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The main goals of the U.S. Antarctic Marine Living Resources Program (AMLR) include the study of the predator/prey interactions and the relationship between physical, chemical, and biological parameters in the marine ecosystem around Elephant Island, Antarctica. As a part of the phytoplankton component of this program, extensive nutrient sampling was carried out to relate chemical characteristics of water masses with phytoplankton distribution and concentration in the study area.
Although inorganic macronutrient concentrations are high (Le Jehan and Treguer 1985, pp. 22-29) and usually do not limit phytoplankton productivity in surface antarctic waters (Sakshaug and Holm-Hansen 1984, pp. 1-18), nutrient depletion has been documented in areas where large phytoplankton blooms occur, especially in coastal regions (Nelson and Smith 1986; Holm-Hansen et al. 1989; Holm-Hansen and Mitchell 1991). Previous studies in the area around Elephant Island have shown that nutrient concentrations are generally high (Silva et al. 1995a). During the AMLR 1995 field season, however, nitrate and phosphate concentrations decreased significantly in response to high concentrations of phytoplankton biomass, which at times exceeded 200 milligrams chlorophyll-a per square meter (Silva et al. 1995b; Villafañe et al. 1995).
The AMLR large-area survey grid, consisting of 91 conductivity-temperature-depth (CTD)/rosette stations, was occupied once during Leg I and once during Leg II. The cruise track and station positions for Legs I and II are given in Martin, Hewitt, and Holt (Antarctic Journal, in this issue). Samples for nutrient analysis were taken from the Niskin bottles attached to the CTD/rosette profiling unit. Water samples from 5 meters (m) depth were taken at all stations; additionally, samples were taken at 20, 50, and 100 m at the 24 stations where primary productivity measurements were made. The water samples (approximately 48 milliliters) were poured into 60-milliliter high-density polyethylene bottles that had been acid-cleaned. The bottles were immediately frozen and maintained at -20°C in an upright position until the time of analyses (1-2 months after collection), which were performed at the Universidad Católica de Valparaíso, Chile, using an autoanalyzer and standard analytical techniques (Atlas et al. 1971).
The distribution of silicic acid ( figure 1) in near-surface waters during both Legs shows a pattern similar to the ones previously found by Silva et al. (1995a). The lowest concentrations (less than 40 micromolar) were in Drake Passage waters; higher concentrations (more than 70 micromolar) occurred toward the southeast (water type V; see Amos, Wickham, and Rowe, Antarctic Journal, in this issue) and south (Bransfield Strait waters) of the sampling grid. This gradient in silicic acid concentrations was present during both Legs and was most pronounced in the zone of the continental shelf-break and the continental slope to the north of Elephant Island.
Phosphate concentrations at 5 m depth ( figure 2) were generally high during both Legs, with values generally ranging from 1.6 to 2.0 micromolar. The lowest phosphate concentrations found during Leg I (less than 1.7 micromolar; figure 2A) were in a large area to the north of Elephant Island. During Leg II (figure 2B), phosphate concentrations were generally higher in this area to the north of Elephant Island, but relatively low concentrations (less than 1.6 micromolar) were found to the south of Elephant Island and to the east of King George Island. These areas of low phosphate concentrations in Leg II coincided with the areas of greatest phytoplankton biomass (see Villafañe et al., Antarctic Journal, in this issue).
The distribution patterns of nitrate concentrations at 5 m depth during both Legs ( figure 3) were similar to those of phosphate concentrations, including the relative changes from Leg I to Leg II. Nitrate concentrations during both Legs ranged from about 22 to 29 micromolar, with the lowest values during Leg I (21 micromolar; figure 3A) existing to the north of Elephant Island, and during Leg II (22 micromolar; figure 3B) to the east of King George.
The variations in inorganic nutrient concentrations shown above reflect different chemical characteristics of the water masses found in the AMLR study area (Silva et al. 1995a) as well as temporal changes related to the assimilation of these nutrients by phytoplankton. The relatively low phosphate and nitrate concentrations during our study period apparently are due to phytoplankton nutrient uptake because high phytoplankton concentrations were observed in these areas (Villafañe et al., Antarctic Journal, in this issue). Similar changes in nutrient concentrations from Leg I to Leg II were observed in the AMLR study area by Silva et al. (1995b), although in that study the minimal phosphate and nitrate concentrations were much lower (less than 1 micromolar phosphate and less than 19 micromolar nitrate). This greater depletion of nutrients during 1995 as compared to values in 1996 is correlated with higher chlorophyll-a concentrations in 1995 (approximately 8 milligrams per cubic meter) as compared to the highest chlorophyll-a concentrations of approximately 5 milligrams per cubic meter in 1996.
This research was supported by National Oceanic and Atmospheric Administration (NOAA) Contract 52ABNF600013. Grateful acknowledgment is extended to the officers and crew of the R/V Yuzhmorgeologiya for their excellent support during all field operations. We also thank M. Hernando for his help onboard ship, the Physical Oceanography group for kindly providing their CTD data, and A. Varas and N. Cáceres for their help in nutrient analysis. Shipboard personnel during Legs I and II included E.W. Helbling, V.E. Villafañe, and T.C. Calvete.
References
Amos, A.F., A.R. Wickham, and C.C. Rowe. 1996. AMLR Program: The 7-year hitch--a meander in the circulation--near Elephant Island. Antarctic Journal of the U.S., 31(2).
Atlas, E.L., L.I. Gordon, S.W. Hager, and P.K Park. 1971. A practical manual for the use of the Technicon Autoanalyzer in seawater nutrient analyses, rev. (Technical Report 71-22). Corvallis: Oregon State University, Department of Oceanography.
Holm-Hansen, O., and B.G. Mitchell. 1991. Spatial and temporal distribution of phytoplankton and primary production in the western Bransfield Strait region. Deep Sea Research, 38(8/9), 961-980.
Holm-Hansen, O., B.G. Mitchell, C.D. Hewes, and D.M. Karl. 1989. Phytoplankton blooms in the vicinity of Palmer Station, Antarctica. Polar Biology, 10(1), 49-57.
Le Jehan, S., and P. Treguer. 1985. The distribution of inorganic nitrogen, phosphorus, silicon and dissolved organic matter in surface and deep waters of the Southern Ocean. In W.R. Siegfried, P.R. Condy, and R.M. Laws (Eds.), Antarctic nutrient cycles and food webs. Berlin: Springer-Verlag.
Martin, J.E., R.P. Hewitt, and R.S. Holt. 1996. The U.S. Antarctic Marine Living Resources (AMLR) Program: 1995-1996 field season activities. Antarctic Journal of the U.S., 31(2).
Nelson, D.M., and W.O. Smith. 1986. Phytoplankton bloom dynamics of the Ross Sea ice edge. II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Research, 33(10), 1389-1412.
Sakshaug, E., and O. Holm-Hansen. 1984. Factors governing pelagic production in polar oceans. In O. Holm-Hansen, L. Bolis, and R. Gilles (Eds.), Marine phytoplankton and productivity. Berlin: Springer-Verlag.
Silva S., N., E.W. Helbling, V.E. Villafañe, A.F. Amos, and O. Holm-Hansen. 1995a. Variability in nutrient concentrations around Elephant Island, Antarctica, during 1991-1993. Polar Research, 14(1), 69-82.
Silva S., N., C. Valenzuela, L. Linacre R., and E.W. Helbling. 1995b. AMLR Program: Inorganic nutrient concentrations near Elephant Island, Antarctica, January, February, and March 1995. Antarctic Journal of the U.S., 30(5), 230-231.
Villafañe, V.E., E.W. Helbling, O. Holm-Hansen, and M.P. Hernando. 1996. AMLR Program: Primary production and distribution of chlorophyll-a near Elephant Island, Antarctica, January to March 1996. Antarctic Journal of the U.S., 31(2).
Villafañe, V.E., E.W. Helbling, O. Holm-Hansen, and M. Montes. 1995. AMLR Program: Horizontal and vertical distribution of phytoplankton biomass near Elephant Island during January, February, and March 1995. Antarctic Journal of the U.S., 30(5), 232-234.