QUANTITATIVE STUDY OF STRUCTURE AND DYNAMICS OF BENTHIC SUBTIDAL ALGAL VEGETATION IN GHANA (TROPICAL WEST-AFRICA)

The subtidal benthic algal vegatation off Tema, Ghana a depths of 8-30 m was surveyed by means of scuba diving. An extensive accumulation over a 7-yr period of presence-absence records was supplemented at the end of the most recent growing season (April 1975) by the collection of material from quadrats. Over 100 spp. were identified from 62 plotless dives and 57 quadrats of 0.25 m2. Normal association analysis and indicator species analysis were performed on the plotless samples. The groupings obtained primarily reflect depth and sampling intensity. Both stands and species were ordinated using reciprocal averaging. The results are not in conflict with those of the classifications, but reveal the importance of substratum as well as depth. The ordination of the quadrat data was more informative than that of the plotless data, because the latter were obtained from mixed substrata. Species-composition based on presence-absence data showed a 10.8% ''turnover'' during a 5-yr interval; there was no net gain or loss in the total number of species. The ''turnover'' is interpreted as representing local shifts in abundance rather than true immigration and extinction. Species diversity was estimated on the basis of biomass data from the quadrat samples. For purposes of comparison the samples from shallower quadrats (depth 10-16 m) were treated separately from those taken from deeper quadrats (those at 22-24 m). The shallower sites have more species, while the deeper sites have greater evenness. The deeper sites have a higher value of Shannon''s entropy H''. The findings are in keeping with expected trends based on the differences between the sites in resources (light supply), environmental stability and heterogeneity of the substratum.