Seasonal patterns of abundance, distributions, and phenology in relation to growth strategies of three Sargassum species

Seasonal patterns of abundance of Sargassum johnstonii Setchell & Gardner, S. herporhizum Setchell & Gardner, and S. sinicola Setchell & Gardner var. camouii (Dawson) Norris & Yensen were observed in the northern Gulf of California where mean monthly sea temperatures range annually from < 14 °C to nearly 30 °C. The three species displayed peaks in size, canopy cover, and fertility in the spring, in between periods of seasonal temperature extremes. Larger plants were the first to develop reproductive structures, suggesting that a threshold in size must be reached before plants become fertile. Fertile receptacles were shed in summer. S. sinicola var. camouii retained a greater portion of its primary stipes during the summer dieback than the other two species and showed a second peak in fertility in fall. Whereas tropical Sargassum are most abundant in winter and temperate-zone Sargassum are most abundant in summer months, the Gulf of California species achieve maximum size and fertility in between seasons of extreme temperatures. A long-term decline in abundance of Sargassum was observed at the study site between 1977 and 1981, possibly due to the detrimental effects on Sargassum of anomalously warm sea temperatures in the Gulf of California in 1978 or to the subsequent effects of warm temperature on food webs in the region.