Subantarctic Zone Research Articles

Climate from dendrochronology: latest developments and results

This review deals with the latest developments in dendroclimatology focused on climate reconstruction. It presents results from research carried out during the period 1992–2001, when both the geographical and chronological extension of tree-ring data were greatly improved. Research projects are presently being carried out in nearly all the main forest land areas of the Subarctic and Subantarctic zones, outside the traditional regions of research in North America and Europe, and about 150 tree-ring chronologies over 1000 years in length have been developed. Special attention is paid to data from Southern Europe and the Mediterranean area, where a detailed temperature reconstruction has been completed back to A.D. 970. Research carried out in Italy and France provides useful information on climate change and CO 2. The aim of this research consists of evaluating whether in the radial tree growth of recent decades, there might be a part unexplained by climate and due directly to increasing atmospheric CO 2 fertilisation. These studies show that many wooden species in various ecosystems show different responses of ring width to increasing atmospheric CO 2. Finally, new scenarios of vegetation distribution and carbon sequestering, due both to positive or negative response from the trees, are offered by studies that reconstruct the expected radial growth of trees in situations of atmospheric CO 2 doubling.

Subsurface structure of interannual temperature anomalies in the Australian sector of the Southern Ocean

A 7 year time series of austral summer expendable bathythermograph (XBT) sections between Tasmania and Antarctica is used to describe the subsurface structure of Southern Ocean interannual temperature anomalies. Comparison of the discontinuous XBT record with a continuous (weekly average) satellite‐based SST data set confirms that the XBT sampling is adequate to resolve interannual variability. Significant correlations are found between changes in surface and subsurface temperature along much of the section. In the Subantarctic zone, surface and subsurface changes are coherent because deep convection in winter homogenizes the upper 600 m, and the summer mixed layer temperature reflects the preconditioning of the previous winter. At other latitudes the regions of high correlation generally extend well beyond the depth of direct atmospheric influence (i.e., the depth of the winter mixed layer). Negative (positive) temperature anomalies at interannual timescales are coherent with poleward (equatorward) displacements of fronts. The simplest explanation for the correspondence between deep, vertically coherent temperature anomalies and the location of ACC fronts is that the observed features reflect meridional shifts of temperature gradients associated with the fronts. The link between temperature anomalies and front displacements suggests Southern Ocean sea surface temperature (SST) anomalies primarily reflect a dynamical response of the ocean to wind or other forcing rather than surface‐trapped features produced by anomalous air‐sea heat flux. The XBT record contains both interannual and longer (unresolved) period signals, which cannot be isolated in the short discontinuous XBT time series. Spectral analysis of the 17 year SST data set reveals roughly equal variability in 2–7 year and >7 year bands.

Sensitivity of δ15N of nitrate, surface suspended and deep sinking particulate nitrogen to seasonal nitrate depletion in the Southern Ocean

We report measurements of the δ15N of nitrate, suspended particulate nitrogen (PN), and sinking PN from cruises and moored sediment traps in the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia. As expected, surface water nitrate δ15N increased as nitrate was consumed during the spring/summer bloom. In contrast, the seasonal cycles of surface water suspended and sinking PN δ15N did not fit expectations from nitrate assimilation alone. Rather than increasing, the δ15N of surface suspended PN was relatively constant in the SAZ (at ∼1‰), and decreased during the summer in the PFZ (from ∼0 to ∼−4‰), most likely due to the production of low 15N PN by summertime ammonium recycling. Deep sediment trap PN δ15N also displayed seasonal decreases (from ∼4 to ∼1‰ in the SAZ, and from ∼3.5 to ∼0.5‰ in the PFZ), which correlated with PON flux magnitude. During high‐flux periods, exported PN δ15N values were close to expectations from nitrate‐based export, but low‐flux periods exhibited higher δ15N, consistent with either a reduction in the isotope effect of nitrate assimilation or more extensive isotopic alteration of the sinking material during low‐flux periods. The mass balance between net nitrate supply and exported PN that links sinking flux δ15N to nitrate utilization requires only that the annually integrated (rather than the seasonally varying) sinking flux of PN δ15N correlates with nitrate depletion. While a correlation between annually integrated sinking PN δ15N to nitrate depletion was observed in both the SAZ and PFZ, the sensitivity of sinking PN δ15N to nitrate depletion was lower than expected. Moreover, the seasonal observations raise the possibility that loss of the summertime high‐flux period represents an alternative explanation to increased nitrate utilization for the high sedimentary PN δ15N observed during glacial periods.

Deep sea sedimentary analogs for the Vostok ice core

Many applications of the Vostok ice core depend critically on the ability to make stratigraphic ties to marine records in the adjacent Southern Ocean. Here we present oxygen isotopic records from high accumulation rate sites in the South Atlantic sector of the Southern Ocean, collected for the purpose of complementing the recently extended δD record from the Vostok ice core. The combination of several planktonic foraminiferal δ18O records from northern subantarctic piston cores demonstrates that all of the millennial‐scale oscillations expressed in the Vostok ice core over the last 60 ky are also present in marine records. The observations also support the assumption that the millennial‐scale oscillations common to both marine and ice archives are synchronous, thus providing a rationale for extending the marine‐ice core comparison through the last 400,000 years, making use of a marine drilled core (ODP Site 1089). By aligning the phase of these common abrupt events, we anchor the Vostok chronology to an orbitally tuned marine sediment chronology—a refinement that allows examination of a variety of paleoclimatological issues such as the relationship between deep ocean variability and Antarctic polar climate. For example, this exercise suggests that, over at least the 4 major deglaciation events, the primary (orbital scale) changes in the chemistry and, most likely, the temperature of the deep Southern ocean were synchronous with changes in atmospheric pCO2 and polar air temperatures. We also find that the deuterium excess in the ice core resembles marine (foraminiferal) δ13C records and that the deuterium excess is synchronous with an “anomalous” foraminiferal δ18O signal (the residual between normalized versions of Vostok δD and foraminiferal δ18O). These observations demand a tight link between the Vostok isotopic record and the air‐sea interaction of the subantarctic zone.

Last glacial sea surface temperatures and sea‐ice extent in the Southern Ocean (Atlantic‐Indian sector): A multiproxy approach

On the basis of the quantitative study of diatom, radiolarian, and planktic foraminiferal assemblages, we estimated summer sea surface temperature (SSST) and sea‐ice extent at 50 sediment core localities in the Atlantic and western Indian sector of the Southern Ocean to reconstruct the last glacial environment at the GLAMAP (18 to 15 ka, equal to 21,500 to 18,000 calendar (cal) years BP) and EPILOG (19.5 to 16.0 ka, equal to 23,000 to 19,000 cal years BP) time slices. Stratigraphic identification of the time slices was accomplished by a combination of AMS 14C measurements, benthic isotope, and siliceous microfossil abundance records. While the SSST estimates reveal greater surface water cooling than reconstructed by CLIMAP [1981], reaching a maximum in the area of the present Subantarctic Zone, the sea‐ice reconstruction indicates that CLIMAP overestimated the expansion of the Antarctic sea‐ice field, especially for austral summer. During winter the sea‐ice field was expanded by 60–70% compared to the present. Last glacial summer sea surface isotherms indicate a northward shift of the zonal bands of the Antarctic Circumpolar Current and a relative expansion of the cold water realm south of the Subantarctic Front by ∼5° in latitude. This coincides with a northward displacement of the zone of enhanced biogenic silica deposition and iceberg occurrence. As a result of northward expansion of Antarctic cold waters and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the area of the present Subtropical Front. The northward displacement of Antarctic cold waters and the related deflection of Southern Ocean waters along the eastern boundary of South America may have resulted in a weakened “cold water route” across the Drake Passage. In contrast, the transport of warm and salty surface water from the Indian into the Atlantic Ocean via the “warm water route” was not blocked allowing continuous but reduced import of heat into the South Atlantic.

Mesozooplankton community structure across the Antarctic Circumpolar Current to the north of South Georgia: Southern Ocean

During four summer seasons mesozooplankton community composition and structure in relation to water mass distribution were investigated along a 735-km transect running across the Antarctic Circumpolar Current (ACC) to the north-west of the island of South Georgia, Southern Ocean. Samples were obtained each year during December–January from the top 200 m of the water column at 22 stations spaced 35 km apart. Cluster analysis revealed four station groupings that were geographically consistent with the different water masses identified on the basis of temperature and salinity properties along the transect. A Sub-Antarctic Zone (SAZ) community characterised by low overall plankton abundance was present at the northernmost end of the transect on three of the four cruises, separated from a Polar Frontal Zone (PFZ) community by the Sub-Antarctic Front (SAF). The PFZ community lay between the SAF and the Polar Front (PF) and was characterised by highest overall abundance and little interannual variability. Two Antarctic Zone (AAZ) communities were found south of the PF that, although taxonomically similar, differed in overall abundance. Although there were significant differences in nutrients (e.g. silicate) and phytoplankton (Chl a) between the different water masses, these factors only weakly correlated with plankton community structure. Copepods were the largest contributors to total abundance within all station groupings (median percentage 83–90% of total) and pteropods were also proportionately abundant in the PFZ (11%). With the exception of pteropods (≤11%) and ostracods (≤3%) all other non-copepod taxa contributed <1% to total abundance. All station groups were characterised by varying proportions of a relatively small subset of species, many of which were present throughout the transect, for example, Oithona similis, Ctenocalanus spp., Euchaeta antarctica, and Rhincalanus gigas. Others were particularly characteristic of different station groups, thus Neocalanus tonsus, Clausocalanus ingens, and Calocalanus spp. were characteristic of the SAZ with few in the PFZ and none in the AAZ. Microcalanus pygmaeus was particularly abundant in the AAZ compared to other regions, as was the polychaete, Pelagobia longicirrata. Other taxa, although widespread, tended to be typical of particular water masses, for example, Calanus simillimus and Limacina helicina in the PFZ, Calanoides acutus, appendicularians and P. longicirrata in the AAZ, and ostracods and chaetognaths in the SAZ. The close physical and biological coupling observed across the ACC confirms the frontal zones and particularly the PF as features across which community properties change in the Atlantic sector of the Southern Ocean.

A 0.55‐Ma paleotemperature record from the Subantarctic zone: Implications for Antarctic Circumpolar Current development

Estimates of summer sea surface temperatures (SSSTs) derived from planktic foraminiferal associations using the Modern Analog Technique and combined with isotopic analyses and determination of ice‐rafted debris, mirror the Pleistocene evolution of the planktic Subantarctic surface waters in the Atlantic Ocean. The SSSTs indicate that the isotherms that define the modern polar front zone and Subantarctic front, were located at more northerly latitudes (up to 7°) during most of the investigated period, which covers the past 550 kyr. Exceptions are during climatic optima in the early Holocene, at marine isotope stages (MIS) 5.5, 7.1, 7.5, 9.3, and presumably during MIS 11.3 when SSSTs exceeded modern values by 1°–5°C. The close similarity between the SSST and the Vostok temperature indicates strong regional temperature correlation. Both records show that MIS 9.3 was the warmest period during the last 420 kyr whereas SSSTs obtained for MIS 11.3 are overestimated due to strong carbonate dissolution. Spectral analysis corroborates that the initiation of warming in southern high latitudes heralds the start of deglaciation on the Northern Hemisphere.

The effect of marginal ice-edge dynamics on production and export in the Southern Ocean along 170°W

We present a synthesis of the rates of gross, new, net andprimary production along with particulate organic carbon (POC) flux at 100 m from four cruises along 1701W in the Southern Ocean. Concurrent satellite pigment data, a primary productivity model, and a nitrate mass balance are used to extrapolate daily production estimates in space and time to seasonal and annual rates. From this analysis, we gain a better understanding of the timing, magnitude and impact of the phytoplankton blooms in this region. One of the dominant features with respect to plankton biomass is the association of high chlorophyll levels with the retreat of the sea ice which begins in October just south of the Polar Front. Our primary production model and satellite pigment data suggest higher production and flux levels characterize the marginal ice zone than previous estimates. Elevatedrates of new production in ice impactedregions are restrictedto relatively short periods following retreat of the ice edge. Export is found to lag the onset of production by up to 1 month. The ratio of POC flux at 100 m to primary production when averaged over the entire season is quite high, increasing from 15% to 25% in the Subantarctic zones to 35–40% near the Polar Front andas high as 50–65% in the southernmost stations, just north of the Ross Sea gyre. Comparisons of phytoplankton community structure andFe stress indicators suggest that blooms at the Polar Front are initially dominated by large centric diatoms, but are replaced by smaller pennate diatom and non-diatom species as Fe levels decrease. Further south, where Fe levels are never as high and large diatoms are not found, we still observe relatively high biomass and elevated production rates andPOC fluxes d uring the short growing season. r 2003 Elsevier Science Ltd. All rights reserved.

Mesoscale surface distribution of biogeochemical characteristics in the Crozet Basin frontal zones (South Indian Ocean)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 249:1-14 (2003) - doi:10.3354/meps249001 Mesoscale surface distribution of biogeochemical characteristics in the Crozet Basin frontal zones (South Indian Ocean) M. Fiala1,*, B. Delille2, C. Dubreuil3, E. Kopczynska4, K. Leblanc3, J. Morvan5, B. Quéguiner3, S. Blain6, C. Cailliau6, P. Conan1, R. Corvaisier6, M. Denis3, M. Frankignoulle2, L. Oriol1, S. Roy3 1Laboratoire Arago, Université P & M Curie, UMR-CNRS 7621, 66651 Banyuls-sur-mer Cedex, France 2Unité d¹Océanographie Chimique, Institut de Physique, Université de Liège, 4000 Sart Tilman, Belgium 3Laboratoire d¹Océanographie et de Biogéochimie, Université de la Méditerranée, UMR-CNRS 6535, Parc Scientifique et Technologique de Luminy, Case 901, 13288 Marseille Cedex 9, France 4Department of Antarctic Biology, Polish Academy of Sciences, Ustrzycka 10, 02141 Warszawa, Poland 5Ecole Normale Supérieure de Chimie de Rennes, Avenue du Général Leclerc, 35700 Rennes-Beaulieu, France 6Laboratoire des Sciences de l¹Environnement Marin, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, UMR-CNRS 6539, 29280 Plouzané, France *Email: mfiala@obs-banyuls.fr ABSTRACT: A mesoscale study was conducted in January and February 1999 in the Crozet Basin frontal zones (43°50¹ to 45°20¹S; 61°00¹ to 64°30¹E) within the southernmost and easternmost convergence area of the Antarctic Circumpolar Current (ACC) and the Agulhas Return Current (ARC). Distribution of biogeochemical parameters was strongly linked to the merged Subtropical (STF) and Subantarctic (SAF) Fronts which mark the border between the cold and less saline subantarctic waters and the warm and more saline subtropical waters. This survey took place during a post-bloom period. Chlorophyll a concentrations were low throughout the study area ranging from 0.2 µg l-1 in the Polar Frontal Zone (PFZ) to 0.4 µg l-1 in the Subtropical Zone (STZ). Maximum chlorophyll a values (0.8 µg l-1) associated with an increase in biogenic silica concentration (from 0.03 to 0.34 µM) and a diatom peak (1.2 x 105 cells l-1) were encountered in the northeastern part of the STF edge. Despite northwardly decreasing concentrations of nitrates from 14 µM in the PFZ to 6 µM in the STZ, they were not the main factor limiting phytoplankton growth. Low silicic acid (mean = 0.6 µM) could have limited diatom development in the PFZ and the STZ where diatom numbers were low. In STZ waters, where average diatom numbers were highest, various species of Nitzschia and Thalassiothrix were common, but Pseudonitzschia spp. were dominant. Throughout the survey area, pico- and nano-sized cells dominated the phytoplankton assemblage, and their number was the highest in the STZ. Cyanobacteria, only present in subtropical waters >12.5°C, were the major component of the picoplankton size-fraction. While dinoflagellate numbers were low in the Subantarctic Zone (SAZ), their abundance and species numbers increased in the STZ, where Oxytoxum laticeps became dominant and several further large-size species of Prorocentrum, Ceratium and Gymnodinium appeared in addition to those at the STF. The distribution of different biogeochemical parameters suggests that the Crozet Basin frontal region is a non-exporting system at the end of summer. During this post-bloom period, biological activity is low and phytoplankton growth severely limited. This is evidenced by the weak dependence of the partial pressure of carbon dioxide (pCO2) on biological activity and the importance of the air-sea exchange in maintaining pCO2 close to saturation. KEY WORDS: Frontal zones · Nutrients · Biogenic silica · Chlorophyll a · pCO2 · Phytoplankton · Bacteria Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 249. Online publication date: March 10, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

N uptake conditions during summer in the Subantarctic and Polar Frontal Zones of the Australian sector of the Southern Ocean

N uptake data obtained from 15N tracer techniques were surveyed in surface waters of the Subantarctic and Polar Frontal Zones southwest of Tasmania during summer 1998. The N flux rates and f ratio were computed using inverse modeling. The approach provides a general framework for the handling of random and systematic variations. Furthermore, factorial experiments based on controlled ammonium additions were used to study the sensitivity of the f ratio relative to perturbations of the regenerated nutrient supply. This was achieved in distinct regions: the Subtropical Convergence Zone, the Subantarctic Zone, the Subantarctic Front, and the Polar Frontal Zone. Overall, the stability of the f ratio appeared closely related to the availability of both dissolved iron and ammonium. The effect of one factor depended on the level of the other one, and the interaction influenced the f ratio patterns. The relationship between f ratio and ammonium supplementation ranged from linear to convex curves, with the steepest slopes in water masses rich in dissolved iron and with low ammonium content. The magnitude of this variation may have important ecological and geochemical consequences. Our results suggest that only versatile systems are able to exhibit events of high new production.

Involvement of polyamines in root development at low temperature in the subantarctic cruciferous species Pringlea antiscorbutica

Polyamine involvement in root development at low temperature was studied in seedlings of Pringlea antiscorbutica R. Br. This unique endemic cruciferous species from the subantarctic zone is subjected to strong environmental constraints and shows high polyamine contents. In the present study, free polyamine levels were modified by inhibitors of polyamine biosynthesis (D-arginine, difluoromethylornithine, cyclohexylammonium, and methylglyoxal-bis-guanylhydrazone) and variations of the endogenous pools were compared to changes in root growth. The arginine decarboxylase pathway, rather than that of ornithine decarboxylase, seemed to play a major role in polyamine synthesis in Pringlea antiscorbutica seedlings. Root, but not shoot, phenotypes were greatly affected by these treatments, which modified polyamine endogenous levels according to their expected effects. A positive correlation was found between agmatine level and growth rate of the primary root. Spermidine and spermine contents also showed positive correlations with primary root growth whereas the putrescine level showed neutral or negative effects on this trait. Free polyamines were therefore found to be differentially involved in the phenotypic plasticity of root architecture. A comparison of developmental effects and physiological concentrations suggested that agmatine and spermine in particular may play a significant role in the control of root development.

Middle Eocene–Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of Site 1090, Agulhas Ridge

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle–Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between ∼78 and ∼71 m composite depth extending from the Early Miocene to the latest Miocene–Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene–Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene–Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.

Ekman Transport Dominates Local Air–Sea Fluxes in Driving Variability of Subantarctic Mode Water

Subantarctic Mode Water (SAMW) is formed by deep convection in winter on the equatorward side of the Antarctic Circumpolar Current. Observations south of Australia show that the SAMW temperature (T) and salinity (S) vary significantly from year to year. The magnitude and density-compensating nature of the temperature and salinity changes cannot be explained by variations in air–sea exchange of heat and freshwater in the subantarctic zone where SAMW is formed. Rather, the T and S variability reflects variations in the equatorward Ekman transport of cool, low salinity water across the subantarctic front. Experiments with a coupled climate model suggest that the observations south of Australia are typical of the subantarctic zone. The model changes in SAMW properties are correlated significantly (at 99% level) with changes in wind stress and northward Ekman transport of cool low-salinity water. In contrast, air–sea heat flux anomalies are mostly a response to changes in SST, and anomalies in precipitation minus evaporation in the subantarctic zone are too small to account for the model SAMW salinity variations. Mode waters provide significant reservoirs of heat and freshwater that extend below the depth of the seasonal thermocline and, hence, can persist from year to year. The fact that wind stress variations can drive changes in mode water properties therefore has implications for climate variability.

Past hydrographic and climatic changes in the Subantarctic Zone of the South Atlantic – The Pleistocene record from ODP Site 1090

Pleistocene summer sea-surface temperatures (SSST) have been reconstructed on a composite core section recovered in the Subantarctic Zone of the Southern Ocean from planktonic foraminifers applying the Modern Analog Technique. The composite consists of Core PS2489-2 and the sections recovered at ODP Site 1090, and documents the last 1.83 Ma. Three distinct climatic periods can be identified that mirror the Pleistocene development of the Southern Ocean hydrography. Cold climatic conditions prevailed at 43°S during glacial as well as during interglacial periods during the early Pleistocene (1.83–0.87 Ma), indicating a northward shift of isotherms that characterize the present-day Polar Front Zone by about 7° of latitude. Evidence shows a strong linkage between Southern Ocean and low latitude climate during that interval time. Between the Mid-Pleistocene Revolution (ca. 0.9 Ma) and the Mid-Brunhes Event (ca. 0.4 Ma), we observe higher amplitude fluctuations in the SSST between glacial and interglacial periods, corresponding to the temperature range between the present Polar Front and Subantarctic Front. These climatic variations have been related to changes in the northern hemisphere ice sheets. The past 0.4 Ma are characterized by strong SSST variations, of up to 8°C, between glacials and interglacials. Only during the climatic optima (stages 11.3, 9.3, 7.5, 7.1, 5.5, and the early Holocene), SSST exceeded present SSST at the core locality (10.2°C). Although the carbonate dissolution record exhibits high variability during the Pleistocene, it can be shown that SSST estimates were not significantly biased. The Mid-Brunhes dissolution cycle as well as the Mid-Pleistocene enhanced carbonate preservation appear to belong to a global long-term variability in carbonate preservation.

The Hepaticae and Anthocerotae of the subantarctic and temperate islands in the eastern Southern Hemisphere (90°E to 0°): an annotated catalogue

On the subantarctic and temperate islands of the eastern Southern Hemisphere (90°E to 0°) 66 Hepaticae and two Anthocerotae species are recorded, among them ten are recorded for the first time. The reports of a further 18 are rejected. Fifteen species and five variety names are newly synonymized. Nineteen species and two subspecies are regarded as endemics. Among the recorded species, 75% are only or mainly known from the Subantarctic Zone. Species otherwise known also from southernmost South America predominate, contrary to its geographical distance, whereas those otherwise known only from the closer Africa are weakly represented and those known only from Australasia are almost lacking.

Physical controls on biogeochemical zonation in the Southern Ocean

The primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilizatio

Physical and biological patchiness of an upper ocean transect from South Africa to the ice edge near the Greenwich Meridian

An undulating CTD (SeaSoar) was towed from the Subtropical Front at 42°S, 12°E to the ice edge at 57.5°S, 2°W. The instrument measured temperature, salinity, pressure, and light in the top 400 m of the water column. Fluorescence and particle counts also were recorded. Fluorescence was calibrated to chlorophylla, and particle counts were converted to biovolume to provide estimates of phyto- and zooplankton, respectively. The size of particles counted covered the range 0.25–8.0 mm. Three major fronts, Subtropical, Subantarctic, and Polar were intersected and identified by their physical characteristics. The Polar Front showed surface and subsurface signatures separated by nearly 300 km. Considerable small-scale structure was observed between the fronts, in particular a large (150-km diameter), warm-core eddy was found in the northern part of the Subantarctic Zone. Both fronts and eddy were associated with significant biological signals. Phyto- and zooplankton correlated well in the frontal regions and south of the Polar Front. The Polar Frontal Zone was associated with elevated concentrations of biomass, whereas concentrations were low at the Subantarctic Front and south of 55°S. In contrast, the warm-core eddy in the Subantarctic Zone contained the highest zooplankton biovolume of the section while chlorophyll concentration was only moderate. We hypothesise that such a distribution results from grazing. The overall distribution of phyto- and zooplankton changed across the Subantarctic Front. The vertical extent of biomass deepened from north to south across the front. Zooplankton tended to be concentrated in relatively narrow, deep-extending bands to the north but were more widely spread to the south. The small-scale hydrographic structure correlated significantly with the distribution of plankton, suggesting strong physical controls on both small zooplankton and phytoplankton.

Modeling the upper ocean dynamics in the Subantarctic and Polar Frontal zones in the Australian sector of the Southern Ocean

A one‐dimensional (1‐D) mixed layer model (the Chen scheme) was applied in the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) to simulate the upper ocean dynamics. The model was forced with 4 years data of the heat fluxes, freshwater fluxes, and wind stresses from the National Centers for Environmental Prediction. In both the SAZ and PFZ, the 1‐D model was capable of reproducing the amplitude of the seasonal sea surface temperature (SST) and the seasonally of the mixed layer depth (MLD). The shallowest MLD was found in January‐February (20 m in the SAZ, 35 m in the PFZ), and the deepest MLD was found between August and October (600 m in the SAZ, 160 m in the PFZ). The summer MLD was shallower in the SAZ than in the PFZ due to the lower wind stress. However, the shallower winter MLD in the PFZ than in the SAZ was due to the strong stratification in the water below the mixed layer. In the SAZ, variability in the wind stress was the dominant term driving the fluctuation in MLD in the summer, but variability in the heat flux was the major factor controlling the timing of the deepening and shoaling of the mixed layer in the winter. In the PFZ both the variability in the wind stress and the heat flux dominated the variability of the MLD in both the summer and the winter.

Seasonal evolution of the mixed layer in the Subantarctic zone south of Australia

The circumpolar Subantarctic Zone (SAZ) is a globally significant region of water mass formation and carbon dioxide uptake from the atmosphere. Here we synthesize the results of nine voyages over 8 years to describe the seasonal variation in mixed layer properties in the SAZ south of Australia for comparison with biogeochemical process studies carried out in late summer (March 1998) as part of the SAZ Project. Winter mixing extends to depths &gt;400 m, resulting in the formation of Subantarctic Mode Water. In summer the mixed layer shoals to 75–100 m, depths which are still sufficiently deep that phytoplankton growth may be light limited. Nitrate and phosphate concentrations are reduced in summer (e.g., nitrate decreases from &gt;15 to &lt;5 μmol kg−1) but remain well above limiting levels. Silicate, in contrast, is low throughout the year (4 μmol kg−1 in winter and &lt;2 μmol kg−1 in summer). Water mass properties along a north‐south hydrographic section in March 1998 suggest that near‐surface waters spread from south to north across the Subantarctic Front (SAF), supplying cool, fresh, nutrient‐rich water to the SAZ. As a consequence, the properties of the southern SAZ differ from those farther north: the mixed layer in the south is cooler, fresher, deeper, higher in nutrients, and bounded below by a halocline (rather than by a seasonal thermocline, as in the northern SAZ). The contrast between the northern and southern SAZ persists throughout the year, suggesting the cross‐front exchange occurs year‐round and likely contributes to the differences in seasonal thermal amplitude and algal biomass accumulation seen in satellite images. Density‐compensated horizontal gradients of temperature and salinity are common in the mixed layer of the SAZ and the northern SAF, consistent with the hypothesis that the vigor of lateral mixing in the mixed layer is a strong function of the magnitude of the lateral density gradient.

Seasonal nutrient depletion and carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean south of Australia

Seasonal surface water nitrate and phosphate depletions were calculated for the Subantarctic Zone (SAZ) and Polar Frontal Zone (PFZ) of the Southern Ocean along the World Ocean Circulation Experiment SR3 repeat section south of Tasmania, Australia, and three other transects slightly to the west. The depletions were calculated between a single winter (July) section and eight other sections from October to March in the years 1991‐1998 and corrected for frontal movements and changes in water masses between the cruises using observed quasi‐linear nutrient‐salinity relationships. Arranged in a seasonal progression, depletion began in approximately October and continued through March, in good agreement with satellite biomass‐based estimates of production. Nitrate depletion in the SAZ (510 mmol N m−2) was approximately twice that of the PFZ (250 mmol N m−2), but phosphate depletions were similar (29 and 24 mmol P m−2 for the SAZ and PFZ, respectively). The SAZ exhibited a Redfield N/P depletion ratio (15.1±2.3), but the PFZ ratio was much lower (8.3±5.4). While the SAZ exhibits greater seasonal nutrient depletion, the relative magnitudes of export in the two zones remain uncertain because supply of nutrients from south of the Polar Front may lower the observed nutrient depletions in the PFZ. However, comparisons with sediment trap and production measurements for the region south of Australia suggest the SAZ and PFZ differences derive from differences in production. Assuming a Redfield C/N ratio (6.6), carbon export for the longest period examined (July‐March) averaged 3400±430 mmol C m−2 in the SAZ, with a range between 2700 and 3900 mmol C m−2 depending on latitude. In the PFZ, carbon export averaged 1600±350 mmol C m−2, with a range of 1100 to 2000 mmol C m−2. These averages represent minimum annual export production estimates because neither resupply nor export outside the July‐March observational period is included.