Hooker (1847) was among the first to report on the association of marine flora with sea ice and the discoloring effect of algal growth on icebergs and pack-ice. He observed,"... they occurred in such countless myriads, as to stain the Berg and the pack-ice a pale ocherous colour." These brown stains are caused by a community of organisms, principally diatoms, living either in the ice or in the brine pockets of the ice. This group of microflora includes over 50 species of mainly neritic diatoms, some chrysophytes, dinoflagellates and green flagellates. Of these, diatoms are ubiquitous and generally dominant. The ice diatom community is composed of both attached species such as Pleurosigma, Nitzschia, Amphi-prora, and Fragilaria, and nonattached species such as Odontella, Coscinodiscus, and Asteromphalus. This community is quite distinct from the planktonic community.
Dinoflagellates are generally rare, although occasionally they may achieve dominance. Phaeocystis, on the other hand, may be present in some areas in enormous numbers at certain times of the year.
Because of the large area that experiences seasonal ice advance and retreat in the Southern Ocean (ice covers on average an area as large as 18.5 x 106 km2 in winter but only about 3.6 x 106 km2 at the end of the austral summer), the influence of the ice-edge on the Antarctic marine ecosystem is expected to be pronounced. One potentially significant impact of the ice-edge is its influence on phytoplankton distribution. Marginal ice zones have been shown to support large blooms of phytoplankton. A dense bloom of Nitzschia curta was encountered near the receding ice-edge off the coast of Victoria Land, Antarctica (Smith and Nelson, 1985). A possible explanation of ice-edge blooms is that the low-density melt-water creates a vertically stable water column.
With the ice microflora serving as a bloom innoculum, phytoplankton accumulation within the euphotic zone is thus promoted. Kozlova (1964) contended that ice conditions determine the quantitative development of diatoms to a greater degree than does the time of year.
Recent studies of the ice-edge zone in the Weddell Sea indicate that, next to the diatoms and Phaeocystis, the protistan choanoflagellates (collared heterotrophic flagellates bearing an extracellular lorica) are the third most abundant group in the water column and are, at times, locally dominant (Buck and Garrison, 1983). These authors provided the first quantitative estimates of the composition of nanoplankton and microplankton in the ice-edge region of the Weddell Sea. The choanoflagellates constitute a conspicuous component of Antarctic nanoplankton, comprising 10-40% of the total number of living nanoplankton (Marchant, 1985). The great abundance of the choanoflagellates and other heterotrophic forms in the ice-edge zone suggests that a tightly coupled food web composed of bacterial, detrital and nano-algal production exists. These microheterotrophs are sometimes as abundant as diatoms and may be an important food source for pelagic consumers such as euphausiids (von Brockel, 1981; Kils, 1982).
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