic acid in surface waters. In contrast, Group 2 is resistant to grazing due to its strong silicification, and its biomass accumulates continuously but relatively slowly throughout the productive period. Group 2 diatoms are concentrated at or near the seasonal pycnocline and thus benefit from upward nutrient fluxes by diapycnal mixing. The decrease in light and the deep convective mixing in the fall produce both light and nutrient limitation leading to a massive carbon export of Group 2 diatoms, a major annual event of the biological pump. This scheme describes the seasonal evolution of plankton communities in surface waters of the Southern Ocean. The scheme could probably be extended to ecosystems that are characterized by a seasonal bloom under influence of iron or other nutrients.
... We expect that zooplankton distribution ranges will move south with the fronts. A sudden large increase in the abundance of foraminiferans has been reported in the Indian sector collected in samples from January 2005 (Takahashi et al., 2010a) Holocene-aged sediments. They found that the modern shell weights are 30-35% lower than those from the sediments, consistent with reduced calcification induced by ocean acidification. ...
... CPR observations are an effective and efficient method for zooplankton monitoring to detect ecosystem change over large ocean scales. On the Umitaka-maru, CPR has contributed significantly to surface zooplankton monitoring in the Southern Ocean (Takahashi et al., 2010Hosie et al., 2014). Regular sampling on the Shirase and the Umitakamaru along single transect provide the opportunity to conduct time-series CPR towing. ...
Article
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This special issue provides an overview of the ten voyages undertaken by the Umitaka-maru from the austral summers of 2002/2003 to 2014/2015 to promote the next phase of study of the ecosystems in the Indian Ocean sector of the Southern Ocean. The voyages by the Umitaka-maru have mainly targeted three areas in the Indian Ocean sector: off Dumont d’Urville Base (France, 140°E transect), off Casey Station (Australia, 110°E transect), and off Syowa Station (Japan, north of Lützow Holm Bay). The findings of Umitaka-maru's research on the krill-independent food web, animal assemblages, community structure and distribution patterns from the epipelagic to the deeper waters provide invaluable information for elucidating the material cycle and predicting future ecosystem changes. Further studies on assessing the influence of sea ice on food webs in the water column are required, which will provide crucial information for predicting ecosystem changes as a result of projected sea ice changes in the near future.
... They remain key organisms of interest in ecology and production studies due to their role in energy transfer from the euphotic zone to deeper ocean layers and in energy exchange from primary producers to higher trophic levels (Calbet et al., 2000). They have been used as effective indicators of global climate change because of their weak swimming ability and short life span (Taylor et al., 2002;Hays et al., 2005) as well as their proximity to the bottom of the food chain (Takahashi et al., 2010). Climate change and associated processes like ocean acidification, overfishing, and eutrophication (Richardson 2008;Mackas et al., 2012) have been altering the community structure as well as carbon flow through marine zooplankton on a global scale (Kjellerup et al., 2012). ...
Article
Estimation of copepod carbon (C) biomass is essential in studies of secondary production and ecology in aquatic systems. The coastal Antarctic region belonging to the Indian Ocean sector of the Southern Ocean is a globally well-known sink for carbon and is extremely sensitive to climate change. During the austral summer, an attempt was made in Prydz Bay to measure copepod prosomal length and use regression equations to derive copepod C-biomass. The technique involved microscopic measurements, by means of a digital imaging device, of copepods collected at four intervals during a period of 48 hr, and the application of appropriate conversion factors to convert these values into C-biomass. Totals of 10.38 mgC m–3 and 25.55 mgC m–3 were recorded in the upper 200 m on day 1 and day 2, respectively. Out of the six copepod species present, Paraeuchaeta antarctica (Giesbrecht, 1902) (31.12 ± 11.10 μg m–3) and Oithona similis (Claus, 1866) (1.01 ± 0.42 μg m–3) represented the highest contributors to C-biomass amongst the calanoids and cyclopoids, respectively. Reports on copepod C-biomass estimates in general are extremely sparse. To our knowledge, this study is the first of its kind in the study area that provides species-specific estimates of C at every time point using a labor-saving and non-destructive method that allows preservation of the samples for further analyses. Based on the C estimates from our study, it can be said that copepod community is an extremely important component of the carbon cycle in coastal Antarctica.
... stratification, mixing, grazing) define the composition, abundance and productivity of the phytoplankton community (Deppeler & Davidson 2017), and subsequently the zooplankton. Most zooplankton sampling in the SO has been conducted with horizontally towed nets (Hunt & Hosie 2003, Takahashi et al. 2011, facilitating detection of changes to surface zooplankton community structure (Takahashi et al. 2010a), seasonal cycles (Hunt & Hosie 2006a) and interannual variation (Takahashi et al. 2010b). To date, there has been little study of the horizontal distribution of zooplankton in the Indian sector of the SO, particularly in the region between 47°E and 57°E. ...
Article
The community composition of zooplankton with an emphasis on copepods was assessed in the frontal zones of the Indian sector of the Southern Ocean (SO) during summer 2013. Copepods were the dominant group in both the bongo net and multiple plankton sampler across the entire region. High zooplankton abundance was recorded along each transect in the Polar Front (PF). Community structure in this front was dominated by common taxa, including Ctenocalanus citer , Clausocalanus spp., Calanoides acutus , Calanus propinquus , Calanus australis and Rhincalanus gigas , which together accounted for > 62% of the total abundance. Calocalanus spp., Neocalanus tonsus and C. propinquus were indicator species in the Sub-Tropical Front (STF), Sub-Antarctic Front and PF, respectively. A strong contrast in population structure and biovolume was observed between then PF and the STF. The community structure of smaller copepods was associated with the high-temperature region, whereas communities of larger copepods were associated with the low-temperature region. Thus, it seems probable that physical and biological characteristics of the SO frontal regions are controlling the abundance and distribution of zooplankton community structure by restricting some species to the warmer stratified zones and some species to the well-mixed zone.
... Consistent with previous CPR studies conducted in various regions of the Southern Ocean, we found that copepods account for most of the zooplankton biomass (Hunt and Hosie, 2003, 2006a, 2006bHosie et al., 2003;Takahashi et al., 2002Takahashi et al., , 2010Takahashi et al., , 2011. Our analyses of copepods, euphausids and Limacina sampled at night permitted to reveal the spatial patterns in the abundance of the different taxa across our study area encompassing the Southern Ocean and the South Indian Ocean. ...
Article
Distinguishing regions based on the geographic distribution of both abiotic factors and living organisms is an old but still actual central issue for biogeographers. In the Southern Ocean, the few existing regionalization studies have been carried out either at very large scales or on the relatively small region around the Sub-Antarctic islands of Kerguelen and the Crozet archipelagos. However, regionalization studies at meso-scales (100–300 km) covering the Indian part of the Southern Ocean and adjacent South Indian Ocean are scarce. These waters, ranging from the Subtropical to the polar region, are home to large populations of well-studied top predators that depend on the biomass of less known mid-trophic level species such as zooplankton. To fill those gaps, our study aims at conducting bioregional analyses of this transition area at the meso-scale based on the distribution of abiotic factors and chlorophyll-a, and to investigate how the abundance of zooplankton varies across the bioregions identified. To that end, we first characterized epipelagic bioregions 30°S in the South Indian Ocean to 65°S in the Southern Ocean and from 40° to 85°E including the islands of Crozet, Kerguelen, Saint-Paul and New Amsterdam. We then determined whether these bioregions correspond to variations in the abundance of zooplankton collected by a Continuous Plankton Recorder. Finally, we analyzed which environmental parameters influence zooplankton abundance. Our analyses evidenced six regions, providing a synthetic overview of a contrasting environment. The spatial variability of zooplankton abundance was explained by most of the environmental variables used in the bioregionalisation and, to a lesser extent, by the bioregions. Copepods are abundant in the colder and physically-energetic regions associated with the Antarctic Circumpolar Current (ACC). Limacina and euphausids are both abundant in regions characterized by a high concentration of chlorophyll-a, although euphausids are also abundant in the subtropical region. This work represents a crucial step forward in the integration of living organism distribution in the regionalization of the Indian part of Southern Ocean and adjacent South Indian Ocean. This can, ultimately contribute to the optimization of marine conservation strategies.
... The dominance of protozooplankton, small copepods and patchy salp blooms in HNLC waters, rather than a diverse and abundant mesozooplankton community as on the northern Kerguelen Plateau, results in control of the low phytoplankton biomass by protist grazing, and efficient remineralization of carbon and nutrients in the upper water column (Figure 1; Landry et al., 2002;Mayzaud et al., 2002;Pakhomov et al., 2002;Atkinson et al., 2004). Considering the omnivorous and detritivorous diet of Oithona similis (Takahashi et al., 2010), it would be expected that the POC export out of the epipelagic zone would be low, as ingestion and fragmentation of sinking particles increase the particle flux attenuation. However, studies such as SAZ-Sense in January/February 2007 have shown a relatively high POC transfer efficiency out of the mixed layer in the HNLC waters around SOTS, in comparison to other sites in the SAZ with higher iron levels or in the PFZ with a diatom-dominated phytoplankton community (Ebersbach et al., 2011). ...
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Marine ecosystems regulate atmospheric carbon dioxide levels by transporting and storing photosynthetically fixed carbon in the ocean’s interior. In particular, the subantarctic and polar frontal zone of the Southern Ocean is a significant region for physically-driven carbon uptake due to mode water formation, although it is under-studied concerning biologically-mediated uptake. Regional differences in iron concentrations lead to variable carbon export from the base of the euphotic zone. Contrary to our understanding of export globally, where high productivity results in high export, naturally iron-fertilized regions exhibit low carbon export relative to their surface productivity, while HNLC (High Nutrient, Low Chlorophyll) waters emerge as a significant area for carbon export. Zooplankton, an integral part of the oceanic food web, play an important role in establishing these main carbon export regimes. In this mini review, we explore this role further by focusing on the impact of grazing and the production of fecal pellets on the carbon flux. The data coverage in the subantarctic region will be assessed by comparing two case studies - the iron-replete Kerguelen Plateau and the HNLC region south of Australia. We then discuss challenges in evaluating the contributions of zooplankton to carbon flux, namely gaps in seasonal coverage of sampling campaigns, the use of non-standardized and biased methods and under-sampling of the mesopelagic zone, an important area of carbon remineralization. More integrated approaches are necessary to improve present estimates of zooplankton-mediated carbon export in the Southern Ocean.
... The cyclopoid O. similis was the most abundant species composing nearly half of the total mesoplankton abundance, which is likely explained by the time period surveyed (late spring-early summer), which is when the abundance of this species is maximal (Hosie Żmijevska, 1987), whereas in other seasons the contribution of this species was significantly lower (e.g., Atkinson & Peck, 1988;Pakhomov et al., 2000;Pakhomov & McQuaid, 1996;Takahashi et al., 2010). Other abundant taxa such as unidentified young calanoid copepodites, euphausiid nauplii, Ctenocalanus citer, and Foraminifera, were dominant in our samples and are known to be common in the Southern Ocean (Voronina, 1984;Vervoort, 1965;Bradford, 1969Bradford, , 1971. ...
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Spatial distribution of zooplankton communities depends on numerous factors, especially temperature and salinity conditions (hydrological factor), sampled depth, chlorophyll concentration, and diel cycle. We analyzed and compared the impact of these factors on mesoplankton abundance, biodiversity, quantitative structure based on proportion of taxa and qualitative structure based on presence/absence of taxa in the Southern Ocean. Samples (43 stations, three vertical strata sampled at each station, 163 taxa identified) were collected with a Juday net along the SR02 transect in December 2009. Mesoplankton abundance in discrete vertical layers ranged from 0.2 to 13,743.6 ind. m ⁻³ , i.e., five orders of magnitude, maximal and minimal values were recorded in the upper mixed and in the deepest layer, respectively. Within the combined 300-m layer, abundances ranged from 16.0 to 1,455.0 ind. m ⁻³ , i.e., two orders of magnitude suggesting that integral samples provide little information about actual variations of mesoplankton abundances. A set of analyses showed that depth was the major driver of mesoplankton distribution (abundance, biodiversity, quantitative structure), hydrological factors influenced two of them (quantitative and qualitative structure), chlorophyll concentration strongly affected only quantitative structure, and diel cycle had an insignificant effect on mesoplankton distribution. Using our current knowledge of the fine structure of the Antarctic Circumpolar Current, we compared effects of four hydrological fronts, i.e., boundaries between different water-masses with distinct environmental characteristics, and eight dynamic jets (narrow yet very intense currents) on mesoplankton distribution. Subtropical, Polar, and Subantarctic Fronts drove quantitative and qualitative structure of mesoplankton assemblages (decreasing in order of influence), while the Southern Boundary affected only qualitative structure. Effects of dynamic jets were insignificant. We suggest that mesoplankton composition is driven by hydrological parameters and further maintained through compartmentalization by fronts. Impact of local eddies and meanders on biodiversity, abundance, qualitative and quantitative structure of mesoplankton is comparable to that of hydrological fronts. Qualitative structure of mesoplankton assemblages mirrors hydrological structure of the Southern Ocean better than quantitative structure and may be recommended for biogeographic analyses of the Southern Ocean. Comparisons with previous reports from the same area retrieved no significant changes in mesoplankton distribution during the period 1992–2009.
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Copepods are one of the most important components of the Southern Ocean food web, and are widely distributed from surface to deeper waters. We conducted discrete depth sampling to clarify the community structure of copepods from the epi- to bathypelagic layers of the oceanic and neritic waters off Adélie and George V Land, East Antarctica, in the austral summer of 2008. Notably high diversity and species numbers were observed in the meso- and bathypelagic layers. Cluster analysis based on the similarity of copepod communities identified seven cluster groups, which corresponded well with water masses. In the epi- and upper- mesopelagic layers of the oceanic zone, the SB (Southern Boundary of the Antarctic Circumpolar Current) divided copepod communities. Conversely, in the lower meso- and bathypelagic layers (500–2000 m depth), communities were consistent across the SB. In these layers, the distributions of copepod species were separated by habitat depth ranges and feeding behaviour. The different food webs occur in the epipelagic layer with habitat segregation by zooplankton in their horizontal distribution ranges.
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Larvae of several marine invertebrates are known to alter their morphology adaptively as a response to changes in conditions, such as food and predator density, within plankton communities. In contrast, nothing is known about plastic re sponses to the density of competitors, which could signal the potential for food limitation or predation risk. We studied effects on trophic morphology of culturing sand dollar pluteus larvae with limiting or saturating food and at low or high density using 1 of 3 potential competitors: feeding conspecifics, feeding heterospecifics, and non-feeding hetero specifics. We hypothesized that feeding competitors would induce a morphological response similar to limiting food, while the effect of non-feeders would depend on whether larvae respond directly to higher density or indirectly to food reduction. Plutei cultured with limited food grew longer feeding arms and shorter stomachs, though only in higher density treatments. Whereas the response to food involved a tradeoff between skeleton and stomach growth, larvae responded to high density by in creasing in vestment in both, reflecting a more complex set of energetic tradeoffs. Similar responses to the presence of feeding and non-feeding larvae implicate a sensory mechanism involving signals from potential competitors rather than a food de cline. Patterns in 2 literature surveys of laboratory experiments and field studies suggest that culture density is a critical but neglected aspect of experimental design in the study of early life-history stages. Larvae are most likely exposed to densities sufficient to induce plasticity following synchronous spawning, highlighting the importance of natural history in understanding density-dependent effects on developmental plasticity.
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"Southern Ocean Continuous Plankton Recorder (SO-CPR) Standards Workshop: SCAR Expert Group on CPR Research" was held at the National Institute of Polar Research (NIPR) on 22-26 November 2011. Twelve participants from four countries attended. The purposes of the workshop were to ensure that consistent and high standards of species identification, methodology, and data quality were being maintained amongst all participants and laboratories in the SO-CPR survey, and to discuss future contributions of the SO-CPR program to a global CPR network. The first three and a half days of the workshop were focused on assessing the accuracy and consistency of species identifications. We concluded that our species identifications and procedures are accurate and uniform, and that the SO-CPR database is of the highest possible standard. Certain taxonomic criteria developed at the workshop will be described in a new laboratory procedures manual. Four major gaps in the database (spatial, temporal, taxonomic, and data analysis gaps) were identified and discussed. Participants concurred that there should be more regular workshops to ensure that the high standards of the SO-CPR program are maintained.
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Historically, most studies about the geographic distribution of zooplankton in the Southern Ocean have been focused on the macro-sized zooplankton (2–20 mm), such as the Antarctic krill and larger-sized copepods. On the other hand, despite the high abundance and biomass, the distribution patterns of micro- (20–200 μm) and meso-sized (200 μm–2 mm) zooplankton communities are little understood. In this study, we investigated the distribution patterns of larger micro-zooplankton (100–200 μm) and meso-zooplankton communities in the seasonal ice zone in the Cosmonaut Sea near Syowa Station and examined the effects of environmental factors and water properties on these communities. The investigation was based on samples collected with 100 μm mesh nets, which are appropriate to estimate the quantitative abundance and community structure of micro- and meso-zooplankton species between 1983 and 1995. Cluster analysis of the samples revealed that the distribution of macro-zooplankton species was influenced by the temperature and salinity of ocean fronts. Among the meso-zooplankton, cyclopoid and small calanoid copepods tended to be ubiquitously distributed. However, among the micro-zooplankton, the distributions of foraminiferans and tintinnids were associated with sea ice extent. The distribution of micro- and meso-zooplankton communities could be used to estimate the impact of environmental changes on the marine ecosystem in the Southern Ocean.
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Many studies using surface plankton data from the Continuous Plankton Recorder (CPR) have suggested that micro- and meso-sized zooplankton species/taxa can serve as indicators of environmental change. But while regular CPR sampling would allow for the detection of changes in zooplankton communities, few intra-annual CPR surveys have been undertaken along single transects. We report the temporal variability of zooplankton community structure using CPR data collected along the Japanese monitoring transect (110° E) in December 2014, January 2015, and March 2015. We found zooplankton abundances in both the Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) to increase from December to January, and mean AZ abundance to peak abruptly in January (381.9 ind. m⁻³) before decreasing to about 25% of that value in March. We attribute changes in total zooplankton abundance to fluctuations in the numbers of the two small dominant copepods, Oithona similis and Ctenocalanus citer. High AZ phytoplankton concentrations in December likely increased surface food availability, influencing population growth, and/or the timing of copepod seasonal ascension. We recommend future CPR surveys include sampling with finer mesh for more accurate determination of abundances of small-sized zooplankton, to better understand the ramifications of climate change on Southern Ocean trophic structure.
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We investigated the composition, distribution and abundance of micro- and mesozooplankton in the Southern Ocean, south of Australia during the austral summer (December-February) of the 2007/08 season using a Continuous Plankton Recorder (CPR). Four CPR tows were conducted during two separate oceanographic voyages under the CEAMARC (Collaborative East Antarctic Marine Census) project. High zooplankton abundance was recorded on each transect in the Polar Frontal Zone (PFZ) and the Inter Polar Frontal Zone (IPFZ). The community structure in these zones was dominated by common taxa including the ubiquitous small calanoid copepods, Oithona similis and Calanus simillimus, accounting for >70% of the total abundance, and copepod nauplii, foraminiferans and appendicularians of the genus Fritillaria spp. also occurred along most of the survey transects. Total zooplankton abundance was comparatively consistent along the four transects, and ranged between 119.8 and 144.7 ind m-3. The results of cluster and IndVal analyses revealed that the dominant species/taxa show similar associations, abundance and distribution patterns on all four transects. There was no evidence of a change of surface zooplankton abundance at the time of towing in this study. Detecting the various distribution patterns of micro- and mesozooplankton species/taxa, and the accumulation of high quality data collected by a consistent methodology will contribute to determining the consequences of climate change impacts on the ecosystem.
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Seasonal cycles can provide insight into the interactions between zooplankton and the environment. However, few intra-annual seasonal studies have been undertaken in the Southern Ocean. We investigated the composition, distribution, and abundance of micro- and meso-zooplankton along the 110°E meridian with three transects in December 2011, January and March 2012 using a Continuous Plankton Recorder. High zooplankton abundance was recorded in the Polar Frontal Zone (PFZ) and the Antarctic Zone (AZ) in both day and night at all transects with 179.0–300.9 ind. m−3. The small copepods Oithona similis, Ctenocalanus citer, and copepodites indet (copepod indeterminable) were dominant in the PFZ and AZ communities. Total zooplankton abundance was comparatively consistent among transects. Nighttime abundance levels remained high in the AZ in March with high abundance of copepodites indet. This seasonal fluctuation appeared to be influenced by recruitment of new populations. Most core species/taxa, except for O. similis, C. citer, and foraminiferans in the AZ area in early January, exhibited a diel decrease in abundance. A multi-ship intra-annual seasonal survey will help detect their various regional and/or seasonal distribution patterns, and the impacts of environmental change on Southern Ocean pelagic ecosystems.
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This paper presents a new and simple method to find indicator species and species assemblages characterizing groups of sites. The novelty of our approach lies in the way we combine a species relative abundance with its relative frequency of occurrence in the various groups of sites. This index is maximum when all individuals of a species are found in a single group of sites and when the species occurs in all sites of that group; it is a symmetric indicator. The statistical significance of the species indicator values is evaluated using a randomization procedure. Contrary to TWINSPAN, our indicator index for a given species is independent of the other species relative abundances, and there is no need to use pseudospecies. The new method identifies indicator species for typologies of species releves obtained by any hierarchical or nonhierarchical classification procedure; its use is independent of the classification method. Because indicator species give ecological meaning to groups of sites, this method provides criteria to compare typologies, to identify where to stop dividing clusters into subsets, and to point out the main levels in a hierarchical classification of sites. Species can be grouped on the basis of their indicator values for each clustering level, the heterogeneous nature of species assemblages observed in any one site being well preserved. Such assemblages are usually a mixture of eurytopic (higher level) and stenotopic species (characteristic of lower level clusters). The species assemblage approach demonstrates the importance of the 'sampled patch size,' i.e., the diversity of sampled ecological combinations, when we compare the frequencies of core and Satellite species. A new way to present species-site tables, accounting for the hierarchical relationships among species, is proposed. A large data set of carabid beetle distributions in open habitats of Belgium is used as a case study to illustrate the new method.
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During the .-rd Japanese Antarctic Research Expedition (JARE) cruise on March +*+, in ,**,, NORPAC net samplings at three stations along a south-north transect, ca. +.*E in the Indian sector were conducted to survey zooplankton commu-nity structure and abundance in the seasonal ice zone (SIZ) of the Southern Ocean. A total of fourteen species/taxa were identified from the three stations. While copepods were numerically dominant at two stations (13.3 and 3-.+ respectively of total abundance), appendicularians were found to be numerically dominant (2..* of total abundance) at the southernmost station. This dominance of Appendicularia at this station suggested that Appendicularia is possibly an integral part of the community structure of the zooplankton in the SIZ. The Southern Boundary (SB) on the +.*E transect was found to be located at 0..-*S and the southernmost station was located south of the SB while the two other stations were located north of the SB. Some species, such as Rhincalanus gigas, Calanus simillimus, Amphipoda, Euphausiacea, and Poly-chaeta, had distribution patterns that correlated with the position of the SB, therefore the SB is considered important in influencing the distribution of the zooplankton and its community structure in the SIZ.
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Between November 2001 and March 2002 an Australian/Japanese collaborative study completed six passes of a transect line in the Seasonal-Ice Zone (south of 62°S) along 140°E. Zooplankton samples were collected with a NORPAC net on 22–28 November, and a Continuous Plankton Recorder on 10–15 January, 11–12 February, 19–22 February, 25–26 February, and 10–11 March. Zooplankton densities were lowest on 22–28 November (ave=61 individuals (ind) m−3), when almost the entire transect was covered by sea ice. By 10–15 January sea surface temperature had increased by ∼2 °C across the transect line, and the study area was ice-free. Total zooplankton abundance had increased to maximum levels for the season (ave=1301 ind m−3; max=1979 ind m−3), dominated by a “Peak Community” comprising Oithona similis, Ctenocalanus citer, Clausocalanus laticeps, foraminiferans, Limacina spp., appendicularians, Rhincalanus gigas and large calanoid copepodites (C1–3). Total densities declined on each subsequent transect, returning to an average of 169 ind m−3 on 10–11 March. The seasonal density decline was due to the decline in densities of “Peak Community” taxa, but coincided with the rise of Euphausia superba larvae into the surface waters, increased densities of Salpa thompsoni, and an increased contribution of C4 to adult stages to the populations of Calanoides acutus, Calanus propinquus and Calanus simillimus. The seasonal community succession appeared to be influenced by the low sea ice extent and southward projection of the ACC in this region. The relatively warm ACC waters, together with low krill biomass, favoured high densities of small grazers during the January/February bloom conditions. The persistence of relatively warm surface waters in March and the seasonal decrease in chlorophyll a biomass provided favorable conditions for salps, which were able to penetrate south of the Southern Boundary.
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Anthropogenic carbon dioxide has been accumulating in the oceans, lowering both the concentration of carbonate ions and the pH (ref. 1), resulting in the acidification of sea water. Previous laboratory experiments have shown that decreased carbonate ion concentrations cause many marine calcareous organisms to show reduced calcification rates. If these results are widely applicable to ocean settings, ocean acidification could lead to ecosystem shifts. Planktonic foraminifera are single-celled calcite-secreting organisms that represent between 25 and 50% of the total open-ocean marine carbonate flux and influence the transport of organic carbon to the ocean interior. Here we compare the shell weights of the modern foraminifer Globigerina bulloides collected from sediment traps in the Southern Ocean with the weights of shells preserved in the underlying Holocene-aged sediments. We find that modern shell weights are 30-35% lower than those from the sediments, consistent with reduced calcification today induced by ocean acidification. We also find a link between higher atmospheric carbon dioxide and low shell weights in a 50,000-year-long record obtained from a Southern Ocean marine sediment core. It is unclear whether reduced calcification will affect the survival of this and other species, but a decline in the abundance of foraminifera caused by acidification could affect both marine ecosystems and the oceanic uptake of atmospheric carbon dioxide.
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Cyclopoid copepods of the cosmopolitan genus Oithona are often the most abundant metazooplankton in both coastal and oceanic waters. Reasons for their ubiquity have been discussed but their feeding habits are not well known. Field observations and results from experiments presented here suggest that Oithona is also coprophagous. Data collected along the eastern shelf of Svalbard (Spitsbergen, Norway) during ice retreat showed that Oithona was the most abundant metazooplankter and a negative correlation between calanoid faecal material (volume) in the water column and Oithona abundance was found, but there was no significant relationship between calanoids and their faeces. In experiments in which calanoid copepods were confined together with Oithona, the latter always removed and presumably ingested a significant amount of calanoid faecal material at rates that appeared independent of ambient food concentrations. About 20 to 30% of the calculated daily carbon requirements of Oithona could have been met from faecal matter. We speculate that Oithona uses chemical cues to find and intercept sinking faecal matter in the water column and that its ability to use this ubiquitous food source efficiently is one of the reasons underlying its extraordinarily widespread occurrence. Although it has been shown that generally only a small percentage of calanoid faeces produced in the surface layer contributes to vertical flux, little is known about the fate of the 'missing' faeces. We argue that metazoans are mainly responsible for retention of faecal material in the surface layer and that cyclopoid copepods are an important part of this 'coprophagous filter' that retards vertical flux in the water column.
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Spatio-temporal variability of zooplankton community structure off east Antarctica (90 to 160°E) was studied from 1988 to 1996 based on samples collected by the Japanese icebreaker 'Shirase'. Three community groups with distinctive species compositions were obtained by cluster analysis. Group 1 was defined as the 'Subantarctic community' because it appeared exclusively north of the Polar Front, and indicator species of the group included Eucalanus longiceps and Limacina retroversa, which are typical subantarctic species. Groups 2a and 2b occurred in the Antarctic Circumpolar Current (ACC), and are thus defined as the 'ACC communities'. All indicator species for Group 2b, including large copepods, Calanoides acutus, Calanus propinquus and Metridia gerlachei, were also common indicators for Group 2a. In Group 2a, smaller copepods (small calanoids,cyclopoids and poecilostomatoids) and non-copepod herbivores were also indicators. Total abundance was markedly high in Group 2a, and copepods numerically dominated all 3 groups (>70% in Group 1, >80% in 2a,>90% in 2b). The longitudinal distribution pattern of Groups 2a and 2b varied between years, although they occurred along the same latitude. Multiple-regression analysis on environmental variables and distribution of the groups demonstrated that Group 2a tended to occur in relatively warm water masses with high chlorophyll a and low silicate concentrations. While Group 2b occurred in colder areas with low chlorophyll a concentrations, the results of this study suggest that the occasional intrusion of northern water further south caused by meandering of the ACC might have been responsible for the observed distribution patterns of the zooplankton community.
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Microphytoplankton distribution in the Atlantic sector of the Southern Ocean was investigated along a transect during the SAAMES II cruise undertaken in late austral summer (January/ February) 1993. Samples were collected at ∼60 km intervals between 34 and 70°S for the analysis of mineral nutrients, and the identification and enumeration of microphytoplankton. Peaks in microphytoplankton abundance were recorded in the neritic waters of Africa and Antarctica, at all major oceanic fronts, and in the marginal ice zone (MIZ). Partial correlation analysis indicated that 45% of the total variance associated with microphytoplankton abundance could be explained by silicate and phosphate concentrations, while temperature accounted for 65% (P<0.001). Cluster and ordination analyses identified two major groups of stations, one north and one south of the Subantarctic Front (SAF). This division appears to be related to differences in temperature and silicate concentrations. Each region comprised distinct microphytoplankton subgroups associated with specific water masses or hydrological features. Indicator species could be identified for some water masses. In the MIZ, microphytoplankton species composition and succession were strongly affected by sea-ice throughout the summer.
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A strategy is presented for analysing marine biological survey data and relating the biotic patterns to environmental data. To avoid circular argument, biotic and environmental data are kept separate. The strategy is illustrated by a worked example using data on the distribution of 182 nematode species in 107 samples in the River Exe estuary. Nineteen stations are grouped Into 4 main clusters using complementary classification and multi-dimensional scaling (MDS) ordination techniques. These are both based on root-root transformed abundance data with the Bray-Curtis measure of similarity. Indicator species characterising each group are extracted using information statistics. Inverse analyses give clusters of co-occurring species which are strongly related to the station groups. Relationships of station groups to environmental variables are revealed by superimposing data for one variable at a time on the MDS plot, showing that some station groups differ in sediment granulometry and others in salinity, for example. Some of the other factors plotted show no difference between station groups. Similarly, physiognomic characteristics of the species are superimposed on the MDS plots of the inverse analysis of species groups, revealing differences in setal length and trophic status between the species groups. Finally, the 4 major station groups and species groups are related to one another in terms of morphological adaptation to the habitat.
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Between October 2001 and March 2002 six transects were completed at monthly intervals in the Sub-Antarctic Zone (SAZ) and Inter-Sub-Antarctic Front Zone (ISAFZ)/Polar Frontal Zone (PFZ) in the Southern Ocean south of Australia. Zooplankton were collected with a Continuous Plankton Recorder and NORPAC net and multivariate analysis was used to analyse the seasonal succession of communities. Despite strong, seasonally consistent, biogeographic differences between the SAZ and ISAFZ/PFZ, community structure in all zones was dominated by a suite of common taxa. These included the ubiquitous Oithona similis, foraminiferans and appendicularians (Core taxa), occurring in >97% of samples and contributing an average of 75% to total sample abundance, and Calanus simillimus, Rhincalanus gigas, Ctenocalanus citer, Clausocalanus brevipes, Clausocalanus laticeps, Oithona frigida, Limacina spp. and chaetognaths (Summer taxa), present in >57% of samples and occurring at seasonally high densities. Because of the dominance of the Core and Summer taxa, the seasonal succession was most clearly evident as a change in zooplankton densities. In October densities averaged
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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.
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The present paper describes latitudinal and vertical changes in the composition, abundance and diversity of copepods in the Indian sector of the Antarctic Ocean, during the end of austral summer along a transect on 66°30′E between 43 and 62°S, within three layers (600–0, 200–0, 100–0 m). Highest copepod densities were noted in the central part of the transect, between the Antarctic Divergence and the Antarctic Convergence, with a maximum in the Antarctic Divergence zone, particularly in the upper levels of the water column. A total number of 80 copepod species were identified over the entire survey area. The south end and the central part of the transect comprised a small number of species. North of the Antarctic Convergence, this number increased markedly with the progressive disappearence of those species characteristic of Antarctic waters and their replacement by temperate and subtropical species. Generally, small copepods, particularly Oithona similis, Oithona frigida and Ctenocalanus citer, dominated in numbers in both Antarctic and sub-Antarctic areas. The contribution of large species to total copepod numbers was much lower, with Calanus simillimus in the central part of the transect, Pleuromamma borealis in the subtropical zone and Calanus propinquus in the southern part. Correspondence analysis showed a marked latitudinal gradient in population structure with four groups of samples and species corresponding to four latitudinal zones. Community structure (species richness, relative dominance index, evenness, Shannon species diversity index) and species abundance patterns (as rank-frequency diagrams) suggested that the maturity and species richness increased gradually from south to north. A low diversity index and evenness were observed in the area of the Antarctic Divergence, whereas the convergence zone showed high diversity and evenness. Conversely, the frontal zone showed high diversity and evenness. Distribution appeared unrelated to chlorophyll concentrations and on the large scale was related to the hydrologic characteristics.
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This paper presents a new and simple method to find indicator species and species assemblages characterizing groups of sites. The novelty of our approach lies in the way we combine a species relative abundance with its relative frequency of occurrence in the various groups of sites. This index is maximum when all individuals of a species are found in a single group of sites and when the species occurs in all sites of that group; it is a symmetric indicator. The statistical significance of the species indicator values is evaluated using a randomization procedure. Contrary to TWINSPAN, our indicator index for a given species is independent of the other species relative abundances, and there is no need to use pseudospecies. The new method identifies indicator species for typologies of species releves obtained by any hierarchical or nonhierarchical classification procedure; its use is independent of the classification method. Because indicator species give ecological meaning to groups of sites, this method provides criteria to compare typologies, to identify where to stop dividing clusters into subsets, and to point out the main levels in a hierarchical classification of sites. Species can be grouped on the basis of their indicator values for each clustering level, the heterogeneous nature of species assemblages observed in any one site being well preserved. Such assemblages are usually a mixture of eurytopic (higher level) and stenotopic species (characteristic of lower level clusters). The species assemblage approach demonstrates the importance of the "sampled patch size," i.e., the diversity of sampled ecological combinations, when we compare the frequencies of core and satellite species. A new way to present species-site tables, accounting for the hierarchical relationships among species, is proposed. A large data set of carabid beetle distributions in open habitats of Belgium is used as a case study to illustrate the new method.
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While a seasonal ice cover limits light penetration into both polar seas for up to ten months a year, its presence is not entirely negative. The mixed layer under sea ice will generally be shallower than in open water at the same latitude and season. Ice forms a substrate on which primary production can be concentrated, a condition which contrasts with the generally dilute nutritional conditions which prevail in the remaining ocean. The combination of a shallow, generally stable mixed layer with a close proximity to abundant food make the under-ice zone a suitable nursery for both pelagic and benthic species, an upside-down benthos for opportunistic substrate browsers, and a rich feeding environment for species often considered to be neritic in temperate environments. Where the ice cover is not continuous there may be a retreating ice edge that facilitates the seasonal production of phytoplankton primarily through increased stability from the melt water. Ice edge blooms similarly encourage secondary production by pelagic animals. Pseudocalanus acuspes, which may be the most abundant and productive copepod in north polar latitudes, initiates growth at the start of the “spring bloom” of epontic algae, reaching sexual maturity at breakup or slightly before. In the Southern Hemisphere, the small neritic copepod Paralabidocera antarctica and adult krill have been observed to utilize ice algae. Calanus hyperboreus breeds in the dark season at depth and its buoyant eggs, slowly developing on the ascent, reach the under-ice layer in April as nauplii ready to benefit from the primary production there. On the other hand, C. glacialis may initiate ontogenetic migrations and reproduction in response to increased erosion of ice algae due to solar warming and melting at the ice-water interface. While the same species in a phytoplankton bloom near the ice edge reproduces actively, those under still-consolidated ice nearby can have immature gonads. Diel migration and diel feeding rhythms under or near the ice have also been observed for several species. In the Northern Hemisphere larger zooplanktonic species may take two, three, or possibly more years to reach maturity, but the grand strategy, apparently used by all, is to assure that their young have reached active feeding stages by the time of maximum primary production in the water column so that maximum growth, often, but not always, with emphasis on lipid storage, can occur during the often brief, but usually intense, summer bloom. The rate of growth of arctic or antarctic zooplankton is not so important as assuring a high level of fecundity when maturity comes. Overwintering is probably not a great hardship and diapause may not be a useful strategy because the environmental temperature is constantly near the freezing point of sea water, and basal metabolism accordingly low. Nonetheless, feeding behaviour and metabolic rates have strong seasonal signals. In the absence of other stimuli, light must be involved in the transformation from winter to summer metabolism and visa versa but the mechanisms still remain obscure.
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The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments. This is important in regulating marine carbon cycling and ocean-atmosphere CO2 exchange. The present rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry. Such changes have been shown to slow down calcification in corals and coralline macroalgae, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica. This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO2 to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO2 levels.
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The first deployment of a Continuous Plankton Recorder (CPR) on board the icebreaker Shirase was conducted during the 41st Japanese Antarctic Research Expedition (JARE) in 1999/2000 austral summer in the Indian sector of the Antarctic Ocean. The CPR was towed horizontally at approximately 10 m depth while the Shirase was steaming at about 14 knots across the Polar Front (PF). Mean total abundance of zooplankton for horizontal five nautical mile sample units was 168.1 (SD: ± 117.18) individuals with the maximum of 456 individuals. Zooplanktons were counted for 34 categories of species/taxa. Copepods occupied more than 90% of the total abundance in numbers. Oithona spp. was the most dominant group among copepods, representing 59% of the total zooplankton. Other numerically important categories were small-sized calanoids (copepodites and adults; 18.4%), and copepodites of Calanoides acutus and Calanus simillimus (8.2%). Latitudinal change of zooplankton abundance coincided with increasing/decreasing tends of temperature and salinity. Two different zooplankton assemblages were identified by cluster analysis and these assemblages seem to be closely related to different water characteristics, such as the of PF and areas of cold water masses. CPR is considered to be an ideal tool for long term monitoring of surface zooplankton communities.
Chapter
Detection of change in Antarctica is of global interest because potential changes in the Antarctic in environment are coupled to the Earth system. An interdisciplinary research theme identified by the SCAR Steering Committee (Weller et al. 1989) for the International Geosphere-Biosphere Program (IGPB) is: “detection of changes of global importance, best observed in Antarctica, which are fundamental to establishing the current nature of trends of change and thereby providing a foundation for understanding the underlying processes.” The goals of Antarctic detection-of-change research should be to determine what is happening now throughout Antarctica and the surrounding seas, attempt to relate the current situation to the past, and understand why changes are occurring.
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The pelagic Southern Ocean contains 3 major zones which, although they interact, may for convenience be considered as having distinct food webs: the ice-free zone of the West Wind Drift dominated by herbivorous copepods, salps and small euphausiids; the seasonal pack-ice zone of the East Wind Drift and adjacent fronts and eddies in which krill Euphausia superba is the dominant element; and the permanent sea-ice zone near the Antartic continent and its ice shelves where zooplankton biomass is low and, thus, predators are scarce. Within the krill-dominated food chain unusually large size steps, and high energy demands for maintenance and swimming, require large concentrations of food to keep the effort for seaching and feeding low during the short summer season. Because survival in the Antarctic is energetically expensive, there is little net gain in the krill system, despite its short food chain, but the large biomass stored in krill swarms and in large vertebrates has made the krill system attractive to exploitation by man.-from Author
Chapter
On a number of recent cruises between Africa and Antarctica closely-spaced measurements were made of hydrographic variables, as well as of primary productivity and chlorophyll-a concentration. We describe the occurrence of high levels of chlorophyll-a at certain specific locations, associated primarily with sea-surface fronts wherever such frontal systems have the characteristics of a convergence. Information gathered by the Coastal Zone Colour Scanner of the Nimbus-7 satellite shows a clear relationship between the horizontal shear edges of currents forming part of the southern Agulhas Current system and biological enhancement. Various possible physical mechanisms which may be responsible for biological enhancement at fronts with different characteristics are discussed.
Chapter
The stature of Antarctic krill within the Antarctic ecosystem is reflected in their scientific name — Euphausia superba, however, they have been called a variety of other terms most of which are somewhat inaccurate. The word krill, itself, is a misnomer since it is derived from the word kril which refers to small fish in Norwegian. This term was then used by North Atlantic whalers to describe the crustaceans found in the stomachs of baleen whales and so has been adopted as a general term for the 85 or so species of euphausiid. Various species of krill have been referred to as: squillae, small red insects, animalcules, pelagic prawns, opposum shrimps and skeleton shrimps, but perhaps the most often used phrase to describe Antarctic krill is “tiny shrimp-like crustaceans”. This phrase is misleading since Antarctic krill are by no means tiny, either considered as crustaceans or as animals in general (Fig. 1). An animal which weighs 1 g as an adult is a medium-sized animal and its size actually makes krill quite large for a crustacean. This is not necessarily a trivial point since there appears to be a direct relationship between the size of an animal and the conservation efforts that are expended on its behalf.
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Previous large-scale surveys of the Prydz Bay region (Antarctica) have identified the continental shelf edge of Prydz Bay as an area of rapid transition between 3 major zooplankton communities. One of these communities is dominated by the Antarctic krill Euphausia superba. This community was located mainly along the continental shelf edge, usually between the offshore main oceanic community dominated by copepods and chaetognaths, and the neritic community dominated by E. crystallorophias. In January to March 1991, the Prydz Bay continental shelf area was the subject of a more intensive mesoscale survey to more accurately define the distribution patterns of the 3 zooplankton communities in that area. Cluster analysis and non-metric multidimensional scaling were used to define the communities, their distribution patterns, indicator species and species affinities. The composition of the communities were much the same as previously defined. E. superba again exhibited a distinct dissociation from all other species. However, the distribution pattern of the krill-dominated community was different from those previously observed. That community did not separate the copepod- and E. crystallorophias-dominated communities in Prydz Bay and apparently was displaced to the west. Temperature was strongly correlated with the zooplankton community distribution patterns, suggesting that temperature has more influence at the mesoscale level than at larger scales. Sea ice patterns, chlorophyll a abundance and salinity were also correlated, to a lesser extent, with the community distribution patterns.
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Available data on the spatial distribution and feeding ecophysiology of Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, in the Southern Ocean are summarized in this study. Antarctic krill and salps generally display pronounced spatial segregation at all spatial scales. This appears to be the result of a clear biotopical separation of these key species in the Antarctic pelagic food web. Krill and salps are found in different water masses or water mass modifications, which are separated by primary or secondary frontal features. On the small-scale (
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A statistical analysis of the surface thermal characteristics of the major oceanic fronts in the Southern Ocean sector south of Africa has been carried out. The mean latitudinal locations, middle temperatures, and temperature gradients are presented for the Agulhas Front, the Sub-Tropical Convergence, the Sub-Antartic Fron, and the Antarctic Polar Front, based on 89 crossings. Wherever possible, the surface and subsurface expressions of fronts have been compared.It is shown that all four fronts have consistent and narrow thermal surface characteristics, making it possible to identify and locate their surface expressions by surface temperature measurements alone. The surface temperature traces across the Sub-Antartic Front, and to a lesser extent across the Antarctic Polar Front, often show inversions north and south of the main horizontal gradient demarcating the front. The subsurface expressions of both the Sub-Tropical Convergence and the Antarctic Polar Front are located predominantly north of the surface expressions. It is shown that the Agulhas Front has only a limited longitudinal distribution and these limits are established.
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Expendable bathythermography, wind, and sea surface temperature data collected along 12 crossings of the Drake Passage were analyzed to describe variations in the structure of the antarctic polar frontal zone within a single austral summer (1976-1977). Collected from icebreakers, research and supply vessels traveling to and from the Antarctic, these data show the development of cold features within the antarctic polar frontal zone, which are most likely expressions of eddies or meanders formed at the polar front. These cold features appear to widen the antarctic polar frontal zone by intensifying the subantarctic front and moving it to the north. The majority of the temperature sections contain such cold features, whose signature can be identified as an inflection in average profiles of 450-m heat content and sea surface temperature. Changes in air temperature and wind direction over the antarctic polar frontal zone observed when cold features are present may reflect the influence of the ocean on the overlying atmosphere.
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Zooplankton abundance, biomass and community structure in the top 300 m layer were studied along the 6°E meridian between 49 and 65°S during a Scandinavian/South African Joint Global Ocean Flux Study (December 1997–January 1998) onboard the S.A. Agulhas. Along the transect, total abundance and biomass of zooplankton ranged from 8 to 384 ind m−3 and 1.4 to 64.1 mg DW m−3, respectively. The general pattern in zooplankton distribution observed was an increase in density between Spring Ice Edge and the Antarctic Polar Front (APF) regions. Copepods numerically dominated zooplankton along the entire transect accounting for 68–97% of the total abundance and 24–90% of the total zooplankton standing stock. Other important contributors to total biomass, particularly within the Winter Ice Edge (WIE) and APF regions, were cnidarians and ctenophores (up to 50%), pteropods (up to 35%), tunicates (up to 30%), chaetognaths (up to 25%) and euphausiids (up to 23%). Numerical analyses identified three major zooplankton groupings, coinciding with the three regions of investigation. A dramatic change in the zooplankton composition occurred at the APF region as several copepod and euphausiid species were found on either side of the front only. A second change in the species composition was observed at the WIE, emphasizing the ecological importance of the frontal zone associated with the winter ice edge.
Article
We investigated the composition, distribution, and abundance of zooplankton in the Drake Passage in February 2000 using a Continuous Plankton Recorder. Zooplankton abundance varied considerably, between 1 and 1537 individuals/segment (where a segment is 5 nautical miles of surface towing) (mean ± SD = 121.3 ± 200.9). The highest abundances were recorded in the vicinity of the Polar Front (PF). Abundances increased at low latitudes north of the Sub-Antarctic Front (SAF) and decreased northward. A positive correlation was observed between zooplankton abundance and chlorophyll a concentration, reflecting the higher abundance of zooplankton north of the SAF, where higher abundances of phytoplankton were also observed. A total of 21 species/taxa of zooplankton were classified. Small calanoid copepods were found throughout the transect and accounted for 57.5% of total zooplankton abundance, followed by the Cyclopoid copepods Oithona spp. (25.9%) and Calanus simillimus (8.4%). The results of cluster analyses reveal that small calanoid copepods were the most important contributors to the high zooplankton abundance around the PF and north of the SAF in the Drake Passage. We suggest that a change in species composition has occurred since Hardy's descriptions in 1927, from abundant larger copepods and chaetognaths to smaller copepods and other species.
In vivo chlorophyll, like many other organic molecules, possesses the ability to fluoresce. This fluorescence was measured continuously with a modified model III Turner fluorometer at sea. Reliable readings were obtained over the range of 0·04–2·0 mg chlorophyll a m−3 while on a 21-day cruise off the coast of Baja California. Since the relationship between fluorescence and chlorophyll was linear on all scales, it should be possible to continuously monitor chlorophyll from 0·04 to between 10 and 15 mg m−3, a range adequate for all open ocean studies.
Article
Large, biomass-dominant Southern Ocean copepod species have been much studied, but small and mesopelagic species also play major rôles in these ecosystems. However, little is known of some basic aspects of their ecology. To address this, the abundances of 23 copepod species and genera were analysed from 72 stations sampled during the Discovery Expeditions in the 1920s to 1950s. Stratified net samples, usually to a depth of 1000 m, provided year-round coverage in the Scotia Sea from the Subantarctic Front to the Weddell-Scotia Confluence. Small copepods (Microcalanus pygmaeus, Ctenocalanus spp., Oncaea spp. and Oithona spp.) formed ∼75% of total copepod abundance in the top 1000 m across all major zones. Oithona spp. composed ∼40% of copepod numbers in the Polar Front area and to its south: further north their importance declined. All mesopelagic taxa except for the warmer-water species Metridia lucens and Pleuromamma robusta, extended throughout the entire study area, with smaller regional differences than for the shallower-living species. The species showed a continuum of temperature ranges, and there was no evidence that the Polar Front was a major biogeographic boundary to their distribution. Indeed, several important species, including Oithona spp. (mainly Oithona similis), Ctenocalanus spp., Metridia lucens and Rhincalanus gigas reached maximum numbers in this area. Total copepod abundance was thus higher in the vicinity of the Polar Front than in any other region. Only two copepod families made pronounced seasonal vertical migrations: Eucalaniidae (Eucalanus longiceps and R. gigas) and Calaniidae (Neocalanus tonsus, Calanoides acutus, Calanus simillimus and Calanus propinquus). Some evidence for a winter descent was found for Ctenocalanus spp. and some deeper-living groups: Euchaeta spp. and the Metridiidae, although their migrations were not so great as for the eucalanids and calanids.
Article
 Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.
Article
In-situ surface chlorophyll concentrations (usually with phaeopigment) in the Pacific and Indian ocean sectors of the Subantarctic water ring (between the Subtropical Convergence and the Polar Front) are collated and reviewed. Offshore and away from hydrographic fronts, most means are only between 0.1 and 0.3 mg m−3, in spite of persistently high nutrients (N, P), and show little seasonality, in contrast to many Antarctic waters and the temperate-subpolar North Atlantic at the same range of latitude. Cell division rates can be expected to be high relative to the temperature-set maximum, and community net production is positive, at least during the summer, with some export of carbon. During the summer half of the year, the pigment concentrations are independent of mixed-layer depth (at least to 275 m), and, hence, of algal division rates. Also the relatively high winter pigment means, most between 0.10 and 0.15 mg m−3 even in the southern parts, show that underwater irradiance does not affect pigment concentration on the seasonal and regional average. Based on inferences from Antarctic bioassays and the enhanced dust supply in the Subantarctic, lack of iron is unlikely to cause directly the low chlorophyll levels, but probably is the reason for the dominance by small-celled phytoplankton. In homology to the offshore Subarctic and equatorial Pacific, grazing is postulated proximately to maintain the low pigment concentrations. These persistently low concentrations, coupled with relatively high cell division rates, entail low phytoplankton production rates that cannot outrun the physical supply of nutrients, thus allowing persistently high nutrient concentrations (N, P) to be maintained. The enhanced rate of primary production during the summer seems only to appear as enhanced concentrations of large zooplankton. In fronts, on island shelves, and downstream from New Zealand, however, phytoplankton blooms of large cells suggest that increased iron supply leads to a different community composition and dynamics and, presumably, at the islands to a temperate seasonal cycle. Much of the Atlantic Subantarctic is apt to be similar to the Indian and Pacific sectors, as is the permanently ice-free outer belt of the entire Antarctic. Thus, the offshore small-celled phytoplankton in these regions, more than a tenth of the global ocean area, appears to consist year-round of “grazer-controlled populations in an iron-limited ecosystem” (phrase used by NM Price and colleagues).
Article
The composition, distribution, abundance, biomass and size structure of mesozooplankton, collected using Bongo nets in the top 300 m layer along a transect between the Antarctic continent and Cape Town, were investigated during the second South African Antarctic Marine Ecosystem Study (SAAMES II) in Jan.–Feb. 1993. Small (<10 mm) and medium (20–50 mm) size groups of zooplankton consistently dominated across the Southern Ocean. The highest zooplankton densities were recorded at the Antarctic Polar Front (APF) and at the Subtropical Convergence (STC). Minor peaks in zooplankton densities were observed in the southern vicinity of the Subantarctic Front (SAF) and APF. Elevated zooplankton stocks were also found within the Marginal Ice Zone (MIZ) and the Polar Frontal Zone. The lowest densities were recorded in the permanently open zone (MIZ–APF) and in the Subantarctic zone (SAF–STC). Copepods were generally important along the entire transect and formed the bulk of zooplankton stock within the MIZ and in the Polar Frontal Zone (APF–SAF), accounting for at least 40–95% of total abundance and biomass. Euphausiids were also a prominent group along the transect. Their contribution was highest (up to 80% of total biomass) between the MIZ and the APF, mainly because of the occurrence of swarms of the Antarctic krill Euphausia superba. Tunicates, Pyrosoma sp. and Salpa fusiformis, were found in great numbers only in the region of the STC and further north, while Salpa thompsoni was abundant at the southern boundary of the APF. Chaetognaths dominated samples numerically and by mass in the Subantarctic Zone. Results obtained from cluster and ordination analyses show that zooplankton community structure was well correlated with the position of various biogeographical zones separated by the main frontal systems of the Southern Ocean. Two major groupings of stations, separated by the SAF, were identified in these analyses. This front separated the Antarctic and the subantarctic/subtropical assemblages, confirming its important role as a biogeographical boundary.
Article
During the late austral summer of 1994, Antarctic waters were characterized by low phytoplankton biomass. Along the 62 degrees E meridian transect, between 49 degrees S and 67 degrees S, chlorophyll (Chl.) a concentration in the upper 150 m was on average 0.2 mg m(-3). However, in the Seasonal Ice Zone (SIZ) chlorophyll a concentrations were higher, with a characteristic deep chlorophyll maximum. The highest value (0.6 mg Chi. a m(-3)) was measured at the Antarctic Divergence, 64 degrees S, corresponding to the depth of the temperature minimum (similar to 100 m). This deep biomass maximum decreased from South to North, disappeared in the Permanently Open Ocean Zone (POOZ) and reappeared with less vigour in the vicinity of the Polar Front Zone (PFZ). In the SIZ, the upper mixed layer was shallow, biomass was higher and the > 10 mu m fraction was predominant. In this zone the > 10 mu m, 2-10 mu m and < 2 mu m size fractions represented on the average 46%, 25.1% and 28.9% of the total integrated Chi. a stock in the upper 100 m, respectively. The phytoplankton assemblage was diverse, mainly composed of large diatoms and dinoflagellate cells which contributed 42.7% and 33.1% of the autotrophic carbon biomass, respectively. Moving northwards, in parallel with the decrease in biomass, the biomass of autotrophic pico- and nanoflagellates (mainly Cryptophytes) increased steadily. In the POOZ, the picoplanktonic size fraction contributed 47.4% of the total integrated Chi. a stock. A phytoplankton community structure with low biomass and picoplankton-dominated assemblage in the POOZ contrasted with the relatively rich, diverse and diatom-dominated assemblage in the SIZ. These differences reflect the spatial and temporal variations prevailing in the Southern Ocean pelagic ecosystem.
Article
The Continuous Plankton Recorder (CPR) Type I was first used in Antarctic waters during the 1925–1927 Discovery Expedition, and has been used successfully for 70 years to monitor plankton in the North Sea and North Atlantic Ocean. Sixty-five years later the CPR as a Type II version returned to Antarctic waters when the Australian Antarctic Division initiated a survey of the Southern Ocean on RSV Aurora Australis south of Australia and west to Mawson. The objectives are to study regional, seasonal, interannual and long-term variability in zooplankton abundance, species composition and community patterns, as well as the annual abundance and distribution of krill larvae. The survey covers a large area from 60°E to 160°E, and south from about 48°S to the Antarctic coast—an area of more than 14 million km2. Tows are conducted throughout the shipping season, normally September to April, but occasionally as early as July (midwinter). The large areal and temporal scale means that it is difficult to separate temporal and geographical variation in the data. Hence, CPRs are now also towed on the Japanese icebreaker Shirase in collaboration with the Japanese Antarctic programme. Shirase has a fixed route and time schedule, travelling south on 110°E in early December and north on 150°E in mid-March each year, and will serve as an important temporal reference for measuring long-term interannual variability and to help interpret the Australian data. Since 1991, over 90 tows have been made, providing over 36,000 nautical miles of records. The most successful seasons to date have been the 1997/1998, 1999/2000 and 2000/2001 austral summers with 20, 31 and 26 tows, respectively. The 1999/2000 season included a unique, nearly simultaneous three-ship crossing of the Southern Ocean along 25° 30’E, 110°E and 157°E. Typical CPR tows show very high abundance of zooplankton in the uppermost 20 m of the permanently open ocean zone between the sea-ice zone and the Sub-Antarctic Front; this is an area thought to be oligotrophic. Appendicularians and small calanoid and cyclopoid copepods dominate the plankton. By comparison the surface waters of the sea-ice zone have low species diversity and abundances. Zooplankton data, and hence distribution patterns, can be time- and geo-coded to GPS data and environmental data collected by the ships’ underway monitoring system (e.g. fluorescence, water temperature, salinity, and meteorological data).
Article
Large-scale features of the Antarctic Circumpolar Current (ACC) are described using all historical hydrographic data available from the Southern Ocean. The geopotential anomaly of the sea surface relative to 1000 db reveals the highly-sheared eastward flow of the ACC and the strong steering of the current by the ridge system around Antarctica. The near-surface property distributions differentiate the ACC waters from the warmer and saltier waters of the subtropical regimes. The Subtropical Front (STF), interrupted only by South America, marks the northern most extent of subantarctic waters. Distributions of properties on isopycnal surfaces show an abrupt end to the characteristic signal of the Upper Circumpolar Deep Water (UCDW), as this water mass shoals southward and is entrained into the surface mixed layer. This sharp water mass boundary nearly coincides with the southernmost circumpolar streamline passing through Drake Passage. To its south are the weakly-sheared circulations of the subpolar regime. Inspection of many hydrographic crossings of this transition reveals that the poleward edge of the UCD W signal is a reasonable definition of the southern boundary of the ACC. At Drake Passage, three deep-reaching fronts account for most of the ACC transport. Well-established indicators of the Subantarctic Front and Polar Front are traced unbroken around Antarctica. The third deep-reaching front observed to the south of the Polar Front at Drake Passage also continues with similar characteristics as a circumpolar feature. It is called here the southern ACC front. Stations from multiple synoptic transects of these circumpolar fronts are used to describe the average property structure within each ACC zone. Between the STF and the southern boundary of the ACC, the shear transport of the circumpolar current above 3000 m is at all longitudes about 100 Sv (1 Sv = 106 m3 s−) eastward.
Article
An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996–January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7–2.3 g m−2 yr−1), and about twice the estimated ocean-wide average (ca. 1 g m−2 yr−1). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11–13 g m−2 yr−1). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g m−2 yr−1) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12–0.05 g m−2 yr−1), reflecting a very low dust input.
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