The relationship between physical variables on topographically simple and complex reefs and algal assemblage structure beneath an Ecklonia radiata canopy

Limestone reefs in South-Western Australia range in topographic complexity at the 1–10 m scale from simple, flat, planar reef structures to topographically complex reefs with many rock walls, overhangs and channels. Algal assemblage structure is known to differ between topographically simple and complex reefs. This research assessed if differences in algal assemblage structure could be associated with the physical environment present on each reef type. Sampling of the physical environment and algal assemblage was carried out on simple, planar reefs and complex, rugose reefs. The physical environment described almost 80% of the algal assemblage structure under an Ecklonia radiata canopy. It was found that topographically simple reefs had greater sediment cover and water depth than adjacent reefs with more complex topography which had more downwelling light, stronger and more turbulent water motion and a greater range of substrate slopes. Greater variation in most physical variables on topographically complex reef shows a greater diversity of microenvironments on that reef type. It is argued that these differences in physical environment between reef types have a considerable influence on algal assemblages by altering key biological processes such as recruitment and interspecific competition.