ALBERT

Description: Breeding grey-headed albatross Thalassarche chrysostoma, tracked from Marion Island (Prince Edward Islands) during November-December 1997 and January-February 1998, showed a strong association with mesoscale oceanographic features, as identified by sea surface height anomalies, in the southern Indian Ocean. During incubation, most birds foraged to the north of the island, at the edges of anomalies created by the Agulhas Return Current in the Subtropical Convergence and the Subantarctic zones. In contrast, during chick-rearing all tracked birds foraged to the southwest of the island, at the edges of anomalies along the South-West Indian Ridge. Previous work in this area has shown that these anomalies are in fact eddies that are created as the Antarctic Circumpolar Current crosses the South-West Indian Ridge. Diet samples taken during the chick-rearing period showed a predominance of fresh specimens of the predatory fish Magnisudis prionosa and the squid Martialia hyadesi. Myctophid fish and amphipods Themisto gaudichaudii, both known prey of M. hyadesi, were also well represented in our samples. Diet samples taken from tracked birds showed birds feeding at edges of positive anomalies returning with fresh specimens of M. prionosa and M. hyadesi. Predatory fish and squid are thus presumably concentrated at these features. Eddies formed at the South-West Indian Ridge have also been shown to drift closer to Marion Island, within the foraging range of penguins and seals breeding on Marion Island. We therefore suggest that these mesoscale oceanographic features may be an important component of the Œlife-support¹ system enabling globally significant populations of seabirds and seals to breed at the Prince Edward Islands.

Description: Ocean circulation and bioproductivity in the Weddell Gyre: geochemical findingsW. GEIBERT1, C. HANFLAND1, J.SCHWARZ1, R.USBECK1,2, A.WEBB3, AND I.ANSORGE31Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany; wgeibert@awi-bremerhaven.de2FIELAX, Company for Scientific Data Management, Bremerhaven, Germany3University of Cape Town, Dept. of Oceanography, Cape Town, South AfricaAtypical distribution of tracersThe Weddell Gyre is one of the key areas for ocean circulation, and consequently for global climate. Here, deep waters reach the surface, and new production of bottom water brings surface signals at depth. This unusual situation is mirrored in the distribution of naturally occuring radionuclides like Radium isotopes or 227Ac in the water column. Not only tracers for ocean circulation show anomalies here. Tracers of particle flux also indicate atypical processes, reaching from the sea surface to the sediment.Atypical productivityResults of an expedition to the Eastern Weddell Gyre (Polarstern Expedition ANT XX/2) now have given a more comprehensive view of the whole Weddell Gyre. Together with measurements of nutrient distribution and other parameters, they reveal that the Weddell Gyre may not only be important in terms of ocean circulation, but it may also represent a site of suprisingly high bioproductivity. Pronounced differences between the Western part of the Gyre and its Eastern part were observed.Here, we present a comprehensive set of radiotracer data from the Weddell Gyre, together with oxygen and nutrient distribution data. Additionally, data of chlorophyll-a illustrate the vertical distribution of phytoplankton, explaining why satellite measurements lead to an underestimation of productivity in the Weddell Gyre. First results for sediment accumulation rates in the Eastern Weddell Gyre will also be presented.

Description: The Eastern Weddell Gyre an overlooked high productivity regionThe Weddell Gyre (WG) is generally known as a region of low bioproductivity. This view has hitherto been supported by relatively low chlorophyll concentrations as detected by satellites, high nutrient concentrations at the sea surface, and very low sedimentation rates in the underlying sediments. However, recent results from inverse modeling suggest a completely different situation for the Eastern part of the Weddell Gyre, from about 10° E to 35° E. The high productivity proposed by the model in the Eastern Weddell Gyre (EWG) is mainly a consequence of a substantial nutrient depletion at the surface, and a corresponding oxygen consumption in deeper layers, as known from the very few data available for that region. Satellite data do not show elevated chlorophyll concentrations in the EWG compared to neighbouring regions.In order to contribute new data from this poorly know region, and to help understanding the contrasting satellite and model data, Polarstern expedition ANT XX/2 into the EWG was carried out in the season 2002/03. A synoptical investigation (Synpart project) of productivity-related ocean properties was performed by the Alfred Wegener Institute together with scientists from the University Cape Town during the cruise. The measurements include fluorimetric detection of chlorophyll in the water column, determination of nutrient and oxygen concentrations, 234Th as a tracer for particle concentration and export, tracers for shallow and deep water circulation (228Ra, 227Ac). In addition, samples from the sediment surface were taken to allow for the determination of particle fluxes into the sediments by means of 230Th. This dataset gives insights into particle flux from the very surface of the ocean to the deep-sea floor. Additionally, data of pCO2 and a comprehensive oceanographic dataset are available from the expedition.The results clearly confirm the strong nutrient depletion in the surface waters of the EWG, in our case observed at 23°E, from about 62°30Ž S to 57°30Ž S. High concentrations of particulate 234Th are found there, in some cases in combination with a depletion of 234Th, which means export of particles from the sea surface. The fluorimetric determination of chlorophyll-a revealed that deep chlorophyll maxima were responsible for the contrasting satellite and model data. In some cases, high nutrient depletion together with deep chlorophyll was found where hardly any chlorophyll was visible at the sea surface by satellite. Compared to other parts of the Weddell Gyre, high opal concentrations in surface sediments of the EWG suggest that at least a fraction part of this productivity reaches the sea-floor (see presentation by Seeberg-Elverfeldt, this meeting). As the feature of deep chlorophyll maxima was not only found in the Eastern Weddell Gyre, but observed throughout the Weddell Gyre, there are implications for the interpretation of satellite images from the entire region. The productivity of the Weddell Gyre, especially of its Eastern part, must be considered to be currently substantially underestimated.

Description: Chlorophyll and other indicators of productivity in the Eastern Weddell Gyre between 0°E and 23°E - Preliminary Results of Polarstern Expedition ANT XX/2, Synpart projectWalter Geibert (1), Regina Usbeck (1), (2), Jill Schwarz (1), Claudia Hanfland (1), Adrian Webb (3) and Isabelle Ansorge (3)(1) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany(2) Now Fa. Fielax, Company for Scientific Data Management, Bremerhaven, Germany(3) University of Cape Town, Dept. of Oceanography, Cape Town, South AfricaAuthor to whom correspondence should be addressed: wgeibert@awi-bremerhaven.deThe Weddell Gyre, extending from the Antarctic Peninsula to about 35°E, is one of the most dynamic regions with respect to the vertical exchange of heat, salt, and nutrients. Deep upwelling supplies large amounts of nutrients to the surface, which results in a large potential productivity, but the region south of the Polar Front is commonly described as a HNLC (High Nutrient, Low Chlorophyll) region, which is partly supported by satellite derived surface chlorophyll values and sediment accumulation rate data. Although the western part of the Weddell Gyre, the Weddell Sea, is relatively well described, there is a lack of data from the Eastern Weddell Gyre. Data from nearby areas as well as results from inverse modelling (Usbeck et al. 2002) point to unexpectedly high export of biogenic silica from the surface waters to intermediate depths. The Synpart project (Synoptical investigation of particle flux in the Eastern Weddell Gyre) was initiated in order to get a comprehensive impression of particle flux in this region. Therefore, the Synpart project comprises SeaWiFS satellite data of chlorophyll a, the measurement of vertical chlorophyll profiles, nutrient and oxygen data, and determination of Thorium export from the surface layer. In the near future, accumulation rates on sediment surface samples taken during ANT XX/2 will be determined via 230Th. Here we present preliminary results of the expedition that give an impression on one of the poorest known regions of the Southern Ocean.

Description: Ocean circulation and bioproductivity in the Weddell Gyre: geochemical findingsW. GEIBERT1, C. HANFLAND1, J.SCHWARZ1, R.USBECK1,2, A.WEBB3, AND I.ANSORGE31Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany; wgeibert@awi-bremerhaven.de2FIELAX, Company for Scientific Data Management, Bremerhaven, Germany3University of Cape Town, Dept. of Oceanography, Cape Town, South AfricaAtypical distribution of tracersThe Weddell Gyre is one of the key areas for ocean circulation, and consequently for global climate. Here, deep waters reach the surface, and new production of bottom water brings surface signals at depth. This unusual situation is mirrored in the distribution of naturally occuring radionuclides like Radium isotopes or 227Ac in the water column. Not only tracers for ocean circulation show anomalies here. Tracers of particle flux also indicate atypical processes, reaching from the sea surface to the sediment.Atypical productivityResults of an expedition to the Eastern Weddell Gyre (Polarstern Expedition ANT XX/2) now have given a more comprehensive view of the whole Weddell Gyre. Together with measurements of nutrient distribution and other parameters, they reveal that the Weddell Gyre may not only be important in terms of ocean circulation, but it may also represent a site of suprisingly high bioproductivity. Pronounced differences between the Western part of the Gyre and its Eastern part were observed.Here, we present a comprehensive set of radiotracer data from the Weddell Gyre, together with oxygen and nutrient distribution data. Additionally, data of chlorophyll-a illustrate the vertical distribution of phytoplankton, explaining why satellite measurements lead to an underestimation of productivity in the Weddell Gyre. First results for sediment accumulation rates in the Eastern Weddell Gyre will also be presented.