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

Abstract

San Dlego Bay contalns a number of harbors and other s~m~lar s~zed embayments Some contaln many boats and others relatively few, prov~ding 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 foullng communit~es (less cover, b~omass, and fewer specles) compared to simllar areas In embayments with few boats. Embayments w ~ t h many boats were characterized by serpulid polychaete worms, filamentous algae, and a solltary tunicate, Ciona jntestinaljs L. These groups apparently tolerated the physical and chem~cal stresses 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 character~st~c of hard bottom communities on many natural reefs. Unl~ke the sessile organisms, the motile crustaceans and polychaetes that nestled among the sesslle groups were strongly associated w ~ t h 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 w ~ t h few boats In comparing embayments with many and few boats, sampling was confined to back-bay areas Hard bottom communltles at the front of embayments were slmllar to back-bay areas where there were few boats The concentrat~on of tributyltin (TBT, a toxic addit~ve to paint) was also hlgher in embayments w ~ t h many boats. We hypothesize that TBT is a cause of the changes in hard bottom communities Hydrograph~c variat~ons among embayments w ~ t h many and few harbors could not explain the cons~stent commun~ty patterns