Changes in hard bottom communities related to boat mooring and tributyltin in San Diego Bay a natural experiment

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Abstract
San Diego Bay contains a number of harbors and other similar-sized embayments. Some contain many boats and others relatively few, providing an unique natural experiment where the ecological impact of mooring many boats was evaluated. Back-bay areas with many boats contained depauperate hard-bottom or fouling communities (less cover, biomass, and fewer species) compared to similar areas in embayments with few boats. Embayments with many boats were characterized by serpulid polychaete worms, filamentous algae, and a solitary tunicate, Ciona intestinalis L. These groups apparently tolerated the physical and chemical stress associated with many boats, but were replaced by other sessile invertebrates, such as mussels, sponges, bryozoans, and other tunicates, in embayments with few boats. These groups are more characteristic of hard bottom communities on many natural reefs. Unlike the sessile organisms, the motile crustaceans and polychaetes that nestled among the sessile groups were strongly associated with microhabitats, such as patches of algae and dense serpulid mats, rather than the presence of many or few boats. However, there were more species of nestling invertebrates in embayments with few boats. In comparing embayments with many and few boats, sampling was confined to back-bay areas. Hard bottom communities at the front of embayments were similar to back-bay areas where there were few boats. The concentration of tributyltin (TBT, a toxic additive to paint) was also higher in embayments with many boats. We hypothesize that TBT is a cause of the changes in hard bottom communities. Hydrographic variations among embayments with many and few harbors could not explain the consistent community patterns.