Chlorophyll/nutrient characteristics in the water masses to the north of South Georgia, Southern Ocean

Chlorophyll a and nutrient concentrations along with temperature and salinity values were measured at 22 CTD stations along a 735-km transect running to the northwest of the island of South Georgia, Southern Ocean. Measurements were repeated during five summer surveys (January and February 1994, January 1996, December 1996, January 1998) and one spring survey (October 1997). The transect sampled Sub-Antarctic Zone water in the north, Polar Frontal Zone water and Antarctic Zone water in the south. Chlorophyll a concentrations were lowest to the north of the transect and frequently high (up to 17 mg m⁻³) in the deep open ocean of the Antarctic Zone. Sub-surface peaks were measured in all zones and chlorophyll a was detectable to a depth of 150 m. There was a clear latitudinal temperature gradient in the near-surface waters (0–50 m), the warmest water occurring in the north (∼12 °C), and the coolest in the Antarctic Zone (∼2 °C). There was also a well-defined latitudinal gradient in summer near-surface silicate concentrations (∼2, 4, and 10 mmol m⁻³ in the Sub-Antarctic Zone, the Polar Frontal Zone and the Antarctic Zone, respectively), increasing to >20 mmol m⁻³ near South Georgia. Distinct differences in silicate concentrations were also evident in all three zones to a depth of 500 m. Near-surface nitrate and phosphate concentrations were relatively low to the north of the transect (∼14 and 1 mmol m⁻³, respectively) and higher in the Polar Frontal Zone and Antarctic Zone (∼18 and 1.4 mmol m⁻³, respectively). Ammonium and nitrite were restricted to the upper 200 m of the water column, and exhibited sub-surface concentration peaks, the lowest being in the Sub-Antarctic Zone (0.68 and 0.25 mmol m⁻³, respectively) and the highest in the Antarctic Zone (1.72 and 0.29 mmol m⁻³, respectively). Surface (∼6 m) spring nutrient measurements provided an indication of pre-bloom conditions; ammonium and nitrite concentrations were low (∼0.27 and 0.28 mmol m⁻³, respectively), while silicate, nitrate and phosphate concentrations were high and similar to previously measured winter values (e.g. ∼26, 23, 2 mmol m⁻³, respectively in the Antarctic Zone). Although the values measured were very variable, and there was some evidence of a seasonal growth progression, the chlorophyll a and nutrient distribution patterns were dominated by intercruise (interannual) factors. Approximate nutrient depletions (spring minus summer) appeared similar in the Polar Frontal Zone and Antarctic Zone for nitrate and phosphate, while silicate showed a marked latitudinal increase from north to south throughout the transect. Highest chlorophyll a concentrations coincided with the highest apparent silicate depletions over the deep ocean of the Antarctic Zone. In this area, relatively warm, easterly flowing Antarctic Circumpolar Current water meets cooler, westerly flowing water that is influenced by the Weddell-Scotia Confluence and is rich in nutrients, especially silicate. Accepted: 27 November 1999     

Movement of oceanic fronts south of Australia during the last 10 ka: interpretation of calcareous nannoplankton in surface sediments from the Southern Ocean

Preservation of calcareous nannoplankton in surface sediment samples from the Southern Ocean south of Australia and adjacent to New Zealand record a single assemblage. The dominant species are Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus. The assemblage varies little in abundance and diversity with minor correlation to present-day overlying surface water masses and oceanic fronts. Increase in abundance of H. carteri and C. pelagicus in the region of the Subtropical Front may reflect higher nutrients associated with this front. The assemblage, although altered by dissolution, represents a warmer climatic interval than present-day with the presence of preferentially dissolved, warm-water species preserved as far south as the Polar Front. The presence of warm-water species under sub-Antarctic waters at the Polar Front is interpreted as a relic population from the Holocene climatic optimum of 10–8 ka. The absence of coccoliths in sediments poleward of the Polar Front suggests an equatorward shift of this front following the climatic optimum, resulting in increased productivity of siliceous phytoplankton associated with the colder waters and increased dissolution of coccoliths. Movement of the Subtropical Front for the same interval is not recorded in the preserved coccoliths. The more heavily calcified form of E. huxleyi which dominates the living assemblage north of the Subtropical Front is subject to dissolution in this region and is poorly preserved in the sediment assemblage.     

A phytoplankton absorption-based primary productivity model for remote sensing in the Southern Ocean

Recent global environmental changes such as an increase in sea surface temperature (SST) are likely to impact primary productivity of phytoplankton in the Southern Ocean. However, models to estimate net primary production using satellite data use SST and uncertain estimation of chlorophyll a (chl-a) concentration. A primary productivity model for satellite ocean color data from the Southern Ocean, which is based on the light absorption coefficient of phytoplankton to reduce uncertainties of sea surface chl-a estimations and bias in optimal values of chl-a normalized productivity derived from SST, has been developed. The new model was able to estimate net primary productivity in the water column (PP _eu) without dependency on temperature when in the range of −2 to 25°C, and it explained 51% of the observed variability in PP _eu with a root mean square error (RMSE) of 0.15. Application of the model revealed that the SST dependent model has overestimated PP _eu in warmer waters around the Subtropical Front, and underestimated PP _eu in colder waters poleward of the Sub-Antarctic Front. This absorption-based primary productivity model contributes to a study of the relationship among spatio-temporal variations in the physical environment, and biogeochemical cycles in the Southern Ocean.     

Southern Ocean Biogeography of Tintinnid Ciliates of the Marine Plankton

Ciliate microzooplankton are important grazers in most pelagic ecosystems and among them, tintinnids, with their largely species‐specific loricas, allow relatively easy assessment of questions of diversity and distributions. Herein, we present the results of a survey of species records of tintinnids from the Southern Ocean (locations below 40°S) reported in 56 publications yielding 2,047 species records (synonyms included) from 402 locations. The 192 species reported can be parsed into two main groups: 32 endemic Southern Ocean species, known only from 40°S and further south, and a second group of 181 widespread species, forms with extensive geographic ranges extending into the Southern Ocean. Widespread species reported from the Southern Ocean can be further divided into a group of 81 species, each recorded multiple times in the Southern Ocean waters and 70 apparent “stray” species which have only been found but once. The endemic and widespread species of the Southern Ocean show both distinct distributional patterns and morphological differences. The assemblage of Southern Ocean endemics is found mostly within the Antarctic zone delimited by the average location of the Polar Front and contains a relatively large portion of wide‐mouthed forms. We give suggestions for future study.     

Spring phytoplankton assemblages in the Southern Ocean between Australia and Antarctica

Variations of phytoplankton assemblages were studied in November–December 2001, in surface waters of the Southern Ocean along a transect between the Sub-Antarctic Zone (SAZ) and the Seasonal Ice Zone (SIZ; 46.9°–64.9°S; 142°–143°E; CLIVAR-SR3 cruise). Two regions had characteristic but different phytoplankton assemblages. Nanoflagellates(<20 μm) and pico-plankton (∼2 μm) occurred in similar concentrations along the transect, but were dominant in the SAZ, Sub-Antarctic Front (SAF), Polar Front Zone (PFZ) and the Inter-Polar Front Zone (IPFZ), (46.9°–56.9°S). Along the entire transect their average cell numbers in the upper 70 m of water column, varied from 3 × 10⁵ to 1.1 × 10⁶ cells l⁻¹. Larger cells (>20 μm), diatoms and dinoflagellates, were more abundant in the Antarctic Zone-South (AZ-S) and the SIZ, (60.9°–64.9°S). In AZ-S and SIZ diatoms ranged between 2.7 × 10⁵ and 1.2 × 10⁶ cells l⁻¹, dinoflagellates from 3.1 × 10⁴ to 1.02 × 10⁵ cells l⁻¹. A diatom bloom was in progress in the AZ-S showing a peak of 1.8 × 10⁶ cells l⁻¹. Diatoms were dominated by Pseudo-nitzschia spp., Fragilariopsis spp., and Chaetoceros spp. Pseudo-nitzschia spp. outnumbered other diatoms in the AZ-S. Fragilaropsis spp. were most numerous in the SIZ. Dinoflagellates contained autotrophs (e.g. Prorocentrum) and heterotrophs (Gyrodinium/Gymnodinium, Protoperidinium). Diatoms and dinoflagellates contributed most to the cellular carbon: 11–25 and 17–124 μg C l⁻¹, respectively. Small cells dominated in the northern region characterized by the lowest N-uptake and new production of the transect. Larger diatom cells were prevalent in the southern area with higher values of N-uptake and new production. Diatom and nanoflagellate cellular carbon contents were highly correlated with one another, with primary production, and productivity related parameters. They contributed up to 75% to the total autotrophic C biomass. Diatom carbon content was significantly correlated to nitrate uptake and particle export, but not to ammonium uptake, while flagellate carbon was well correlated to ammonium uptake, but not to export. Diatoms have contributed highly to particle export along the latitudinal transect, while flagellates played a minor role in the export.     

Shearwater Foraging in the Southern Ocean: The Roles of Prey Availability and Winds

Background

Sooty (Puffinus griseus) and short-tailed (P. tenuirostris) shearwaters are abundant seabirds that range widely across global oceans. Understanding the foraging ecology of these species in the Southern Ocean is important for monitoring and ecosystem conservation and management.

Methodology/Principal Findings

Tracking data from sooty and short-tailed shearwaters from three regions of New Zealand and Australia were combined with at-sea observations of shearwaters in the Southern Ocean, physical oceanography, near-surface copepod distributions, pelagic trawl data, and synoptic near-surface winds. Shearwaters from all three regions foraged in the Polar Front zone, and showed particular overlap in the region around 140°E. Short-tailed shearwaters from South Australia also foraged in Antarctic waters south of the Polar Front. The spatial distribution of shearwater foraging effort in the Polar Front zone was matched by patterns in large-scale upwelling, primary production, and abundances of copepods and myctophid fish. Oceanic winds were found to be broad determinants of foraging distribution, and of the flight paths taken by the birds on long foraging trips to Antarctic waters.

Conclusions/Significance

The shearwaters displayed foraging site fidelity and overlap of foraging habitat between species and populations that may enhance their utility as indicators of Southern Ocean ecosystems. The results highlight the importance of upwellings due to interactions of the Antarctic Circumpolar Current with large-scale bottom topography, and the corresponding localised increases in the productivity of the Polar Front ecosystem.     

Increased intrusion of warming Atlantic water leads to rapid expansion of temperate phytoplankton in the Arctic

The Arctic Ocean and its surrounding shelf seas are warming much faster than the global average, which potentially opens up new distribution areas for temperate‐origin marine phytoplankton. Using over three decades of continuous satellite observations, we show that increased inflow and temperature of Atlantic waters in the Barents Sea resulted in a striking poleward shift in the distribution of blooms of Emiliania huxleyi, a marine calcifying phytoplankton species. This species' blooms are typically associated with temperate waters and have expanded north to 76°N, five degrees further north of its first bloom occurrence in 1989. E. huxleyi's blooms keep pace with the changing climate of the Barents Sea, namely ocean warming and shifts in the position of the Polar Front, resulting in an exceptionally rapid range shift compared to what is generally detected in the marine realm. We propose that as the Eurasian Basin of the Arctic Ocean further atlantifies and ocean temperatures continue to rise, E. huxleyi and other temperate‐origin phytoplankton could well become resident bloom formers in the Arctic Ocean.     

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     

Microzooplankton grazing in the surface water of the Southern Ocean during an austral summer

Microzooplankton grazing was investigated in surface waters of the Indo-Pacific and Atlantic sectors of the Southern Ocean by the dilution method. Phytoplankton growth varied mainly between 0.1 and 0.4 day⁻¹, and microzooplankton grazing between 0.0 and 0.3 day⁻¹. Great fluctuations in phytoplankton growth rate were observed at one station within 3 weeks and between closely spaced stations. Microzooplankton grazing rates were similar to phytoplankton growth rate despite the variation of phytoplankton growth rates, although in some cases, phytoplankton growth overwhelmed microzooplankton grazing. These observations suggest that microzooplankton are the main consumers of primary producers, and that steady state between phytoplankton growth and microzooplankton grazing is usually established in the Southern Ocean in austral summer. Received: 5 November 1996 / Accepted: 4 March 1997     

Phytoplankton and particulate matter at the Weddell / Scotia Confluence (47°W) in summer 1989, as a final step of a temporal succession (EPOS project)

During January 1989, phytoplankton biomass and species composition were studied in a north / south transect at the Weddell / Scotia Confluence (47°W), between 57° and 61°30′S. Results showed a diatom bloom in the Scotia Sea (chlorophyll a 1.9 μg l⁻¹, particulate organic carbon 239 μg l⁻¹), dominated by Fragilariopsis cylindrus, Dactyliosolen antarcticus and Chaetoceros dichaeta. Low chlorophyll a / phaeopigments ratios (about 1.4) and silicate concentrations (15 μmol l⁻¹) suggested that this was an advanced bloom phase, probably linked to high grazing pressure. Minimum chlorophyll a values of 0.1–0.2 μg l⁻¹ and particulate organic carbon 46 μg l⁻¹ were found at the Weddell / Scotia Front and in a subsurface layer of the Weddell Sea Water. In the southern part of the transect (61°30′S), in the Weddell Sea, a second surface maximum was found (chlorophyll a 0.9 μg l⁻¹, particulate organic carbon 120 μg l⁻¹), but with a different species composition, with Cryptomonas sp. dominant. Our results show a succession within the diatom community in the Weddell / Scotia Confluence Waters when comparing the three EPOS legs. In the Weddell Sea from spring to summer, nanoflagellates, with only a minor contribution from diatoms, persist over a long period with little change in the community structure. We suggest that the frontal system, together with the receding ice edge and the grazing pressure of either krill or protozooplankton, are mainly responsible for the phytoplankton distribution patterns found. Received: 3 July 1996 / Accepted: 3 November 1996     

Optical fractionation of chlorophyll and primary production for coastal waters of the Southern Ocean

Our objective was to quantify the potential variability in remotely sensed chlorophyll a (Chl a) and primary productivity in coastal waters of the Southern Ocean. From data collected throughout the springs/summers of 1991–1994, we calculated the proportion of water column Chl a and primary productivity within the upper optical attenuation length (K ⁻¹ _par) and the satellite-weighted depth. The temporal variability was resolved every 2–3 days and was observed to be greater within years than between years. Three-year averages (n=223) revealed that 10.2 ± 3.6% of total Chl a and 14.8 ± 6.5% of production occurred within satellite-weighted depth in predominantly Case I waters. The average values were twice as high within K ⁻¹ _par, 24.1 ± 8% of total Chl a and 34 ± 9% of production respectively. Masked in these long-term averages are very large changes occurring on short time scales of seasonal blooms. We observed that the patterns of Chl a vertical distribution within blooms are also subject to taxonomic influence and dependent upon the physiological state of the phytoplankton. Highest proportions of water column Chl a in the first optical depth were measured during the rapid onset of surface cryptophyte blooms each year, i.e. 50% within K ⁻¹ _par and 30% above the satellite-weighted depth. Lowest fractions, 6% and 2% of biomass within K ⁻¹ _par and satellite-weighted depth respectively, were associated with peak bloom conditions independent of taxonomy. Our analyses suggest that satellite-dependent models of Chl a and subsequent chlorophyll-dependent primary production will be challenging to develop for the near-shore Southern Ocean, especially given the potentially high natural variability in the vertical distribution of Chl a driven by physical forcing, the photoadaptive abilities of polar phytoplankton, and taxonomic influences. Accepted: 27 August 1999     

Phytoplankton biomass and primary production in the marginal ice zone of the northwestern Weddell Sea during austral summer

During the austral summer of 1995, distributions of phytoplankton biomass (as chlorophyll a), primary production, and nutrient concentrations along two north-south transects in the marginal ice zone of the northwestern Weddell Sea were examined as part of the 8th Korean Antarctic Research Program. An extensive phytoplankton bloom, ranging from 1.6 to 11.2 mg m⁻³ in surface chlorophyll a concentration, was encountered along the eastern transect and extended ca. 180 km north of the ice edge. The spatial extent of the bloom was closely related to the density field induced by the input of meltwater from the retreating sea ice. However, the extent (ca. 200 km) of the phytoplankton bloom along the western transect exceeded the meltwater-influenced zone (ca. 18 km). The extensive bloom along the western transect was more closely related to local hydrography than to the proximity of the ice edge and the resulting meltwater-induced stability of the upper water column. In addition, the marginal ice zone on the western transect was characterized by a deep, high phytoplankton biomass (up to 8 mg Chl a m⁻³) extending to 100-m depth, and the decreased nutrient concentration, which was probably caused by passive sinking from the upper euphotic zone and in situ growth. Despite the low bloom intensity relative to the marginal ice zone in both of the transects, mean primary productivity (2.6 g C m⁻² day⁻¹) in shelf waters corresponding to the northern side of the western transect was as high as in the marginal ice zone (2.1 g C m⁻² day⁻¹), and was 4.8 times greater than that in open waters, suggesting that shelf waters are as highly productive as the marginal ice zone. A comparison between the historical productivity data and our data also shows that the most productive regions in the Southern Ocean are shelf waters and the marginal ice zone, with emerging evidence of frontal regions as another major productive site. Accepted: 27 September 1998     

The distribution and nutrient status of phytoplankton in the Southern Ocean between 20° and 70° W

Summary The distribution of phytoplankton along transects amounting to about 10,000 nautical miles in the sector of the Southern Ocean between 20° and 70°W was determined during the austral summer of 1978/79. Chlorophyll a concentration was monitored by the continuous measurement of in vivo fluorescence (IVF). Surface samples were collected for the determination of temperature, salinity, chlorophyll a concentration, carbon fixation rate and species of the phytoplankton. Phytoplankton distribution was found to be extremely patchy both locally and regionally. High phytoplankton concentrations were often associated with either hydrographic features, such as upwelling or the presence of sea-ice, or with bathymetric features, such as shelf breaks, submarine mountain ranges or islands. Enrichment experiments, in which the effects of various nutrient additions on the rate of ¹⁴C fixation by the natural phytoplankton were compared, and bioassay experiments, in which the growth of Thalassiosira pseudonana (Hustedt) Hasle and Heimdal in enriched water samples was measured, were carried out using water samples collected at various stations throughout the study area. Although these techniques were effective in demonstrating nutrient limitation elsewhere, the results suggest that availability of nitrate, phosphate, silicate, trace metals or vitamins exerts no primary control over phytoplankton abundance south of the Polar Front.     

The satellite-derived distribution of chlorophyll-a and its relation to ice cover, radiation and sea surface temperature in the Barents Sea

The response of oceanic phytoplankton to climate forcing in the Arctic Ocean has attracted increasing attention due to its special geographical position and potential susceptibility to global warming. Here, we examine the relationship between satellite-derived (sea-viewing wide field-of-view sensor, SeaWiFS) surface chlorophyll-a (CHL) distribution and climatic conditions in the Barents Sea (30–35°E, 70–80°N) for the period 1998–2002. We separately examined the regions north and south of the Polar Front (∼76°N). Although field data are rather limited, the satellite CHL distribution was generally consistent with cruise observations. The temporal and spatial distribution of CHL was strongly influenced by the light regime, mixed layer depth, wind speed and ice cover. Maximum CHL values were found in the marginal sea-ice zone (72–73°N) and not in the ice-free region further south (70–71°N). This indicates that melt-water is an important contributor to higher CHL production. The vernal phytoplankton bloom generally started in late March, reaching its peak in late April. A second, smaller CHL peak occurred regularly in late summer (September). Of the 5 years, 2002 had the highest CHL production in the southern region, likely due to earlier ice melting and stronger solar irradiance in spring and summer.     

Chla-specific productivity of picophytoplankton not higher than that of larger phytoplankton off the South Shetland Islands in summer

Size-fractionated chlorophyll a (Chla)-specific productivity (μgC μgChla ⁻¹ h⁻¹) was measured at 11 stations off the northern coast of the South Shetland Islands during summer. The Chla-specific productivity of the 2- to 10 or 10- to 330-μm fraction was highest at 100% and 23% light depths. The Chla-specific productivity of the 2- to 10-μm fraction was generally highest, and that of the <2 or 10- to 330-μm fraction was sometimes highest at 12% and 1% light depths. Temperature was less than 3°C within the euphotic zone at all stations. The hypothesis of Shiomoto et al., according to which Chla-specific productivity of picophytoplankton (<2 μm) is not significantly higher than that of larger phytoplankton (>2 μm) in water colder than 10°C, was supported on condition that light is not limited for larger phytoplankton. Received: 16 September 1997 / Accepted: 8 December 1997     

Variation of the phytoplankton community across the subtropical convergence zone in the west pacific sector of the southern ocean during early austral Summer 1995/6

Phytoplankton of the Southern Ocean, 140–148°E and 40–53°S, was sampled from early austral summer Nov. 1995 to Dec. 1995 to examine cell abundance, cell volume and biomass (cell carbon) distribution across the fronts. A total of 90 phytoplankton taxa were identified. They were 50 diatoms, 37 dinoflagellates, 2 silicoflagellates, and 1 prymnesiophyte. 73 species were observed from north of the subtropical convergence zone and 71 species from south of the subtropical convergence zone.Pseudonitzschia spp. was the most widely distributed species. Nanoplankton predominated cell number of phytoplankton throughout the stations. The abundance of diatoms was higher than that of dinofiagellates. Total biomass profiles were dependent to microphytoplankton biomass. Maximum cell number and biomass were observed from subsurface layer. Phytoplankton community changed across the subtropical convergence zone and 50–53°S (antarctic convergence zone), and physicoehemical factors seem to controll the distribution.     

Seasonal change in foraging areas and dive depths of breeding king penguins at Heard Island

The seasonal variation in the foraging behaviour of king penguins (Aptenodytes patagonicus) was studied at Heard Island (53°05′S, 73°30′E) during 1992/1993. On seven occasions throughout the breeding cycle, time-depth-light recorders were deployed on breeding adults to record the dive activities and foraging. Foraging locations changed with season: in autumn and spring 1992, adults foraged between 48–52°S and 74–78°E, about 370 km NNE of Heard Island close to the Polar Front. Two penguins tracked in winter travelled 2220 km east of Heard Island (95°E) along the northern ice limit, and 1220 km south of Heard Island to approximately 65°S, respectively. In spring (October), the penguins again foraged further north than during winter. The foraging area utilised in October overlapped the area where the penguins foraged in March/April. The penguins' diving behaviour also varied seasonally: the modal depth of deep dives (>50 m) increased from about 100 m in February to 220 m in October. Mean dive depths increased from 70 ± 52 m in March 1992 to 160 ± 68 m in August 1992. Penguins dived deep (>50 m) only during daylight hours (16 h in February, 9 h in July). Mean dive durations ranged from 2.9 ± 1.1 min in March 1992 to 5.1 ± 1.2 min in August 1992. Associated with changes in foraging location and dive behaviour was a change in diet composition: during summer the penguins ingested mainly myctophid fish (>90%) while in winter the most important diet item was squid. Accepted: 19 October 1998     

Phytoplankton production and biomass at frontal zones in the Atlantic sector of the Southern Ocean

A high resolution study of chlorophyll a and primary production distribution was carried out in the Atlantic sector of the Southern Ocean during the austral summer of 1990–91. Primary production (¹⁴C assimilation) and photosynthetic capacity levels at frontal systems were among the highest recorded during the cruise (2.8–6.3 mgC·m^–3·h^–1, and 1.3–4.7mgC·mgChl a ^–1·h^–1, respectively). Blooms at ocean fronts were strongly dominated by specific size classes and species. This suggests that the increase in biomass was probably the result of an enhancement of in situ production by selected components of the phytoplankton assemblage, rather than accumulation of cells through hydrographic forces. This hypothesis is supported by the high variability of photosynthetic capacities at adjacent stations along the transects. Blooms (ca 2.7–3.5 mg Chl a·m^–3) were found at three oceanic fronts (the Subtropical, Subantarctic and Antarctic Polar Fronts) during the early summer. These were equivalent to, or denser than, blooms in the Marginal Ice Zone and at the Continental Water Boundary. Seasonal effects on phytoplankton community structure were very marked. In early summer (December), netphyto-plankton (>20 m) was consistently the major component of the frontal blooms, with the chain-forming diatoms Chaetoceros spp. and Nitzschia spp. dominating at the Subantarctic and Antarctic Polar Fronts, respectively. During late summer (February), nanophytoplankton (1–20 m) usually dominated algal communities at the main frontal areas. Only at the Antarctic Polar Front did netphytoplankton dominate, with the diatom component consisting almost exclusively of Corethron criophilum. An early to late summer shift of maximum phytoplankton biomass from north to south of the Antarctic Polar Front was observed. Spatial covariance between silicate levels and water-column stability appeared to be the main factor controlling phytoplankton production at the Antarctic Polar Front. Low silicate concentrations may have limited diatom growth at the northern edge of the front, while a deep mixed layer depth reduced production at the southern edge of the front.     

Bacterioplankton in Antarctic Ocean Waters During Late Austral Winter: Abundance, Frequency of Dividing Cells, and Estimates of Production

Bacterioplankton productivity in Antarctic waters of the eastern South Pacific Ocean and Drake Passage was estimated by direct counts and frequency of dividing cells (FDC). Total bacterioplankton assemblages were enumerated by epifluorescent microscopy. The experimentally determined relationship between in situ FDC and the potential instantaneous growth rate constant (μ) is best described by the regression equation ln μ = 0.081 FDC − 3.73. In the eastern South Pacific Ocean, bacterioplankton abundance (2 × 10⁵ to 3.5 × 10⁵ cells per ml) and FDC (11%) were highest at the Polar Front (Antarctic Convergence). North of the Subantarctic Front, abundance and FDC were between 1 × 10⁵ to 2 × 10⁵ cells per ml and 3 to 5%, respectively, and were vertically homogeneous to a depth of 600 m. In Drake Passage, abundance (10 × 10⁵ cells per ml) and FDC (16%) were highest in waters south of the Polar Front and near the sea ice. Subantarctic waters in Drake Passage contained 4 × 10⁵ cells per ml with 4 to 5% FDC. Instantaneous growth rate constants ranged between 0.029 and 0.088 h⁻¹. Using estimates of potential μ and measured standing stocks, we estimated productivity to range from 0.62 μg of C per liter · day in the eastern South Pacific Ocean to 17.1 μg of C per liter · day in the Drake Passage near the sea ice.