Plankton community structure in the physical environment surrounding the Prince Edward Islands (Southern Ocean)

The results are presented of a macroscale physical and biological oceanographic survey conducted during the second Marion Island Offshore Study in the upstream and downstream regions of the Prince Edward Islands in the austral autumn (April/May) 1997. Upstream of the islands, the Sub-Antarctic Front appeared to combine with the Antarctic Polar Front to form an intensive frontal feature. Closer to the islands, the fronts appeared to separate. Influenced by the shallow topography of the southwest Indian ridge, the Sub-Antarctic Front was steered northwards around the islands while the Antarctic polar front appeared to meander eastwards, where it was again encountered in the southeastern corner of the survey grid. Downstream of the islands, an intensive cold-core eddy within the Polar Frontal Zone was observed. Its exact genesis is unknown but it is possibly generated by instabilities within the meandering Antarctic polar front as its surface signature was characteristic of Antarctic surface water masses found south of the Antarctic polar front. The cold-core eddy appeared to displace the sub-Antarctic front northwards. South of the eddy, a warm patch of sub-Antarctic surface water was observed; its position appeared to be controlled by the meandering Antarctic Polar Front which lay on either side of this feature. No distinct microphytoplankton groupings could be distinguished by numerical analyses, although four distinct zooplankton groupings were identified. These corresponded to the sub-Antarctic surface waters, Antarctic surface waters and the polar frontal zone waters. The fourth grouping comprised those stations where the lowest zooplankton abundances during the entire investigation were recorded and, as a consequence, does not reflect any spatial patterns. These results suggest that the species composition and distribution of plankton in the vicinity of the islands were consistent with the prevailing oceanographic regime. Accepted: 15 March 1999     

Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (April/May) 1989 are presented. During the investigation, the Subantarctic Front, upstream of the islands, was shown to lie initially south at 46°38′S, while downstream, the front remained in a northern position of approximately 46°S. Surface expressions of the front show that the Subantarctic Front forms a zonal band, while the subsurface expressions (200 m) show a distinct meander in both regions. In the upstream region of the islands, the northern branch of the Antarctic Circumpolar Current, the Subantarctic Front, influenced by the shallow bathymetry, was deflected around the northern edge of the islands. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (7°C, 33.75) to Antarctic Surface Water (5°C, 33.70) as the Polar Frontal Zone was crossed. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surface Water from north of the Subantarctic Front was advected southwards across the Polar Frontal Zone, while cooler waters, which had been modified in the transitional band of the Polar Frontal Zone, were advected northwards. In the downstream region a warm eddy consisting of Subantarctic Surface Water was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution patterns during the investigation were consistent with the prevailing oceanographic regime. Four distinct groupings of stations were identified by numerical analysis. These corresponded to stations found north of the Subantarctic Front, within the warm eddy, located in the Polar Frontal Zone, and those stations associated with the meander. The groupings were separated by the Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddies in the downstream region of the islands may represent an effective mechanism for transporting warm water species across the Subantarctic Front. Accepted: 19 August 1998     

Biogeographic study of the planktonic communities of the Prince Edward Islands (Southern Ocean)

Microphytoplankton and zooplankton composition and distributionin the vicinity of the Prince Edward Islands and at the Sub-antarcticFront (SAF) were investigated in late austral summer (April/May)1996. Samples were collected for analysis of chlorophyll a concentration(Chi a), microphytoplankton and zooplankton abundance. Generally,the highest Chl a concentrations (up to 2.0 µg l^–1)and zooplankton densities (up to 192 ind. m^–3) were recordedat stations within the inter-island area while the lowest values(<0.4 µg l^–1) were observed at stations upstreamof the islands. High Chl a and zooplankton biomass values werealso associated with the SAF. Microphytoplankton were dominatedby chain-forming species of the genera Chaetoceros (mainly C.neglectus),Fragilariopsis spp. and the large diatom Dactyliosolen antarcticus.The zooplankton assemblages were always dominated by mesozooplanktonwhich at times contributed up to 98% of total zooplankton abundanceand up to 95% of total biomass. Among mesozooplankton, copepods,mainly Clausocalanus brevipes and Metridia lucens numericallydominated. Among the macrozooplankton euphausiids, mainly Euphausiavallentini, E.longirostis and Stylocheiron maximum, and chaetognaths(Sagitta gazellae) accounted for the bulk of abundance and biomass.Cluster and ordination analysis did not identify any distinctbiogeographic regions among either the microphytoplankton orzooplankton.     

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

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

Structure and grazing impact of the protozooplankton community in the waters surrounding the Prince Edward Islands (Southern Ocean)

Fish <1 year old were sampled during 1 year using nets inshore at South Georgia. Some fish were kept in aquaria. Growth rates were estimated using the exponential model. During June to October 1980, field growth rates of Parachaenichthys georgianus and Champsocephalus gunnari were 0.33 and 0.48% SL day⁻¹, respectively. Gobionotothen marionensis (1979 cohort) grew at 0.40% SL day⁻¹ during June to November in the field, and 0.34% SL day⁻¹ in the laboratory from September to March. Notothenia coriiceps grew at 0.28% SL day⁻¹ in the laboratory during September to March. During November to December, Artedidraco mirus grew at 0.82% SL day⁻¹ in the field. The 1980 cohort of G. marionensis grew at 1.39% SL day⁻¹ during November to January in the field. During January, the field growth rate of G. gibberifrons was 1.39% SL day⁻¹. Growth rates increased three-fold from winter to summer. Temperature can only explain one-half of this range in growth rates, whereas all of this range can be explained by food availability. Therefore, seasonal food resource limitation has a major effect on Antarctic fish growth. Received: 30 June 1997 / Accepted: 7 September 1997     

Morphometric relationships of the caridean shrimp Nauticaris marionis Bate, 1888 at the Prince Edward Islands (Southern Ocean)

The relationships between three measurements of body size (carapace length, total body length and eye diameter) and three measurements of mass (wet, dry and ash-free dry weights) of the caridean shrimp Nauticaris marionis are presented. It is argued that a precisely defined carapace length is the most accurate indicator of body size. A conversion formula relating carapace length to total body length is also provided, as are conversion equations relating the different measures of mass. Accepted: 17 May 1999     

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

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

Time-series analysis of hydrological and biological variability on the Prince Edward Island (Southern Ocean) shelf

Eulerian time-series were completed at three sites on the south eastern shelf of the Sub-Antarctic Prince Edward Islands (PEI), from 9 to 27 April 2000, to investigate the spatial and temporal variability of mesozooplankton communities in relation to on-shelf hydrodynamics. Surface temperature and phytoplankton biomass were recorded daily at all three sites, while two Bongo net tows and vertical temperature profiles were collected daily at two sites. Contrary to predictions of on-shelf water retention at low ambient current velocities, cross-correlation analysis identified a northward advection of ∼8.23 cm s⁻¹ over the study area. Day to day changes in both physical and biological parameters were associated with cross-shelf advection. However, sea temperature had a narrow range of 6.2–7.4°C, while average between sample Bray–Curtis similarity in zooplankton communities was high, exceeding 75%. The relatively consistent average sample biomass amounted to ∼494 tonnes dry weight of mesozooplankton per day being advected onto the south eastern sector of the PEI shelf. We hypothesise that the observed stability of both physical and biological parameters was due to the Sub-Antarctic Front, with its high volume transport and steep physical gradients, being far to the north of the islands. Spatial and temporal variability of hydrodynamics on the PEI shelf, and particularly its relationship to shifting frontal positions, is highlighted as a future research area necessary to quantify the gross delivery of allochthonous biomass to the PEI ecosystem.     

The plankton of the Prince Edward Islands

Summary The Prince Edward Islands (47°S, 38°E) are situated very close to the border between the antarctic and subantarctic zones. There is, indeed, some difference of opinion whether this border, the Antarctic covergence or Polar front, should be drawn at 46° or 50°S (see Discussion). Deacon (1983) claims that the complexity of the currents in this area leads to more interchange and less clear gradation between Antarctic and subantarctic waters than in most longitudes. This is a study of the species composition, and a very rough estimate of the standing stock, of the plankton fauna of the waters surrounding the Prince Edward Islands during two surveys conducted by us in 1982 and 1983. They are compared with each other and with a third survey by Grindley and Lane in 1979 and data presented by Miller (1985). The plankton is mainly antiboreal (Systematic Account and Table 1). However, entrants of tropical, sub-tropical and antarctic origin are noted. The means of advection of these aliens into the region are hypothesized and it is further speculated that they may be, at times, relics of decayed biocoenoses.     

Trophic importance of the hyperiid amphipod, Themisto gaudichaudi, in the Prince Edward Archipelago (Southern Ocean) ecosystem

The feeding dynamics and predation impact of the hyperiid amphipod, Themisto gaudichaudi, on the zooplankton community in the waters surrounding the Prince Edward Archipelago were investigated at 30 stations in late austral summer (April/May) 1998. Ingestion rates of T. gaudichaudi were estimated using two approaches, the gut fullness index and in vitro incubations. Throughout the investigation mesozooplankton, comprising copepods, pteropods and chaetognaths, numerically and by biomass dominated Bongo samples. Zooplankton abundances and biomass ranged from 8 to 271 ind. m⁻³ and between 1.01 and 7.47 mg dwt m⁻³, respectively. Densities of T. gaudichaudi during the study were low, never exceeding 0.4 ind. m⁻³. Gut content analysis (n=61) indicates that T. gaudichaudi is a non-selective, opportunistic carnivore generally feeding on the most abundant copepod and chaetognath species. Peaks in feeding activity were recorded at sunrise and sunset, corresponding to their diel vertical migration patterns. Daily rations estimated from in vitro incubations and gut fullness index were equivalent to 1.2–8.7% and between 11.5 and 19.8% of body dry weight, respectively. The predation impact of T. gaudichaudi averaged over the upper 300 m of the water column was low, accounting for <0.4% of the mesozooplankton biomass or <3% of the mesozooplankton secondary production. Indeed, the predation impact is likely to be lower as the contribution of the smaller copepods (e.g. Oithona spp.) to total zooplankton was underestimated due to the sampling gear employed. The low predation impact recorded during this study can be related to low abundances of T. gaudichaudi. It is likely that the importance of T. gaudichaudi as a secondary production consumer in the waters surrounding the Prince Edward Archipelago demonstrates a high degree of spatio-temporal variability. Accepted: 6 December 1999     

New records of Acari from the sub-Antarctic Prince Edward Islands

Sixty species of Acari are recorded from the sub-Antarctic Marion and Prince Edward Islands (the Prince Edward archipelago). Twenty of the 45 species collected on recent expeditions are new and currently undescribed. Other new taxa include a family of Mesostigmata, four new genera, and the first sub-Antarctic records of Cillibidae (Mesostigmata) and Eryngiopus (Prostigmata). Fifteen of the 31 species previously reported from the islands are confirmed, although eight of the previous accounts remain doubtful. The fauna, which shows a distinction between the shoreline and terrestrial components, comprises endemic, South Indian Ocean Province and sub-Antarctic mite species. Accepted: 18 July 1998     

Killer whale predation on subantarctic fur seals at Prince Edward Island, Southern Indian Ocean

Killer whales at subantarctic Marion Island have been subjected to several scientific studies. In contrast, up until recently, there has been only one record of these animals documented for neighbouring Prince Edward Island. We here report on killer whale observations at Prince Edward Island during March 2012. During 3?days of opportunistic observations at a fur seal colony on the island, killer whales were sighted on six occasions. These probably represented three different pods numbering 11 individuals in total. During all sightings, individuals were seen hunting subantarctic fur seal pups, with four successful predatory events observed.     

Holothurians (Echinodermata) from Marion and Prince Edward Islands: new and little-known species

Two new species Mesothuria edwardensis sp.n. and Paradota marionensis sp.n., and four little-known species Cucumaria kerguelensis Théel, 1886, Cladodactyla crocea croceoides (Vaney, 1908), Psolidium incertum (Théel, 1886) and Synallactes challengeri (Théel, 1886) of holothurians are described from Marion and Prince Edward Islands.     

The distribution and abundance of white-chinned petrels (Procellaria aequinoctialis) breeding at the sub-Antarctic Prince Edward Islands

The white-chinned petrel (Procellaria aequinoctialis) is the seabird most often killed on longlines in the Southern Ocean and is listed as vulnerable to extinction. We estimated the population breeding at the Prince Edward Islands, the last breeding site for the nominate subspecies that lacks a recent population estimate. White-chinned petrel burrows are largely confined to deep, muddy soils, usually on slopes below 200?m, but locally up to 420?m. After correcting for count bias, Marion Island has an estimated 29,900 nests (95?% CI 27,700–32,400). Burrow occupancy rates at the start of the incubation period were 65?% during one-off surveys, but repeat surveys found that at least 73?% of burrows were occupied and 87?% of burrows showed signs of occupancy. This suggests that there were roughly 24,000 occupied nests on Marion Island (95?% CI 20,000–28,000). A more cursory survey on Prince Edward Island yielded 14,700 burrows, suggesting that there are 9,000–15,000 occupied nests. The nominate subspecies of white-chinned petrel occupies approximately 974,200 nests (95?% CI 678,000–1,286,000), with the Prince Edward Islands, the third most important breeding site, after South Georgia and Kerguelen. Assuming that populations breeding at islands in the Atlantic and Indian Oceans winter in different regions, the impact of fishery bycatch is likely to have had a greater impact on the Indian Ocean population. The Marion Island survey provides a baseline against which future population changes can be assessed.     

Mesozooplankton interactions with the shelf around the sub-Antarctic Prince Edward Islands archipelago

Mesozooplankton surveys were conducted in April/May for fourconsecutive years (1996–1999) in the vicinity of the PrinceEdward Islands (PEIs), Southern Ocean. The PEIs are locatedin the Polar Frontal Zone, directly in the path of the east-flowingAntarctic Circumpolar Current. Zooplankton were collected byoblique tows using a Bongo net fitted with 300 µm mesh.The abundance, biomass and average size of the mesozooplanktonin the upstream (USR), inter-island (IIR) and downstream (DSR)regions indicated that some groups and species were significantlyaffected by their interaction with the shallow shelf watersof the PEIs. Total mesozooplankton abundance and biomass weretypically highest in the DSR, but no consistent pattern wasevident in the USR and IIR. Copepods, euphausiids and fish weregenerally of a low average size in the IIR. This small sizewas largely attributed to the reduced abundance, or completeabsence, of mesopelagic species from the shelf region. Of totalbiomass, the mesopelagic species Euphausia longirostris, Euphausiasimilis, Pleuromamma abdominalis, Paraeuchaeta biloba and Oncaeaantarctica together contributed an average of 16% to the USR,2% to the IIR and 15% to the DSR. Conversely, epipelagic speciesshowed no consistent pattern of abundance and biomass distributionbetween regions. The low incidence of mesopelagic species overthe island shelf was attributed mainly to reduced advectionof deep water into the shelf region (average depth = 200 m),rather than predation, particularly during the through-flowmode between the islands. This resulted in substantial regionaldifferences in euphausiid community structure. The epipelagicspecies Euphausia vallentini and Thysanoessa vicina completelydominated the IIR, comprising on average 89% of total euphausiidbiomass in this region. However, predation may be importantduring the water-trapping mode between the islands. Advectionof zooplankton into the IIR appeared to be affected by the proximityof the Subantarctic Front (SAF). In 1996, when the SAF was farnorth of the PEIs, reduced current velocities resulted in somedegree of water retention over the shelf and an increased predationimpact. Conversely, when the SAF was close to the PEIs in 1999,more large plankton were transported over the island shelf.High current velocities and productivity associated with theSAF appear to increase the biomass and size of allochthonouszooplankton/nekton advected into the IIR, and consequently mayhave increased the availability of prey to land-based predators.The long-term southward movement of the SAF recently observedin the vicinity of the PEIs may therefore have important implicationsfor the ecosystem of these islands.     

Patterns in the benthic communities on the shelf of the sub-Antarctic Prince Edward Islands

Summary Benthic communities at the sub-Antarctic Prince Edward Islands were sampled qualitatively with an epi-benthic sled at 57 stations over a five year period. Additional information on these communities was obtained from replicate underwater photographs. In total, 546 macrofaunal species were found. Despite the extreme isolation and geological youth of the islands, there seems to have been a diverse colonisation. A cluster-analysis based on species abundance aggregated most stations into 8 groups. Each of these groups had a unique depth and substratum combination. Thourella variabilis (Octocoralea), and Cabarea darwinii (Bryozoa) characterised deep rocky assemblages and Serpula vermicularis, Lanice conchilega (Polychaeta) and Magellania kerguelenensis (Brachiopoda) characterised soft substrata. Indicator species separating each of the 8 clusters are given. Trends in percentage cover and densities of major taxa with respect to depth and substratum, as shown by photographs, are described. Deep, rocky substrata were dominated by Porifera, Bryozoa and Cnidaria, while soft-substrata were dominated by Polychaeta, Bivalvia and Brachiopoda. Errant forms consisted predominantly of Echinodermata, with Crustacea being important in shallower habitats. Filter-feeders dominated the benthos of the islands in all habitats.     

Day and night mesopelagic fish assemblages off the Kerguelen Islands (Southern Ocean)

Mesopelagic fish assemblages were investigated in the Polar Frontal Zone off the Kerguelen Islands during summer 1995, in parallel with a king penguin tracking study. During the day, the upper offshore water layers (0–200 m) have low potential prey diversity and abundance with only three fish species: a lanternfish, Krefftichthys anderssoni, a member of the Muraenolepididae, Muraenolepis marmoratus, and the early stages of the nototheniid, Lepidonotothen squamifrons. The mesopelagic fish community, including the typical myctophids, first appears in the deeper layer (300 m). At night, the surface layer (50 m) is invaded by the mesopelagic Myctophidae Electrona antarctica, Gymnoscopelus braueri, G. piabilis, G. fraseri, G. nicholsi, Protomyctophum bolini and P. tenisoni. Deeper (>100 m), a cline of species assemblages from the coast to offshore is observed. Accepted: 4 August 1999     

Diatom communities from the sub-Antarctic Prince Edward Islands: diversity and distribution patterns

During an extensive survey of the freshwater and moss-inhabiting diatoms of the Prince Edward Islands, a total of 214 taxa belonging to 60 genera were found. Three main communities can be found on the two islands. A large part of the samples was grouped into an aquatic group, bringing together all samples from lentic and lotic waterbodies. A second, contrasting, group was formed by all dry samples whereas in a third group all habitats with higher mineral contents were found. The habitats differed clearly in taxa composition reflecting that way possible environmental differences. Although the diatom composition between the two islands of this island group (Marion and Prince Edward Island) showed some minor qualitative differences, no marked quantitative differences have been observed. The diatom flora shows a distinct similarity with the other islands in the southern Indian Ocean, such as the Crozet archipelago and the Kerguelen Islands forming a biogeographical entity in this part of the ocean, distinctly separated from the non-marine diatom flora in other parts of the (sub-)Antarctic Region.