Effects of anthropogenic disturbances of tropical soft-bottom benthic communities

The benthic ecosystem of the lagoon surrounding Tahiti, the most populated island of French Polynesia, was investigated to assess the impacts of terre strial runoff on these benthic communities. Five lagoonal zones based on po pulation densities around the coast of Tahiti were identified, and within e ach zone a transect from the fringing reef to the barrier reef was sampled, a total of 18 stations. Only large macrofauna collected on a 2 mm sieve we re considered in this study. Multivariate analysis using total biomass and environmental factors showed that the stations formed 3 main groups which w ere related to sediment characteristics, including percentage of silt, orga nic matter and phaeopigment levels. The distribution of the major feeding g roups was related to the amounts of terrestrial inputs and distance from th e shore. The stations on the barrier reef and those in zones adjacent to lo w population areas were not impacted by these terrestrial inputs. Deposit-f eeding communities of capitellid polychaetes were dominant in the channel p arts of the lagoon, which acted as decanting ponds. Chaetopterid polychaete s played an important role in recycling sediments of terrigenous origin in the fringing ecosystem. The patterns of diversity, density and biomass of t he benthos around the lagoon revealed that some areas were impacted by mode rate terrigenous inputs. It appears that the intermediate disturbance hypot hesis explains the functioning of the parts of the benthic lagoonal ecosyst em which are subjected to human impact. The organic enrichment by terrestri al inputs contrasts with food limitation that appears to occur in the non-d isturbed areas. Despite high organic loads, the benthic communities present at the harbour station were not depauperate, as bioturbation by burrowing alpheids and callianassids prevented anoxic conditions from developing.