CORRELATION BETWEEN INTERTIDAL SEAWEED COMMUNITY COMPOSITION AND SEA-WATER TEMPERATURE PATTERNS ON A GEOGRAPHICAL SCALE

Quantitative seaweed community data were collected from the lower intertidal region of 21 semi-exposed shores along ca 600 km of coastline around the southern tip of Africa. There is a gradient of ambient sea water temperature regimes at these sites, with annual means from 12.9 to 16.7 °C, and a difference between maximum and minimum monthly means of from 1.8 to 5.8 °C. An analysis by Reciprocal Averaging has revealed a community gradient which follows closely the gradient of temperature change around the coastline. Values for the first axis of the analysis correlate significantly with annual mean, maximum monthly mean and minimum monthly mean sea temperatures. Community samples from the warmest region of False Bay, where temperatures are raised, are similar to those from geographically distant sites with comparable temperature regimes. Apart from one site with a very different community caused by sand-scouring effects, there was no effect of substratum on community composition on the different rock types sampled. It is concluded that not only the absolute ranges of seaweed species distributions, but also the structure and composition of seaweed communities on a biogeographical scale can be correlated with seawater temperature regime. © 1990, Walter de Gruyter. All rights reserved.