Subtidal understorey algal community structure in kelp beds around the Cape Peninsula (Western Cape, South Africa)

The subtidal understorey seaweed communities were studied along a coastal d istance of 104 km around the Cape Peninsula, which is situated in an overla p region between two marine provinces and characterized by a considerable t emperature gradient. Sampling was carried out at six sites (4 to 10 quadrat s per site) around the Cape Peninsula. For each of the quadrats, biomass of each species, grazing, and environmental variables such as temperature, wa ve exposure and sand cover were determined. The data were analysed using ca nonical correspondence analysis (CCA) and two way indicator species analysi s (TWINSPAN). A total of 142 seaweed taxa were found at the six sites (21 C hlorophyta, 14 Phaeophyta and 107 Rhodophyta). The two sides of the Peninsu la have a very different biomass-composition of Chlorophyta, Phaeophyta and Rhodophyta. The biomass of Rhodophyta in the Atlantic sites is much higher than in the Bay, and the biomass of Chlorophyta is higher in False Bay tha n on the west coast. A change in floristic composition of subtidal algal co mmunities around the Cape Peninsula can be observed and is principally rela ted to seawater temperature and wave exposure. Next to these physical facto rs, grazing is demonstrated to be important in determining species composit ion. A lower degree of wave exposure might result in a higher number of gra zers in False Bay. The occurrence of a higher cover of encrusting coralline s in the Bay is probably a consequence of the higher grazing pressure. Dist inct community types can be recognized from TWINSPAN and CCA.