INFLUENCE OF SARGASSUM SPP. ATTACHED TO RHODOLITHS ON SAMPLING EFFORTAND DEMOGRAPHIC-ANALYSES OF SARGASSUM SPP. (SARGASSACEAE, PHAEOPHYTA)ATTACHED TO A REEF

Sargassum species at Rottnest Island, Western Australia form extensive subtidal beds on a limestone reef. Thalli are also attached to rhodol iths which collect in hollows in the limestone reef. Size frequency hi stograms demonstrate that the sample population consisted of two discr ete components, thalli attached to rhodoliths and thalli attached to t he reef. Sargassum thalli on rhodoliths were a small (2-4%) but persis tent component of the total adult Sargassum population. Recruitment on to rhodoliths was also lower (30%) than on to the reef and mortality was higher. Despite their low abundance and contagious distribution, S argassum attached to rhodoliths contributed to biasing estimates of po pulation density, reproductive output and recruitment potential for Sa rgassum attached to the limestone reef. Sargassum plants attached to r hodoliths also appear to move thus they may have implications for disp ersal and gene flow of Sargassum populations and for other organisms f ound on limestone reefs.