ZONATION AND PHENOLOGY OF THREE SPECIES OF SARGASSUM IN THE INTERTIDAL ZONE OF THE NORTHERN GULF OF CALIFORNIA.

Abstract

Three species of Sargassum are the P10st abundant intertidal macroalgae at Puerto Penasco, Sonora, Mexico. Sargassum johnstonii Setchell & Gardner, S. herporhizum Setchell & Gardner, and S. sinicola Setchell & Gardner var. camouii (Dawson) Norris & Yensen are zoned on emergent reef in low intertidal areas. Sargassum johnstonii occurs in a zone above dense stands of S. herporhizum, and scattered patches of S. 8inicola occur on the lowest emergent reef. Sargassum sinicola, the most abundant species, predominates in pools throughout the intertidal zone. In mid-intertidal pools the species show the same zonation with respect to water depth that they do on emergent reef. Ecological separation is clear, the species occurring in different vertical zones or different habitats (pools or emergent reef). At some sites where S. herporhizum is rare or absent, the upper limit of emergent S. sinicola plants shifts upward probably because of a combination of physical and biological factors. The three species in this highly seasonal region reach maximum size and canopy cover in early spring. All produce fertile receptacles in the spring and shed their branches and die back in summer. Surviving S. sinicola persist through the summer at larger sizes, recommence growth and produce a second crop of receptacles in the fall; the other two species grow but are not fertile until the following spring. The species differ in allocation of biomass to vegetative and sexual reproductive structures. Sargassum herporhizum invests a high proportion of its wet and dry biomass into extensively branched, rhizoidal holdfasts. The holdfasts of the other two species are smaller relative to their upper branches, and are not rhizoidal. Experimental clearings showed that S. herporhizum was the most effective at recovering continuous space after disturbance and also after normal summer dieback. Sargassum johnstonii and S. sinicola produce large volumes of sexual receptacles on buoyant branches, which have the potential for wide dispersal, whereas S. herporhizum produces a relatively small volume. A trade-off between short-range vegetative encroachment abilities and potential for long-range dispersal of sexual propagules may have occurred in the evolution of reproductive strategies of these Sargassum species.