The phenology of four species of Sargassum at Magnetic Island, Australia

The seasonal variation in density, length, biomass and reproduction for four species of Sargassum living on reefs at Magnetic Island, Queensland, Australia was quantified monthly for a period of 21 months. All our species showed seasonal peaks in size and reproduction. Sargassum tenerrimum, S. fissifolium and S. oligocystum all showed biomass and length peaks in summer (December-February) followed by a late summer peak in reproduction (March-May) while S. linearifolium had earlier peaks in both size (June-September) and reproduction (September-January). Density of adult plants in mixed species aggregations varied unpredictably with no obvious seasonal pattern. Tagging of adult plants suggests that individuals may persist for one of more seasons. Causal hypotheses to explain the observed temporal fluctuations for these species of Sargassum are discussed