Disturbance and organisms on boulders

Summary

Several aspects of disturbance by waves to intertidal boulders were monitored for one year high and low on the shore on two rock platforms on the east coast of Australia. These boulders were also subjected to burial by sand, and this disturbance was monitored in the same places for two years. The mobile and sessile organisms on the tops and bottoms of the rocks were sampled six times during the same period in order to determine patterns in community structure and assess the role of disturbance. Small rocks were overturned more often and buried to a greater extent than large rocks, but rocks of all sizes were moved similar distances with similar frequency. These contrasting results were probably caused by the fact that small rocks were rounder than large rocks. Rocks were buried in sand much more at Long Reef than at Cape Banks, but differences among places in disturbance by waves were not great.

Patterns in community structure were complex but suggested that disturbance was only important for sessile organisms on the undersides of boulders. The number of sessile species present was always related to the size of the rock but the total cover of algae on the tops of rocks never was. Some particular species were most abundant on large rocks but others showed the reverse pattern, covering more space on the smaller boulders. This latter pattern may have been due to the shorter period of exposure during low tide in this situation, though grazers were also abundant on these rocks in some places. In contrast, nearly all species on the undersides of rocks reached greatest abundance on the medium to large rocks, consistent with effects of disturbance. Space was in short supply low on the shore and overgrowth apparently reduced diversity but this was not the case elsewhere. Overall, the results indicate that disturbance was frequent and important, but not for all species nor in all places.