Distribution of six major copepod species around South Georgia during an austral winter

Summary A net sampling survey was conducted around the island of South Georgia during July/August 1983. This study compares the age structure and vertical and horizontal distributions of the dominant copepods Calanoides acutus, Calanus simillimus, C. propinquus, Rhincalanus gigas, Metridia lucens and M. gerlachei. The chief physical and biological factors affecting the distributions of these species are assessed and the results are compared with those from a similar survey around the island carried out in early summer (1981/1982). The survey grid lay within the Polar Front during the winter survey, and horizontal changes in copepod abundance corresponded well to the temperature gradient across the front. This pattern was interrupted by the South Georgia shelf where the seasonal migrants (Calanoides acutus, Rhincalanus gigas and Calanus simillimus) occurred in high abundance. The concentration of these migrants over the shelf relative to the oceanic surface layer was attributed to the shelf having prevented their seasonal migration. Within the oceanic area the copepods occupied differing depths, with Calanoides acutus and Metridia gerlachei living deeper than Calanus simillimus, C. propinquus and M. lucens. The populations also tended to live deeper in the warmer (NW) portion of the oceanic survey area. In contrast to the summer survey the age structure of each species showed little variation throughout the survey area. This was attributed mainly to the decreased rates of copepod growth and metabolism in winter.     

Summer-winter differences in copepod distribution around South Georgia

Zooplankton was sampled on a synoptic grid of stations centered on South Georgia during the austral summer of 1981/82 and winter 1983. Within the top 1000 m layer at oceanic stations, copepods averaged 48% of the total biomass in summer and winter, but outnumbered all other taxa combined by a factor of 10. In winter the mean zooplankton biomass within the top 1000 m was 68% of its summer level. Copepod biomass was 77% of its summer level. During both surveys, the large and abundant Calanoides acutus and Rhincalanus gigas dominated the copepod biomass and, with several other species, showed a marked downwards seasonal migration out of the top 250 m layer in winter. Antarctic epipelagic species predominated around the island during summer but tended to be replaced by sub-Antarctic and cosmopolitan species during the winter. Factors likely to influence our estimates of overall copepods abundance and changes in species composition include seasonality of reproduction, net mesh selection and differences in water mass distribution. The observed trends are attributed mainly to variation in the position of the Polar Front which lay north of the island during the summer survey yet lay across the survey area in winter. This resulted in a greater influence of sub-Antarctic water around South Georgia in winter and the displacement of Antarctic species.     

Diets and feeding selectivity among the epipelagic copepod community near South Georgia in summer

During the austral midsummers of 1989/1990 and 1990/1991, seven grazing experiments were undertaken near South Georgia. The copepods spanned 3 orders of magnitude in body mass, from young copepodids of small pseudocalanids to adult females of Rhincalanus gigas. Incubations were in natural sea-water and feeding rates were determined by microscope counts of food items (size range 7–1200 m). Daily rations of the smallest copepods were up to 120% body carbon per day. These high rations contrast with values of less than 10% for large copepods (older copepodids of Calanoides acutus and R. gigas). All sizes of copepods could ingest the full size spectrum of measured particles. However, maximum filtration rates of small copepods were on cells <100 m whereas the large grazers cleared the largest cells (usually long diatoms) at maximum rates. Motile, non-diatom taxa (mainly heterotrophic, aloricate dinoflagellates and ciliates) did not appear to have been eaten in preference to diatoms of similar size, but their abundance and high calorific value meant that they comprised a median of 43% of carbon intake across the experiments. These motile, mainly <50 m cells were of a suitable size for ingestion by small copepods and consequently featured more prominently in their diets. The ability of small copepods to feed heavily on cells <50 m, before, during or after blooms, may be important in their life cycles, leading to reduced competition with their larger relatives.     

Distribution of cephalopods recorded in the diet of the Patagonian toothfish (Dissostichus eleginoides) around South Georgia

The cephalopod component of the diet of Patagonian toothfish, Dissostichus eleginoides, around South Georgia was analysed from stomach contents collected between March and May 2000. Cephalopods occurred in 7% of D. eleginoides stomachs. A total of 363 cephalopod beaks were found, comprising 16 cephalopod species, of which 15 had not been previously recorded in the diet. Octopodid A (probably Pareledone turqueti) was the most important cephalopod species by number of lower beaks (36 beaks; 20.2% of the lower beaks) and Kondakovia longimana was the most important in terms of estimated mass (76% of the cephalopod component). When the cephalopod component of D. eleginoides was compared with other predators between March and May 2000, D. eleginoides fed more on octopods (25% of the lower beaks) than black-browed and grey-headed albatrosses (<1% of the lower beaks). The low frequency of the squid Martialia hyadesi in the diet of D. eleginoides around South Georgia was also noticed in the diet of albatrosses, and suggests that M. hyadesi was not present in these waters in 2000 (probably due to migratory movements or reproduction failure), despite being a candidate for commercial exploitation. The presence of the squid Mesonychoteuthis hamiltoni in the diet of D. eleginoides and being caught by a longline hook whilst presumably feeding on D. eleginoides, may indicate that juveniles of Mesonychoteuthis hamiltoni are prey of D. eleginoides adults, and when Mesonychoteuthis hamiltoni reach a large size as adults, they become the predator.     

The stomach contents of Patagonian toothfish around South Georgia (South Atlantic)

The proportion of Patagonian toothfish Dissostichus eleginoides , caught around South Georgia in the south-west Atlantic, with empty stomachs was much lower in fish caught in pots compared to longlines (28 and 91%, respectively, of examined individuals). It is hypothesized that pots caused reduced levels of stress on capture. Stomach content data examined from pot-caught fish will probably therefore be more comprehensive than that from fish caught using longlines. A wide range of prey items was identified in the stomachs of D. eleginoides and stomach contents of individuals caught using the two fishing methods were significantly different. The most common prey item for D. eleginoides caught using pots was the decapod prawn Nauticaris sp., which was restricted in location and depth. However, prawns were not common in the stomachs of D. eleginoides caught from the same location using longlines. Stomach contents from the two fishing methods remained significantly different when Nauticaris sp. were eliminated from the assessment, although fishes then dominated the diet of D. eleginoides caught using either fishing gear. The study confirms that D. eleginoides is an opportunistic carnivore, and indicates that feeding habits depend on the local availability of food items, as well as factors such as depth and predator size. The potential ecological impacts of fishing for D. eleginoides on the South Atlantic ecosystem are discussed.     

Benthic fauna species diversity in six central Florida lakes in summer

The summer macroinvertebrate fauna was studied in six central Florida monomictic lakes during July, 1973. No direct relationships were found between physiochemical conditions and species diversity. Of the 22 species of benthic organisms recorded, only four species were found in all the lakes. Mean species diversity (d) values for the lakes ranged from 0.735 to 1.731.     

Microphytoplankton assemblages in the waters surrounding South Georgia, Antarctica during austral summer 1994

The composition and distribution of microphytoplankton assemblages in the vicinity of South Georgia were examined during a krill survey conducted during voyage 119 the RV Africana in austral summer (January/February) 1994. Microphytoplankton distribution was studied at 83 stations along a zig-zag transect in the waters surrounding South Georgia. Oceanographic and zooplankton data suggest that conditions were anomalous during the investigation, in that sub-Antarctic surface waters had probably been advected southwards and were present around the island. The two most widely distributed species were Corethron criophilum and Eucampia antarctica, which were recorded at all stations sampled. Also well-represented amongst the microphytoplankton assemblages were temperate, neritic species such as Chaetoceros atlanticus, Proboscia alata and Odontella weissflogii. This can probably be related to the intrusion of sub-Antarctic surface waters unusually far south. Cluster and ordination analysis identified three distinct groupings of stations in the waters surrounding the islands. These were found to the east, west and north of the islands. The diatom groupings identified during this investigation broadly correspond to three water masses previously described in the vicinity of South Georgia. Despite the anomalous hydrographical conditions that existed during the investigation, the differences in phytoplankton were negligible when compared to normal years. Small mesoscale features are, however, important in imposing biogeographic patterns on microphytoplankton. Received: 21 February 1996 / Accepted: 3 November 1996     

The distribution of the Euchaetidae (Copepoda: Calanoida) around South Georgia

Summary Fourteen species of the genus Euchaeta (Copepoda: Calanoida) were encountered during two cruises undertaken around South Georgia during November–December 1981 (Summer) and July–August 1983 (Winter). All 14 species were present in summer but only nine in winter. The distribution of the majority of species centered on the mesopelagic (500–1000 m) and bathypelagic (1000–2000 m) depth horizons. During both cruises, four species, Euchaeta Antarctica, E. biloba, E. rasa and E. farrani were numerically dominant. Although all four species had a wide depth distribution, E. Antarctica and E. biloba had distributions centered around the 250–500 m (epi-mesopelagic) and mesopelagic depth horizons. E. rasa was most abundant in the mesopelagic whereas E. farrani, the largest species encountered, was centered on the mesopelagic and bathypelagic depth horizons. Two species, E. Antarctica and E. biloba commonly occurred in the surface 250 m, the former being far more abundant especially over the shelf and shelf break areas, with a high abundance of copepodite stages IV and V being present in summer and adults in winter.     

Distribution of dominant calanoid copepod species in the Greenland sea during late fall

Summary Between 6 November and 12 December 1988, vertical distributions of Calanus finmarchicus, C. hyperboreus, C. glacialis and Metridia longa were studied at three stations in the Arctic water of the Greenland Sea Gyre (GSG) and compared with two stations in the Atlantic water (AW) of the Westspitsbergen Current. Nine depth strata down to 3,000 m were sampled. C.finmarchicus was most abundant in AW, C. hyperboreus in GSG, M. longa showed no preference and C. glacialis was rare everywhere. Stage composition differed with species and water mass. Vertical distribution varied also with water mass in all species but C. hyperboreus, which was always centered between 1,000 and 1,500 m. The other species were concentrated in the upper 300 m in AW and between 1,000 and 1,500 m in GSG, although not all stages followed this trend. Ontogenetic vertical migration leads to significant dislocations of organic matter from the euphotic zone to great depth in the Greenland Sea. In egg production experiments, C. hyperboreus spawned up to 149 eggs female^–1 day^–1. Implications of stage composition, sex ratio, and gonad maturation on copepod life cycles are discussed.     

The summer zooplankton community at South Georgia: biomass,vertical migration and grazing

Zooplankton abundance and biomass were determined during January 1990 at two stations to the north-west of South Georgia using a Longhurst Hardy Plankton Recorder (LHPR). At both shelf and oceanic station sites, zooplankton biomass, (excluding Euphausia superba), was found to be ca. 13 g dry mass m^–2. Copepods and small euphausiids dominated the catches. These estimates are over 4 times higher than values generally reported for the Southern Ocean and may reflect firstly, the high productivity of the study area, secondly, the time of year, summer, when biomass for many species is maximal, and thirdly, the high sampling efficiency of the LHPR. Principal components analysis disclosed similarities and differences between adjacent depth strata in terms of abundance, biomass and species composition. At both stations most variability occurred in the mixed layer (0–60 m) and thermocline (60–120 m) with depth horizons below this being more homogeneous. Diel migrations were observed for most taxa with abundance increasing in the mixed layer at night. At the oceanic station, species and higher taxa belonging to the mesopelagic community were generally well spread throughout this domain and, with the exception of Pleuromamma robusta and Metridia curticauda, showed little evidence of migration. The grazing impact of the epipelagic community (copepods and small euphausiids) was estimated to remove 3–4% of the microbial standing stock day^–1 and a conservative 25% and 56% of daily primary production at the oceanic and shelf stations respectively.     

Water-use strategies of six co-existing Mediterranean woody species during a summer drought

Drought stress is known to limit plant performance in Mediterranean-type ecosystems. We have investigated the dynamics of the hydraulics, gas exchange and morphology of six co-existing Mediterranean woody species growing under natural field conditions during a drought that continued during the entire summer. Based on the observed minimum leaf water potentials, our results suggest that the six co-existing species cover a range of plant hydraulic strategies, from isohydric to anisohydric. These differences are remarkable since the selected individuals grow within several meters of each other, sharing the same environment. Surprisingly, whatever the leaf water potentials were at the end of the dry period, stomatal conductance, photosynthesis and transpiration rates were relatively similar and low across species. This result contradicts the classic view that anisohydric species are able to maintain gas exchange for longer periods of time during drought stress. None of the plants showed the expected structural acclimation response to the increasing drought (reduction of leaf-to-sapwood area ratio), thereby rejecting the functional equilibrium hypothesis for our study system. Instead, three of the six species increased photosynthetic area at the branch level. The observed dissimilar patterns of gas exchange, hydraulics and morphology across species seem to be equally successful given that photosynthesis at the leaf level was maintained at similar rates over the whole dry period.     

Distribution of post-weaning Antarctic fur seal Arctocephalus gazella pups at South Georgia

Potentially some of the biggest gaps in our knowledge about the ecology of Antarctic fur seals (Arctocephalus gazella) relate to juvenile animals. We investigated the at-sea distribution of five male and five female fur seal pups post-weaning. The study was carried out at Bird Island, South Georgia during two successive winters using satellite-linked platform transmitter terminals (PTTs). Our results are analysed in relation to pup sex and the physical environment and productivity of those 2 years, as well as in the context of our present knowledge of where post-breeding females and males forage. The available physical and biological data during both of the winters of this study suggest that both years were not unusual. We report marked differences between the sexes with male pups foraging significantly further away from land and their birth site than do females. The pups foraged in areas to the East of Bird Island seldom reported as foraging areas for the adult population. Also as winter progressed they showed a more oceanic distribution leaving the continental shelf, possibly to exploit a different prey source that was more readily available in the upper water column.     

Antioxidant levels from different Antarctic fish caught around South Georgia Island and Shag Rocks

Antarctic fish have been isolated for over several million years in an environment with a very low and constant temperature and high oxygen concentration. In such conditions the oxidative stress might be an important factor affecting their metabolic adaptive strategies. Activity of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), vitamin E levels and total antioxidant capacity (TRAP) were measured in liver, gill, heart and muscle homogenates of red-blooded (Nototheniidae) and white-blooded (Channichthyidae) Antarctic fish. SOD activity was also measured in blood samples. Gill SOD activity was threefold higher in channichthyids than in nototheniids while CAT and GPx were significantly higher in the gills of channichthyids. The increased SOD activity of channichthyids probably reflects the large PO₂ gradient across their gills. The H₂O₂ produced seems to be preferentially eliminated by diffusion, according to the low levels of CAT and GPx found in the gills of these species. In contrast, blood SOD was about fivefold higher in the latter group, which possesses erythrocytes and thus a much higher oxygen-carrying capacity. CAT activity was always higher in nototheniids except in muscle. However, vitamin E did not show clear differences between families except for the pattern observed in muscle. The higher content of vitamin E in this tissue shown in channichthyids is related to the higher volume density of mitochondria reported for this group, since vitamin E is responsible for preventing membrane lipid peroxidation. Accordingly, TRAP (representative of hydrosoluble antioxidant capacity) was also higher in muscle of channichthyids. This is probably related to the role of ascorbic (a hydrosoluble compound) acid in regenerating vitamin E. Accepted: 4 September 1999     

Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994

Measurements of adult Antarctic krill (Euphausia superba) gutcontents, evacuation and egestion rates, as well as digestiveefficiency, were carried out during February-March 1994 in thevicin ity of South Georgia to estimate in situ daily ration.These were combined with acoustically derived biomass data tocalculate the grazing impact of Antarctic krill and its contributionto the carbon flux in the region. Individual levels of gut pigmentconcentrations and evacuation rates ranged from 27 to 1831 ngchlorophyll a-eq. ind.^–1 and from 0.133 to 0.424 h^–1,respectively. Losses of pigment fluor escence during digestionwere very high, ranging from 58 to 98% of the total pigmentdigested. Daily carbon consumption estimated using the gut fluorescencemethod varied from 0.234 to 0.931 mg C ind.^–1 day^–1(or 0.4–1.7% of body carbon), compared to {small tilde}2.73mg C ind.^–1 day^–1 (or {small tilde}5% of body carbon)using the faecal pellet production data. The 3-fold higher dailyration estimated using egestion rate data may be explained bypredation on micro-and mesozooplankton. Maximum krill grazingimpact ranged from 0.4 to 1.9% of the total phytoplankton stockor from 10 to 59% of the total daily primary production. However,grazing impact on the microphytoplankton (>20 µm) wassubstantially higher, at times exceeding 100% of the daily microphytoplanktonproduction. It is suggested that to meet its energetic demands,kriil must consume a substantial proportion of heterotrophiccarbon. ³Present address Zoology Department, University of Fort Hare,P/Bag X1314, Alice, 5700, South Africa     

Respiratory metabolism in two species of carabid beetle from the sub-Antarctic island of South Georgia

The influence of feeding state on cold-adapted metabolism was investigated in the adults of two carabid beetles, Trechisibus antarcticus and Oopterus soledadinus (Coleoptera: Carabidae), which have been introduced to sub-Antarctic South Georgia. The metabolic rates in both fed and starved O. soledadinus and T. antarcticus were determined at eight temperatures ranging from 0 to 35°C, using a Servomex 570A oxygen analyser. There was no significant difference in the metabolic rates between the fed and starved animals of each species. In T. antarcticus this ranged from 0.28 to 3.84 ml O₂ g⁻¹ h⁻¹, and in O. soledadinus from 0.19 to 2.80 ml O₂ g⁻¹ h⁻¹ at 0 and 35°C, respectively. In each of the four experimental groups there was a strong positive correlation between metabolic rate and temperature, with the highest increase occurring between 0 and 5°C. In contrast, the metabolic rate was significantly negatively correlated with initial live weight of the beetles at most temperatures. The results are discussed comparatively with other species and against a background of the ecology of the two carabids at South Georgia. Received: 26 August 1996 / Accepted: 3 February 1997     

Egg production rates of eight calanoid copepod species during summer 1997 off Newport, Oregon, USA

Measurements of hydrography, water transparency, chlorophyll (Chl) a and egg production rates (EPRs) by females of Calanus marshallae Frost, Calanus pacificus Brodsky, Eucalanus californicus Johnson, Epilabidocera longipedata Sato, Pseudocalanus mimus Frost, Centropages abdominalis Sato, Acartia longiremis Lilljerborg and Paracalanus parvus (Claus) were estimated at weekly intervals between 17 July and 2 September 1997. Production of eggs was determined in 24 h incubations to examine the effects of environmental variability on EPR, to detect the possibility of food limitation of EPR, and to evaluate the hypothesis that growth rates of females are size dependent. During the study, an anomalous downwelling event occurred, possibly in response to the 1997 El Niño, which allowed us to determine how El Niño events affect EPRs of coastal copepods. The larger copepods Calanus marshallae, Calanus pacificus and Centropages abdominalis showed the highest egg production and specific growth rates during the period of active upwelling (18 July-13 August, water temperatures 8-13C, Chl a concentration 4.7-16.2 l^-1 and water transparency 3-5 m). After 27 August, the 1997-98 El Niño arrived off Oregon, creating a downwelling situation. Upwelling winds ceased, the thermocline intensified, temperature and transparency increased (to >18°C and 16 m), and Chl a declined to <2 g l^-1. Densities of the common coastal species declined greatly as well. Paracalanus parvus became the dominant species, and Eucalanus californicus, Epilabidocera longipedata and Corycaeus anglicus became common in our samples. EPRs for the larger boreal copepods (Calanus and Centropages) declined greatly during El Niño;; the smaller copepods, Pseudocalanus mimus, A.longiremis and Paracalanus parvus, showed low but relatively constant egg production and specific growth rates during both upwelling and downwelling events. Over the entire study period, only three species produced eggs at its maximum rate (24-28 eggs female^-1 day^-1), Pseudocalanus mimus which averaged 4 eggs day^-1. All other species had EPRs that were two or five times below their maximum EPR. Thus, EPRs were not related to body size, contrary to our expectations. Hatching success was variable among species. Values as low as 20-40% were found for all species at least once during the study period, suggesting that occasionally a substantial portion of egg production may not be viable.      

Collembola of sub-Antarctic South Georgia

Qualitative samples of Collembola were obtained from a range of substrates near Husvik, Stromness Bay, South Georgia, between January and March 1996. Collections made at Hope Point near Grytviken (Cumberland East Bay) in 1980/1982 and 1997 were also examined. Fifteen species of Collembola were recorded around Husvik; most were widely distributed. Two of these, Friesia sp. nov. and Cryptopygus badasa, represent additions to the previously recognised fauna of 17 species. A new record of an introduced species, Hypogastrura purpurescens, was identified in collections from Hope Point in 1980/1982, bringing the total South Georgian fauna to 20 species. A key to South Georgian Collembola is included. H. purpurescens and the congeneric Hypogastrura viatica, both cosmopolitan invasive species, have also been recognised on other sub-Antarctic islands and have displaced resident species from some habitats. Their presence on South Georgia, and the dominance of H. viatica in some habitats, highlight the importance of strict quarantine measures to avoid the introduction of further alien invertebrates. Accepted: 30 November 1998