Rachael S. Shreeve

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

Copepod hatching success in marine ecosystems with high diatom concentrations

Diatoms dominate spring bloom phytoplankton assemblages in temperate waters and coastal upwelling regions of the global ocean. Copepods usually dominate the zooplankton in these regions and are the prey of many larval fish species. Recent laboratory studies suggest that diatoms may have a deleterious effect on the success of copepod egg hatching. These findings challenge the classical view of marine food-web energy flow from diatoms to fish by means of copepods. Egg mortality is an important factor in copepod population dynamics, thus, if diatoms have a deleterious in situ effect, paradoxically, high diatom abundance could limit secondary production. Therefore, the current understanding of energy transfer from primary production to fisheries in some of the most productive and economically important marine ecosystems may be seriously flawed. Here we present in situ estimates of copepod egg hatching success from twelve globally distributed areas, where diatoms dominate the phytoplankton assemblage. We did not observe a negative relationship between copepod egg hatching success and either diatom biomass or dominance in the microplankton in any of these regions. The classical model for diatom-dominated system remains valid.

Response of the copepod community to a spring bloom in the Bellingshausen Sea

During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m−3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northern regions of the Southern Ocean. However, the estimated grazing impact of the copepod community at all the stations was negligible, rising to a maximum of only 8.4% of daily primary production at the most northerly bloom station. These low values result from the very low numbers of copepods in the epipelagic, particularly under the ice. Of the copepods measured, grazing was mainly by the adult female population of Oithona spp. before the bloom, and appeared to be mainly by Oithona spp. and C. acutus during the bloom.

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.81C and 0.51C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s � 1 ) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s � 1 . Chlorophyll a biomass was generally low (o 1m g m � 3 ) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m � 3 ) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m � 3 ) associated with the frontaljet. Increased zoopl ankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north

Distribution of pelagic larvae of benthic marine invertebrates in the Bellingshausen Sea

During November and December 1992, plankton samples were collected using a ring net of mesh size 200 μm vertically hauled through a 600 m water column, at five stations along a transect running north from the Allison Peninsula in the Bellingshausen Sea. Three stations were located over the continental shelf; two of these were ice bound, whilst the third was at the ice edge. Two other stations were in deeper, ice-free water. Sixteen different larval and juvenile types were found representing seven phyla: Echinodermata, Nemertea, Coelenterata, Mollusca, Annelida, Arthropoda and Bryozoa, of which the first two were the most abundant. Larval numbers and types decreased with distance offshore and away from permanent sea ice. The presence of many stages of nemertean larval development within a short time scale, in an area where developmental tends to be slow, suggests that reproduction occurs over an extended period and that the larvae have a long planktonic phase. The increased size of later developmental stages of the nemertean larvae indicates they obtain nutrition within the water column during winter, when little particulate food is present.

The spring mesozooplankton community at South Georgia: a comparison of shelf and oceanic sites

Abstract Mesozooplankton (predominantly 200–2000 μm) were sampled at a shelf and an oceanic station close to South Georgia, South Atlantic, during austral spring (October/November) 1997. Onshelf zooplankton biomass was extremely high at 10–16 g dry mass m−2 (0–150 m), 70% comprising the small neritic clausocalaniid copepod Drepanopus forcipatus. Large calanoid species, principally Calanoides acutus and Rhincalanus gigas, contributed only 8–10%. At the oceanic station, biomass in the sampled water column (0–1000 m) was ∼6.5 g dry mass m−2 and 4–6 g dry mass m−2 in the top 200 m. Here, large calanoids composed 40–50% of the standing stock. Antarctic krill (Euphausia superba) occurred in low abundances at both stations. Vertical profiles obtained with a Longhurst Hardy Plankton Recorder indicated that populations of C. acutus and R. gigas, which overwinter at depth, had completed their spring ascent and were resident in surface waters. Dry mass, carbon and lipid values were lower than found in summer but were consistent with overwintered populations. Phytoplankton concentrations were considerably higher at the oceanic station (2–3 mg chlorophyll a m−3) and increased over the time on station. In response to this, egg production of both large calanoid species and growth rates of R. gigas approached those measured in summer. Onshelf phytoplankton concentrations were lower (<1 mg m−3), and low egg production rates suggested food limitation. Here phytoplankton rations equivalent to 6% zooplankton body C would have been sufficient to clear primary production whereas at the oceanic station daily carbon fixation was broadly equivalent to zooplankton carbon biomass.

Egg production in three species of Antarctic Calanoid Copepods during an austral summer

Egg production in three species of calanoid copepods Rhincalanus gigas, Calanoides acutus and Calanus simillimus was investigated via incubations of females and recovery of eggs from net hauls made around South Georgia during January 1993. Average daily egg production was highest for the sub-Antarctic C. simillimus, (15.5 eggs female−1 d−1). This species normally spawns in the spring in the central part of its geographical range but was apparently delayed by the colder waters found around South Georgia. For R. gigas and C. acutus egg production averaged 8.9 and 6.0 eggs female−1 d−1, respectively. The former species appeared to be undergoing protracted recruitment while the population of the latter was preparing to overwinter. Considerable interstation variability existed, although no relationships were apparent between surface chlorophyll concentrations and either egg production in experiments or in the numbers of eggs recovered by the nets. Clutch size (eggs produced spawning female−1 d−1) did not differ significantly between the three species although the maximum clutch size recorded for R. gigas (94 eggs) was almost twice that of C. simillimus. Samples taken from the Bellingshausen Sea during the latter part of 1992 indicated that recruitment of R. gigas and C. acutus commenced in early December in this region when adult females were concentrated in the surface 250 m and a diatom bloom was developing. Egg numbers were highest in the surface 50 m (up to 350 m−-3) at both the Bellingshausen and South Georgia stations. At the latter site females migrated into these surface layers at night; thus it would appear that spawning is largely nocturnal and linked to diurnal migratory behaviour.

The distribution of acoustic backscatter from zooplankton compared with physical structure, phytoplankton and radiance during the spring bloom in the Bellingshausen Sea

The distribution of zooplankton as indicated by acoustic backscatter was compared with that of the phytoplankton biomass, with the degree of the shading caused by the presence of the bloom, with the density structure of the top 250 m of the water column and the current flow in the Bellingshausen Sea in the Austral spring 1992. High concentrations of phytoplankton were observed in an area between 67–68.5°S and 84–88°W. The relative importance of these parameters to the distribution of the zooplankton was observed to vary at different depths between 0 and 300 m. Near the surface and below the pycnocline the light field and the phytoplankton concentrations were more important, but in the intermediate region of the greatest vertical density differences sigmaO was the most important parameter. The spatial spectra of the zooplankton abundance at different depths in the water column were compared.

Biogeochemistry of a Southern Ocean plankton ecosystem: Using natural variability in community composition to study the role of metazooplankton in carbon and nitrogen cycles

large euphausiid Antarctic krill may range from ca. 2 to 150 g fresh mass (FM) m � 2 . When krill biomass is low, copepod biomass may be correspondingly higher and overall zooplankton biomass remains more or less unchanged. Krill are omnivorous, feeding facultatively either as grazers on microplankton or as predators on smaller zooplankton. This leads to complex feedbacks within the plankton. A simple model of the phytoplankton–copepod–krill system is used to simulate two scenarios of zooplankton composition. For the ‘‘low krill-high copepod’’ scenario, the model predicts higher phytoplankton biomass and production, lower mixed layer (ML) ammonium, nitrate and silicate concentrations, and higher detrital carbon production than in the ‘‘high krill-low copepod’’ scenario. Nitrogen cycling provides the most explicit demonstration of the differences between the scenarios. For the ‘‘low krill-high copepod’’ scenario, ML ammonium concentration decreased by 25% over 20 days, but excretion by metazooplankton supplied 30% of phytoplankton nitrogen demand. In the ‘‘high krill-low copepod’’ scenario, ML ammonium only declined by 10% over 20 days, but metazooplankton excretion was much lower, at 10% of phytoplankton N demand. These predictions are compared with data from several surveys covering krill biomass in the range 10–55 g FM m � 2 . Phytoplankton chlorophyll biomass is negatively related to krill biomass, and ML nutrients are positively correlated with krill biomass in these data. Both observations and model results suggest that variation in biogeochemical carbon and nitrogen cycles in the South Georgia pelagic ecosystem is determined largely by changes in zooplankton community composition and its impact on phytoplankton dynamics. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 4207 Oceanography: General: Arctic and Antarctic oceanography; 4215 Oceanography: General: Climate and interannual variability (3309); 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics; 4842 Oceanography: Biological and Chemical: Modeling; KEYWORDS: interannual variability, Southern Ocean, pelagic ecosystem, biogeochemistry, phytoplankton, grazing control

Egg hatching times of Antarctic copepods

Abstract Egg hatching times were determined at a range of temperatures for four species of commonly occurring Antarctic copepods. At a given temperature the eggs of Rhincalanus gigas took longest to hatch, up to 9 days at 0°C, followed by those of Calanoides acutus, Calanus propinquus and Calanus simillimus. A Bělehrádeks temperature function with the parameter b fixed at −2.05 accounted for >95% of the variance for each species. There was an approximate doubling in hatching times between 5°C and 0°C for R. gigas and for the other species the increase in embryonic duration was 40–50% at the lower temperature.