Factors Governing Primary Production

The factors governing variability in primary production in Antarctic waters have been discussed by Holm-Hansen et al. (1977) and Fogg (1977), and more recently by El-Sayed (1984,1987) and Sakshaug and Holm-Hansen (1986). A brief account of the factors influencing primary production of the Southern Ocean is given below.

Light

It is well known that variations in incoming solar radiation between summer and winter in the Antarctic are extreme. Mean monthly values of radiant energy plotted against UC uptake and chlorophyll a concentration (Fig. 8.12) show clearly that the rise and fall of phytoplankton biomass and primary production in Antarctic waters are in direct response to the amount of incident radiation.

In their study of the effect of radiant energy on the photosynthetic activity of the phytoplankton in the Ross Sea, Holm-Hansen et al. (1977) found that, on days when light intensity was high, photosynthetic rates were low in surface waters, and maximal at depths corresponding to between 25% and 50% of incident radiation. On the other hand, when incident light was low during the in situ incubation period, photosynthetic rates remained fairly constant in the upper waters of the euphotic zone, or they were highest at the surface (Fig. 8.13). These results are

Fig. 8.11. Seasonal variations in surface and integrated chlorophyll a and 14C carbon uptake in the Pacific Sector of the Southern Ocean (from El-Saycd, 1970a).

most probably due to photo-inhibition, whereby high light intensity causes a decrease in photosynthetic rates. The threshold of photo-inhibition for Antarctic phytoplankton was calculated to be in the range of 40-50 cal/cm2/half-light day.

Light penetration in Antarctic waters is determined not only by the intensity and angle of incidence of light, surface reflectance, and absorption by suspended particles, but also by the presence of thick fast-ice and pack-ice, which appreciably reduces the amount of submarine illumination.

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