Retrospective analysis of epiphyte assemblages in relation to seagrass loss in a eutrophic coastal embayment

Abstract

This is a retrospective study of shifts in species composition of epiphytic macroalgae that occurred during a period of seagrass loss in Cockburn Sound, Western Australia. Detailed analysis of data on relative abundances of 108 epiphytic taxa collected over 2 yr from 27 widely dispersed sites around Cockburn Sound showed differences in epiphyte composition between sites with loss of sea- grasses (Loss sites) and those with no loss in the the long-term (No Loss sites). Future seagrass losses, however, were not predicted by shifts in species composition. Most species were rare and varied with season of sampling. Hence, varying assemblages of species contributed to differences on each sam- pling occasion. Only 6 of the 108 species were strong, consistent contributors to differences between Loss and No Loss sites (Sphacelaria sp., Laurencia sp., Enteromorpha intestinalis, E. prolifera, Ulva lactuca, Acrochaetium sp.) and could be considered as 'indicator species'. Only 2 species (Sphacelaria sp. and Laurencia sp.) were present consistently in high abundances at No Loss sites. Foliose green algae (Ulva sp., Enteromorpha spp.), commonly considered to signal eutrophication, were found con- sistently in higher abundances at Loss sites. Shifts in epiphyte species composition correlated with patterns of nutrient loading and seagrass decline, in addition to the increases in biomass reported earlier, but did not predict future seagrass loss.