Effects of propagule settlement density and adult canopy on survival of recruits of Sargassum spp. (Sargassaceae: Phaeophyta)

The effects of increased densities of settled propagules and the presence of adults on survlval of recruits were lnvest~gated to determine whether patterns of propagule settlement would pel slst and be expressed as distribution of recruits for Sargassum spln~l l lgeru~n S d ~ s t l c h u n ~ and S podacanthum in a subtidal mixed specles bed at Rottnest Island, Western Australia The effect of highei densities of settled propagules (recruit-recruit lnteractions) was studied uslng l~mestone settlement plates seeded with known densities of propagules Density-dependent n ~ o r t a l ~ t y occurred at settlement densities < 105 propagules m A large component of recruit mortality was density independent (57 "/;> of total mortality) Vanabllity in recruit survival at any one settlement density was also great Growth of recruits was negatively density dependent Two distinct sizes of recru~ts were observed (< 10 and > 10 mm in height) The effect of adult canopy (recruit-adult interactions) on recruit sur\ilval was studied using a cleanng expenment where adult thalli were removed from 4 m2areas For the flrst 6 mo the presence of adults had llttle effect on recruit survival Aftei 6 mo, the presence of adults leduced recruit suivival3-fold and surv~val was highly vanable either under canopies or in cleared areas Distr~butions of dispersed propagules can persist as distnbutions of adults ivlthin beds of Sargassum spp at Rottnest Island More likely this would happen where patches within the bed are recolonized or where the bed is extended at ~ t s edges, because orders of magn~ tude differences in density of settled propagules would occur in these areas over dlstanccs of meters Recruit survival at densities < lo5 ind m-' are equally influenced by density-dependent and density-independent processes Presence of adult canopy reduces survival of recruits > 6 mo old, but the amount that survi\~al is reduced by canopy is not within the same range as differences in denslty of propagule settlement The causes of high densityindependent mortality and its effect on patterns of propagule settlement need to be studied further