Confounding response of macrofauna from a confluence of impacts: brine and sewage pollution

Throughout the shores of the world, a confluence of different sources of pollution is common, e.g., through outlets releasing brine, industrial, and domestic sewage. In this study, we assessed whether a combined disposal of domestic sewage and brine altered the patterns of abundance and assemblage structure of subtidal macrofauna inhabiting sandy sea beds on the south coast of Gran Canaria (Canary Islands, NE Atlantic Ocean). Samples were collected in two surveys (May 2008 and January 2009) at three distances (0, 15, and 30 m) away from the discharge point. Macrofaunal abundances showed different patterns with varying proximity from the disposal point in the two surveys. In May 2008, lower abundances were observed at 0 m (53.89±46.82 ind.) than at 15 m (120±91.7 ind.) and 30 m (283.89±189.33 ind.) away from the disposal point. In January 2009, however, higher abundances were observed at 0 m (131.33±58.69 ind.) than at 30 m (100±24.44 ind.) and 15 m (84.78±58.39 ind.) away from the disposal point. Significant differences in macrofaunal assemblage structure were found with varying proximity from the discharge point in both surveys. The effect of proximity from the disposal point was confounded by changes in sedimentary composition between surveys, which contributed to explain differences in macrofaunal abundances and assemblage structure with varying proximity from the discharge point. As a result, confounding drivers of macrofaunal assemblage structure, here, changes in sediment composition, accounted for changes in macrofaunal abundances and assemblage structure with varying proximity from a combined disposal point of brine and sewage.