Plankton Under The Ross Ice Shelf

Until recently, the existence of life in the relatively deep cul-de-sacs beneath the large Antarctic ice shelves was primarily a topic of speculation. Because the thickness of the Ross Ice Shelf (400-500 m) precludes photosynthesis in the underlying water column, it was originally suggested that a sub-ice biota might be absent altogether or that a highly specialized, novel assemblage might exist in which chemosynthesis (formation of organic compounds from inorganic substances using energy derived from oxidation reactions) rather than photosynthesis takes place. Efforts by the scientific community to put this speculation to test resulted in the successful undertaking of the Ross Ice Shelf Project (RISP). One of the objectives of the RISP was to examine interactions between the biota of the Ross Sea and that underneath the Ross Ice Shelf, if found. In 1977 and 1978, a hole was successfully drilled at a site about 430 km from the open Ross Sea

(82°22.5'S, 168°37.5'W) through 420 m of ice which was underlain by a 237 m water column (Clough and Hansen, 1979). At all depths studied, investigators found that the water had sparse populations of micro- and macro-organisms, components of which might comprise a food web. Sampled organisms included bacteria, algae, microzooplankton, and large zooplankton.

Samples from three depths in the water column (20, 110 and 200 m) were concentrated on 35 |im mesh Nitex and were examined for microplankton. Pen-nate diatoms accounted for about 80% of the microplankton at 20 m. Forms of several genera, including Amphiprora, Fragilaricrpsis, Navicula, Nitzschia, Pinnularia and Pleurosigma, were observed. No pennate diatoms were seen at 200 m. Centric diatoms were represented by a few empty frustules of Coscinodiscus and Triancria found at 20 and 110 m.

The only dinoflagellates recognized were species of Peridinium (Protoperidin-ium), principally P. depressum and P. antarcticum. A few naked dinoflagellates (10-20 |0.m in length) and a few specimens of the silicoflagellate Distephanes speculum, including several which retained their protoplast, were observed (Azam et al., 1979).

Metazoan forms observed included naupliar and postnaupliar copepods. In addition to the copepods, two specimens, probably of polychaete larva, were found in the sample from 20 m (Azam et al., 1979).

Based on the available data, RISP investigators could not determine whether these microbial organisms represent an indigenous population or if they represent the remnants of populations advected from the Ross Sea. For this reason, no conclusions can be drawn as to whether the organisms found constitute a functioning in situ food web. However, based on 14C fixation experiments carried out in the water beneath the Ross Ice Shelf, it is concluded that the food web is ultimately dependent on water advected from the Ross Sea (Horrigan, 1981).

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