VARIABILITY AT DIFFERENT SPATIAL SCALES BETWEEN A SUBTIDAL ASSEMBLAGE EXPOSED TO THE DISCHARGE OF SEWAGE AND 2 CONTROL ASSEMBLAGES

Environmental disturbances can alter the variability of assemblages of organisms in impacted sites compared to control sites. It has therefore been proposed that increased variability might be an important feature of stressed populations. Increased variability may be due to changes in the population structure of individual species or changes in the suite of species. In this study, spatial variances of shallow subtidal assemblages of organisms inhabiting vertical cliff-faces were compared among two control locations and one location that had for many yeats been exposed to the discharge of sewage. These assemblages covered nearly all available space on the substratum and consisted primarily of encrusting and foliose macro-algae and numerous filter-feeding animals, such as ascidians, sponges and bryozoans. Mean differences in abundances between these locations were investigated using Beyond BACI designs. In addition, these locations were used to examine the model that assemblages are more variable in disturbed than undisturbed environments and to try to distinguish differences in variability due to differences in the population structure of individual species from that due to changes in species composition. The assemblages were sampled at two spatial scales at each of three depths in each location. There were significant differences between the polluted location and one or other of the control locations in the mean abundances of some organisms, the variances of certain species (or recognisable types) at each spatial scale and for multivariate measures of species composition. There was, however, no evidence to support the prediction of increased variability in the apparently polluted location compared to the control locations. Importantly, for many measures of abundance and variability, the control locations were as different from each other as they were from the polluted location, suggesting that the latter fell within the range of natural subtidal assemblages separated by these spatial scales. These findings emphasise the need to include more than one control location in any study of a potential environmental impact, so that any effects of that impact can be distinguished from the range of natural variability.