Open Southern Ocean Research Articles

Size-fractionated primary production in the open Southern Ocean in austral spring

Size-fractionated primary production was measured by carbon-14 uptake incubations on three transects between 47°S and 59°30′S along 6°W in October/November 1992. Open Antarctic Circumpolar Current and ice-covered Weddell Gyre water showed comparable low productivity (∼0.3 gCm−2 day−1) and size distribution. Picoplankton ( 20 μm) phytoplankton was only minor. Productivity in the Polar Front Zone north of 50°S, with higher water column stability, was up to 10 times higher with microplankton (>20 μm) being predominant. No ice-edge bloom occurred over the 2 months study period; this is explained by non-favourable hydrographic conditions for blooming and the lack of melt-water lenses upon ice retreat. Picoplankton tended to make higher contributions at lower water column stability, and microplankton to make higher contributions at higher stability. Mixing, together with light climate, are discussed as the driving forces for Antarctic primary production and for its size structure.

Interannual scales of variation of pigment concentrations from coastal zone color scanner data in the Benguela Upwelling system and the Subtropical Convergence zone south of Africa

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the coastal zone color scanner (CZCS) measurements from the Nimbus 7 satellite over the Benguela Upwelling region near Cape Town, South Africa, during the period 1978 to 1981 (Shannon, 1985). We give a brief overview of this research and then concentrate on a more recent analysis of level III CZCS data obtained from NASA for the region 10–60°S, 10–100°E. This area includes the Benguela Upwelling system on the continental shelf and the southern ocean with the Subtropical Convergence zone south of Africa. High annual concentrations of chlorophyll (5 mg m−3) typical of other upwelling systems in the world ocean occurred in the Benguela shelf region, and the data show a strong interannual signal in the 7 years of composited data from 1978 to 1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguete regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m−3), with the strongest signal (1.5 mg m−3) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder Subantarctic water to the south. The high levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. There appears to be no clear seasonal variation in the Subtropical Convergence data. Reasons for the strong signal in the surface chlorophyll concentrations at the front between the Agulhas Return Current and the southern ocean are discussed, and it is shown that the Agulhas Plateau sets up a topographic Rossby wave in the Agulhas Return Current, which can be clearly identified in the CZCS signal.

The significance of Marginopora vertebralis (Foraminifera) in surficial sediments at Esperance, Western Australia, and in last interglacial sediments in northern Spencer Gulf, South Australia

Surficial sediments at Esperance, on the southern coast of Western Australia, are predominantly bioclastic carbonates in which tests of foraminifera are a major constituent. The megascopic species Marginopora vertebralis is prolific, occupying relatively protected areas of algal turf and seagrasses. The abundance of this normally tropical foraminifer at southern latitudes can be attributed to the Leeuwin Current, which brings warm waters from the tropics to southwestern Australia, and then eastwards, past Esperance into the Great Australian Bight. The Leeuwin Current is significantly warmer than the cool interfacing waters of the Southern Ocean that otherwise characterise the southern Australian continental shelf. Last interglacial sediments of the Glanville Formation, in northern Spencer Gulf of South Australia, host an assemblage of fossil foraminifera which is similar to that of the modern environment at Esperance, especially the presence of M. vertebralis. Two bivalve molluscs, Anadara trapezia and Pinctada carchariarium, both characteristic of warm waters, are also present in the Glanville Formation. Marginopora vertebralis and other faunal elements migrated to South Australia by means of the last interglacial equivalent of the Leeuwin Current, which apparently had greater definition at that time. It may therefore be inferred that, during other times of global warming and corresponding higher sea levels, the palaeo-Leeuwin current had greater ecological influence across the continental shelf of Southern Australia. Thus, warm-water marine fossils in the Glanville Formation do not necessarily signify last interglacial water temperatures for the open Southern Ocean, beyond the continental shelf. Similarly, it cannot be assumed that these fossils necessarily indicate a last interglacial climate for terrestrial southern Australian that was warmer than presently prevails.

The response of Antarctic icebergs to ocean waves

The heave, tilt, and strain responses of three Antarctic tabular icebergs to ocean waves were measured during a 1980–1981 cruise of HMS Endurance to the South Atlantic. The three icebergs, located near the South Sandwich and South Orkney islands, were instrumented with accelerometers, tiltmeters, and wire strainmeters, while a Waverider buoy was used to record the ocean wave field. The thickness of the icebergs was surveyed by a helicopter‐borne radio echo sounder. The heave response occurred mainly at the swell period but with outbreaks of bobbing which lasted for a few cycles at a resonant period (about 40 s), which agreed well with the predictions of a numerical finite element model. The roll response occurred mainly at a long resonant period (40–50 s), which again agreed well with the model, but there was also a significant response at ocean wave periods (5–20 s), which exceeded predictions. The strain response had a component at very long periods, which is unexplained by theory, while the surface strain at ocean wave periods agreed with the simple analytical model of Goodman et al. (1980). Using this model it is possible to predict a wave height and period that will cause breakup of the icebergs, and we conclude that swell‐induced breakup is likely to occur during major storms in the open southern ocean.

Aquatic primary production in the Antarctic

A review is presented of studies, including recent work by members of the British Antarctic Survey, on the primary productivity of plankton, ice-flora and benthos in both marine and freshwater habitats in the Antarctic. Those members of the flora so far studied have low compensation points enabling slow growth in low light intensities but otherwise show no apparent adaptation to temperatures around freezing point. Certain sea areas, mostly inshore, have dense standing crops with daily and annual productivities as high as those of productive areas elsewhere in the oceans but in the open Southern Ocean productivity seems generally low even although nutrient concentrations are high, probably because of excessive turbulence carrying plankton out of the photic zone. There is as yet insufficient data to show whether, as a whole, the photosynthetic efficiency of the Southern Ocean is greater or less than that of other sea areas. Antarctic algae liberate extracellular products of photosynthesis but there is no definite evidence that these are reassimilated to support growth when light intensities are low and it may be that this material, carried by currents, supports heterotrophic production in other parts of the ocean. Comparison of data for adjacent marine and freshwater systems shows that their rates of primary production are much the same but the marine phytoplankton shows characteristics of shade-adapted cells consistent with the greater turbulence to which it is exposed.