The response of encrusting coralline algae to canopy loss : an independent test of predictions on an Antarctic coast

We assessed whether published observations ofthe ecology of encrusting coralline algae (Rhodophyta) from tropical and temperate coasts could be used to predict patterns and responses on a polar coast where such knowledge does not exist. On subtidal rocky coasts near Casey, East Antarctica, we detected a strong positive association of understorey encrusting coralline algae with canopies formed by the endemic alga Himantothallus grandifolius. The experimental removal of H. grandifolius caused corallines to bleach from red to pink/white concomitant with a decline in their photosynthetic activity. The magnitude of this decline (mean ± SE= 56.85±8.43%) was remarkably similar to that observedon temperate coasts (45.98±5.91%). Positive effects of nutrient enrichment of the surrounding water, hypothesized to alleviate the negative effects of canopy loss on encrusting corallines, were not detected. Removing H. grandifolius increased the intensity of photosynthetically active radiation and ultra-violet radiation reaching the substratum by three orders of magnitude, providing the basis for models invoking enhanced irradiance as the primary cause of the negative effects of canopy loss. Striking similarities among our results and those from tropical and temperate coasts suggest that responses of encrusting corallines to loss of canopies may have predictive properties across large distances and environmental gradients (tropical–temperate–polar).