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 abundance of intertidal organisms at 2 decommissioned outfalls were compared with control (sewage outfalls that remained in operation) and reference (natural) areas. Two intertidal levels (mid- and low-shore) were sampled 5 times 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 would decrease through time at the decommissioned outfalls, while no noticeable changes in the number of species and the abundance of green algae were predicted at the reference and control locations, The 4 outfalls (2 decommissioned and 2 controls) were analysed separately with an asymmetrical ANOVA to identify differences between the outfall and the average of their 2 respective reference locations. Non-metric multidimensional scaling ordination using Bray-Curtis similarity was used to identify differences between outfalls and reference locations in the structure of the assemblages at the 2 heights on the shore. During the first (6 mo after closure) and second sampling period, fewer species and greater abundance of green algae were observed at every outfall than at their respective reference locations in low-shore areas. In the subsequent sampling periods, the number of species at the decommissioned outfalls increased through time while there were consistently fewer species at the control outfalls, The opposite pattern was observed for the abundance of green algae (i.e., decreases through time at the decommissioned outfalls). Assemblages at control outfalls never clustered with their reference locations. At midshore levels, no pattern was observed at any sampling date. Furthermore, at the first time of sampling, the number of species was not smaller at the outfalls than at reference locations. These results suggest that sewage outfalls have little impact on the number of midshore species. The results of univariate analysis were similar to those from multivariate analyses. Assemblages of species in low-shore areas at the decommissioned outfalls were different from those at reference locations at the first sampling date, but became more similar by the last sampling date. Some exceptions were observed at the site closest to the point of discharge. Again no differences in abundances of organisms were observed at any time for any outfall. The results showed a recovery of the benthic assemblages in the low-intertidal zone after the closure of 2 sewage outfalls. Furthermore, this study supports the importance of using more than 1 reference and control areas to measure recovery of a site without ambiguity.