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During a combined hydroacoustic-trawling survey at the Prince Edward Arch~pelago, a kelp bed of Macrocystis laevis growing at a maximum depth of 68 m was repeatedly observed in the open shelf area This is the deepest population ever described for any species of giant kelp. Nutrient availability and temperature in the area were found to have no significant impact on kelp production and distribution. Likewise, measurements of photon flux density showed that neither plant growth nor sporophyte formation was likely to have been light-inhibited during the period of the observations. However, since irradiance levels in the deep kelp bed were close to the lower threshold for growth, the absence of plants below 68 m depth was probably determined by light availability. Around Marion Island, where M. laevis is found only within the 20 m isobath, the offshore limit of the kelp beds is controlled by availability of suitable rock substrata. The inshore limit appears to be set by the maximum height of storm-induced waves. Four species of Macrocystis are generally recognized: M, integrlfolia, M, angustifolja, M. pyrifera and the recently described M. laevis (Womersley 1954, Neushul 1971, Hay 1986, Brostoff 1988). Macrocystis kelps are found in cold temperate oceans with surface temperatures varying from 0 to 20 OC, and their main distribution is circumpolar in the Southern Hemisphere, between 40 and 60" S (Table 1; Cribb 1954, Michanek 1983). They are also present in the major coastal upwelling areas. On the west coast of South Africa they occur between 33"24' and 34"8'S and on the Pacific coast of South America extend almost to the equator, reappearing in North America along Baja California and reaching southern Alaska to the north (Table 1). Only 2 species of Macrocystis are represented in the Southern Ocean: M. pyrifera and M. laevis. While M. pyrifera is widely distributed from subantarctic South America to New Zealand, and occurs at most islands in the Atlantic and Indian sectors of the Southern Ocean, M. laevis is endemic to the Prince Edward Archipelago

Deep occurrence of the giant kelp Macrocystis laevis in the Southern Ocean