Antarctic Euphotic Zone

* Surface values

** Integrated values for the euphotic zone

* Surface values

** Integrated values for the euphotic zone integrated values for the Atlantic Sector are not markedly higher than those for the Pacific (15.94 mg/m2 and 12.62 mg/m2, respectively; El-Sayed, 1968). Caution should be exercised in interpreting these results, however. While a great many of the stations occupied in the Atlantic Sector were located in relatively productive coastal regions, most of the stations occupied in the Pacific Antarctic were in oceanic regions noted for their lower productivity (El-Sayed, 1970a).

Primary Production

The rate at which photosynthetic organisms synthesize organic matter from inorganic substances (primary productivity) is generally measured in oceanic waters by the UC uptake method (Steemann Nielsen, 1952). Primary productivity in the Pacific Sector generally correlates very well with the distribution of the standing crop of phytoplankton. As with chlorophyll a concentration, primary productivity shows wide point-to-point variation. However, rates of photosynthesis are generally low (< 5.0 mgC/m3/hr), with an average integrated value for the euphotic zone of only 0.134 gC/m2/day (El-Sayed and Turner, 1977; Holm-Hansen et al., 1977). This production value is similar to that reported for other open waters of the world's oceans (Steemann Nielsen, 1954).

In general, maximum photosynthetic activity (like chlorophyll a) occurs at depths corresponding to between 25% and 50% of surface light intensity. Carbon uptake often extends well below the depth of 1% surface illumination. At some stations in the Ross Sea, primary production below the euphotic zone was slightly in excess of 25% of the total production in the water column (El-Sayed et al., 1983). However, in other parts of the Pacific Sector, the percentage could be much lower (< 10%).

As in the case of the phytoplankton standing crop, the primary productivity of surface water samples in the Atlantic Sector was about four times the Pacific value (5.25 mgC/m3/hr compared to 1.22 mgC/m3/hr, respectively). In terms of inte-

Fig. 8.6. Stations occupied by U.S.N.S. Eltanin in the Pacific Sector of Antarctic during Cruises 19-28, less Cruise 27 (after El-Sayed, 1970b).
Fig. 8.7. Vertical distribution of chlorophyll a at several stations occupied during Eltanin Cruise 27. Position of arrows indicates depth of euphotic zone (after El-Sayed, 1970a).

Fig. 8.8. Vertical distributions of total and nanoplankton (< 10 (im) primary production and chlorophyll a at stations occupied during Eltanin Cruise 51 (after El-Sayed and Turner, 1977).

ho KJ O

Fig. 8.8. Vertical distributions of total and nanoplankton (< 10 (im) primary production and chlorophyll a at stations occupied during Eltanin Cruise 51 (after El-Sayed and Turner, 1977).

grated values, however, the Atlantic stations were not substantially more productive than the Pacific stations (50.70 and 32.01 mgC/m2/hr, respectively; El-Sayed, 1968).

Although the Southern Ocean has always been described in terms which reflect its proverbial richness, the results of the primary productivity measurements made in recent years have clearly demonstrated that the high productivity of Antarctic waters is real only with regard to the inshore waters, and not with respect to the oceanic regions. According to Holm-Hansen et al. (1977), the average primary productivity of the Southern Ocean is 0.134 gC/m2/day which is about the same as that of the oligotrophic Gulf of Mexico (El-Sayed and Turner, 1977). This together with the recognition that the productive season in the Antarctic seas is about 120 days per year, forces us to qualify earlier emphasis on the high productivity of the Southern Ocean.

Latitudinal and Longitudinal Variations

Due to the vast expanses of the Antarctic and sub-Antarctic waters in the Pacific Sector of the Southern Ocean, it is instructive to study the latitudinal and longitudinal variations in the phytoplankton standing crop and primary productivity in these regions. When data from Eltanin Cruises 19-28 (less 22) are averaged over 5° of latitude and plotted (Fig. 8.9), it is apparent that values of chlorophyll a and 14C uptake in surface water samples between 40° and 60°S show little variation with latitude. However, substantial increases in these parameters are noted south of 70°S. Averaging over 10° of longitude (Fig. 8.10) shows that the western section of the Pacific Sector of the Southern Ocean between 140°E and 170°W (excluding 150o-160oE) has conspicuously higher values of chlorophyll a and 14C uptake than the central and eastern sections. This richness can be attributed, in part, to the proximity of the observation sites to New Zealand, Australia, and Tasmania, and the so-called "landmass effect" proposed by Doty and Oguri (1956).

The region to the west of the Antarctic Peninsula is also noted for its rich phytoplankton populations; surface chlorophyll a concentrations as high as 18 mg/m3 were recorded by the author from the Gerlache Strait (El-Sayed, 1967). Even higher values (25 mg/m3) are reported by Burkholder and Mandelli (1965) from the same Strait.

Seasonal Variations

The seasonal variations of pigment and primary productivity values collected in the Pacific Sector during nine cruises of the Eltanin are shown in Fig. 8.11. Standing crop and photosynthetic activity of the primary producers were substantially higher during the austral spring and summer cruises than during the autumn and winter cruises. Hart (1942) observed similar seasonal variations in the phytoplankton population and related these variations to seasonal changes in the physico-chemical environment. Such seasonal variations in abundance are most pronounced for the diatoms, which reach their maximum density in mid-

Fig. 8.9. Latitudinal distribution of chlorophyll a and I4C uptake in surface water samples collected during Eltanin Cruises 19-28 (less Cruise 22). Dashed lines indicate average values; numbers in parentheses refer to number of observations (after El-Sayed, 1968).
Fig. 8.10. Longitudinal distribution of chlorophyll a and 14C uptake in surface water samples collected during Eltanin Cruises 19-28 (less Cruise 22). Dashed lines indicate average values; numbers in parentheses refer to number of observations (after El-Sayed, 1968).

January in what Hart referred to as the Intermediate Zone (i.e. c. 55°50'-60°00'S) or at the end of January and in the first half of February further south (Hasle, 1969). The dinoflagellates, in general, parallel the diatoms in their seasonal variations.

Was this article helpful?

0 0

Post a comment