Fig. 8.12. Monthly variation of radiation energy (with average cloudiness) at "Maudheim" station (71°03'S, 10°56'W) compared with monthly changes in surface chlorophyll a and 14C uptake in Antarctic waters (after El-Sayed, 1971b).
Temperature is commonly listed as one of the factors, if not the main factor (Saijo and Kawashima, 1964), influencing the rate of primary production in Antarctic waters. Antarctic phytoplankton appear to be obligate psychrophiles (cold-loving) since they grow well at low temperatures, but show a sharp drop in photosynthetic rate at temperatures above 10°C. Neori and Holm-Hansen (1982) concluded that temperature limits primary production rates at times when light intensity is saturating the photo-chemical apparatus of the cell. Since the phytoplankton are saturated by a light intensity approximately 10-15% of that generally incident upon the sea surface, it is apparent that temperature can be a ratecontrolling factor in the upper 10-20 m of the water column.
The numerous observations on nutrients (phosphate, nitrate and silicate) in Antarctic waters clearly show that these nutrients appear to be in excess of phytoplankton requirements. There are no data available to suggest that phytoplankton
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