Subantarctic Southern Ocean Research Articles

Variations of terrestrial input and marine productivity in the Southern Ocean (48°S) during the last two deglaciations

Various biomarkers (n‐alkanes, n‐alcohols, and sterols) have been studied in a piston core TSP‐2PC taken from the Southern Ocean to reconstruct the paleoenvironmental changes in the subantarctic region for the last two deglaciations. Mass accumulation rates of terrestrial (higher molecular weight n‐alkanes and n‐alcohols) and marine (dinosterol and brassicasterol) biomarkers increased significantly at the last two glacials and stayed low during interglacial peaks (early Holocene and the Eemian). These records indicate that the enhanced atmospheric transport of continental materials and the increased marine biological productivity were synchronously linked in the Southern Ocean at the last two glacials. This suggests that increased glacial dust inputs have relieved iron limitation in the subantarctic Southern Ocean. These two processes, however, were not linked at the cooling phase from the Eemian to marine isotope stage (MIS) 5d. During this period, paleoproductivity may have been influenced by the latitudinal migration of the high‐production zone associated with the Antarctic Polar Front.

Limitation of algal growth by iron deficiency in the Australian Subantarctic Region

In March 1998 we measured iron in the upper water column and conducted iron‐ and nutrient‐enrichment bottle‐incubation experiments in the open‐ocean Subantarctic region southwest of Tasmania, Australia. In the Subtropical Convergence Zone (∼42°S, 142°E), silicic acid concentrations were low (< 1.5 µM) in the upper water column, whereas pronounced vertical gradients in dissolved iron concentration (0.12‐0.84 nM) were observed, presumably reflecting the interleaving of Subtropical and Subantarctic waters, and mineral aerosol input. Results of a bottle‐incubation experiment performed at this location indicate that phytoplankton growth rates were limited by iron deficiency within the iron‐poor layer of the euphotic zone. In the Subantarctic water mass (∼46.8°S, 142°E), low concentrations of dissolved iron (0.05‐0.11 nM) and silicic acid (< 1 µM) were measured throughout the upper water column, and our experimental results indicate that algal growth was limited by iron deficiency. These observations suggest that availability of dissolved iron is a primary factor limiting phytoplankton growth over much of the Subantarctic Southern Ocean in the late summer and autumn.

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

Dimethylsulphide is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP) which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the microphysical properties of clouds relevant to climate change are sensitive to CCN density, it has been postulated that marine sulphur emissions may play a role in climate regulation. Here we examine the DMS cycle in the subantarctic Southern Ocean west of the atmospheric baseline station at Cape Grim, NW Tasmania, where a long time series of atmospheric data has been collected. The Southern Ocean is relatively free of anthropogenic sulphur emissions and thus sulphate aerosols will be mainly due to the biogenic source of DMS. An atmospheric general circulation model has been used to provide meteorological forcings under current and doubled atmospheric CO2 conditions. We have used an existing DMS production model to investigate the sensitivity of the sea-to-air flux to the simulated changes in temperature and wind speed at the ocean surface. Under doubled CO2 conditions sea-surface temperature is simulated to increase by 4 °C throughout the year. Annual mean surface wind speed is simulated to decrease by about 3%. The changes in temperature and wind speed cause a net increase of DMS transfer velocity in the range 3-11%. The increase in temperature will also increase the growth rate of phytoplankton. The annual integrated DMS flux is simulated to increase between 2% and 8% under doubled CO2 conditions. A median change of + 5% in DMS flux corresponds to an increase in the range 2-4% in cloud condensation nuclei (CCN) concentration and a perturbation to radiative forcing of — 0.29 W m-2, confirming a minor role for DMS-derived aerosols in climate regulation.

Cd/Ca in Planktonic Foraminifera of the Glacial Southern Ocean

Since the industrial revolution, 200 years ago, atmospheric carbon dioxide levels have risen by 90 ppmV, an amount equalling the natural variation between the last glacial maximum and pre-industrial values. To predict the fate and influence of anthropogenic carbon dioxide, it is vital to understand the natural controls on the glacial-interglacial variations in atmospheric composition. The biological pump is known to be an important draw-down of carbon dioxide today due to the complete removal of nutrients from the euphotic zone of most surface waters and regeneration at depth. An exception exists in the Southern Ocean today, where the surface waters have high phosphate concentrations indicating incomplete removal of the nutrients either due to very rapid upwelling rates or iron limitation. There is huge potential for glacial carbon dioxide draw-down if the efficiency of the biological pump had been enhanced in the glacial Southern Ocean. Particle proxies suggest that export productivity increased in the glacial subantarctic Southern Ocean (1) but there has been no clear evidence of a lowering of the glacial surface-water nutrients (2). fil3c in planktonic foraminifera has been found to be unreliable as a nutrient tracer in surface waters due to a number of factors such as susceptibility to isotopic fractionation during air-sea exchange (3). Cd/Ca in foraminifera, a reliable deep-water nutrient tracer, has been uncalibrated for surface waters and is often present at levels too low for determination by existing methods. We have developed a new isotope dilution, mass-spectrometric method for low-level determination of Cd/Ca in foraminifera which has allowed reliable and accurate planktonic analyses to be made. We have investigated the application of Cd/ Ca in planktonic foraminifera for the reconstruction of past surface water phosphate levels. Down-core records of Cd/Ca in G.Bulloides covering the last complete glacial cycle have been obtained from a latitudinal transect of cores spanning the present day Subtropical Convergence Zone (SCZ), at approximately 40~ and the Antarctic Polar Front (APF), at approximately 50~ in the Indian Ocean sector of the Southern Ocean. In each core there is a striking correlation between the Cd/Ca and the 5180 of G.Bulloides over the last glacial cycle indicating low surface water nutrient levels during the glacials and high nutrient levels during the interglacials. The greatest reduction in nutrient levels occurs in the subantarctic zone of the Southern Ocean, with little change south of the APF, consistent with the export productivity conclusions. There are two possible explanations for this glacial lowering of surface water nutrients. Firstly the rate of upwelling remains constant but the efficiency of nutrient removal is increased, possibly by iron fertilization due to an increased atmospheric dust fallout in glacial times. Secondly, the glacial Southern Ocean water column became stratified with a stable thermocline altowing the surface water productivity to deplete the nutrients without constant replenishment from upwelling. It is important to distinguish between these two mechanisms as each has a differing potential for glacial draw-down of atmospheric carbon dioxide. A simple box-model following some of the concepts in (4) has been run to resolve which of these two mechanisms is more consistent with the new surface water nutrient data and the existing export productivity conclusions.

Use of remote sensing and mathematical modelling to predict the flux of dimethylsulfide to the atmosphere in the Southern Ocean

An existing ecological model of DMS production has been extended and applied to the spring-summer period in the Subantarctic Southern Ocean. The model predicts that production of phytoplankton and dissolved DMS will increase during spring to reach a maximum in summer consistent with the atmospheric data collected at the Cape Grim baseline station. Archival Coastal Zone Color Scanner satellite imagery has been used to define the seasonal range in phytoplankton concentration in the study region and validate model predictions. Local measured wind and sea temperature data have been used to calculate the DMS transfer velocity which is used to compute the sea-to-air flux of DMS. The seasonal trend in predicted DMS flux is in good agreement with the flux estimates made from observations.

Independent marine and atmospheric model estimates of the sea‐air flux of dimethylsulfide in the Southern Ocean

Marine and atmospheric models have been combined with data collected at the Cape Grim atmospheric baseline station, to estimate the flux of dimethylsulfide to the atmosphere during the spring‐summer period in the Subantarctic Southern Ocean in the vicinity of the Cape Grim baseline atmospheric monitoring station. The marine model predicts that production of phytoplankton and of dissolved DMS will increase during spring to reach a maximum in summer consistent with the data on atmospheric DMS. Local wind and sea temperature data have been used to calculate the DMS transfer velocity which was used to compute the sea‐to‐air flux of DMS. Independent predictions of the DMS flux using an atmospheric model and Cape Grim data are in excellent agreement with the marine based prediction.

Ecology of the limpet Nacella delesserti (Philippi) at Marion Island in the sub-Antarctic southern ocean

The ecology of Nacella delesserti (Philippi) is described from studies of its biomass, habitats, feeding, growth and its rôle as a central prey species at sub-Antarctic Marion Island. N. delesserti is the most abundant macro-invertebrate on the shores of Marion Island. Mean densities in the intertidal boulder fields and in the subtidal zone are 936 and 158·m −2, respectively, with an overall mean of 209·m −2 and peak densities of 3866 and 1700·m −2 in the intertidal and subtidal zones. In subtidal areas where it exceeds ≈500·m −2 N. delesserti forms stacks or towers throughout the year, up to 42 being found one on top of the other. Mean biomass was 86.7g·m −2 and production 96.5g·m −2·yr −1, both an order of magnitude higher than previously described for any Antarctic or sub-Antarctic limpet. The main diet of N. delesserti is algal spores and sporelings, although it also grazes on the fronds of attached macroalgae. Loose fronds of the kelp Durvillaea antarctica (Chamisso in Choris) Hariot and other algae are also eaten and are probably important in sustaining the dense limpet populations. Both diet and habitat are very broad and unspecialized, probably related to the fact that Nacella delesserti is the only large invertebrate grazer on the shores of the island. Limpets held in cages and fed kelp fronds provided data on grazing rates, growth, and assimilation efficiency. Assimilation efficiency ranged from 0.14 in small limpets to 0.06 in large animals. N. delesserti is estimated to consume 779 g dry algal material·m −2·yr −1, and plays a major rôle in controlling algal distribution and abundance. Growth of free-living animals was calculated from the Von Bertalanffy equation. N. delesserti reaches ≈35 mm in the first year of growth, attaining 60 mm after ≈5 yr and living for a maximum of 8 to 10 yr. In the nearshore region mortality is ≈75% per annum in the first 1.5 yr, dropping to 40% in animals of 1.5 to 5 yr old and 15% in older limpets. The rolling of boulders is a major but unquantified source of mortality in the intertidal and shallow inshore areas. Size-specific predation by the fish Notothenia coriiceps (Richardson), starfish Anasterias rupicola (Verrill) and kelp gull Larus dominicus (Lichtenstein) accounts for, respectively, 15 to 30, 40, and 20% mortality per annum, and Nacella delesserti constitutes 27, 90, and 45% of their respective diets. Limpets with shell lengths between 25 and 50 mm are particularly vulnerable, those 〉50 mm attaining a relative refuge from predation. As a herbivore and prey species, N. delesserti is clearly a key species in the nearshore and intertidal zone of Marion Island.