COLONIZATION AND DEVELOPMENT IN ENCRUSTING COMMUNITIES FROM THE ANTARCTIC INTERTIDAL AND SUBLITTORAL

A total of 985 rocks with a wide range of surface areas were examined from six locations at Signy Island in the maritime Antarctic. The shal lowest site was intertidal and the deepest at 42 m. The probability of coralline algae occurring was found to increase with surface area and depth of rocks, implying decreased levels of turnover or physical dis turbance with increasing rock size and depth. Percent area colonised, number of animal phyla, bryozoan species and bryozoan colonies all inc reased both with rock surface area and depth. The largest rocks in the intertidal had broadly similar levels of colonisation and community d evelopment as did the smallest at 42 m. Thus, because of depth-depende nt ice-scour, community development in the Antarctic sublittoral may b e followed along a pseudo-time sequence by using two axes (substratum size and depth) of environmental stability. Frequent disturbance appea rs to be responsible for maintaining the level of diversity and preven ting monopolisation. Bryozoans and polychaetes were the most abundant encrusting animal groups, although tunicates and sponges were the domi nant overgrowth competitors. The faunal elements of the colonising bio ta were almost entirely confined to the undersurfaces of rocks whereas algae dominated upper surfaces. In most bryozoan species the proporti on of colonies occurring on the upper surfaces of rocks increased with depth to 34 m and then decreased at 42 m where silt deposition appare ntly became a major influence. Such a shift in distribution may reflec t decreasing current velocities, and therefore reduced disturbance to animal feeding, and/or decreasing growth of coralline algae due to red uced light availability.