Colpomenia bullosa crust (Phaeophyceae) stressed less by grazing than by abiotic factors in high intertidal rockpools in southeast Australia

The influence of stresses on the crust phase of Colpomenia bullosa, originally thought to be Ralfsia verrucosa, inhabiting rockpools above the lowest high water on Kainga Reef, SE New South Wales, was investigated. The mean densities of grazing gastropods Nerita atramentosa and Austrocochlea porcata were 107 and 59 animals m^-2, respectively, with no seasonal variation. Some pools were more attractive to Austrocochlea than others. Herbivore impact on C. bullosa crust was assessed by collecting and examining feces and counting the number of crust filaments. Seasonal variations in grazing and in crust cover were not related in a way that would suggest a causal relationship. The calculated annual consumption was 7% of the crust. Grazing probably kept the crust clean of epiphytes. When local conditions isolated the pools from the sea for days, some crusts survived a known salinity of 127 psu and an estimated 175 psu. The crust of C. bullosa on pool sides survived up to 58 h of emersion, but complete drying on pool bottoms was lethal within 14 h presumably as a result of saturating salinity preceding complete drying. UVB-absorbing substances, assumed to be phlorotannins, were released by the crust during pool drying. It is concluded that high temperatures were the likely cause of both the slow growth rate and the high mortality of C. bullosa crust on Kainga Reef in summer.