Influence of Ecklonia radiata kelp canopy on structure of macro-algal assemblages in Marmion Lagoon, Western Australia

Abstract

A recent study of the influence of a wave exposure gradient on macroalgal assemblages associated with kelp stands in Marmion Lagoon, Western Australia found macroalgal communities had high spatial heterogeneity. Much of this heterogeneity was between replicate quadrats within sites and exposure level. The cause of such spatial heterogeneity is investigated further. Ninety 0.25 m2 quadrats were sampled from 9 sites (10 quadrats from each site) during the Australian autumn (April-May) 1996. The sites were nested in groups of 3 across a gradient in wave exposure. The quadrats were also grouped by density of adult Ecklonia radiata thalli, which is the local canopy forming kelp. Three categories were used: 4 kelp thalli m−2. Eighty two species were observed, but only 19 species occurred in >10% of quadrats and a further 13 species in 5–10% of quadrats. This suggests that species were patchily distributed among quadrats. This patchy distribution was found to be greatest in quadrats with Ecklonia radiata densities 4. The major taxa contributing to this patchy distribution were the brown algae Lobophora variegata, Sargassum spinuligerum and Sargassum spp. juveniles, and the red algae Amphiroa anceps, Chauviniella coriifolia, Dictymenia sonderi, Heterodoxia denticulata, Jeannerettia pedicellata, Pterocladia lucida and Rhodymenia sonderi. Many of these species only occurred or were most abundant in areas of low kelp density. The results demonstrate that assemblage structure at local scales, between replicate quadrats, was influenced by density of the kelp canopy just as much as gradients in exposure to ocean swells between reef lines. Many species were influenced both by kelp density and exposure to swells.