OSMUND HOLM-HANSEN and CHRISTOPHER D. HEWES,Polar Research Program, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202
JENNY MATURANA, Escuela de Ciencias del Mar, Universidad Católica de Valparaiso, Valparaiso, Chile
LORENA RIOS, Instituto de Investigaciones Oceanologicas, Universidad Autonoma de Baja California, Ensenada, Baja California, Mexico
GASTON GONZALEZ-RODAS, Department of Oceanography, Texas A&M University, College Station, Texas 77843
One of the major objectives of the phytoplankton component of the Antarctic Marine Living Resources (AMLR) program is to improve understanding of the factors that influence the distribution and concentration of phytoplankton throughout the study area. The AMLR large-area survey grid around Elephant Island ( see Martin, Hewitt, and Holt, Antarctic Journal , in this issue) is particularly interesting and challenging in this regard because it includes diverse water types and associated frontal mixing zones, in addition to both relatively shallow coastal and deep pelagic waters.
The distribution of phytoplankton in surface waters was estimated continuously during the AMLR cruise by sensors (fluorometer and transmissometer) on a clean water-intake line, as well as in depth profiles (0 to 200 meters) at every conductivity-temperature-depth (CTD)/carousel station. The CTD/carousel unit included a submersible fluorometer and eleven 10-liter Niskin bottles, which were closed at standard depths between 5 and 200 meters. Concentrations of chlorophyll- a were measured on samples from the Niskin bottles by concentration of the phytoplankton onto GF/F glass fiber filters, extracting the photosynthetic pigments in absolute methanol (Holm-Hansen and Riemann 1978), and measurement of fluorescence (Holm-Hansen et al. 1965). During Survey A (27 January to 10 February 1997), 106 CTD/carousel stations were occupied as described by Martin et al. ( Antarctic Journal , in this issue).
Chlorophyll- a concentrations in surface waters varied greatly; the lowest concentrations (0.1 to 0.3 micrograms/liter) were in the northwest and southeast regions of the survey area, and the highest concentrations (4 to 7 micrograms/liter) were in the region between King George Island and Elephant Island and also in the northeast corner of the survey area (figure 1). The pattern of integrated chlorophyll- a values for the upper 100 meters of the water column was similar to that for 5-meter chlorophyll- a values; the lowest values were 15 to 25 milligrams per square meter, and the highest were 250 to 350 milligrams per square meter. The mean integrated chlorophyll- a (milligrams per square meter) values in the various water zones as delineated by Amos, Wickham, and Rowe ( Antarctic Journal , in this issue) were 30.5 (Water Zone I), 65.9 (Water Zone II), 66.4 (Water Zone III), 86.1 (Water Zone IV), and 49.2 (Water Zone V). On the basis of both physical and biological characteristics, stations in Water Zone I have been divided into Water Zone Ia, which shows little or no mixing with waters from the other four zones, and Water Zone Ib, which shows considerable mixing with waters from Water Zones II or III (Holm-Hansen et al. 1994). The mean integrated chlorophyll- a values for the 33 stations in Water Zone Ia and the six stations in Water Zone Ib were 25.6 and 43.6 milligrams per square meter, respectively.
The profile of distribution of chlorophyll- a with depth (figure 2) varies considerably in the water zones described by Amos et al. ( Antarctic Journal , in this issue). Stations in Water Zone Ia (Drake Passage waters) have the lowest chlorophyll- a concentrations in surface waters and a deep chlorophyll- a maximum between 50 and 100 meters. Chlorophyll- a concentrations in Water Zone Ib are also low but show maximal values at or close to the surface. Stations in Water Zone V (Weddell Sea water) also show relatively low surface chlorophyll- a concentrations, but concentrations slowly decrease with depth. Chlorophyll- a concentrations in Water Zones II, III, and IV are all much higher than in Water Zones I and V and have maximal values at or close to the surface.
Figure 3 shows the geographical extent of the AMLR stations which are characterized by relatively high concentrations of chlorophyll- a between 50 and 100 meters as compared to the concentrations from 5 to 30 meters. The zones of deep distribution of chlorophyll- a are found in Water Zone I waters, where phytoplankton growth is thought to be limited by available iron (Holm-Hansen et al. 1994), and also in Water Zone V waters, which are nutrient-rich but characterized by physical instability of the upper water column.
This research was supported by National Oceanic and Atmospheric Administration (NOAA) contract number 50ABNF600013. Grateful acknowledgment is extended to the officers and crew of the R/V Yuzhmorgeologiya for their excellent support during all field operations. We thank the Physical Oceanography group for kindly providing their CTD data. Shipboard personnel included J. Maturana, C.D. Hewes, L. Rios, G. Gaston-Rodas, and O. Holm-Hansen.
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