Colonisation 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 shallowest 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 disturbance with increasing rock size and depth. Percent area colonised, number of animal phyla, bryozoan species and bryozoan colonies all increased both with rock surface area and depth. The largest rocks in the intertidal had broadly similar levels of colonisation and community development as did the smallest at 42 m. Thus, because of depth-dependent ice-scour, community development in the Antarctic sublittoral may be followed along a pseudo-time sequence by using two axes (substratum size and depth) of environmental stability. Frequent disturbance appears to be responsible for maintaining the level of diversity and preventing monopolisation. Bryozoans and polychaetes were the most abundant encrusting animal groups, although tunicates and sponges were the dominant overgrowth competitors. The faunal elements of the colonising biota were almost entirely confined to the undersurfaces of rocks whereas algae dominated upper surfaces. In most bryozoan species the proportion of colonies occurring on the upper surfaces of rocks increased with depth to 34 m and then decreased at 42 m where silt deposition apparently became a major influence. Such a shift in distribution may reflect decreasing current velocities, and therefore reduced disturbance to animal feeding, and/or decreasing growth of coralline algae due to reduced light availability.