Résumé
Storm-water run-off is an important source of contamination and cause of water quality degradation in urbanised coastal areas. Although laboratory tests show that run-off is toxic for some organisms, its effects in the field remain uncertain. We investigated the effects of run-off on assemblages of mobile invertebrates within kelp beds in Sydney Harbour, Australia. We first tested patterns of differences in assemblages that are (1) directly associated with kelp holdfasts and those that are (2) developing on artificial units of habitat (AUHs) between locations with and without storm-water drains. We then transplanted assemblages on AUHs from locations without to locations with storm-water drains, predicting that assemblages would (1) change from those in the original location and (2) become more similar to those in the receiving location. Despite the initial pattern of differences in assemblages on AUHs, no clear effects of run-off were observed in the transplantation experiment. While transplanted assemblages changed in one location, no differences were detected in a second location. In a third location, there were experimental artefacts in the assemblage responses to transplantation. Total abundance (except for 1 location) and number of taxa did not change. Our results show the challenges of detecting potential impacts of disturbances in the field. First, manipulative experiments are necessary since the existence of patterns of differences can easily be observed, but does not necessarily indicate a causal relationship. Second, appropriate procedural controls are required as the experimental techniques can cause stress on organisms or can elicit different responses from them as a result of spatial variability.