RAPID CHANGES IN ENCRUSTING MARINE ASSEMBLAGES EXPOSED TO ANTHROPOGENIC POINT-SOURCE POLLUTION - A BEYOND BACI APPROACH

Cover of and the number of species in encrusting macrobenthic assembla ges inhabiting temperate rocky reefs in the vicinity of an ocean outfa ll changed rapidly following the discharge of secondary treated sewage effluent. Within 3 mo of the commissioning of the outfall, significan t reductions in the cover of crustose and foliose algae were apparent when this outfall area was compared to 2 reference locations. The cove r of several species of sponge, including Cymbastela concentrica, Geod inella sp, and Spongia sp., also underwent marked declines coincident with the commissioning of the outfall. Only 1 category of cover increa sed significantly at the outfall; this was a nondescript matrix compri sing silt and microorganisms, which doubled its representation to almo st 60 %. We did not detect significant declines in the cover and numbe r of species of sponges or total fauna, however. A 'Beyond BACI' exper imental design was used to determine the environmental impact because of the great spatial and temporal variability in these shallow water ( similar to 20 m) encrusting communities. Photographic samples were tak en in 3 periods, the first pre-commissioning and the other 2 post-comm issioning. Multivariate analyses revealed marked shifts in the structu re of the assemblage al the outfall relative to the reference location s; these shifts were clearly depicted by a non-metric multi-dimensiona l scaling (nMDS) plot. A SIMPER analysis confirmed that the overall co mposition of the community at the outfall changed from one in which al gae and sponges were well represented to an assemblage dominated by si lt and ascidians.