ALBERT

Notes: Abstract Microzooplankton grazing and community structure were investigated in the region of the Subtropical Convergence (STC) during three cruises of the South African Antarctic Marine Ecosystem Study (SAAMES) in austral summer (January/February 1993; December 1994/January 1995) and winter (June/July 1993). Chlorophyll a concentrations were consistently dominated by the 〈20 μm size fraction during all three cruises, while the contribution of the microphytoplankton (〉20 μm) to total chlorophyll a concentrations varied considerably between cruises. Microzooplankton communities were numerically dominated by protozoans comprising ciliates (aloricates and tintinnids) and dinoflagellates. Instantaneous growth coefficients of phytoplankton in the vicinity of the STC showed no seasonal trends. However, marked seasonal differences were observed in the size structure of the phytoplankton. The grazing impact of microzooplankton was highest when the 〈20 μm chlorophyll fraction contributed 〉95% of the total. Under these conditions, the instantaneous grazing rates ranged between 0.15 and 0.66 d-1. These correspond to daily losses of 14 to 48% of the inntial standing stock and between 45 and 81% of the potential primary production. At stations where microphytoplankton contributed significantly (∼-20%) to total chlorophyll concentrations, the grazing coefficients were lower, ranging between 0 and 0.53 d-1. This corresponds to a loss of 〈41% of the initial standing stock, or between 0 and 56% of the potential production. Our data suggest that microzooplankton represent the main grazing sink for production when the 〈20 μm chlorophyll size-class dominates total chlorophyll. These facts suggest that the efficiency of the biological pump may vary over time.

Notes: Abstract Combined trawling (rectangular midwater trawl, RMT-8 net) and acoustic (120 kHz) surveys in the upstream and downstream region of the Prince Edward Islands were undertaken during April and May 1996 and 1997. A total of 49 species of macroplankton and micronekton were encountered within the region investigated. Mesopelagic fishes, euphausiids, chaetognaths and tunicates dominated numerically and by biomass. Average abundance and biomass of macroplankton/micronekton in the top 300 m layer were low, 18 individuals 1000 m−3 and 288 mg dry wt 1000 m−3, respectively. Numerical analysis revealed the presence of two major groups of stations broadly corresponding to the offshore (upstream and downstream) and inshore (inter-island) realms. Planktonic samples and acoustic measurements revealed that elevated densities of macroplankton/micronekton were associated with the subantarctic front region and in close proximity to the island plateau. Based on trawling and acoustic survey results, it is suggested that large plankton and micronekton are mostly washed around rather than across the inter-island shelf region. This is in contrast with the “replenishing hypothesis” previously proposed for this region by other investigators.

Notes: Abstract The predation impact of the two chaetognaths Eukrohnia hamata and Sagitta gazellae on mesozooplankton standing stock were investigated in three depth layers during two 24 h stations occupied in the vicinity of Marion Island in late austral summer (April/May) 1986. The zooplankton community at both stations was dominated by small copepods (Oithona spp., Microcalanus spp.), which accounted for 〉95% of total zooplankton abundance. Chaetognaths comprised 〈2% of total zooplankton abundance. E. hamata constituted 〉95% of the total chaetognath stock. The general trend in both species was decreasing abundance with increasing depth, which appeared to be correlated to the distribution of copepods (r 2 = 0.45; P 〈0.05). Gut-content analysis showed that copepods (mainly Oithona spp., Calanus spp. and Rhincalanus gigas) and ostracods were the main prey of both species, accounting for 87 and 61% of the total number of prey in E. hamata and S.␣gazellae stomachs, respectively. In the guts of S.␣gazellae, pteropods (Limacina spp.) and chaetognaths were also well represented. The mean number of prey items (NPC) for E. hamata ranged from 0.02 to 0.06 prey individual−1 which corresponds to an individual feeding rate (Fr) of between 0.05 and 0.12 prey d−1. For S.␣gazellae, the NPC values were higher, varying between 0.04␣and 0.20 prey individual−1, or between 0.15 and 0.76 prey d−1. The daily predation impact of the two chaetognaths was estimated at between 0.3 and 1.2% of the copepod standing stock or between 7 and 16% of the daily copepod production. Predation by S. gazellae on chaetognaths accounted for up to 1.6% of the chaetognath standing stock per day.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract The structure and grazing dynamics of the protozooplankton community at ten stations in the upstream and downstream region of the Prince Edward Islands (46°38′S, 37°57′E) were investigated during early austral autumn (April/May) 1997. Total chlorophyll-a (chl-a) concentrations throughout the investigation were 〈0.6 μg l−1 and were always dominated by nano- and picophytoplankton (〈20 μm). Although microphytoplankton (〉20 μm) generally contributed 〈10% of the total chl-a, its contribution to total was significantly higher at downstream stations (F= 1.03; P 〈 0.05). Nanoflagellates (〈20 μm) numerically dominated the protozooplankton community while the 〉20 μm fraction was dominated by dinoflagellates. Instantaneous growth rates of phytoplankton at upstream stations ranged from 0.01 to 0.18 day−1 and between 0.01 and 0.19 day−1 at downstream stations. Instantaneous grazing rates of protists on phytoplankton upstream of the islands ranged from 0.03 to 0.07 day−1, which corresponds to a loss of up to 8% (mean = 5%) of the initial phytoplankton standing stock or up to 71% (mean = 37%) of the potential primary production per day. Downstream of the islands, although the grazing rates of the protists were within the same range (0.01–0.07 day−1), the grazing impact was lower, corresponding to up to 7% (mean = 3%) of the initial standing stock or up to 47% (mean = 25%) of the potential primary production per day. The spatial difference in the grazing impact of protozooplankton appears to reflect a shift in the size structure of the phytoplankton community resulting from the “island mass effect”.

Notes: Abstract Net sampling and continuous acoustic measurements within the Antarctic Polar Frontal Zone (APFZ) and in the vicinity of the Prince Edward Islands were conducted during austral autumn (April/May) 1997 to describe the composition and distribution of macrozooplankton and micronekton, and to investigate their relations to the prevailing oceanographic regime in the area. Two major circulation patterns associated with the Subantarctic (SAF) and Antarctic Polar (APF) Fronts existed in the oceanic environment surrounding the Prince Edward Islands, promoting high cross-frontal mixing both upstream and downstream of the islands. Average abundance and biomass of macroplankton/micronekton in the top 300-m layer were 21 ind. 1000 m−3 and 467 mg DW 1000 m−3, respectively. Pelagic crustaceans (euphausiids and amphipods), fish, chaetognaths and gelatinous zooplankton dominated numerically and by biomass. Continuous acoustic measurements displayed elevated pelagic biomass at the SAF and APF. Although four groupings of stations were identified using cluster analysis, a single macroplankton/micronekton community was recognized in the top 300-m layer throughout the offshore area of the APFZ. A modification of the APFZ community was observed within the inter-island region. Subantarctic species dominated zooplankton samples throughout the APFZ, although subtropical species were also well represented at stations occupied in the northern region of the APFZ. A biological response reflected in macroplankton community composition, resulting from an extensive cross-frontal mixing, was observed within the APFZ around the Prince Edward Islands.

Notes: Abstract 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.