Sargassum beds at Rottnest Island: species composition and abundance

Abstract

The change in structure of intertidal and subtidal mixed species Sargassum beds were studied at 3 locations on Rottnest Island, Western Australia. Eight species of Sargassum were found in these mixed beds. The intertidal and subtidal Sargassum beds studied were dominated by the S. spinuligerum-distichum-podacanthum complex, but other species (S. linearifolium, S. ligulatum, S. tristichum, S. decurrens and an unidentified Sargassum sp.) were also recorded growing juxtaposed within the same bed. The relative abundances of all species changed with location. At Crayfish Rock and Nancy Cove at all depths, and in the intertidal at North Point, the S. spinuligerum-distichum-podacanthum complex was the numerical dominant (3-27 individuals 0.25 m⁻² ). In the subtidal at North Point, S. tristichum was dominant (2.5- 3.2 individuals 0.25 m⁻² ), whereas at Crayfish Rock and Nancy Cove it occurred in densities of <1 individual 0.25 m⁻². Species also showed variations with depth. Sargassum decurrens and S. tristichum were found mainly subtidally, whereas S. ligulatum was found intertidally. Densities of most species varied little with the seasons. One exception was S. ligulatum, which appears to be an annual. The seasonal timing of reproduction was also different between locations. Sargassum spinuligerum reproduced in spring and summer at Nancy Cove, Crayfish Rock, and the intertidal at North Point, but in the subtidal at North Point, low levels of reproduction were recorded year round with maximum density of reproducing thalli recorded in autumn-winter. Such variability in presence and abundance with location, depth, and time of the year makes each Sargassum bed unique. I propose that the unique combinations of species present and their abundances in Sargassum beds at Rottnest Island are the product of separate long distance dispersal events (possibly of whole thalli) combined with a highly localized pattern of dispersal of Sargassum propagules. This combination would result in different species present, different dominants, and different timing of reproduction within species (as observed in subtidal populations of S. spinuligerum) between locations.