THE INFLUENCE OF PLANT STRUCTURE ON THE SPECIES RICHNESS, BIOMASS AND SECONDARY PRODUCTION OF MACROFAUNAL ASSEMBLAGES ASSOCIATED WITH WESTERN AUSTRALIAN SEAGRASS BEDS

Considerable differences were found in the species of macrofauna present at Cliff Head and Seven Mile Beach, two coastal seagrass-dominated sites with differing wave exposure in Western Australia. The differences in the faunas between sites were generally much greater than differences between the various habitats within each site. The species richnesses of the faunal assemblages in different seagrass habitats nevertheless followed consistent patterns at both sites, with rankings of habitats from species richest to poorest as: turf, Amphibolis frond, Posidonia, Halophila/Heterozostera, Amphibolis rhizome, detached macrophyte and unvegetated sediments. The faunas associated with artificial seagrass clumps placed in three habitats at each site showed similar patterns of species richness, with significant differences between habitats and between sites but with no site-habitat interaction. Only one environmental factor, the diversity of food resources, was found to correspond with these patterns of faunal species richness. Macrofaunal abundance, biomass and production were all much greater in vegetated than unvegetated habitats. These faunal parameters nevertheless varied greatly between habitats and between sites. Amongst the vegetated habitats, estimated annual secondary production was highest in theAmphibolis rhizome (47.2 g AFDW · m-2 · yr-1) and least in the Halophila/Heterozoslera (24.3 g AFDW · m-2 · yr -1) habitats at Cliff Head. The converse situation was found at Seven Mile Beach, with the Halophila/Heterozostera habitat (42.2 g AFDW · m-2 · yr-1) having nearly four times the estimated macrofaunal production of the Amphibolis rhizome habitat (11.7 g AFDW · m-2 · yr-1). These differences between sites are explained by reference to the distribution of plant debris bound at the sediment surface. Similar relationships between secondary production, the biomass of debris and the biomass of live plant material were independently formulated for the two sites, with the general equation for both sites being P = 13.8 + 0.22 · D + 0.039 · S (n = 12, r2 = 0.61) where P is annual macrofaunal production (g · m-2 · yr-1), D is the mean biomass of debris (g· m-2) and S is the mean biomass of live plant material (g · m-1). Neither the biomass of seagrass rhizomes nor the biomass of drifting debris were significantly correlated with secondary production. © 1990.