Reefs as contributors to diversity of epiphytic macroalgae assemblages in seagrass meadows

Abstract

The links between macroalgae-covered reefs and diversity and biomass of epiphytic algae assemblages in adjacent seagrass meadows were investigated. Algae assemblages were sam- pled in 3 habitats: on-reef, seagrass meadow near-reef ( 3000 m), off the coast of Fremantle, Western Australia. We interpreted differences among these habitats as evidence that proximity to reefs influences epiphytic algae assemblages. Propagules in the water column, epiphytes recruiting onto artificial seagrass units (ASU) and assem- blages on natural Posidonia sinuosa were sampled in each habitat. For propagules, ASU and natural P. sinuosa, there were significant differences in the structure of algae assemblages near and distant from reefs, while assemblages near reefs tended to resemble those found on reefs. Multivariate analysis (ANOSIM) confirmed that propagule assemblages near reefs and on reefs were similar, but those distant from reefs differed from those on reefs. Near-reef habitat recorded the highest total number of algal taxa growing on ASU (53), followed by on-reef (44) and distant-from-reef (37) habi- tats. ANOSIM again confirmed that assemblages at on- and near-reef sites were not significantly dif- ferent, but both differed from those at sites distant from reef. We identified 59 species of algal epi- phytes growing on mature P. sinuosa leaves at near-reef and distant-from-reef sites, with only 19 species being common to both habitats, and multivariate analyses were suggestive of differences in assemblage structure. Biomass was only recorded for the ASU epiphyte assemblages and was great- est in the near-reef habitat (0.63 ± 0.11 g shoot -1 ) followed by the on-reef habitat and the distant- from-reef habitat (0.18 ± 0.04 and 0.11 ± 0.01 g shoot -1 , respectively). Our results demonstrate that diversity and biomass of macroalgae epiphytes in seagrass ecosystems differ between habitats of varying proximity to reefs.