Summary

Subtidal reefs in temperate waters support a great variety of marine life that lives attached to the rock surfaces. Algae (seaweeds), sponges and ascidians (sea squirts) are usually the main groups represented. Within these groups, the particular species found on any given reef depend on such factors as depth, reef structure and the amount of wave action. Some species are present all year round, while others are only seasonally abundant.

In this study, the spatial and temporal patterns of variability in ascidian assemblages were investigated on horizontal subtidal rocky reefs at Port Stephens, on the mid-north coast of NSW. The surveys (one in summer and one in winter) were designed to provide a baseline dataset on ascidian diversity and distribution patterns for an area that is now part of the Port Stephens – Great Lakes Marine Park. The composition of ascidian assemblages between exposed oceanic island reefs and sheltered reefs within Port Stephens, and between two depth zones within each reef system, were measured from direct counts made by SCUBA divers and by an analysis of underwater photographs. Twenty-one different ascidian species were recorded, with sheltered reefs having more species (14 – 15) and greater abundances than exposed reefs (11 – 12 species). Ascidian assemblages were highly variable between individual reef sites and particularly between depth zones, but temporal variation was only observed for a few species. Such highly variable spatial patterns in the distribution and abundance of marine organisms is not unusual. The large size of the Port Stephens – Great Lakes Marine Park and the representation of many different types of reef habitats within it, however, means that the full range of ascidians and other rocky reef biodiversity is likely to be adequately protected into the future.