The US Southern Ocean Joint Global Ocean Flux Study: an introduction to AESOPS

The United States Southern Ocean Joint Global Ocean Flux Study (JGOFS), als o known as AESOPS (Antarctic Environment and Southern Ocean Process Study), focused on two distinct regions. The first was the Ross-Sea,continental sh elf, where a series of six cruises collected a variety of data from October : 1996 through February 1998. The second area was the southwest Pacific sec tor of the Southern Ocean, spanning the Antarctic Circumpolar Current (ACC) at similar to 170 degreesW. Data were collected within this region during five cruises from September 1996 through March 1998, as well as during sele cted transits between New Zealand and the Ross Sea. The first results of th ese cruses are described in this issue. The Ross-Sea investigation extensiv ely sampled the area along 76 degrees 30'S to elucidate the temporal patter ns and processes that contribute to making this one of the Antarctic's most productive seas. Hydrographic distributions confirm that stratification is initiated early in October within the polynya, generating an environment t hat is favorable for phytoplankton growth. Significant spatial variations i n mixed-layer depths, the timing of the onset of stratification, and the st rength of the stratification existed throughout the growing season. Nutrien t concentrations reflected phytoplankton uptake, and reached their seasonal minimal in early February. Chlorophyll concentrations were maximal in earl y January, whereas productivity was maximal in late November, which reflect s the temporal uncoupling between growth and biomass accumulation in the re gion. Independent estimates of biogenic export suggest that majority of the flux occurred in late summer and was strongly uncoupled from phytoplankton growth, The ACC region exhibited seasonal changes that in some cases were greater than those observed in the Ross Sea. Sea ice covered much of the re gion south of the Polar Front in winter, and retreated rapidly in late spri ng and early summer. Mixed layers throughout the region shoaled in summer d ue to surface heating, while the addition of freshwater from melting sea ic e enhanced stratification in the Seasonal Ice Zone, creating conditions fav orable for phytoplankton growth. For example, silicic acid concentrations d ecreased from initial values as high as 65 to less than 2 muM within approx imately 100 km (from 65.7 to 64.8 degreesS). Fluorescence values, however, showed less than a two-fold variation over the same distance, The vertical Aux of carbon in the Polar Front area is substantial, and marked variations in the composition of exported material exited over the region. The result s provide a means whereby the controls of phytoplankton growth and organic matter flux and remineralization can be analyzed in great detail. Additiona l results of the AESOPS project are discussed. (C) 2000 Published by Elsevi er Science Ltd.