HORIZONTAL ZONATION OF EPIPHYTIC ALGAE ASSOCIATED WITH AVICENNIA-MARINA (FORSSK) VIERH PNEUMATOPHORES AT BEACHWOOD MANGROVES NATURE-RESERVE, DURBAN, SOUTH-AFRICA

A sampling technique designed to study the distribution of macroalgae associated with Avicennia marina (Forssk.) Vierh. pneumatophores was applied. Eighteen species of algae were recorded growing epiphytically on the pneumatophores. A variety of environmental factors were measured simultaneously. Resultant data were subjected to Correspondence Analysis and grouping patterns which influenced the two major axes on the scatterplot revealed the presence of ecological gradients through the mangrove swamp. Algal cover was dominated by Bostrychia moritziana (Sender) J. Ag., B. radicans (Mont.) Mont. and Caloglossa leprieurii (Mont.) J. Ag. Clustering of these species on the scatterplot corresponded to a salinity gradient through the swamp from isosalinity in the south to hyposalinity towards the north. A second gradient, lateral in influence, corresponding to wetting frequency was apparent from the permanent water supply (the creek) outwards to the drier outer pneumatophore perimeter. Microcoleus chthonoplastes Thuret ex Gomont and Lyngbya confervoides C. Ag. ex Gomont were the most abundant Cyanophyceae, but were usually inconspicuous within the mass of Rhodophyceae. Rhizoclonium implexum (Dillw.) Kuetz. and R. riparium (Roth) Harvey, which were visually obvious in the field when clustered together in macroscopic colonies, were the most abundant members of the Chlorophyceae. Distribution of the blue-green and green algae was similar to that of the red algae, although they tended to be more apparent in the drier areas as the red algae became less dominant. Salinity, tidal inundation and the related wetting frequency appeared to be the main environmental factors governing horizontal zonation of epiphytic algae associated with mangroves in this estuarine ecosystem.