The density, standing stock, reproductive periodicity and alginate yields have been investigated for a mixed Sargassum species population at Reunion Rocks, KwaZulu-Natal, South Africa. The population consists of two dominant components, Sargassum elegans Suhr and a second species, here termed Sargassum sp. 1. Sargassum elegans typically makes up more than 70% of the population which is heterogeneous in both biomass and density distribution. These species share the same macro-scale distribution within the site and display the same periods of growth and senescence. Standing stock dropped in the cooler winter months and reached a maximum in the hot summer months. Biomass was correlated to one month antecedent minimum, maximum and mean air temperature. Density patterns followed a similar pattern and biomass increased with density. Alginate yield of both species peaked in the warm summer months and was lowest during the cooler winter months. Reproduction peaks were recorded i n the cooler months after the peak in biomass and density.