Temporal variation in the structure of intertidal assemblages following the removal of sewage

Following the closure of 2 outfalls, changes in the number of species and a bundance of intertidal organisms at 2 decommissioned outfalls were compared with control (sewage outfalls that remained in operation) and reference (n atural) areas. Two intertidal levels (mid- and low-shore) were sampled 5 ti mes over 2 yr following the closure of these outfalls. It was proposed that the number of species would increase and the abundance of green algae woul d decrease through time at the decommissioned outfalls, while no noticeable changes in the number of species and the abundance of green algae were pre dicted at the reference and control locations, The 4 outfalls (2 decommissi oned and 2 controls) were analysed separately with an asymmetrical ANOVA to identify differences between the outfall and the average of their 2 respec tive reference locations. Non-metric multidimensional scaling ordination us ing Bray-Curtis similarity was used to identify differences between outfall s and reference locations in the structure of the assemblages at the 2 heig hts on the shore. During the first (6 mo after closure) and second sampling period, fewer species and greater abundance of green algae were observed a t every outfall than at their respective reference locations in low-shore a reas. In the subsequent sampling periods, the number of species at the deco mmissioned outfalls increased through time while there were consistently fe wer species at the control outfalls, The opposite pattern was observed for the abundance of green algae (i.e., decreases through time at the decommiss ioned outfalls). Assemblages at control outfalls never clustered with their reference locations. At midshore levels, no pattern was observed at any sa mpling date. Furthermore, at the first time of sampling, the number of spec ies was not smaller at the outfalls than at reference locations. These resu lts suggest that sewage outfalls have little impact on the number of midsho re species. The results of univariate analysis were similar to those from m ultivariate analyses. Assemblages of species in low-shore areas at the deco mmissioned outfalls were different from those at reference locations at the first sampling date, but became more similar by the last sampling date. So me exceptions were observed at the site closest to the point of discharge. Again no differences in abundances of organisms were observed at any time f or any outfall. The results showed a recovery of the benthic assemblages in the low-intertidal zone after the closure of 2 sewage outfalls. Furthermor e, this study supports the importance of using more than 1 reference and co ntrol areas to measure recovery of a site without ambiguity.