Algal canopy as a proxy for the disturbance history of understorey communities in East Antarctica

Much of the macroalgal zonation on Antarctic coasts is thought to be maintained by ice scour. The frequency and severity of ice scour typically decrease with depth, which is hypothesized to drive the zonation of two canopy-forming macroalgae, Desmarestiamenziesii and Himantothallusgrandifolius. If true, understorey communities should share the same history of ice scour as their respective canopies, and their composition should vary accordingly. To evaluate this prediction we collected boulders from under each canopy species at two depths, 6 and 12 m, at two sites on the coast of East Antarctica. We examined the hard-substrate communities growing on boulders and tested for differences in community composition with respect to canopy species, surface orientation, and depth. Communities under the different canopies showed some variation consistent with the hypothesized difference in disturbance history. Those under H. grandifolius accommodated a greater abundance and diversity of sponges, which is usually characteristic of older, later successional communities. Differences were subtle, however, suggesting that canopies might be maintained by ice disturbance over large temporal scale relative to those at which understorey communities develop, and/or that canopies themselves influence understorey composition. This study describes patterns associated with one of the most prominent examples of bathymetric zonation in shallow Antarctic benthos, and experimental work is now needed to partition the processes at work.