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Fig. 8.13. Vertical distribution of photosynthesis (PS) measured in |ig/litre/hr and temperature at Station 14 (left) and Station 16 (right) of Eltanin Cruise 51 (January/February 1972) in the Ross Sea (after Holm-Hansen et al., 1977).

Fig. 8.13. Vertical distribution of photosynthesis (PS) measured in |ig/litre/hr and temperature at Station 14 (left) and Station 16 (right) of Eltanin Cruise 51 (January/February 1972) in the Ross Sea (after Holm-Hansen et al., 1977).

growth in the Antarctic is limited by nutrient deficiency (Hayes et al., 1984). Even at the peak of phytoplankton growth, the concentration of nutrients remains well above limiting values. For instance, during the heavy bloom of Phaeocystis pouchetii in the Ross Sea (El-Sayed et al., 1983), the nitrate concentration was still high, with euphotic zone values averaging 17 |ig at./l. It is therefore unlikely that nutrients are sufficiently low at any one time to become limiting factors to the growth of the phytoplankton. However, according to Walsh (1971) and Allanson et al. (1981), patterns of silicate distribution indicate that Si03 may be the most limiting of the major nutrients for the growth of Antarctic diatoms, thus corroborating a similar conclusion arrived at earlier by Hart (1942). With regard to the trace elements, Jacques (1983) carried out enrichment experiments using Zn, Mo, Co, Mn, and Fe, and showed that they are not limiting factors. It is, however, possible that organic factors, e.g., vitamin B12 and thiamin (see Carlucci and Cuhel, 1977), may alter the species composition of the phytoplankton without changing the overall rate of primary production. It is also possible that the availability of trace micronutrients may be altered by pack-ice and iceberg melt-water, thus affecting the productivity or species composition of the waters in the vicinity of pack-ice and icebergs.

Water Column Stability

Several investigators have drawn attention to the importance of the stability of the water column in controlling production (Braarud and Klem, 1931; Gran, 1932; Sverdrup, 1953; Pingree, 1978). Of the several processes which have been suggested to be important in initiating and sustaining near-ice blooms, the most significant is the vertical stability induced by melt-water. According to this suggestion, first proposed by Marshall (1957) for Arctic waters, the low salinity of melt-water contributes to the stability of the near-ice water column, thus helping to retain the phytoplankton near the surface and promoting a bloom. Corroborative evidence that this mechanism is important in the initiation of ice-edge blooms in the Antarctic is furnished by Jacobs and Amos (1967), El-Sayed (1971a) and Smith and Nelson (1985). Stability of the upper water layers may therefore play an important role in the development of Antarctic phytoplankton blooms. Sakshaug and Holm-Hansen (1986) report that for all "bloom stations", where chlorophyll a was > 2 mg/m3, the pycnocline (zone where water density changes markedly with depth) was between 20 m and 40 m deep. They speculate that 50 m is the maximum pycnocline depth for a bloom to develop. The presence of an homogeneous (i.e., isothermal) water column reaching to a depth of 50-100 m during most of the year therefore hinders the development of blooms and contributes to the low primary production of Antarctic waters.

Grazing

In the Southern Ocean, euphausiids (principally Euphausia superba, "krill"), which may constitute half of the Antarctic zooplankton biomass (Holdgate, 1970), are the dominant herbivores. Several investigators have observed that areas of high krill concentration are usually noted for their low standing crop of phytoplankton. In 1981, during the First International BIOMASS Experiment (FIBEX), Polish investigators found that, in the central parts of the Bransfield Strait, areas of dense krill concentration exhibited low chlorophyll a values at the surface (< 0.5 mg/m3) and in the water column (< 50 mg/m2) (Rakusa-Suszczewski, 1982). Chilean scientists also reported similar findings during FIBEX. For example, in the region to the south of the South Shetland Islands, Uribe (1982) measured low values of chlorophyll a (< 0.5 mg/m3) in the central part of the Bransfield Strait where high krill concentrations in the 10-200 m layer were found. According to Uribe (1982), the poverty of the phytoplankton was not due to nutrient limitation (phosphate : 1.4 ng at./l; nitrate : 17.9 (ig at./l) but was most likely due to intensive krill feeding. In addition, data from the R.V. Melville cruise (also during FIBEX) demonstrated, on a temporal basis, the inverse relationship often noted between phytoplankton biomass and zooplankton (mainly krill) abundance (Holm-Hansen and Huntley, 1984).

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