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The area around Elephant Island, the study site of the U.S. Antarctic Marine Living Resources (AMLR) Program, is especially interesting because its physical, chemical, and biological characteristics have been reported to be extremely variable in regard to both spatial and temporal considerations (El-Sayed 1988, pp. 101-119; Helbling, Villafañe, and Holm-Hansen 1995; Silva et al. 1995; Villafañe, Helbling, and Holm-Hansen 1995). This region is also very productive in regard to the commercial harvesting of the antarctic krill, Euphausia superba Dana (Marr 1962), particularly over the continental shelf-slope north of Elephant Island (Macaulay, English, and Mathisen 1984). In this article, we report on the distribution of chlorophyll-a and primary production parameters in the AMLR study area during the austral summer of 1996.
The large-area survey grid, which consisted of 91 conductivity-temperature-depth (CTD)/rosette stations, was surveyed once during Leg I and once during Leg II. In addition, three cross-shelf transects were done, and extensive sampling was performed down to 2,000 meters (m) or to within 10 m of the bottom at shallower stations. More detailed information about the location of stations and transects is given in Martin, Hewitt, and Holt (Antarctic Journal, in this issue). Water samples were taken at 11 standard depths (5, 10, 15, 20, 30, 40, 50, 75, 100, 200, and 750 m or within 10 m of the bottom at shallow stations) from the 10-liter Niskin bottles mounted on the rosette. The following sensors were attached to the rosette to obtain continuous profile data of important variables in the upper water column:
Chlorophyll-a analyses were performed with samples taken from the upper 200 m depth following standard fluorometric techniques (Holm-Hansen et al. 1965) using a Turner Designs fluorometer. Samples of 100 milliliters were filtered through a GF/F Whatman glass fiber filter and the photosynthetic pigments extracted in absolute methanol; after at least 1 hour of extraction, chlorophyll-a concentrations were determined from the fluorescence of the extract (Holm-Hansen and Riemann 1978). The chlorophyll-a content of the nanoplankton fraction (cells <20 micrometers) was obtained in the same fashion, but the sample was first prefiltered through Nitex mesh with a pore size of 20 micrometers.
Samples for primary productivity measurements were taken at 5, 10, 15, 20, 30, 40, 50, and 75 m depth, placed in 50-milliliter polycarbonate tubes, and inoculated with 5 microcuries of sodium bicarbonate (NaH14CO3). The tubes were then placed in an incubator, which had neutral density screens to simulate the irradiance conditions existing at the depth from which the samples had been taken. The temperature of the samples in the incubator was maintained close to surface-water temperatures by pumping surface seawater through the incubator. After 6-8 hours of incubation under direct solar radiation, the samples were filtered (GF/F filter), exposed to hydrogen chloride (HCl) fumes, dried overnight, and the assimilated radiocarbon determined using standard liquid scintillation techniques. Incident solar radiation (PAR) was continuously recorded during both Legs using a 2-pi sensor mounted on the ship's superstructure.
The distributions of chlorophyll-a at 5 m depth during both Legs are shown in figure 1. A general increase of phytoplankton biomass from Leg I to Leg II occurred, and chlorophyll-a values were less than 2.5 milligrams per cubic meter (mg m-3) throughout the study area during Leg I but reached values up to 5 mg m-3 to the east of King George during Leg II. In general, the lowest surface chlorophyll-a values were found in the northwest portion of the grid, whereas the highest chlorophyll-a values were found in Bransfield Strait waters and in the northeast portion of the sampling grid. The mean chlorophyll-a values at 5 m depth during Legs I and II were 0.85 and 1.5 mg m-3, respectively. The proportion of nanoplanktonic cells was high during both Legs, in general accounting for more than 80 percent of the chlorophyll-a.
Figure 2 shows the depth distribution of chlorophyll-a along the three cross-shelf transects. In transect 1 (figure 2A), an area of relatively high chlorophyll-a values (more than 0.8 mg m-3) was observed in the southern portion of the transect, and a deep chlorophyll-a maximum was evident at about 80 m depth. Transect 2 (figure 2B) had a more homogeneous distribution of chlorophyll-a, exhibiting low values (less than 0.8 mg m-3) throughout almost the entire transect. The depth distribution of chlorophyll-a in transect 3 (figure 2C) was different from the other two, as it was characterized by high chlorophyll-a values in surface waters and diminishing concentrations with depth. The highest values observed (more than 2 mg m-3 from the surface to about 50 m depth) were found over the shelf drop-off.
The photosynthesis vs. irradiance (P vs. I) characteristics of phytoplankton in the AMLR sampling grid are shown in figure 3. The mean maximum values for assimilation numbers were slightly higher during Leg I than during Leg II (2.3 and 1.9 milligrams of carbon fixed per milligram chlorophyll-a per hour, respectively). The data in figure 3 illustrate that as irradiance increased, photosynthetic rates were somewhat inhibited. Because the samples were contained in polycarbonate tubes which absorb essentially all radiation below 360 nanometers, this inhibition could have been due to PAR, to ultraviolet radiation between 360 to 400 nanometers in wavelength, or to both. The values for mean daily PAR radiation during Legs I and II were 38 and 28 Einsteins per square meter per day, respectively.
This research was supported by National Oceanic and Atmospheric Administration (NOAA) contract number 52ABNF600013. Grateful acknowledgment is extended to the officers and crew of the R/V Yuzhmorgeologiya for their excellent support during all field operations.
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