The role of recruitment in establishment and persistence of crustose coralline algae (Corallinales, Rhodophyta) has received little study. Studies of crustose coralline algae have focussed mainly on the ecology of adult populations. This thesis investigates the role of recruitment in the persistence of crustose coralline algae at Rottnest Island, Western Australia.

Crustose coralline algae occupy large areas of rocky substrata in both intertidal and subtidal habitats in southwest Australia. Twelve species of crustose coralline algae belonging to seven genera covered most limestone substrata on reefs in kelp, seagrass and reef pavement habitats at Rottnest Island. The species were a mixture of tropical and temperate taxa reported from Australian waters. The most diverse genus was Hydrolithon (Foslie) Foslie containing four species. The genera Mesophyllum Lemoine and Lithophyllum Philippi were each represented by two species. Sporolithon Heydrich, Lithothamnion Heydrich, Metamastophora Setchell and Spongites Kiltzing each contained only one species. A key to these common genera for use by ecologists with limited knowledge of crustose coralline anatomy is presented using gross, conspicuous morphological characters that delineate the genera.

An understanding of spatial variation in recruitment is required so hypotheses can be posed at relevant spatial scales, but this is not often studied. Spatial variation in recruitment of crustose coralline algae was determined at scales from 5 mm to 2 km, in reef pavement, seagrass and kelp habitats at Rottnest Island. Small-scale spatial pattems (scales 5-25 mm) in recruit density was non-random when recruit densities were high, implying density dependent processes influenced the distribution of recruits. Random distributions of recruits occurred when densities were low in each habitat. Significant spatial variation in recruitment was demonstrated at scales <2 m suggesting that recruitment in crustose coralline algae was strongly influenced by within-habitat, local processes. Large amounts of variation were also found between northern and southern shores of Rottnest Island (< 2 km) indicating that regional-scale processes may also influence recruitment in reef pavement and seagrass habitats. Significant variation in recruitment at scales < 0.5 m (i.e. terracotta tiles) suggested that, 50 x 50 mm areas on 100 x100 mm terracotta tiles were appropriate for sampling recruitment in assemblages of crustose coralline algae at Rottnest Island.

Demography and reproductive phenology of a mixed species assemblage of adult crustose coralline algae were determined at Nancy Cove, Rottnest Island during April 1997 (Autumn), June 1997 (Winter), September 1997 (Spring) and January 1998 (Summer). The relative abundance of eight common species varied over one year (Clark's R = 0.63, p<0.01). Tetrasporangial and bisporangial thalli were most common while gametophytes were rare and only present for some species. The density of tetrasporangial conceptacles also varied significantly among sample times (Clark's R = 0.481, p<0.001). Greatest conceptacle densities occurred in winter and the lowest in autumn. Numbers of tetra- and bisporangia per conceptacle varied during the study (Clark's R = 0.802, p<0.001) with lowest spore numbers occurring in summer and the highest numbers in winter and spring. Overall reproductive potential of the assemblage (total sporangia per unit area for all species) was greatest in winter. The most abundant species influenced these patterns more strongly than less abundant species.

Recruitment was also determined on four occasions between April 1997 and March 1998. Terracotta tiles were deployed during the sampling of adult thalli for demography and reproductive phenology to compare seasonal patterns of recruitment with reproduction. Numbers of crustose coralline macrorecruits after three months followed similar seasonal patterns to the reroductive potential of adult thalli three months earlier i.e. recruitment was highest in winter and lowest in summer. There was no clear relationship between reproduction and microrecruitment after one month. This suggested that there might be a lag period between propagule availability and measurable recruitment in crustose coralline algae at Rottnest Island. Assemblage-scale studies of recruitment and reproduction accounted for temporal trends in these processes for all species after three months, however large variation in reproduction of individual species indicated that recruitment should be studied separately for species of crustose coralline algae in the future.

Crustose coralline algae colonise space by lateral growth from broken thalli as well as recruitment from propagules. Recruitment from spores and lateral regrowth in a mixed species crustose coralline assemblage was investigated in a kelp bed at Nancy Cove, Rottnest Island. In contrast to several other studies, kelp was found to have no significant direct effect on recruitment and lateral growth of crustose corallines at the study site. After approximately 3.5 months, secondary effects associated with competing foliose and turf algae, significantly negatively influenced the cover of crustose coralline recruits. A conceptual model was proposed in which rapid recruitment of crustose coralline algae enables them to monopolise free space shortly after it becomes available. Post-recruitment coexistence and competition then occur until such time that a kelp canopy is restored removing foliose species through canopy effects. The final structure of adult assemblages is proposed to result from overgrowth competition among crusts beneath a dense kelp canopy.

This research has provided new information about the role of recruitment in the persistence of crustose coralline algae. Complex mosaics consisting of several species, and difficulties in identifying recruits limited the scope of this study to the assemblage-level. These results have highlighted the need for species level measurement of recruitment in crustose coralline assemblages. The assemblage-level approach has demonstrated that local processes such as reproduction and dispersal were important to recruitment in crustose coralline algae. These ideas contradict the commonly held view that marine populations are demographically open and as such future research should be aimed at measuring dispersal of propagules. Recruitment plays a fundamental role in establishment and regulation of crustose coralline assemblages, but post-recruitment processes such as competition limit the development, and strongly influence the final structure, of these assemblages.