Dispersal of propagules of Sargassum spp. (Sargassaceae: Phaeophyta): Observations of local patterns of dispersal and consequences for recruitment and population structure

Propagule dispersal in Sargassum spp. was studied from reproductive adults as a single point source of reproductive adults and distributed naturally throughout a bed. Dispersal from single sources was investigated by staining 80 reproductive thalli, tying them together at one location, and sampling released propagules by suctioning the substratum with a venturi suction pump at distances up to 2 m from the stained thalli. Dispersal from many reproductive adults within the bed was determined using limestone settlement plates placed within and outside a subtidal Sargassum bed containing reproductive thalli that were releasing propagules. Numbers of settled propagules were subsequently compared with the numbers of macroscopic recruits visible 2-3 months after propagule release and then with mortality of these recruits.From a single source, dispersal of propagules was highly localized with most propagules settling within 1 m (approximate to 98%). Densities of settled propagules declined exponentially with distance from their source. The rate of decline was variable and significantly different (at p = 0.05) between repeated experiments. Propagule settlement was greater within Sargassum beds than beyond them and the source of many of the propagules settling beyond the Sargassum bed appears to be the local bed itself. Mortality of settled propagules was very high with 0.0045% surviving to visible recruits. Further exponential losses occurred such that only 0.0001% survived for 12 months.Propagules of Sargassum disperse locally, most settling within their bed of origin. Highly localized propagule dispersal and settlement could lead to patchy distributions of recruits within and outside a Sargassum bed. The high mortality of recently settled propagules suggests that recruitment into the local adult population could at times be uncoupled from local propagule dispersal. The causes of high mortality of recently settled propagules is unknown and deserves further study.