The shading effects of Dictyota sp . algae ( Dictyotales , Phaeophyta ) on Acropora aspera coral on the Heron Island reef flat

A major threat to coral reefs today is the phase shift to algaldominated reefs. Algae can stress corals by overgrowing and smothering them, blocking sunlight needed for photosynthesis, and enhancing microbial growth. However, the mechanisms underlying algae-coral interactions are still poorly understood. This study examined how the tumbleweed algae Dictyota sp. affects the photophysiology of the branching coral Acropora aspera at Heron Island, southern Great Barrier Reef, where both species are now commonly found. Algae-covered and algae-free samples from the reef flat and from a three-day aquaria experiment were compared. Dinoflagellate density and chlorophyll a content increased in algae-covered coral branches. Therefore, A. aspera covered by Dictyota was shade-acclimated, which may temporarily protect A. aspera from light stress. Future research should address long-term effects, as this study raises the possibility that Dictyota shading could either against or enhance thermal bleaching.