WALKER O. SMITH, JR., ANN-MAREE WHITE, SCOTT POLK, and SYLVIE MATHOT, Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996
AESOPS (Antarctic Environment Southern Ocean Process Study) is a multidisciplinary program designed to address three broad objectives: to quantify the net flux of carbon dioxide between the ocean and atmosphere and its seasonal variability, to evaluate the factors that lead to phytoplankton blooms in both the Ross Sea and the antarctic polar front, and to understand the controls on production and fate of biogenic material in these regions. To complete these objectives, a series of four process studies on the R/V Nathaniel B. Palmer were conducted on the Ross Sea continental shelf (as well as a benthic cruise). The cruises were completed during the winter-spring transition, the spring bloom, summer conditions, and the autumn-winter transition. This article describes preliminary results from the first process study.
The Ross Sea polynya has been identified as the region supporting the southern oceans' most spatially extensive and earliest phytoplankton bloom (Arrigo and McClain 1994; Smith and Gordon 1997). The causes of the bloom's initiation remain elusive, however. We hypothesized that deep convective mixing in the polynya is reduced by both low-density water from melting ice as well as decreased winds, which allow the phytoplankton to grow at near maximal rates. We reached the study area (figure 1) on 18 October 1996. Ice cover was near 100 percent, although the ice thickness was about 30-50 centimeters. Mixed layers at station Orca were initially about 154 meters (m) (figure 2) and chlorophyll levels were uniformly low [less than 0.1 microgram per liter (g L-1); figure 2]. The phytoplankton consisted of single-celled Phaeocystis sp. cells. At the end of the cruise the mixed layer at the same site was about 67 m (figure 3), and chlorophyll levels had increased to 0.5 g L-1 (figure 3). Phaeocystis sp. continued to dominate and began to form small colonies.
We also conducted sampling at a number of stations situated along an east-west transect at 76°30'S. Mixed layers along this transect ranged from 48 m to more than 600 m during our first occupation and from 24 m to 600 m during our last occupation of this line. Chlorophyll levels ranged from 0.001-0.380 g L-1 during mid-October to 0.021-2.371 g L-1 during early November. Nitrate concentrations had decreased minimally [approximately 1 micromolar (mM ) reduction] despite the fact that the photoperiod extended to 24 hours by 28 October and maximal irradiances approached 2,000 micromole quanta per square meter per second on clear days. Winds were modest for much of the time in the Ross Sea.
We believe that we observed the initiation of the seasonal phytoplankton bloom in the Ross Sea (chlorophyll concentrations exceeded more than 10 mg L-1 in December). Phytoplankton growth appeared to be restricted by deep vertical mixing, which was driven by surface cooling and convective overturn. As the mixed layer shoaled, the mean irradiance encountered by the phytoplankton assemblage increased (Nelson and Smith 1991). Given that the phytoplankton were nutrient saturated (with both macronutrients such as nitrate and micronutrients such as iron) during this period, and that the assemblages were adapted to extremely low photon flux densities (5 micromole quanta per square meter per second or less), phytoplankton growth began early in the year, under ice, and under extremely low irradiance conditions. Furthermore, we believe that as maximum growth was attained, biomass increased rapidly until growth became limited by trace metals in late spring and early summer. A complete assessment of the temporal changes of the controls of phytoplankton growth awaits the synthesis of all data from all process cruises.
This research was supported by National Science Foundation grant OPP 95-31990.
Arrigo, K.R., and C.R. McClain. 1994. Spring phytoplankton production in the western Ross Sea. Science , 266, 261-263.
Nelson, D.M., and W.O. Smith, Jr. 1991. Sverdrup revisited: Critical depths, maximum chlorophyll levels, and the control of southern ocean productivity by the irradiance-mixing regime. Limnology and Oceanography , 36(8), 1650-1661.
Smith, W.O., and L.I. Gordon. 1997. Hyperproductivity of the Ross Sea (Antarctica) polynya during austral spring. Geophysical Research Letters , 24, 233-236.