THE STRUCTURE OF SUBTIDAL ALGAL AND INVERTEBRATE ASSEMBLAGES AT THE CHATHAM ISLANDS, NEW-ZEALAND

We examined the distribution and abundance of organisms on subtidal ro cky reefs at nine sites around the Chatham Islands, a remote group 780 km east of southern New Zealand. We sampled five depth strata ranging from 1 to > 16m to identify spatial patterns in the abundance of alga e and invertebrates and to assess their variation within and among sit es. This information is used to discuss hypotheses concerning communit y structure at this remote locality. Several patterns were apparent. T he immediate subtidal was occupied by the southern bull kelp Durvillae a spp. A suite of 11 fucalean species were dominant to a depth of 10 m with an average abundance of 28 m(-2), while one species, Carpophyllu m flexuosum, occurred mostly in deeper water. Only two laminarian spec ies of algae were present at the islands. The indigenous Lessonia thol iformis was abundant at 2.5 to 15 m and was not found in deeper water, while the giant kelp Macrocystis pyrifera was abundant at two sites i n 12 to 18 m. The commercially valuable abalone Haliotis iris was extr emely abundant in shallow water, with an overall mean of 6 m(-2) at 5 m. The sea urchin Evechinus chloroticus was common, but reached high d ensities only in small (< 25 m(2)) patches. The characteristic urchin- dominated zones reported in kelp beds world-wide were not seen. There was considerable site-to-site variation in the occurrence and abundanc e of individual species. Some differences between sites were associate d with shelter from swell (e.g. M. pyrifera was found only in sheltere d sites) and physical habitat (e.g. juvenile H. iris were found only b eneath boulders inshore), but much of the variation could not be expla ined by physical or depth-related factors alone. We hypothesize that t he differences in these kelp bed assemblages compared to mainland New Zealand are partially due to the high degree of endemism at the Chatha m Islands. Local variation cannot be explained by herbivory, and is mo st likely the result of the various life-history characteristics of th e major habitat-forming species, the large brown algae.