Biodiversity and structure of the suprabenthic assemblages from South Shetland Islands and Bransfield Strait,Southern Ocean

During the austral summer 1995, suprabenthic samplings were carried out at 24 stations (depth range 45–649 m) located around Livingston Island, within the caldera of Deception Island and in the Bransfield Strait. At each station, the near-bottom motile fauna was simultaneously collected with a multinet Macer-GIROQ sled in three water layers above the bottom. This study presents original data on the occurrence, diversity, vertical distribution and abundance of suprabenthic taxa in this near-bottom environment. The most speciose taxa were amphipods (at least 140 spp.), followed by isopods (66 spp.), pycnogonids (31 spp.) and mysids (19 spp.). Total abundances ranged between 31 ind./100 m² (Bransfield Strait, 361 m depth) and 6817 ind./100 m² (South Livingston Island, 163 m depth). According to stations, the groups numerically dominant and more frequent were amphipods (17 stations) or mysids (seven stations). Four suprabenthic assemblages were discriminated in the study area, apparently more structured by the degree of shelter-exposure and development of sessile epifauna than by water depth or sediment features.     

Suprabenthic assemblages from South Shetland Islands and Bransfield Strait (Antarctica): preliminary observations on faunistical composition, bathymetric and near-bottom distribution

During austral summer 1995 suprabenthic assemblages were investigated at 24 stations located around Livingston Island, Deception Island and Bransfield Strait, at depths ranging from 45 to 650 m. Suprabenthos was collected on R/V Hesperides using a Macer-GIROQ sledge with an automatic opening and closing system. This study presents data on the occurrence and relative abundance of the main suprabenthic groups collected in the water layer immediately adjacent to the bottom (10–140 cm above bottom). Commonest groups were Amphipoda, Mysidacea, Isopoda, Cumacea and Euphausiacea. The distribution and spatial separation in the suprabenthic habitat of the main groups are discussed. Received: 20 February 1997 / Accepted: 14 July 1997     

Quantitative analysis of macrobenthic soft-bottom assemblages in South Shetland waters (Antarctica)

Macrobenthic assemblages were investigated at 26 stations located around Livingston Island, Deception Island and the Bransfield Strait at depths ranging from 42 to 671 m. Representatives of 30 major taxa were found. The maximal density was 5,260 specimens·m⁻² at Livingston Island; the mean abundance per station ranged from 160 to 4,380 specimens·m⁻². The total biomass of the macrozoobenthos declined with depth, with mean values of 3,201 g·m⁻² at shallower depths (< 100 m) and 210 g·m⁻² further down (> 100 m). After multivariate analysis (cluster analysis, MDS) based on Bray-Curtis dissimilarities, most stations could be assigned to one of three groups on the basis of distinct biomass differences between sites. The first cluster with a rich Ascidiacea biomass is common on shallower bottoms. The second, with Ophiuroidea as a characteristic group, is common on deeper bottoms. The absence of an ‘indicator’ taxon is characteristic of the remaining cluster of those stations with the lowest biomass values. No significant correlations were detected between macrobenthic biomass and any sediment parameters measured, probably because part of the benthos (i.e. the epifauna) could be better explained by the coupling with a highly productive water column. The role of the epi-infauna sensu Gallardo as the main factor structuring benthic assemblages in the investigated area is discussed. Received: 7 March 1996/Accepted: 26 July 1996     

Larval fish distribution and abundance in the Antarctic Peninsula region and adjacent waters

The spatial distribution, species composition and abundance of ichthyoplankton in the Bransfield Strait and adjacent waters were studied during two cruises in the Antarctic spring 1991/92 and summer 1992/93 seasons. A multiple plankton net (Bioness) and a Bongo net were used to collect samples at 35 stations in 1991/92 and 75 stations in 1992/93. Early larval stages (14 species) and juveniles (13 species) representing the known Bransfield Strait ichthyofauna were present in the water column. The nototheniids predominated in the entire study area. The greatest species diversity was found in the uppermost 200 m of the water column in the Bransfield Strait. Notothenia gibberifrons and Nototheniops larseni dominated in spring, whilst in summer Pleuragramma antarcticum dominated in association with N. larseni. The dominant species in the Gerlache Strait were P. antarcticum and Notothenia kempi, while P. antarcticum and Trematomus scotti were predominant in the Bellingshausen Sea area.     

Ophiuroid community structure of the South Shetland Islands and Antarctic Peninsula region

During the BENTART 95 survey, epifauna was sampled with an Agassiz trawl at 24 stations ranging from north of Livingston Island (South Shetland Islands) to the Antarctic Peninsula at depths from 40 to 850 m. Four assemblages of ophiuroids were identified and correspond to Deception Island, Trinity Island, and the southern and northern zones of Livingston Island. In addition to these regions, gradients related to depth and filter-feeder biomass were found. The brittle-star assemblages were dominated by Ophionotus victoriae Bell, 1902, which to a large extent determine the biogeographic structure in the South Shetland Islands.     

Effect of the abundance of three predominating copepod species on adequate sample volume and sample size in Bransfield Strait (Antarctic Peninsula) and waters north of the Weddell Sea

Summary The effect on adequate sample size and sample volume of the abundances of three predominant copepod species, Metridia gerlachei, Calanus propinquus and Calanoides acutus, were studied in Bransfield Strait (Antarctic Peninsula) in the austral summer of 1988–1989 and waters north of the Weddell Sea in 1989–1990. Copepod abundances were higher in the area north of the Weddell Sea, with the exception of Metridia gerlachei, which was evenly distributed over both areas. Local (intra-station) patchiness was not found, indicating random distribution over small areas. In the assessment of inter-station variability in Bransfield Strait, with standard error of the mean set arbitrarily at 20% of the average abundance and a sampling volume of 150 m³, the theoretical minimum sample size (number of sampling stations) ranged from 6 to 17 for juvenile copepods and from 11 to 25 for adults. The minimum number of stations in the area north of the Weddell Sea reached from 5 to 7, and from 7 to 10 respectively.     

Molecular taxonomy of Osedax (Annelida: Siboglinidae) in the Southern Ocean

We report the discovery of three new species of Osedax in the deep Southern Ocean, expanding the diversity and geographical range of this genus of bone‐eating worms. Osedax rogersi sp. n. and Osedax crouchi sp. n. were found on a whale skeleton at 1444 m in the Kemp Caldera in the East Scotia Sea during the Chemosynthetic Ecosystems of the Southern Ocean (ChEsSo) project. The recently described species, Osedax antarcticus, found on whale bones implanted at a depth of 550–650 m off Smith Island in the Bransfield Strait, was also found inhabiting the whale skeleton in the Kemp Caldera. Osedax crouchi as well as another new species Osedax nordenskjoeldi sp. n. have also been found on the implanted whale bones off Smith Island. These two localities are approximately 1800 km apart demonstrating a wide distribution of species within the genus. We describe the three new species, O. rogersi, O. crouchi and O. nordenskjoeldi and report the second record of O. antarcticus. We also present a new phylogenetic analysis for Osedax, including data examining genetic connectivity between the Scotia Arc and the Bransfield Strait.     

Electrophoretic investigation of genetic variation in two krill species Euphausia superba and E. crystallorophias (Euphausiidae)

Summary Specimens of Euphausia superba and of E. crystallorophias from different locations were analyzed electrophoretically for protein variation. The present study extends previous genetic studies on E. superba by analyzing samples from the East Wind Drift and repeat samples from the Bransfield Strait, Elephant Island and the Weddell Sea. E. crystallorophias was collected in the Weddell Sea and around the Antarctic Peninsula in order to provide information on the breeding structure of the species in this region. For all loki taking all sampling sites together for both species except GPI in E. crystallorophias no significant deviation of phenotype distributions from random mating expectations was observed. Furthermore, the allele distributions in all polymorphic loci for both species were found to be homogeneous. Estimates of genetic distance between samples within each species are low (0.0001 to 0.0003 in E. superba and 0.0001 to 0.0002 in E. crystallorophias), and are consistent with results expected for samples from a single interbreeding population. Estimate of genetic distance between these two species was 0.9729. These results suggest that for each species specimens from all locations investigated in the Bransfield Strait and Weddell Sea belong to a single genetically homogeneous population. A possible mechanism for maintaining such homogeneity and the implications for fishery management are discussed.     

Semi-quantitative study of macrobenthic fauna in the region of the South Shetland Islands and the Antarctic Peninsula

During the BENTART 95 Expedition, 24 Agassiz trawls for macrozoobenthos sampling were carried out at depths of 40–850 m, from north of Livingston Island to the Antarctic Peninsula. The samples were analysed using a semi-quantitative method, and with the resulting numerical data, transformed into a six-point scale, we constructed a Bray-Curtis similarity matrix. A total of 74,624 specimens, belonging to 38 taxonomic groups, were collected. The most abundant group was Polychaeta Sedentaria, with 36% of the total, whereas the highest relative masses were from Ascidiacea (23%), Echinoidea Regularia (18%) and Ophiuroidea (16%). The maximum number of specimens recorded was 15,600 ind./50 l. Cluster analysis separated stations located in Foster Bay (Deception Island), characterised by low taxonomic richness and high relative mass (average: 26.7 kg). A zonation of ascidians, regular sea urchins and ophiuroids was observed at Deception Island, clearly related to depth and substratum type. The remaining stations were separated into two groups. The first one comprised the shallowest stations (40–130 m), dominated by sessile active filter-feeders, belonging to Ascidiacea, Demospongia and Bryozoa, and probably related to high primary production zones. The second group comprised deeper stations and was dominated by classes exhibiting a diversity of trophic strategies: Ophiuroidea and Asteroidea, to 400 m, and Polychaeta Sedentaria at greater depths. Received: 20 February 1997 / Accepted: 8 September 1997     

Radiolarian taxa from Southern Ocean sediment traps (Atlantic sector)

Summary This study gives a first inventory of radiolarian taxa collected with sediment traps in different areas of the Southern Ocean (Drake Passage, Powell Basin and Bransfield Strait). It includes 66 taxa or taxa groups of which 46 were already described. Two previously described species groups and 20, yet undescribed, taxa are documented. The name Protocystis bicornis (Haecker) is replaced by P. spinosus as it is a later homonym of P. bicornis (Borgert). The occurrence pattern of the radiolarian taxa indicates distinct differences in the species composition between neritic environments (Bransfield Strait and Powell Basin) and pelagic, open ocean conditions (Drake Passage).     

Suprabenthic euphausiids and mysids from the South Shetland Islands and the Bransfield Strait, Southern ocean (BENTART-95 cruise)

During the austral summer 1995, the suprabenthic assemblages from 24 stations (45–649 m water depth) located around the South Shetland Islands and in the Bransfield Strait were sampled with a Macer-GIROQ sled. The purpose of this paper is to describe the structure of two main crustacean groups (euphausiids and mysids) in the near-bottom water layers sampled by the sled and to focus on the life history of two dominant mysid species from these areas (Mysidetes posthon and Antarctomysis maxima). The distribution and the main factor structuring populations in the investigated area are discussed.     

Macrobenthic mollusc assemblages and diversity in the West Antarctica from the South Shetland Islands to the Bellingshausen Sea

Although frequent studies have been made on the benthos assemblages living in the Southern Ocean, few studies have dealt with molluscan assemblages and ecology. During the research programmes BENTART carried out in austral summers of 2003 and 2006 in a study area including Low Island, West Antarctic Peninsula, Bellingshausen Sea and Peter I Island, 45 stations were sampled using quantitative box-corer, Agassiz trawl, epibenthic sledge and rock dredge, at depths of 53–3,304 m. These data were used to search and analyse the malacological assemblages. A total of 2,889 living specimens of molluscs belonging to 139 species of shelled Gastropoda, Bivalvia and Scaphopoda were recoded. Gastropods were the best-represented in species richness, but the bivalves accounted the major abundance and Cyamiocardium denticulatum (460 individuals) was the most abundant species. Species richness varied from 1 to 37 species and diversity showed great variations at different stations. Four groups of stations of low similarities and some differences in faunistic composition were observed: one that gathers the Antarctic Peninsula and Peter I Island, one that also includes the Low Island, and two composed by stations of Bellingshausen Sea.     

Environmental influences on zooplankton and micronekton distribution in the Bransfield Strait and adjacent waters

The spatial distribution, biomass and size structure of key mesozooplankton species and micronekton in the Bransfield Strait (Antarctica) are described in relation to environmental variables during the austral summer 2001. Stratified (BIONESS) biological sampling at five depth-ranges and CTD casts were performed at 40 stations, including a cross-Strait transect. Six families, 11 genera and 16 species composed the total catch of larval and juvenile fish, which were more abundant in the upper 75 m. Trematomus scotti was the most abundant fish and large individuals dominated at depths >75 m. The fish distribution patterns were associated with the main water masses in the area and with bottom depth. Spatial segregation in density and biomass of krill (Euphausia superba) and salps (Salpa thompsoni) was observed at a relatively small scale, in relation to the main water masses. Also, size-related spatial segregation was found in krill. The present study suggests that the spatial distribution of krill, salps and early life stages of fish, are influenced by the main water masses in the Strait, and that species may adapt their reproductive strategies not only to seasonal production peaks, but to transport processes within water bodies that maximise their fitness through optimum temperature and/or trophic environments.     

Evidence of high annual growth rate for lichens in the maritime Antarctic

On Livingston Island (South Shetland Islands, Antarctica) young moraines, now roughly 45 years old, were investigated in 1991 and 2002. More than 500 thalli of the 6 most abundant species, Acarospora macrocyclos, Bellemerea sp. Buellia latemarginata, Caloplaca sublobulata, Rhizocarpon geographicum and Usnea antarctica, previously measured in 1991, were measured again in 2002, which allowed an accurate measure of thallus growth rates. From our results, we believe it important to emphasize that: (1) lichen colonization probably took place at the end of the 1950s, soon after the last glacier retreat. (2) No relation between thallus growth rate and boulder size could be established. (3) Higher annual growth rates than those previously estimated were observed for R. geographicum and Bellemerea sp. and the former had one of the highest growth rates ever reported for this species. We speculate whether this high growth rate may be related to the rapid retreat of the Livingston Island ice cap and glaciers during the last recent years, and which have been attributed to climate warming on the Antarctic Peninsula region.     

A year round comparative study on the population structures of pelagic Ostracoda in Admiralty Bay (Southern Ocean)

The population structures of the three dominant planktonic halocyprid Ostracoda species in Admiralty Bay (King George Island, Antarctic Peninsula) were followed throughout the course of a year in zooplankton samples collected once every three weeks from February 1993 to January 1994. The sampling was conducted at two stations: A in the central part of Admiralty Bay (400–0 m) and B in the entrance to the Bay from the Bransfield Strait (400–0 m). The samples were taken using a WP-2 net (square mouth opening of 0.196 m² and 200 μm mesh) hauled vertically from the bottom to the surface. Changes in the age structures of the populations of three species Alacia belgicae, Alacia hettacra and Metaconchoecia isocheira were tracked. Their population structures differed. The changes in A. belgicae suggested that it reproduces year-round, whereas both A. hettacra and M. isocheira probably complete their life cycles within a year. The cycle in A. hettacra probably starts earlier in the year than that of M. isocheira. Populations of A. belgicae and M. isocheira were more advanced in their development at station A, than at station B, but A. hettacra was more advanced at the latter. Advection appears to play a role in maintaining the populations in the shelf waters. Comparisons between populations in the shelf area (Admiralty Bay) and in open ocean waters (Croker Passage) show that the M. isocheira population is older in shelf water whereas the age structure of A. belgicae population is not influenced by the locality.     

Radiolarian flux in Antarctic waters (Drake Passage,Powell Basin,Bransfield Strait)

Summary The study of radiolarians collected during sediment trap experiments in the Drake Passage, the northern Powell Basin, and the King George Basin of the Bransfield Strait provides new information on the fluxes of radiolarian shells in Antarctic waters, on the annual flux pattern, the species distribution and its ecological significance, and on alteration processes of the radiolarian shells in the water column and at the sediment/water interface. A 28-month monitoring with time-series sediment traps in the Bransfield Strait indicates an annual flux pattern characterized by short-term flux pulses during austral summer, which reach daily fluxes of up to 5 × 10³ shells m^–2 and which account for more than 90% of the total annual flux. The distinct seasonal variations are linked to variations in the sea ice coverage. Other controlling factors are the production of phytoplankton and the impact by zooplankton grazers, e.g., krill. During the summer flux pulses the vertical fluxes of radiolarians range between ca. 3 and 21 × 10⁴ shells m^–2, values that are one or more orders of magnitudes lower than fluxes observed at sites in the tropical and northern high-latitude ocean. Significant lateral transport of radiolarians was documented during the austral summer in the Bransfield Strait by a factor of 10 increase of the radiolarian flux in the lower portion of the water column and the species composition trapped in deeper waters. Radiolarian assemblages associated with pelagic and neritic environments characterized by typical Antarctic taxa (Antarctissa spp.) and a group of species with bipolar distribution (e.g. Plectacantha oikiskos, Phormacantha hystrix), respectively, are distinguished. While the signal of polycystine radiolarians is relatively well recorded in the sediments, the shells of phaeodarians, which were observed at fluxes of up to 1 × 10³ shells m^–2day^–1 in the upper portion of the water column, are almost completely dissolved during settling through the water column.     

Mesoscale spatial distribution of chaetognaths along hydrographic gradients in the South Scotia Sea (Antarctica)

Mesoscale distribution of chaetognaths in the South Scotia Sea was studied in January 1994. Three transects (ice margin to Elephant Island, ice margin to the Weddell-Scotia Confluence, and the Drake Passage) and a station grid in the East Bransfield Strait were laid out, seeking the greatest possible heterogeneity in hydrographic conditions. Six species were collected, with the most abundant species, Eukrohnia hamata, accounting for 83% of the total number of individuals, followed by Sagitta gazellae (9.7%). High spatial variability was observed both along the transects and over the mesoscale station grid. The station grid allowed two assemblages to be distinguished on the basis of species abundance and demographic population structure. Both assemblages were distributed through areas with distinct hydrographic characteristics. Use of a smaller sampling scale and a more detailed definition of development stages than in earlier studies revealed a higher level of spatial heterogeneity than previously suggested for chaetognaths in the Southern Ocean. Accepted: 11 April 1999     

Winter aspects of the ichthyoplankton community in Antarctic Peninsula waters

Summary Ichthyoplankton was sampled from the Antarctic Peninsula area of the South Polar Ocean in early winter (May and June 1986). A total of 153 eggs from two species and 1368 larvae or juvenile stages from 12 species were obtained. These included pelagic species, and demersal species with a long pelagic larval or juvenile phase. Most abundant were larvae of Pleuragramma antarcticum and Notothenia kempi, and eggs of Notothenia neglecta. The distribution of notothenioid and paralepidid larvae was apparently unaffected by ice cover, whereas myctophid larvae were confined to ice-free waters. Areas where newly hatched Chionodraco hamatus occurred coincided with dense aggregations of Euphausia superba (Krill) furcilia larvae which is a potential food resource during winter. The hatching of icefish larvae during winter is apparently independent of the seasonal production cycle. Epipelagic eggs of Notothenia neglecta were found during the spawning season, which suggests that eggs ascend to the surface after demersal spawning and that development takes place near the sea surface during winter. Larvae of Notothenia kempi were chiefly confined to shelf and slope waters to the west of the Antarctic Peninsula, with larger larvae found in coastal shelf areas. Pleuragramma antarcticum occurred in the coastal waters off the Biscoe Islands, in the Gerlache Strait, and in the northern Bransfield Strait. The smallest larvae were found in the northern Bransfield Strait, whereas those at the Biscoe Islands and in Gerlache Strait waters were larger and of a similar size. A cyclonic gyre to the west of the Antarctic Peninsula observed in the austral summer was likely to have affected the larval drift of Pleuragramma antarcticum and Notothenia kempi. Differences in the timing of spawning and hatching and the vertical distribution of these larvae will lead to different transport and spatial distribution patterns. It is hypothesized that early winter conditions do not imply severe limitations on the year-class success of larval fish. Dispersal and increased mortality may occur during the second half of the winter.     

Ophiuroid biodiversity patterns along the Antarctic Peninsula

Benthic ecological surveys using standardized methods are crucial for assessing changes associated with several threats in the Southern Ocean. The acquisition of data on assemblage structure over a variety of spatial scales is important to understand the variation of biodiversity patterns. During the ANT XXIX/3 (PS81) expedition of RV Polarstern, three different regions at the tip of the Antarctic Peninsula were sampled: the northwestern Weddell Sea, the Bransfield Strait, and the northern boundary of the South Shetland Archipelago in the Drake Passage. The aim of this study was to characterize the distribution and biodiversity patterns of ophiuroid assemblages in these regions and depths. We quantified different community parameters in terms of the number of species, abundance, and biomass. Additionally, we calculated various components of species diversity (alpha, beta, and gamma diversity) over the three regions. Based on the benthic surveys, we collected 3331 individuals that were identified to species level (17 species). Overall, species diversity, as measured based on rarefaction, species richness and evenness estimators, was higher in the Bransfield Strait compared to the Weddell Sea and Drake Passage. Two deep stations in the Weddell Sea showed high dominance only of Ophionotus victoriae. Significant differences in the patterns of alpha diversity were found among the regions but not between depth zones, whereas beta diversity showed no differences. Regarding the resemblance among the ophiuroid assemblages of each region, there was a significant gradient from east to west with a maximum distance between the stations in the Drake Passage and the Weddell Sea. This study provides a baseline for detecting potential effects related to climate change, and it furnishes a basis for the implementation of monitoring schemes of Antarctic assemblages.     

Geographical distribution and abundance of postlarval and juvenile Pleuragramma antarcticum (Pisces,Notothenioidei) off the Antarctic Peninsula

Summary The early life history of Pleuragramma antarcticum off the Antarctic Peninsula is described from extensive material collected during four expeditions between 1975/76 and 1982 by means of an RMT 1+8. Postlarvae occurred frequently on the northern continental shelf in February and March except during the 1977/78 season when they were almost absent. Abundance varied mainly between 1.8 and 14.7 postlarvae per 10³ m³ with maxima between 32.6 and 143.3 per 10³ m³. Juveniles were less abundant with 0.1–4.9 and maxima between 9.8 and 28.3 per 10³ m³, but formed a consistent, predominant proportion (83–97%) of the pelagic juvenile fish fauna throughout the years of the study. They were distributed over the whole Bransfield Strait and adjacent waters in the top 200 m. Postlarvae and juveniles showed different patterns of vertical distribution; more than 70% of postlarvae occurred in the top 135 m of the water column whereas 83% of juveniles were encountered between 70 and 200 m with increasing abundance towards the lower depth. Three length groups were identified corresponding to age groups 0, 1, and 2. Hatching was estimated to occur in December. Postlarval distribution patterns and the prevailing transport of water masses suggest that coastal waters south of the tip of the Antarctic Peninsula and in the north-eastern Bellingshausen Sea are probable spawning areas. The Bransfield Strait and adjacent waters represent nursery grounds for the early stages of P. antarcticum from these sites.