JENNY MATURANA and NELSON SILVA, Escuela de Ciencias del Mar, Universidad Católica de Valparaiso, Valparaiso, Chile
CHRISTOPHER D. HEWES and OSMUND HOLM-HANSEN, Polar Research Program, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202
The essential macroinorganic nutrients nitrogen, phosphorus, and silicon are generally not limiting phytoplankton growth in antarctic waters. The likely exceptions are in protected coastal areas where phytoplankton blooms over 25 micrograms of chlorophyll- a have been recorded (Holm-Hansen et al. 1989) and in the region of the Polar Front, where silicon concentrations are low (Tréguer and Jacques 1992). In the AMLR study area ( see Martin, Hewitt, and Holt, Antarctic Journal , in this issue), concentrations of inorganic nitrogen, phosphorus, and silicon are generally well above the concentrations thought to be limiting phytoplankton growth. The main interest in measuring inorganic nutrient concentrations in the AMLR large-area survey grid involves the use of nutrients to characterize different water masses, as well as to indicate the extent of nutrient recycling in the upper water column.
Water samples for nutrient analyses were obtained from the 10-liter Niskin bottles attached to the conductivity-temperature-depth (CTD)/carousel profiling unit. Acid-cleaned high-density polyethylene bottles of 50-milliliter capacity were rinsed four or five times with water directly out of the Niskin bottle before being filled with approximately 35 milliliters. The sample bottles were then frozen in an upright position and maintained at -20°C or lower until time of analysis. The samples were analyzed at the Universidad Católica de Valparaiso with an autoanalyzer following the techniques described by Atlas et al. (1971). Nitrate and nitrite were not determined separately, so that the term "nitrate" refers to the sum of nitrate plus nitrite. The concentration of nitrite, however, is very low as compared to that of nitrate; it averages approximately 1 percent of the nitrate concentration in antarctic surface waters (Biggs et al. 1982).
The concentrations of nitrate, phosphate, and silicic acid at 5 meters depth throughout the AMLR survey grid are shown in figure 1. The lowest concentrations for nitrate and phosphate are found in the relatively shallow region between King George Island and Elephant Island and also in the northeastern portion of the survey grid. These areas correspond to the regions of relatively high phytoplankton biomass (Holm-Hansen et al., Antarctic Journal, in this issue), and hence, the low nutrient concentrations reflect nutrient uptake by primary producers. The pattern of silicic acid distribution is different; the lowest values are in the northwestern portion of the survey grid.
Figure 2 shows the concentrations of silicic acid (106 stations, 5 meters depth) in the five water zones as determined by temperature and salinity characteristics ( see Amos, Wickham, and Rowe, Antarctic Journal , in this issue). Water Zone I (Drake Passage waters) has the lowest silicon concentrations, whereas Water Zone V (Weddell Sea water) has the highest concentrations. The two stations in Water Zone I with relatively high silicon concentrations are A74 and A129, which lie on the periphery of Water Zone I and apparently are mixed to some extent with waters from Water Zones II and IV, respectively.
The concentrations of nitrogen, phosphorus, and silicon in the upper 200 meters of the water column at seven stations are shown in figure 3. The most dramatic difference between the different water zones is that stations in Water Zone I have much lower concentrations of silicon throughout the upper 100 meters as compared to stations in Water Zones II or V. The concentration of silicon at 50 meters at Station A63 is seen to resemble Water Zone I waters. It is to be noted that Station A63 is in Water Zone II, but that the temperature and salinity values suggest considerable mixing with Water Zone I.
The nutrient concentrations reported above are much higher than the concentrations generally required to permit maximal growth rates of phytoplankton. This strongly suggests that the major inorganic nutrients (nitrogen, phosphorus, and silicon) are not limiting rates of primary production in the AMLR study area. There is evidence, however, that phytoplankton growth rates in Water Zone I waters are limited by availability of iron (Holm-Hansen et al. 1994).
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, and O. Holm-Hansen.
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