Subantarctic Southern Ocean Research Articles

High-resolution view of the spring bloom initiation and net community production in the Subantarctic Southern Ocean using glider data

AbstractIn the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

Sea surface temperature variability in the Pacific sector of the Southern Ocean over the past 700 kyr

In spite of the important role played by the Southern Ocean in global climate, the few existing paleoceanographic records in the east Pacific sector do not extend beyond one glacial‐interglacial cycle, hindering circumpolar comparison of past sea surface temperature (SST) evolution in the Southern Ocean. Here we present three alkenone‐based Pleistocene SST records from the subantarctic and subtropical Pacific. We use a regional core top calibration data set to constrain the choice of calibrations for paleo SST estimation. Our core top data confirm that the alkenone‐based UK37 and UK′37values correlate linearly with the SST, in a similar fashion as the most commonly used laboratory culture‐based calibrations even at low temperatures (down to ∼1°C), rendering these calibrations appropriate for application in the subantarctic Pacific. However, these alkenone indices yield diverging temporal trends in the Pleistocene SST records. On the basis of the better agreement withδ18O records and other SST records in the subantarctic Southern Ocean, we propose that the UK37 is a better index for SST reconstruction in this region than the more commonly used UK′37 index. The UK37‐derived SST records suggest glacial cooling of ∼8°C and ∼4°C in the subantarctic and subtropical Pacific, respectively. Such extent of subantarctic glacial cooling is comparable to that in other sectors of the Southern Ocean, indicating a uniform circumpolar cooling during the Pleistocene. Furthermore, our SST records also imply massive equatorward migrations of the Antarctic Circumpolar Current (ACC) frontal systems and an enhanced transport of ACC water to lower latitudes during glacials by the Peru‐Chile Current.

Interannual pteropod variability in sediment traps deployed above and below the aragonite saturation horizon in the Sub-Antarctic Southern Ocean

Anthropogenic inputs of CO2 are altering ocean chemistry and may alter the role of marine calcifiers in ocean ecosystems. Laboratory research and ocean models suggest calcifiers in polar waters are especially at risk, particularly pteropods: pelagic aragonite-shelled molluscs. However, baseline data for natural populations of pteropods are limited, especially for polar and sub-polar waters. In order to establish baseline data on diversity, preservation state and shell flux of in situ populations of Sub-Antarctic Southern Ocean pteropods, we deployed sediment traps above (1,000 m) and below (2,000 m) the aragonite saturation horizon (ASH) (currently at 1,200 m) from 1997 to 2006 at 47°S, 142°E. We identified seven pteropod taxa. We applied a shell opacity index to each shell collected and found 50% of shells collected above the ASH to be in pristine condition but only 3% of the shells collected below the ASH showed such a high degree of preservation. We estimated pteropod shell mass fluxes for the region (0.17–4.99 mg m−2 day−1), and we identified significant reductions in shell flux for Limacina helicina antarctica forma rangi and Clio recurva to the trap series above the ASH and for Limacina helicina antarctica forma rangi and Limacina helicina antarctica forma antarctica to the trap series below the ASH over the interval 1997–2006. Our data establish a temporal and vertical snapshot of the current Sub-Antarctic pelagic pteropod community and provide a baseline against which to monitor Southern Ocean pteropods responses, if any, to changing ocean conditions projected for the region in the coming decades.

Grazing suppression of dimethylsulphoniopropionate (DMSP) accumulation in iron-fertilised, sub-Antarctic waters

The impact of in situ iron fertilisation on the production of particulate dimethylsulphoniopropionate (DMSPp) and its breakdown product dimethyl sulphide (DMS) was monitored during the SOLAS air–sea gas exchange experiment (SAGE). The experiment was conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-Antarctic Southern Ocean (46.7°S 172.5°E) to the south-east of New Zealand, during March–April, 2004. In addition to monitoring net changes in the standing stocks of DMSPp and DMS, a series of dilution experiments were used to determine the DMSPp production and consumption rates in relation to increased iron availability. In contrast to previous experiments in the Southern Ocean, DMS concentrations decreased over the course of the 15-d iron-fertilisation experiment, from an integrated volume-specific concentration in the mixed layer on day 0 of 0.78 nM (measured values 0.65–0.91 nM) to 0.46 nM (measured values 0.42–0.47 nM) by day 15, in parallel with the surrounding waters. DMSPp, chlorophyll a and the abundance of photosynthetic picoeukaryotes exhibited indiscernible or only moderate increases in response to the raised iron availability, despite an obvious physiological response by the phytoplankton. High specific growth rates of DMSPp, equivalent to 0.8–1.2 doublings d −1, occurred at the simulated 60% light level of the dilution experiments. Despite the high production rates, DMSPp accumulation was suppressed in part by microzooplankton grazers who consumed between 61% d −1 and 126% d −1 of the DMSPp production. Temporal trends in the rates of production and consumption illustrated a close coupling between the DMSP-producing phytoplankton and their microzooplankton grazers. Similar grazing and production rates were observed for the eukaryotic picophytoplankton that dominated the phytoplankton biomass, partial evidence that picoeukaryotes contributed a substantial proportion of the DMSP synthesis. These rates for DMSPp and picoeukaryotes were considerably higher than for chlorophyll a, indicating higher cycling rates of the DMSP-producing taxa than for the bulk phytoplankton community. When compared to the total phytoplankton community, there was no evidence of selection against the DMSP-containing phytoplankton by the microzooplankton grazers; the opposite appeared to be the case. SAGE demonstrated that increased iron availability in the HNLC waters of the Southern Ocean does not invariably lead to enhanced DMS sea–air flux. The potential suppression of DMSPp accumulation by grazers needs to be taken into account in future attempts to elevate DMS emission through in situ iron fertilisation and in understanding the hypothesised link between levels of Aeolian iron deposition in the Southern Ocean, DMS emission and global albedo.

Organic‐rich mud on the western margin of southern Africa: Nutrient source to the Southern Ocean?

The biological pump plays a major role in the transfer of CO2 from the atmosphere to the deep Southern Ocean, a transfer which is largely controlled by the supply of iron and which may partially explain glacial to interglacial variations in pCO2. Analogous to the well‐documented, smaller‐scale “island mass effect,” we propose that the lateral advection of iron by south flowing intermediate waters along the southern African margin may sustain high‐productivity blooms of the Subtropical Convergence Zone (SCZ) between 10 and 70°E. We assess the present‐day interglacial (Holocene) reservoirs and fluxes of organic carbon (OC) and terrigenous mud on the western margin of southern Africa in order to estimate the potential supply of Fe to the Southern Ocean. The highly productive Benguela Upwelling System (BUS) appears to be a relatively inefficient coastal biological pump. Repeated sediment resuspension by wave and tidal energy dissipation limits OC burial to <0.2% of net primary production (NPP) in the southern BUS and to between 0.2 to 2.4% in the northern BUS. Productivity and OC‐rich mud accumulation are focused on the inner portion of the 100–200 km wide shelf which, combined with south flowing bottom currents, limits the export of OC beyond the shelf break to 1.2–8.4% of NPP. However, winnowing of 1 million tons yr−1 of clay particles and the potential early diagenetic benthic (dissolved) Fe flux may supply 10 times more Fe than is transported by dust to the open ocean biological pump of the SCZ. Lowering sea level during glacial periods disperses interglacial mud deposits off the shelf and increases particulate Fe export by as much as a factor of 4. Glacial pulses of margin export may enhance the efficiency of the subantarctic Southern Ocean biological pump and contribute to the initial as well as glacial maximum drawdown in pCO2.

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north-south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol(-1) was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

Conservation and management frameworks for sub-Antarctic international waters

The waters of rhe sub-Antarctic Southern Ocean are legally distinctive from those which fall within the Antarctic Treaty area extending to 60 degrees South, not only because the Treaty does not apply in those waters but also because sovereignty over sub-Antarctic islands is largely uncontested. As a result, many of the countries claiming sub-Antarctic possessions are able to assert the full range of maritime claims, resulting in sub-Antarctic waters being a mixture of state-controlled waters and those which fall under either international fisheries regimes such as the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) or are high seas. Conservation and management of sub-Antarctic waters other than with respect to fishing have not received the same priority as they have in waters around the continent. However, the same issues arise and are in fact magnified due to the greater accessibility of the northern reaches of the Southern Ocean and potential for greater environmental and resource impact. The extent of the international waters of the Sub-Antarctic Southern Ocean is clarified here, the applicable international management regimes are reviewed and the contemporary issues confronting the region are considered. Proposals are considered for enhanced internationalisation of the region.

Control of the Antarctic ice sheet by ocean–ice interaction

The Antarctic ice cap is the largest ice sheet of modern times. It is of considerable importance to predict the sea level variability due to the associated changes in ice volume. We present the results of a simple grounded ice sheet model, developed from Oerlemans [Oerlemans, J., 2002. Global dynamics of the Antarctic Ice Sheet, Climate Dynamics 19, 85–93.], in which the net oceanic evaporation influences the ice cap volume in two ways, through changes in: (i) the accumulation rate, and (ii) the mean sea level. The net evaporation changes are driven by the sea surface temperature (SST) anomaly time series of Howard [Howard, W.R., 1997. A warm future in the past, Nature, 388, 418–419.] for the subantarctic Southern Ocean over the period 220 kyr to the present. The effect of the waxing and waning of the northern hemisphere ice sheets is integrated into the model using an independent model, in which ice melting depends on the SST anomaly and ice calving depends on the sea level anomaly. A series of analytical expressions are derived for the related properties of the coupled ocean–ice system applicable over time scales of 100 kyr, which show, in particular, that the Antarctic ice cap volume changes are due mainly to the effects of the northern hemisphere ice sheets on sea level (which influences ice calving), rather than directly to changes in SST, and hence the ice cap volume is greatest during interglacial periods. This conclusion, which is independent of the specification of the ice melting regime for the northern hemisphere ice sheets, strongly suggests that the changes in accumulation flux estimated from the Vostok proxy temperature data and used in other studies of the Antarctic mass balance have been overestimated. A simple expression is also presented for the lag of ice cap volume to SST, and it is found that the predictions for the mean sea level variability are similar to observations for a melting flux of the northern hemisphere ice sheets about twice their accumulation flux due to the net oceanic evaporation, except during major deglaciations when these two fluxes appear to be of similar magnitude.

Mid-latitude Southern Indian Ocean response to Northern Hemisphere Heinrich events

The lack of temporal resolution and accurate chronology of Southern Ocean marine cores has hampered comparison of glacial millennial-scale oscillations between the Southern Ocean, Antarctic ice and other records from both hemispheres. In this study, glacial climate variability is investigated over the last 50 ka using a multi-proxy approach. A precise chrono-stratigraphy was developed on the high-sedimentation rate core MD94-103 (Indian Southern Ocean, 45°35′S 86°31′E, 3560 m water depth) by geomagnetic synchronization between the later core and NAPIS75, and 14C dates. High-resolution time-series of δ18O in planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma, and sea surface temperatures (SSTs) estimated from the alkenone UK′37 index and foraminifera assemblages have been generated. Temporal evolution of the two temperature proxy records is notably different during the last glacial period. While foraminifera data indicate a consistent cooling towards the last glacial maximum, anomalous warm glacial alkenone temperatures suggest a strong advection of 〈warm〉 “detrital” alkenones by surface waters of the Agulhas current. Superimposed to this general trend, during Heinrich events, foraminiferal SSTs point to warmer surface waters, while concurrent alkenone SSTs exhibit apparent coolings probably caused by enhanced local alkenone production. By analogy to modern observations, possible influence of ENSO-like conditions on the subantarctic Southern Ocean SSTs is discussed.

Middle Eocene to early Miocene environmental changes in the sub-Antarctic Southern Ocean: evidence from biogenic and terrigenous depositional patterns at ODP Site 1090

During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation. The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates (≤2 cm ka −1). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm ka −1) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient- and silica reservoir established during the pre-late Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation. The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene.

The sensitivity of dimethyl sulfide production to simulated climate change in the Eastern Antarctic Southern Ocean

Dimethyl sulfide (DMS) is a radiatively active trace gas produced by enzymatic cleavage of its precursor compound, dimethyl sulfoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. Once ventilated to the atmosphere, DMS is oxidised to form non-sea-salt sulfate and methane sulfonate (MSA) aerosols, which are a major source of cloud condensation nuclei (CCN) in remote marine air and may thus play a role in climate regulation. Here we simulate the change in DMS flux in the Eastern Antarctic ocean from 1960–2086, corresponding to equivalent CO 2 tripling relative to pre-industrial levels. Calibration to contemporary climate conditions was carried out using a genetic algorithm to fit the model to surface chlorophyll from the 4-yr SeaWiFs satellite archive and surface DMS from an existing global database. Following the methodology used previously in the Subantarctic Southern Ocean, we then simulated DMS emissions under enhanced greenhouse conditions by forcing the DMS model with output from a coupled atmospheric–ocean general circulation model (GCM). The GCM was run in transient mode under the IPCC/IS92a radiative forcing scenario. By 2086, the change simulated in annual integrated DMS flux is around 20% in ice-free waters, with a greater increase of 45% in the seasonal ice zone (SIZ). Interestingly, the large increase in flux in the SIZ is not due to higher in situ production but mainly because of a loss of ice cover during summer–autumn and an increase in sea-to-air ventilation of DMS. These proportional changes in areal mean flux (25%) are much higher than previously estimated for the Subantarctic Southern Ocean (5%), and point to the possibility of a significant DMS–climate feedback at high Southern latitudes. Due to the nexus between ice cover and food-web structure, the potential for ecological community shifts under enhanced greenhouse conditions is high, and the implications for DMS production are discussed. DOI: 10.1034/j.1600-0889.2003.00077.x

Temporal variations in the diet of the squid Moroteuthis ingens at Macquarie Island: stomach contents and fatty acid analyses

The onychoteuthid squid Moroteuthis ingens was collected as by-catch from a commercial trawl fishery in the vicinity of Macquarie Island, within the Pacific sector of the Southern Ocean. Squid were collected during 3 austral summers and 1 austral winter between January 1995 and June 2000. Stomach contents and fatty acid profiles of both mantle and digestive gland tissues were used to determine if the diet of M. ingens was subject to temporal variations in this region. Discriminant analysis (DA) of stomach contents data and digestive gland fatty acid data indicated that the diet varied significantly on an interannual basis, most likely due to an increase in the consumption of the myctophid Gymnoscopelus nicholsi during the summer of 1999 relative to the summers of 1995 and 2000. Comparisons with oceanographic data reveal that the summer of 1999 was a period of warmer sea-surface temperatures and reduced primary production compared to the summers of 1995 and 2000. Fluctuations in oceanographic conditions may have underpinned variations in the availability of prey during the study period. DA of digestive gland fatty acid data also indicated that the diet varied significantly between the summer and winter of 2000. Stomach contents data indicate that the myctophid Electrona carlsbergi replaced Krefftichthys anderssoni as a key prey species for M. ingens during the winter period, as has been observed for other marine predators in the sub-Antarctic Southern Ocean. A comparison of methods reveals that, while fatty acid analyses greatly aid the interpretation of stomach contents data, both techniques are subject to limitations and are best used in combination.

Coupling between cycles of phytoplankton biomass and aerosol optical depth as derived from SeaWiFS time series in the Subantarctic Southern Ocean

The Subantarctic Southern Ocean is a high‐nutrient low‐chlorophyll region, and it has been suggested that primary production is limited by deep mixing and the availability of iron. Australian dust is high in iron content and can be transported over the Subantarctic Southern Ocean, particularly during the austral spring and summer when dust storm frequency in southern Australia is maximal. We present evidence for a coupling between satellite‐derived (SeaWiFS) aerosol optical thickness and chlorophyll concentration in the upper ocean. The coupling is evident at monthly, weekly and daily timescales. Although the monthly coherence is likely to be due to other covarying factors, the coupling at weekly and daily timescales supports the hypothesis that the episodic atmospheric delivery of iron is stimulating phytoplankton growth. We also discuss the impact of oceanic dimethylsulfide production on aerosol concentration in the study region.

Quantification of algal iron requirements in the Subantarctic Southern Ocean (Indian sector)

Abstract Shipboard iron-addition incubation experiments were carried out in the Indian sector of the Subantarctic Southern Ocean during the Antares -IV campaign in late January–February 1999. The aim of these experiments was to estimate the dissolved iron requirements of the native phytoplankton community in this oceanic region, in order to improve the parameterisation of iron as a limiting nutrient in a coupled 1D physical–biogeochemical ocean model. The experiments were conducted with plankton collected from the upper water column (∼20 m depth) at three sites in the Crozet Basin between 43–46°S and 61–65°E: (1) the Polar Front Zone (PFZ, dissolved Fe=0.33 nM), (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF, dissolved Fe=0.29 nM), and (3) the southern Subtropical Zone (STZ, dissolved Fe=0.09 nM). Experimental results from each site indicate that algal community growth rates varied as a function of added iron concentration. Monod saturation functions fitted to the experimental data yield estimates for the community half-saturation constant for growth with respect to iron ( K μ ) of 0.41–0.45 nM Fe (PFZ), 0.055–0.086 nM Fe (SAF/STF) and 0.092–0.093 nM Fe (STZ, with macronutrients added), each of which has an estimated uncertainty of ±20%. The K μ estimate for the SAF/STF site reflects the mixed algal assemblage (diatoms+nanoplankton+dinoflagellates) that grew in the experimental incubations, whereas the K μ estimates for the PFZ and STZ sites probably reflect the Fe requirements of the small pennate diatoms such as Pseudo-nitzschia spp., which dominated the algal biomass produced in these experiments. The fact that there are significant differences between the K μ estimates for the PFZ and STZ sites suggests that similar diatom assemblages may have quite different iron requirements, perhaps due to differences in environmental conditions (e.g., light, macronutrient levels). We also examine the sensitivity of a one-dimensional coupled physical–biogeochemical model to the choice of K μ for iron, using time-series observations from the KERFIX station close to the Polar Front. The model was best able to simulate the KERFIX observations using a diatom K μ value of ∼0.1 nM Fe, which is considerably less than our experimental estimate of ∼0.4 nM Fe for the PFZ. This discrepancy probably reflects differences in the iron requirements of diatom populations immediately north and south of the Polar Front in the Kerguelen region, due to differences in diatom species composition, availability of light and silicic acid, or the environmental and physiological histories of the diatom populations.

Resource limitation of phytoplankton growth in the Crozet Basin, Subantarctic Southern Ocean

In January–February 1999, we performed shipboard iron- and macronutrient-addition experiments in the Crozet Basin, Indian sector of the Subantarctic Southern Ocean, to evaluate the sufficiency of ambient iron and macronutrient concentrations for algal growth. Experiments were conducted with near-surface seawater collected from three locations in a narrow latitudinal band characterized by relatively low algal biomass (o0.7m gl � 1 chlorophyll a), low dissolved iron concentrations (o0.33 nM), and strongmeridional g radients in temperature, salinity and macronutrient concentrations: (1) the Polar Frontal Zone (PFZ) near 461S, 651 E( B19mM nitrate and 1.2mM silicic acid); (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF) near 44112 0 S, 63123 0 E( B5.4mM nitrate and 0.5mM silicic acid); and (3) the southern Subtropical Zone (STZ) near 43118 0 S, 62131 0 E( o0.1mM nitrate and B1.4mM silicic acid). Our experimental results reveal three distinct regimes of resource limitation of phytoplankton growth. In the PFZ, iron availability exerted the primary limitation on nitrate drawdown and biomass accumulation, thus community growth, with silicic acid availability exerting a secondary limitation on diatom growth and biogenic silica production. Within the SAF/STF, iron deficiency was also the primary limitation on algal community growth; however, here we observed evidence of secondary limitation of nitrate drawdown and biomass accumulation by silicic acid deficiency, via control of algal community structure—such that iron addition preferentially stimulated the growth of non-diatom nanoplankton—suggesting that the algal community was poised close to co-limitation by iron and silicic acid. As expected, our experimental results indicate that macronutrients (nitrate/phosphate) were the primary limitation on community growth in the STZ waters; however, our results also suggest that iron deficiency imposed a significant secondary limitation on community growth, particularly diatom growth, such that the algal community was poised close to co-limitation by macronutrients and iron. We conclude that these same regimes of resource limitation are likely to regulate phytoplankton growth and export production over much of the open-ocean Subantarctic region during the

Control of phytoplankton growth by iron and silicic acid availability in the subantarctic Southern Ocean: Experimental results from the SAZ Project

Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterized by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ∼0.05 nM) and silicic acid (Si, <1 μM). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8°S, 142°E) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 μM added silicic acid (+Si); and 1.9 nM added iron plus 9 μM added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth and species composition. Both nutrients together are needed to elicit a maximum growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community.

Control of phytoplankton growth by iron supply and irradiance in the subantarctic Southern Ocean: Experimental results from the SAZ Project

The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47°S, 142°E) and south (54°S, 142°E) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fυ/Fm<0.3). Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid‐replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I0 at 47°S and 9% I0 at 54°S because of MLDs of 40 (47°S) and 90 m (54°S), when these stations were occupied. The greater MLD at 54°S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47°S experiment, chlorophyll a levels increased to >1 μg L−1 only in the high‐Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54°S experiment there were increases in chlorophyll a (to >2 μg L−1), cell abundances, silica production, and carbon fixation only in the high‐light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54°S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47° and 54°S sites appear to represent minimum and maximum MLDs, where Fe and Fe/irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.

Response of marine bacterial community composition to iron additions in three iron‐limited regimes

In high‐nutrient low‐chlorophyll (HNLC) regimes, iron additions consistently result in primary productivity increases, and the phytoplankton community shifts from small species toward large diatoms. Heterotrophic bacterial production and abundance also increase in HNLC Fe addition experiments, but whether changes in bacterioplankton community composition also occur when Fe is added is unknown. We used trace metal clean shipboard incubation experiments, and molecular biological methods to examine this question in three Fe‐limited environments: the subarctic Pacific, the subantarctic Southern Ocean, and the California coastal upwelling region. After Fe additions and subsequent phytoplankton community shifts, changes in bacterial community composition were examined using denaturing gradient gel electrophoresis and terminal restriction fragment length polymorphism of polymerase chain reaction—amplified bacterial 16S rRNA genes. Responsive bacterial phylotypes in either +Fe or control treatments were classified using phylogenetic analyses of DNA sequences. In general, iron‐mediated changes in bacterial communities in all three environments were surprisingly minor compared to the changes in phytoplankton community composition. Responsive phylotypes were mostly γ‐proteobacteria in the subarctic and California HNLC areas, but no changes were noted in the subantarctic experiments. Although bacterial growth and biomass are closely linked to phytoplankton‐derived carbon supplies, our results suggest that on the time scale of our experiments (4–5 d), species composition of algal and bacterial communities can be decoupled in Fe‐limited waters.

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) 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 micro-physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS-climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea-to-air DMS flux for the period 1960 to 2080, corresponding to equivalent CO₂ tripling relative to pre-industrial levels. The results confirm a minor but non-negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) 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 micro-physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS-climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea-to-air DMS flux for the period 1960 to 2080, corresponding to equivalent CO2 tripling relative to pre-industrial levels. The results confirm a minor but non-negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.