Young-Hyang Park

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Variations in behavior and condition of a Southern Ocean top predator in relation to in situ oceanographic conditions

Responses by marine top predators to environmental variability have previously been almost impossible to observe directly. By using animal-mounted instruments simultaneously recording movements, diving behavior, and in situ oceanographic properties, we studied the behavioral and physiological responses of southern elephant seals to spatial environmental variability throughout their circumpolar range. Improved body condition of seals in the Atlantic sector was associated with Circumpolar Deep Water upwelling regions within the Antarctic Circumpolar Current, whereas High-Salinity Shelf Waters or temperature/salinity gradients under winter pack ice were important in the Indian and Pacific sectors. Energetic consequences of these variations could help explain recently observed population trends, showing the usefulness of this approach in examining the sensitivity of top predators to global and regional-scale climate variability.

Frontal structure, water masses, and circulation in the Crozet Basin

Due to topographic steering by the Crozet and Kerguelen Plateaus, the Antarctic Circumpolar Current (ACC) core in the Crozet Basin area is shifted to its northernmost position in the Southern Ocean, along the southern limb of the South Indian subtropical gyre. Here the jet-like current is embedded within a narrow band ( 26.5 kg/m3) in the western half of the basin and heavier mode (γθ < 26.7 kg/m3) in its eastern half. These different varieties of Crozet Basin Mode Water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone. The area north of the Kerguelen Plateau appears to be an important source of water mass modification. Antarctic Intermediate Water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher, oxygen-richer, colder subantarctic water. Energetic eddy activities observed along the frontal zone likely enhance such cross-frontal exchange of water masses. Equatorward spreading of the modified thermocline water masses (Mode Water, Antarctic Intermediate Water) occurs as part of the anticyclonic circulation in the South Indian subtropical gyre which is centered within the basin. Strong injection of North Indian Deep Water into the ACC south of Amsterdam Island is clearly evidenced for the first time. This deep oxygen minimum water centered at about 3000 m can be traced to the east of Madagascar, from where it spreads southward through the eastern part of the Crozet Basin. Due to the influence of this water mass, North Atlantic Deep Water characteristics of the Circumpolar Deep Water are significantly altered in Kerguelen-Amsterdam passage. Our results indicate a cyclonic deep-level circulation pattern in the Crozet Basin, with a northward flow (9 Sv) of Circumpolar Deep Water / Antarctic Bottom Water as a deep western boundary current and a partially compensating southward flow (3 Sv) of North Indian Deep Water through the eastern part of the basin. It is concluded that the Crozet Basin constitutes an important passage for the exchange of water masses between the Southern Ocean and the Indian Ocean.

Reliability of flipper-banded penguins as indicators of climate change

In 2007, the Intergovernmental Panel on Climate Change highlighted an urgent need to assess the responses of marine ecosystems to climate change. Because they lie in a high-latitude region, the Southern Ocean ecosystems are expected to be strongly affected by global warming. Using top predators of this highly productive ocean (such as penguins) as integrative indicators may help us assess the impacts of climate change on marine ecosystems. Yet most available information on penguin population dynamics is based on the controversial use of flipper banding. Although some reports have found the effects of flipper bands to be deleterious, some short-term (one-year) studies have concluded otherwise, resulting in the continuation of extensive banding schemes and the use of data sets thus collected to predict climate impact on natural populations. Here we show that banding of free-ranging king penguins (Aptenodytes patagonicus) impairs both survival and reproduction, ultimately affecting population growth rate. Over the course of a 10-year longitudinal study, banded birds produced 39% fewer chicks and had a survival rate 16% lower than non-banded birds, demonstrating a massive long-term impact of banding and thus refuting the assumption that birds will ultimately adapt to being banded. Indeed, banded birds still arrived later for breeding at the study site and had longer foraging trips even after 10 years. One of our major findings is that responses of flipper-banded penguins to climate variability (that is, changes in sea surface temperature and in the Southern Oscillation index) differ from those of non-banded birds. We show that only long-term investigations may allow an evaluation of the impact of flipper bands and that every major life-history trait can be affected, calling into question the banding schemes still going on. In addition, our understanding of the effects of climate change on marine ecosystems based on flipper-band data should be reconsidered.

Southern Ocean frontal structure and sea-ice formation rates revealed by elephant seals

Polar regions are particularly sensitive to climate change, with the potential for significant feedbacks between ocean circulation, sea ice, and the ocean carbon cycle. However, the difficulty in obtaining in situ data means that our ability to detect and interpret change is very limited, especially in the Southern Ocean, where the ocean beneath the sea ice remains almost entirely unobserved and the rate of sea-ice formation is poorly known. Here, we show that southern elephant seals (Mirounga leonina) equipped with oceanographic sensors can measure ocean structure and water mass changes in regions and seasons rarely observed with traditional oceanographic platforms. In particular, seals provided a 30-fold increase in hydrographic profiles from the sea-ice zone, allowing the major fronts to be mapped south of 60°S and sea-ice formation rates to be inferred from changes in upper ocean salinity. Sea-ice production rates peaked in early winter (April–May) during the rapid northward expansion of the pack ice and declined by a factor of 2 to 3 between May and August, in agreement with a three-dimensional coupled ocean–sea-ice model. By measuring the high-latitude ocean during winter, elephant seals fill a “blind spot” in our sampling coverage, enabling the establishment of a truly global ocean-observing system.

King penguin population threatened by Southern Ocean warming

Seabirds are sensitive indicators of changes in marine ecosystems and might integrate and/or amplify the effects of climate forcing on lower levels in food chains. Current knowledge on the impact of climate changes on penguins is primarily based on Antarctic birds identified by using flipper bands. Although flipper bands have helped to answer many questions about penguin biology, they were shown in some penguin species to have a detrimental effect. Here, we present for a Subantarctic species, king penguin (Aptenodytes patagonicus), reliable results on the effect of climate on survival and breeding based on unbanded birds but instead marked by subcutaneous electronic tags. We show that warm events negatively affect both breeding success and adult survival of this seabird. However, the observed effect is complex because it affects penguins at several spatio/temporal levels. Breeding reveals an immediate response to forcing during warm phases of El Niño Southern Oscillation affecting food availability close to the colony. Conversely, adult survival decreases with a remote sea-surface temperature forcing (i.e., a 2-year lag warming taking place at the northern boundary of pack ice, their winter foraging place). We suggest that this time lag may be explained by the delay between the recruitment and abundance of their prey, adjusted to the particular 1-year breeding cycle of the king penguin. The derived population dynamic model suggests a 9% decline in adult survival for a 0.26°C warming. Our findings suggest that king penguin populations are at heavy extinction risk under the current global warming predictions.

Cross-frontal exchange of Antarctic Intermediate Water and Antarctic Bottom Water in the Crozet Basin

Abstract The hydrographic survey made across the Antarctic Circumpolar Current (ACC) in the southwestern Crozet Basin during the April–May 1993 ANTARES-1 cruise revealed unprecedented strong cross-frontal injections of newly formed Antarctic Intermediate Water (AAIW) into the subtropical zone north of the ACC. The ACC in this area is mostly associated with the Subantarctic Front (SAF) and Subtropical Front (STF), which are tightly merged into one structure bordered to the north with the eastward extension of the Agulhas Return Current. The oxygen and salinity extrema of the newly injected AAIW, at a depth of 1200 m, well north of the STF, reach as high as 6.17 ml l−1 and 34.15. These values coincide almost exactly with those in the AAIW source, found at a depth of 500 m, just south of the SAF, but surpass considerably (oxygen by + 1.35 ml l−1 and salinity by −0.23 psu) those values corresponding to the old AAIW at the nearby subtropical stations. Such spectacular cross-frontal injections of new AAIW were not continuous in space and time, but were found to be rather local, intermittent and impulsive. This was closely correlated with the highly perturbed, meandering frontal zone due to the frequent passages of strong atmospheric depressions during the cruise. There is also clear evidence of the northward penetration of Antarctic Bottom Water (AABW) through the Crozet-Kerguelen gap and its northward flow along the deep western boundary of the Crozet Basin, supporting previous observations. Moreover, our observations indicate more precisely the bottom water pathway, which is concentrated against the continental rise west of 56°E, following roughly the 4000 m isobath. It is concluded that the Crozet Basin frontal zone acts as a barrier only for the upper layer water masses, and that it is dynamically transparent for the cross-frontal exchange of intermediate and deeper layer water masses of potential density anomalies greater than σϱ = 27.1, including AAIW and AABW.

Movements of foraging king penguins through marine mesoscale eddies

Despite increasing evidence that marine predators associate with mesoscale eddies, how these marine features influence foraging movements is still unclear. This study investigates the relationship of at-sea movements of king penguins to mesoscale eddies using oceanographic remote sensing and movement data from 43 individual trips over 4 years. Simultaneous satellite measurements provided information on gradients of sea surface temperature and currents associated with eddies determined from altimetry. Penguins tended to swim rapidly with currents as they travelled towards foraging zones. Swimming speed indicative of foraging occurred within mesoscale fronts and strong currents associated with eddies at the Polar Front. These results demonstrate the importance of mesoscale eddies in directing foraging efforts to allow predators to rapidly get to rich areas where high concentrations of prey are likely to be encountered. When returning to the colony to relieve the incubating partner or to feed the chick, the birds followed a direct and rapid path, seemingly ignoring currents.

Large-scale circulation and its variability in the south Indian Ocean from TOPEX/POSEIDON altimetry

The first 18 months of altimeter data from the TOPEX/POSEIDON (T/P) mission were analyzed to study the large-scale mean surface circulation and its variability in the south Indian Ocean. A T/P sea level time series corrected for ocean tides using the Texas model (Eanes, 1994) was first validated against in situ sea level measurements from a bottom pressure recorder near Amsterdam Island. T/P data are a little contaminated by a 60-day tidal aliasing effect, very near the alias periods of M2 and S2 constituents. When this 60-day effect is removed, the data agree well with the bottom pressure-derived sea level within 2.0 cm rms difference. This indicates that only low-frequency ocean signals of periods greater than 60 days can be safely studied from T/P data. A T/P-derived mean dynamic topography relative to the Ohio State University 91A geoid (Rapp et al., 1991) was compared to the output of the fine resolution Antarctic model (Webb et al., 1991) and to historical hydrographic data. The altimetric solution shows excellent agreement with the numerical model solution and indicates an anticyclonic subtropical gyre north of the Antarctic Circumpolar Current and two cyclonic subpolar gyres south of this current, respectively, west and east of the Kerguelen Plateau. The western subpolar gyre corresponding to the eastern boundary of the Weddell Gyre appears to extend as far east as 60°E in the Weddell-Enderby Basin. However, the hydrography-derived dynamic topography does not reveal any clear evidence of these gyre features because of its inability to detect the barotropic component of currents. Finally, the T/P-derived sea level variability reveals clearly the well-known, strong current-topography interaction in the Southern Ocean. There appear no significant interannual variations of the mean position and magnitude of the major current systems in the south Indian Ocean.

Frontal structure and transport of the Antarctic Circumpolar Current in the south Indian Ocean sector, 40–80°E

Abstract Based on recent conductivity-temperature-depth (CTD) data at the Kerguelen-Amsterdam passage plus historical expendable bathy thermograph (XBT) and hydrographie data from the Crozet Basin, the frontal structure and volume transport of the Antarctic Circumpolar Current (ACC) in the south Indian Ocean sector are described. Recent findings on sea-level variability from GEOSAT altimeter data are reviewed as well as barotropic transport variability from bottom pressure measurements at Kerguelen and Amsterdam Islands. The ACC here is mostly concentrated in a narrow frontal zone of 2–3° of latitude, without any noticeable multi-band structure, as a result of the confluence of the Subantarctic and Subtropical Fronts. The Polar Front in this sector is not associated with any prominent current core. This regional particularity of the ACC was further supported by GEOSAT altimetry data. About 80% of the ACC transport (106Sv; 1 Sv=106 m3s−1) passes north of Kerguelen, concentrated along the northern flank of the Kerguelen Plateau; only 30 Sv passes south of Kerguelen. The bottom-pressure-derived barotropic transport variability at Kerguelen-Amsterdam passage was estimated to lie between 10 and 30 Sv, consistent with findings at Drake Passage. It is suggested that the study area constitutes a prominent ACC observatory during the WOCE (World Ocean Circulation Experiment).