Measuring restoration in intertidal macrophyte assemblages following sewage treatment upgrade

Understanding the effectiveness of pollution mitigation actions in terms of biological recovery is essential if the environmental protection goals of management policies are to be achieved. Few studies, however, have evaluated the restoration of seaweed assemblages following pollution abatement. This study aimed to investigate the response of macroalgal vegetation to the upgrade of a wastewater treatment plant using a "Beyond BACI" experimental design. Temporal differences in vegetation structure between the outfall and two control locations over a 10-year period were assessed. Improvement in sewage treatment was found to lead to increases in diversity, cover of morphologically complex algae and spatial heterogeneity. The multivariate composition of assemblages at the outfall location became more similar to that at the controls; however, their complete recovery may depend on factors other than pollution removal. Our findings also suggest that the extent of restoration and the time required to detect it are largely predetermined by the response variables we choose to assess recovery.