Disturbance of intertidal soft sediment assemblages caused by swinging boat moorings
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The impact of swinging boat moorings on intertidal benthic assemblages was investigated in a small estuary on the south coast of England. Mooring buoys fixed near low water mark on a muddy shore were attached to 5 m of galvanised steel chain and had not been let for 12 months. Core samples for macro-invertebrates and sediments were taken both within and outside the chain radius of each buoy. The assemblage structure, biomass and abundance of selected bird prey species were examined at a range of scales. The study revealed variation in the impact of mooring buoys relative to control areas at two different times of sampling. Prior to the removal of buoys, the assemblage structure within areas affected by the buoys was found to be significantly different from unaffected areas. The abundance of the amphipod Corophium volutator, an important bird prey species, was significantly less in the areas affected by the buoys. In the second sampling programme (15 months after removal of buoys), the impact of extant buoys remaining in commission was not detectable. Assemblage structure in areas from which buoys had been removed was distinct from control areas which had never had buoys. The removal of mooring buoys clearly affected the assemblage, yet convergence with control areas, indicative of recovery, was not complete after 15 months. It is suggested that the effect of swinging mooring chains scraping over the mud surface may modify sediments favouring the greater prominence of larger particles such as gravel and shell fragments. The ecological impact of swinging moorings on estuarine benthic assemblages in designated protected areas is discussed in the context of other spatial and temporal disturbances.