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 assemblages inhabiting temperate rocky reefs in the vicinity of an ocean outfall changed rapidly following the discharge of secondary treated sewage effluent. Within 3 mo of the commissioning of the outfall, significant reductions in the cover of crustose and foliose algae were apparent when this outfall area was compared to 2 reference locations. The cover of several species of sponge, including Cymbastela concentrica, Geodinella sp, and Spongia sp., also underwent marked declines coincident with the commissioning of the outfall. Only 1 category of cover increased significantly at the outfall; this was a nondescript matrix comprising silt and microorganisms, which doubled its representation to almost 60 %. We did not detect significant declines in the cover and number of species of sponges or total fauna, however. A 'Beyond BACI' experimental 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 taken in 3 periods, the first pre-commissioning and the other 2 post-commissioning. Multivariate analyses revealed marked shifts in the structure of the assemblage al the outfall relative to the reference locations; these shifts were clearly depicted by a non-metric multi-dimensional scaling (nMDS) plot. A SIMPER analysis confirmed that the overall composition of the community at the outfall changed from one in which algae and sponges were well represented to an assemblage dominated by silt and ascidians.