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

Abstract

The United States Southern Ocean Joint Global Ocean Flux Study (JGOFS), also known as AESOPS (Antarctic Environment and Southern Ocean Process Study), focused on two distinct regions. The "rst was the Ross-Sea continental shelf, where a series of six cruises collected a variety of data from October 1996 through February 1998. The second area was the southwest Paci"c sector of the Southern Ocean, spanning the Antarctic Circumpolar Current (ACC) at &1703W. Data were collected within this region during "ve cruises from September 1996 through March 1998, as well as during selected transits between New Zealand and the Ross Sea. The "rst results of these cruses are described in this issue. The Ross-Sea investigation extensively sampled the area along 76330 S to elucidate the temporal patterns and processes that contribute to making this one of the Antarctic's most productive seas. Hydrographic distributions con"rm that strati"cation is initiated early in October within the polynya, generating an environment that is favorable for phytoplankton growth. Signi"cant spatial variations in mixed-layer depths, the timing of the onset of strati"cation, and the strength of the strati"cation existed throughout the growing season. Nutrient concentrations re#ected phytoplankton uptake, and reached their seasonal minimal in early February. Chlorophyll concentrations were maximal in early January, whereas productivity was maximal in late November, which re#ects the temporal uncoupling between growth and biomass accumulation in the region. Independent estimates of biogenic export suggest that majority of the #ux 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.0967-0645/00/$ -see front matter 2000 Published by Elsevier Science Ltd. PII: S 0 9 6 7 -0 6 4 5 ( 0 0 ) 0 0 0 5 9 -X Sea ice covered much of the region south of the Polar Front in winter, and retreated rapidly in late spring and early summer. Mixed layers throughout the region shoaled in summer due to surface heating, while the addition of freshwater from melting sea ice enhanced strati"cation in the Seasonal Ice Zone, creating conditions favorable for phytoplankton growth. For example, silicic acid concentrations decreased from initial values as high as 65 to less than 2 M within approximately 100 km (from 65.7 to 64.83S). Fluorescence values, however, showed less than a two-fold variation over the same distance. The vertical #ux of carbon in the Polar Front area is substantial, and marked variations in the composition of exported material exited over the region. The results provide a means whereby the controls of phytoplankton growth and organic matter #ux and remineralization can be analyzed in great detail. Additional results of the AESOPS project are discussed.

Phytoplankton and iron limitation of photosynthetic efficiency in the Southern Ocean during late summer

“…Macronutrients and micronutrients exhibited different patterns of variability, some details of which have been previously described Smith et al, 2000a;Measures and Vink, 2001;Morrison et al, 2001). Macronutrient concentrations varied systematically on the Polar Front cruise, generally increasing towards the south, most dramatically for silicate.…”

“…Everywhere, except north of 551S, nitrate and phosphate concentrations exceeded 18 and 1 mM, respectively. An abrupt silicate gradient was present near 651S by early summer and was still evident in late February; north of this location surface layer concentrations dropped below 10 mM, but southward they consistently exceeded 50 mM (Smith et al, 2000a). Iron concentrations also varied, but in contrast to the macronutrient distributions, dissolved iron concentrations in surface waters were lowest south of B651S (o0.15 nM; Fig.…”

Surface chlorophyll concentrations in relation to the Antarctic Polar Front: seasonal and spatial patterns from satellite observations

“…Higher dissolved iron concentrations observed within the PF were likely a contributing factor in bloom development (de Baar et al, 1995;Smetacek et al, 1997;Löscher et al, 1997;Lancelot et al, 2000). The PF and surrounding waters were also surveyed intensely along 170jW in the Pacific sector during AESOPS (Smith et al, 2000). During AESOPS, a bloom was observed within the PF and also at the SACCF during December 1997 (Moore et al, 1999b;Abbott et al, 2000Abbott et al, , 2001Mengelt et al, 2001;Barth et al, 2001).…”

“…During fall, winter, and early spring months, when mixed layers are deep, light limitation of phytoplankton growth rates is likely (Mitchell et al, 1991;Veth et al, 1997;Lancelot et al, 2000;Smith et al, 2000;Boyd et al, 2001). In addition, given the importance of iron for photoadaptation, iron-light co-limitation is also possible (Sunda and Huntsman, 1997;Boyd et al, 1999Boyd et al, , 2000Boyd et al, , 2001.…”

An intermediate complexity marine ecosystem model for the global domain

“…For the ROSS site we compare with all data from the transects along B761S. Due to the latitudinal gradients in nutrient concentrations across the PF, we restrict our comparison to waters at the PF and just to the south (168-1721W, 60.5-62.51S, see Smith et al, 2000). At the NABE site, we compare with available in situ data from the region 17-211W by 46-481N.…”