Subantarctic Zone Research Articles

Wintertime trace metal (Zn, Cu, Ni, Cd, Pb and Co) and nutrient distributions in the Subantarctic Zone between 40–52°S; 155–160°E

In high nitrate low chlorophyll (HNLC) oceanic regions, trace metals play a key role in regulating phytoplankton productivity; however large knowledge gaps remain in the biogeochecmical cycling of trace metals in these areas. Here I present the first comprehensive winter dataset (0–1000 m depth) for dissolved zinc, cadmium, copper, nickel, cobalt, and lead along with the macronutrinets phosphate and silicon, between 40°S–52°S, which traverses the Subantarctic zone (SAZ), east of Australia. Dissolved lead concentrations were conservative with depth with values ranging between 12 and 33 pmol kg − 1 . Surface dissolved concentrations for cadmium, copper, nickel, cobalt and phosphate showed large north–south gradients, with lower concentrations at the Subtropical Front (∼ 43°S) and higher concentrations at the Subantarctic Front (∼ 51°S). Profiles of dissolved cadmium, copper, nickel and cobalt concentration increased with depth in a manner similar to that of phosphate. Dissolved cadmium was strongly correlated to phosphate at concentrations greater than 0.74 µmol kg − 1 ; below this concentration cadmium was depleted (< 40 pmol kg − 1 ). The depletion of cadmium before that of phosphate is consistent with the hypothesis that the ‘kink’ in the cadmium–phosphate relationship is linked to iron limitation in Southern Ocean waters. For zinc and silicon, surface dissolved concentrations were low across the transect. Profiles of dissolved zinc and silicon concentration increased with depth and were similar in nature, indicating a close coupling between these two elements. The coupling of zinc and silicon implies that diatoms may control the global zinc–silicon relationship. These results also suggest that mode water and intermediate waters, which are important conduits for the export of silicon depleted waters to low latitudes, play major roles in regulating the concentration of zinc in the main thermocline outside of the Southern Ocean.

Spatial and temporal distribution of Fe, Ni, Cu and Pb along 140°E in the Southern Ocean during austral summer 2001/02

The distribution of dissolved (D) and acid-dissolvable (AD) Fe, Ni, Cu and Pb in the upper water column (0–300 m depth) was determined in the Australian sector of the Southern Ocean (140°E meridian) during three cruises conducted between November 2001 and March 2002. For Ni and Cu, there was no significant difference in concentration between dissolved and acid-dissolvable species. DNi and DCu showed significant ( P = 0.01) positive correlations with silicate, phosphate and nitrate, reflecting their strong nutrient-type behaviour. For Fe and Pb, the acid-dissolvable concentration mostly exceeded the dissolved concentration, reflecting the importance of labile particulate species for these elements. DPb decreased between January and February in the Polar Frontal Zone and in Antarctic continental shelf water. ADPb maxima occurred in the Antarctic Zone, resulting in a maximum AD/D ratio of 7. The mean DFe concentration in the surface mixed layer was 0.3 nM in the sub-Antarctic zone, 0.4 nM in the Polar Frontal Zone, 0.5 nM in the Antarctic Zone and increased southward beyond the Antarctic Divergence and towards the continent. DFe did not show a clear temporal change in its horizontal distribution, which was in contrast to the other nutrients and trace metals. ADFe substantially increased in Antarctic continental shelf water where the AD/D ratio reached 11. The following conclusions can be drawn from these data. (1) Ni and Cu exist exclusively as dissolved species and their distributions are mainly controlled by their biogeochemical cycling, similar to those of the major nutrients. (2) Pb is dominated by particulate species. The distribution of DPb is temporally and spatially variable due to a sporadic source and strong scavenging. (3) DFe is rather a minor fraction of total Fe in Antarctic continental shelf water where shelf sediments and Antarctic sea-ice appear to be strong sources for Fe. There is substantial temporal variation in the supply of Fe to the upper water column. DFe in the mixed layer of the open Southern Ocean is maintained at low concentrations throughout summer due to uptake by phytoplankton and scavenging.

Winter‐time dissolved iron and nutrient distributions in the Subantarctic Zone from 40–52S; 155–160E

In HNLC oceanic regions, iron (Fe) controls phytoplankton productivity yet large gaps remain in our understanding of iron's biogeochemical cycle. Here we present the first comprehensive winter dataset for dissolved Fe (dFe) and nitrate (NO3) distributions (0–1000 m depth) between 40S–52S, which transects the Subantarctic zone (SAZ), west of New Zealand. Surface concentrations (&lt;0.2 nmol Fe kg−1) were conspicuously low, i.e., probably biologically limiting even at winter‐reserve levels, at frontal zones between 43S (Subtropical Front) and ∼51S (Subantarctic Front). A fivefold range in Fe:NO3 molar ratios was observed along the transect, with Subtropical waters, where blooms occur, having the highest ratios in subsurface waters. The major wintertime supply of dFe in the SAZ is from Ekman advection of waters from the south (but calculated source water dFe is ∼0.2 nmol Fe kg−1), suggesting that mixed‐layer dFe concentration is controlled by how long these southern waters remain at the surface (∼3 years).

Vertical and lateral variations in coccolithophore community structure across the subtropical frontal zone in the South Atlantic Ocean

In this study the coccolithophore compositions of 11 plankton depth stations along a N–S transect from the South Atlantic Subtropical Gyre to the Subantarctic Zone were examined qualitatively and quantitatively. The lateral and vertical distribution patterns of not only the most abundant taxa but also of the morphotypes of distinct species complexes, such as Calcidiscus leptoporus, Emiliania huxleyi, and Umbellosphaera tenuis were the focus. Geographic variation among morphotypes mirrors different ecological affinities of the members of a species complex. Multivariate statistics were used to infer the relationship between a set of known environmental data and species concentrations. The results of the detrended Canonical Correspondence Analysis (CCA) revealed the presence of distinct species assemblages. The Subtropical Gyre assemblage within the upper 50 m of the photic zone is mainly composed of Umbellosphaera irregularis, U. tenuis types III and IV, Discosphaera tubifera, Rhabdosphaera clavigera, S. pulchra and E. huxleyi var. corona, adapted to warm and oligotrophic conditions. In the deeper photic zone abundant Florisphaera profunda, Gephyrocapsa ericsonii and Oolithotus spp. are encountered, benefiting from higher nutrient concentrations in the vicinity of the nutricline. A well-defined Subtropical Frontal Zone (STFZ) association is clearly dominated by E. huxleyi types A and C throughout the upper 100 m of the water column. Secondary contributors in the upper photic zone are Syracosphaera spp. (mainly S. histrica, S. molischii), Michaelsarsia elegans, Ophiaster spp. and U. tenuis type II. This assemblage is associated with cooler, nutrient-rich waters. E. huxleyi type B is found deeper in the water column. Here it is accompanied by Algirosphaera robusta, G. muellerae, and S. anthos indicating a tolerance of lower light availability in environments with elevated productivity. C. leptoporus spp. leptoporus shows relatively high cell numbers in all sampled water levels throughout the STFZ. Interestingly, its coccoliths are often smaller 5 μm in lith diameter. The mean coccolithophore assemblages of a station were compared to the underlying surface sediment assemblages. For the most part, the distribution of the morphotypes is reflected in the sedimentary archive, thus proving their potential as paleoecological proxies.

Inter-annual variability of the carbon dioxide oceanic sink south of Tasmania

Abstract. We compiled a large data-set from 22 cruises spanning from 1991 to 2003, of the partial pressure of CO2 (pCO2) in surface waters over the continental shelf (CS) and adjacent open ocean (43° to 46° S; 145° to 150° E), south of Tasmania. Climatological seasonal cycles of pCO2 in the CS, the subtropical zone (STZ) and the subAntarctic zone (SAZ) are described and used to determine monthly pCO2 anomalies. These are used in combination with monthly anomalies of sea surface temperature (SST) to investigate inter-annual variations of SST and pCO2. Monthly anomalies of SST (as intense as 2°C) are apparent in the CS, STZ and SAZ, and are indicative of strong inter-annual variability that seems to be related to large-scale coupled atmosphere-ocean oscillations. Anomalies of pCO2 normalized to a constant temperature are negatively related to SST anomalies. A reduced winter-time vertical input of dissolved inorganic carbon (DIC) during phases of positive SST anomalies, related to a poleward shift of westerly winds, and a concomitant local decrease in wind stress is the likely cause of the negative relationship between pCO2 and SST anomalies. The observed pattern is an increase of the sink for atmospheric CO2 associated with positive SST anomalies, although strongly modulated by inter-annual variability of wind speed. Assuming that phases of positive SST anomalies are indicative of the future evolution of regional ocean biogeochemistry under global warming, we show using a purely observational based approach that some provinces of the Southern Ocean could provide a potential negative feedback on increasing atmospheric CO2.

Pronounced mid-Pleistocene southward shift of the Polar Front in the Atlantic sector of the Southern Ocean

Abstract Ocean Drilling Program (ODP) Site 1094, situated in the Atlantic sector of the Antarctic region below the Polar Front (PF), is dominated by silica-rich sediments during the Pleistocene, but does contain occasional calcium carbonate-rich intervals. The analysis of the coccolithophore assemblages of one of these intervals in the mid-Pleistocene allows us to interpret a prominent southward displacement of the PF. An age model constructed using paleomagnetic and biostratigraphic information places this event around 1 Ma (close to the Jaramillo subchron) in an interval of strong eccentricity and high amplitude in the insolation record. We distinguish five intervals based on coccolithophore assemblage and abundances. According to the age model used, this calcium carbonate-rich period should include Marine Isotope Stage 31, the most significant interglacial period recorded at this time; however, this model cannot decipher if adjacent isotope stages are partially represented. Interval V (the oldest) is interpreted as an episode with progressive abundance of coccolithophores with a characteristic assemblage from the Polar Front Zone (PFZ) but with dominance of siliceous micro-organisms. Interval IV contains the maximum abundance of coccolithophores with an assemblage characteristic of the Subantarctic Zone (SAZ), with peaks in abundance of Calcidiscus leptoporus , Coccolithus pelagicus , Helicosphaera carteri and Syracosphaera spp. related with the proximity of the Subantarctic Front (SAF). The southward displacement of the frontal system, linked to an increase in sea-surface temperature produced increases in diversity and in productivity. Conversely, we consider that the presence of some coccolithophore taxa, linked in other environments with anomalies in salinity, could reflect a pulse in the melting of West Antarctic Ice Sheet, as suggested previously for this interval. Interval III is characterized by a PFZ assemblage reflecting again a northward shift of the PF. Interval II is considered in this context as another southward pulse, but less important than Interval IV and with the absence of SAF and/or melting-related taxa. Interval I is considered a transition from a situation similar to the present-day PFZ to the Antarctic Zone conditions as consequence of a northward displacement of PF and Weddell Gyre/Antarctic Circumpolar Current boundary.

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), 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.

Zooplankton nutrition, storage and fecal lipid composition in different water masses associated with the Agulhas and Subtropical Fronts

Fecal pellets of zooplankton can be important carriers of organic matter from surface to deep waters of the oceans. The summer cruise ANTARES 4 gave us the opportunity to collect zooplankton from the frontal subantarctic zone (station 7 or s-ant) and adjacent subtropical waters (station 8 or s-trop). Triglyceride fatty acid profiles were used to assess the trophic position of two copepods ( Metridia lucens and Pleuromamma spp.), three euphausiids ( Euphausia spinifera, Nematoscelis megalops, and Thysanoessa sp) and a decapod Acanthephyra sp. The euphausiid E. spinifera was studied at the two sites and its reaction to contrasting trophic environments and hydrology was highlighted by variations in its lipid composition. Most species showed various degree of omnivory: E. spinifera from subantarctic showed the highest degree of herbivory on phytoplankton (high proportion of 16:4(n-1), 18:5(n-3), 16:1(n-7)), while the same species at the subtropical station, as well as Thysanoessa sp and M. lucens from the subantarctic station displayed more omnivorous feeding characteristics, partly linked to the abundance of microzooplankton cells. Acanthephirasp, N. megalops and Pleuromammasp from station 8 (s-trop) appear to be essentially carnivorous (high 18:1n-9/18:1n-7 ratio, high levels of 20:1(n-9) or 22:1(n-11)). The trophic position influenced the composition of feces, with higher PUFA percentages in fecal pellets from carnivorous species. Hence, the surface faunal assemblage and the season controlled not only the quantity but also the quality of the food supply as PUFA exported towards the benthic domain.

Primary productivity, differential size fraction and pigment composition responses in two Southern Ocean in situ iron enrichments

Two in situ iron-enrichment experiments were conducted in the Pacific sector of the Southern Ocean during summer 2002 (SOFeX). The “north patch,” established within the Subantarctic Zone (∼56°S), was characterized by high nitrate (∼21 mmol m −3) but low silicic acid (2 mmol m −3) concentrations. North patch iron enrichment increased chlorophyll (Chl) by 12-fold to 2.1 mg m −3 and primary productivity (PP EU) by 8-fold to 188 mmol C m −2 d −1. Surprisingly, despite low silicic acid concentrations, diagnostic pigment and size-fraction composition changes indicated an assemblage shift from prymnesiophytes toward diatoms. The “south patch,” poleward of the Southern Boundary of the Antarctic Circumpolar Current (SBACC) (∼66°S), had high concentrations of nitrate (∼27 mmol m −3) and silicic acid (64 mmol m −3). South patch iron enrichment increased Chl by 9-fold to 3.8 mg m −3 and PP EU 5-fold to 161 mmol C m −2 d −1 but, notably, did not alter the phytoplankton assemblage from the initial composition of ∼50% diatoms. South patch iron addition also reduced total particulate organic carbon:Chl from ∼300 to 100; enhanced the presence of novel non-photosynthetic, but fluorescent, compounds; and counteracted a decrease in photosynthetic performance as photoperiod decreased. These experiments show unambiguously that in the contemporary, high nitrate Southern Ocean increasing iron supply increases primary productivity, confirming the initial premise of the Martin Iron Hypothesis. However, despite a 5-fold increase in PP EU under iron-replete conditions in late summer, the effect of iron on annual productivity in the Southern Ocean poleward of the SBACC is limited by seasonal ice coverage and the dark of polar winter.

Southern Ocean control on the extent of denitrification in the southeast Pacific over the last 70 ka

Temporal changes in oceanic denitrification, the bacterial reduction of nitrate under suboxic conditions, highlight the potential importance of N inventory changes and the production of N 2O on the climate system. At the same time, the cause of the globally observed variation in denitrification remains unclear. High-resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from ODP Site 1234 on the Chile Margin record integrated denitrification changes within the Peru–Chile Upwelling system over the last ∼70 ka. Denitrification changes in the southeast Pacific are coherent with Antarctic climate changes recorded by the Byrd ice core δ 18O record, and lead northern hemisphere climate events. The southern-hemisphere character of the Chile margin δ 15N record suggests that episodes of reduced denitrification in the SE Pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of Subantarctic Mode Water (SAMW), which forms in the Subantarctic zone of the Southern Ocean and feeds into the low-latitude thermocline.

Euphausiid population structure and grazing in the Antarctic Polar Frontal Zone — austral autumn 2004

The euphausiid community structure and grazing dynamics were investigated in the West Indian sector of the Polar Frontal Zone during the austral autumn 2004. Subsurface (200m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Subantarctic Front and the Antarctic Polar Front bisected the survey area into two distinct zones, the Subantarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll a (Chl a) biomass was typical for the region (<25mg Chl a m−2), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 m−3 to 3.1 m−3 and from 0.1mg dry weight m−3 to 8.1mg dry weight m−3 respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). A multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, and (2) restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. Euphausia triacantha had the highest daily ingestion rate, ranging from 1 226.1ng pigment (pigm) ind−1 day−1 to 6 029.1ng pigm ind−1 day−1, whereas the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4–943.0ng pigm ind−1 day−1). The total grazing impact of selected euphausiids ranged from <0.1μg pigm m−2 day−1 to 20.1μg pigm m−2 day−1, corresponding to <0.15% of the areal Chl a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggest that phytoplankton represent a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements.

Summer and winter air–sea CO 2 fluxes in the Southern Ocean

The seasonal variability of the carbon dioxide (CO 2) system in the Southern Ocean, south of 50°S, is analysed from observations obtained in January and August 2000 during OISO cruises conducted in the Indian Antarctic sector. In the seasonal ice zone, SIZ (south of 58°S), surface ocean CO 2 concentrations are well below equilibrium during austral summer. During this season, when sea-ice is not obstructing gas exchange at the air–sea interface, the oceanic CO 2 sink ranges from −2 to −4 mmol/m 2/d in the SIZ. In the permanent open ocean zone, POOZ (50–58°S), surface oceanic fugacity fCO 2 increases from summer to winter. The seasonal fCO 2 variations (from 10 to 30 μatm) are relatively low compared to seasonal amplitudes observed in the subtropics or the subantarctic zones. However, these variations in the POOZ are large enough to cross the atmospheric level from summer to winter. Therefore, this region is neither a permanent CO 2 sink nor a permanent CO 2 source. In the POOZ, air–sea CO 2 fluxes calculated from observations are about −1.1 mmol/m 2/d in January (a small sink) and 2.5 mmol/m 2/d in August (a source). These estimates obtained for only two periods of the year need to be extrapolated on a monthly scale in order to calculate an integrated air–sea CO 2 flux on an annual basis. For doing this, we use a biogeochemical model that creates annual cycles for nitrate, inorganic carbon, total alkalinity and fCO 2. The changing pattern of ocean CO 2 summer sink and winter source is well reproduced by the model. It is controlled mainly by the balance between summer primary production and winter deep vertical mixing. In the POOZ, the annual air–sea CO 2 flux is about −0.5 mol/m 2/yr, which is small compared to previous estimates based on oceanic observations but comparable to the small CO 2 sink deduced from atmospheric inverse methods. For reducing the uncertainties attached to the global ocean CO 2 sink south of the Polar Front the regional results presented here should be synthetized with historical and new observations, especially during winter, in other sectors of the Southern Ocean.

Barium cycling along WOCE SR3 line in the Southern Ocean

Spring and summer profiles of dissolved and particulate barium (Ba) from WOCE SR3 line (145°E) in the Southern Ocean are compared and seasonal evolutions discussed. Fluxes are estimated from mass conservation equations and from differences in reservoir contents between seasons. Subtraction of barium is observed at mesopelagic depths (upper 600 m) and appears to exceed up to tenfold the combined local build-up and the deep-ocean fluxes of particulate Ba, pointing towards significant dissolution to take place in intermediate and deep waters. Although regression analyses identify silicate as the major predictor of dissolved Ba, Ba and silicate are clearly uncoupled in surface waters where Ba behaves more similar to nitrate, excluding diatoms as users of Ba. Moreover, we observe a southward decrease in the Ba vs. silicate regression slopes driven by the conditions in intermediate and deep waters and mainly marked by the location of the Polar Front. These findings corroborate existing knowledge about the predominant control by barite formation and dissolution on the oceanic Ba cycle. It suggests a decreased dissolution of barite south of the Polar Front as compared to the situation in the Polar Front Zone and the Subantarctic Zone. This is in agreement with the fact that, except for deep waters, the Antarctic Circumpolar Current water column is oversaturated with respect to barite, in contrast to the situation north of the Polar Front, where the whole water column is undersaturated [Jeandel C., B. Dupré, G. Lebaron, C. Monnin and J.F. Minster, 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. I, 43 (1), 1–31; Monnin, C., C. Jeandel, T. Cattaldo and F. Dehairs, 1999. The marine barite saturation state of the world's ocean. Mar. Chem., 65, 253–261].

Nitrogen isotope constraints on subantarctic biogeochemistry

We report nitrate (NO3−) nitrogen isotope ratios for seawater samples collected in the Subantarctic Zone of the Southern Ocean during both winter and summer as part of the Australian Antarctic CRC Subantarctic Zone (SAZ) Project. The concentration and 15N/14N of the wintertime surface nitrate are very close to those of the subantarctic thermocline. The 15N/14N of nitrate in the surface increases sharply into the summer even though there is little seasonal change in nitrate concentration. There are two possible end‐member explanations for this observation. First, there may be significant equatorward nitrate transport during the summer, including a supply from the Antarctic surface. Second, the isotope effect of algal nitrate assimilation may be higher than has been estimated elsewhere, for example, for the seasonal sea ice zone of the Antarctic. We use a simple geochemical box model of the SAZ surface mixed layer as it evolves over the course of the summer to simulate salinity, nitrate concentration, and the 15N/14N of nitrate and sinking N. Our results suggest that a significant portion (∼30%) of the summertime SAZ nitrate is supplied from south of the Subantarctic Front and that N export is ≥3.5 mmol N m−2 d−1. Our approach also identifies the necessity of an isotope effect for nitrate assimilation in the SAZ of ≥7‰ and probably 8–9‰. Comparison to laboratory results suggests that this relatively high isotope effect may result from light limitation of algal growth in the SAZ.

Testing the silica leakage hypothesis with sedimentary opal records from the eastern equatorial Pacific over the last 150 kyrs

We have measured 230Th‐normalized opal fluxes in several cores from the eastern equatorial Pacific (EEP) to test the validity of the “silica leakage” hypothesis, which purports that redistribution of silicic acid from the Southern Ocean to the low latitudes was responsible for a significant portion of the reduction in atmospheric carbon dioxide (CO2) during the last glacial period. The silica leakage hypothesis predicts higher opal fluxes in the EEP and lower opal fluxes in the Southern Ocean during periods of low atmospheric CO2. These predictions are not borne by the sedimentary record during glacial oxygen isotope stage 2 (OIS 2, 13–27 kyrs B.P.). However, we find a prominent opal flux maximum in the EEP in the middle of OIS 3 (ca. 40–60 kyrs BP) coinciding with low opal fluxes in several cores from the subantarctic zone. This observation is consistent with silica leakage from the Southern Ocean to the equatorial upwelling region during OIS 3, when both low dust flux and extended sea ice could have contributed to limiting diatom productivity in the Southern Ocean. Since this event is not associated with a clear minimum in the Vostok ice record of CO2, its impact on atmospheric CO2 appears to be small.

Atypical δ15N variations at the southern boundary of the East Pacific oxygen minimum zone over the last 50 ka

We report a nitrogen isotope record (ODP Site 1233) from the southern Chile margin at 41°S. The site is located slightly south of the southern boundary of the Peru–Chile upwelling system and the associated oxygen minimum zone off Peru and northern Chile. We show that our nitrogen isotope record, from the time interval 0–50 calendar kiloyears before present (ka B.P.), bears an atypical pattern both in shape and timing when compared with records obtained from either the continental margin of the eastern Pacific or the Subantarctic Zone (SAZ) of the Southern Ocean. The δ 15N values at Site 1233 are relatively high throughout the record, varying between 9‰ and 13‰. The major features are a pronounced δ 15N increase at the beginning of the deglaciation, a maximum from 19 to 10 ka B.P.; thereafter a large decrease during the early Holocene, and millenial scale oscillations showing an Antarctic timing. We propose that the record results from an amalgam of low-latitude and high-latitude processes. Low-latitude processes, including a stronger advection signal of heavy nitrates from the denitrifying zones off Peru and northern Chile, would explain the timing of the deglaciation rise and the heaviest values found over this interval, excluding the Antarctic Cold Reversal period. The overall differences between site 1233 and records from Peru and northwest American margins suggest however that the origin of the δ 15N signal off Chile is largely controlled by hydrologic and climatic changes in the Southern Ocean. We propose that the interplay between nutrient demand in the SAZ and latitudinal shifts of hydrologic fronts controlled both the concentrations and the isotopic signature of the remaining nitrate delivered to the Chile margin. Then, the glacial surface waters of the southern Chile margin were likely lower in nitrate concentration and bear a higher δ 15N than during interglacial periods.

The seasonal succession of zooplankton in the Southern Ocean south of Australia, part II: The Sub-Antarctic to Polar Frontal Zones

Between October 2001 and March 2002 six transects were completed at monthly intervals in the Sub-Antarctic Zone (SAZ) and Inter-Sub-Antarctic Front Zone (ISAFZ)/Polar Frontal Zone (PFZ) in the Southern Ocean south of Australia. Zooplankton were collected with a Continuous Plankton Recorder and NORPAC net and multivariate analysis was used to analyse the seasonal succession of communities. Despite strong, seasonally consistent, biogeographic differences between the SAZ and ISAFZ/PFZ, community structure in all zones was dominated by a suite of common taxa. These included the ubiquitous Oithona similis, foraminiferans and appendicularians (Core taxa), occurring in >97% of samples and contributing an average of 75% to total sample abundance, and Calanus simillimus, Rhincalanus gigas, Ctenocalanus citer, Clausocalanus brevipes, Clausocalanus laticeps, Oithona frigida, Limacina spp. and chaetognaths (Summer taxa), present in >57% of samples and occurring at seasonally high densities. Because of the dominance of the Core and Summer taxa, the seasonal succession was most clearly evident as a change in zooplankton densities. In October densities averaged <15 ind m −3, rising to 52 ind m −3 (max=92 ind m −3) in November, and subsequently increasing slowly through to January (ave=115 ind m −3; max=255 ind m −3). Densities peaked abruptly in February (ave=634 ind m −3; max=1593 ind m −3), and remained relatively high in March (ave=193 ind m −3; max=789 ind m −3). A latitudinal lag in seasonal development was observed with peak densities occurring first in the SAZ (February) and then in the ISAFZ/PFZ (March). The seasonal community succession was strongly influenced by species population cycles. The role of zooplankton in biogeochemical cycling in the SAZ and ISAFZ/PFZ was discussed in the light of past sediment trap data collected from the study area.

Determining prey distribution patterns from stomach‐contents of satellite‐tracked high‐predators of the Southern Ocean

The distribution of many cephalopod, crustacean and fish species in the Southern Ocean, and adjacent waters, is poorly known, particularly during times of the year when research surveys are rare. Analysing the stomach samples of satellite‐tracked higher predators has been advocated as a potential method by which such gaps in knowledge can be filled. We examined the viability of this approach through monitoring wandering albatrosses Diomedea exulans at their colony on Bird Island, South Georgia (54°S, 38°W) over the winters (May–July) of 1999 and 2000. At this time, these birds foraged in up to three different water‐masses, the Antarctic zone (AZ), the sub‐Antarctic zone (SAZ) and the sub‐Tropical zone (STZ), which we defined by contemporaneous satellite images of sea surface temperature. A probabilistic model was applied to the tracking and diet data collected from 38 birds to construct a large‐scale map of where various prey were captured. Robustness/sensitivity analyses were used to test model assumptions on the time spent foraging and relative catch efficiencies and to evaluate potential biases associated with the model. We were able to predict the distributions of a wide number of cephalopod, crustacean of fish species. We also discovered some of the limitations to using this type of data and proposed ways to rectify these problems.

Trophodynamics of selected mesozooplankton in the west-Indian sector of the Polar Frontal Zone, Southern Ocean

Mesozooplankton community structure and grazing impact were investigated at 15 stations in the west-Indian sector of the Polar Frontal Zone during the third dynamics of Eddie impacts on Marion’s ecosystem cruise, conducted during April 2004. An intense frontal feature, likely the convergence of the Sub-Antarctic and Antarctic Polar Fronts, was identified running in a north-eastward direction across the survey area. Total integrated chlorophyll-a (chl-a) biomass ranged from 4.15 mg m−2 to 22.81 mg m−2 and was dominated by picophytoplankton at all stations. Mesozooplankton abundances ranged from 163.84 ind m−2 to 2,478.08 ind m−2 and biomass between 6.70 mg Dwt. m−2 and 23.40 mg Dwt. m−2. The mesozooplankton community was dominated almost entirely by copepods, which contributed between 35% and 79% (mean=63%; SD=±12%) of the total numbers. The pteropoda, Limacina retroversa, contributed up to 30% (mean=10%; SD=± 8%) of the total numbers. Numerical analysis identified two distinct mesozooplankton communities separated by the intense frontal feature, namely the Antarctic and the Sub-Antarctic Zone Groups. Ingestion rates of the four numerically dominant copepod species (Calanus simillimus, Clausocalanus spp., Ctenocalanus spp. and Oithona similis) and the pteropod, L. retroversa, were estimated using the gut fluorescence technique. Total grazing impact ranged from 0.156 mg (pigm) m−2 to 2.958 mg (pigm) m−2 or between 1% and 29% of the available chl-a per day. The four copepods contributed approximately 36% of the total daily grazing impact, while the pteropod contributed to a mean of 64%, indicating that this zooplankton group may play an important role in the Southern Ocean carbon cycle. In general, the highest daily grazing impact was exhibited in the Antarctic Zone Group (mean=12% phytoplankton standing stock per day).

Fe and Zn effects on the Si cycle and diatom community structure in two contrasting high and low-silicate HNLC areas

We compared the importance of Fe, Zn and Si availability for diatom growth and silicification through microcosm enrichment experiments in two contrasting HNLC systems of the Sub-Arctic and Sub-Antarctic Pacific. The Bering Sea was characterized by low Fe and Zn concentrations (<0.2 nM) but relatively high silicic acid (5.6–15.9 μM). The addition of 0.25 nM Fe induced a 2–7-fold increase in diatom cell abundance ( Pseudo-nitzschia sp. and Cylindrotheca closterium), an increase in Chl a, biogenic silica, and particulate organic carbon and nitrogen, and a 2–3-fold decrease in the average cellular Si content. Zn had no impact on biomass parameters or diatom community structure in this region. The Sub-Antarctic Zone (SAZ) was a low Si-HNLC system, with initial silicic acid levels of 0.45 μM and Fe and Zn concentrations <0.03 nM. Si was the proximate limiting factor controlling diatom growth, followed by a secondary role for Fe on non-siliceous phytoplankton. In this region, we also found evidence for Zn-mediated changes in diatom community structure. The presence of Zn (+1 nM) shifted the community away from a large colonial pennate ( Pseudo-nitzschia sp.) towards a smaller and less silicified solitary pennate ( Cylindrotheca closterium), potentially prone to more rapid silica dissolution in the surface layer. Despite the dominance by the same two diatom genera, these two high-latitude regimes exhibited different nutrient limitation scenarios. Diatom growth in the Bering Sea was strongly Fe-limited, while the SAZ was mainly limited by Si and only secondarily by Fe.