Association of epilithic algae with sediment distribution on a windward reef in the northern Great Barrier Reef, Australia

Epilithic algal communities (EACs) can trap sediments on hard substrata of coral reefs. The distribution of sediments and the relationship between EAC s and sediment loads were investigated on windward reef zones near Lizard I sland in the northern Great Barrier Reef, Australia. The load (accumulated weight) and grain size composition of sediments within EACs differed signif icantly among zones. EACs on the reef crest contained consistently low load s of sediments (1-2 g dm(-2)) which were poorly sorted containing even prop ortions of sediment grains from silt to coarse sand size. With leeward dist ance from the crest, sediments increased in average load (up to 56 g dm(-2) ) and became locally patchy and relatively well sorted. The modal size clas s was predominantly 500-1000 mu m. Sediment load on substrata was related t o algal functional groups; coverage of crustose and small branching forms d isplayed negative and positive correlations with sediment load respectively . The heights of EACs were significantly correlated with sediment loads (r = 0.83; P < 0.001). Sediment loads were reduced during a period of rough we ather, with corresponding reductions in the heights of EACs. The results in dicate that physical features of EACs influence sediment distribution on co ral reef substrata and may mediate the effects of sedimentation on reef bio ta.