BIOMASS, COVER AND ZONATION PATTERN OF SUBLITTORAL MACROALGAE AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS, ANTARCTICA

Antarctic macroalgae have been studied taxonomically and biogeographically, but vegetational zonation patterns are still described intuitively. Quantitative studies on the macroalgal vegetation at Signy Island, Antarctica, are scarce. The aims of this study were to provide a more complete species list, to collect information of the distribution and zonation pattern of the algae by studying standing crop and percentage cover of the macroalgae in relation to depth and site, and to quantify the influence of environmental variables on the species composition of macroalgal communities. Transects at two rocky sites, one sheltered and one exposed, were studied in detail using both a photographic and a harvest sampling method. Thirty-six species were identified. Both the number of species and macroalgal biomass were low compared with sub-Antarctic regions. The vertical zonation found was: an ice-abraded zone characterized by Iridaea cordata, a zone 5 to 14 m depth dominated by Desmarestia anceps and Desmarestia menziesii and a zone 15 to 25 m characterized by Himantothallus grandifolius. Of the four environmental variables studied (depth, substratum, slope, exposure) only depth and substratum were significantly related to the species composition of the algal vegetation. No species were found with an optimum at depths greater than 20 m and the lowest depth of occurrence for Himantothallus grandifolius was predicted at 35 m. A consistent shift was found between the two sites studied: sample plots of the sheltered site corresponded with plots roughly 1 to 2 m deeper at the exposed site. Depth-response models of the macroalgae indicated a higher probability of occurrence for Desmarestia anceps, Himantothallus grandifolius and Rhodophyta towards shallower depths in the more sheltered site. The probability of finding Desmarestia anceps deeper than 29 m is nil, while the possibility of finding Himantothallus grandifolius is still 20% at 29 m. Important factors in explaining. the growth of macroalgae higher in the sublittoral at the sheltered site might be the light conditions and ice scouring at the start of the growing season. In summer, differences in exposure and biological factors might be of more importance in explaining the biomass differences in macroalgae between the two sites.