MEPS 194:39-53 (2000)  -  doi:10.3354/meps194039

ABSTRACT: The benthic ecosystem of the lagoon surrounding Tahiti, the most populated island of French Polynesia, was investigated to assess the impacts of terrestrial runoff on these benthic communities. Five lagoonal zones based on population densities around the coast of Tahiti were identified, and within each 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 were considered in this study. Multivariate analysis using total biomass and environmental factors showed that the stations formed 3 main groups which were related to sediment characteristics, including percentage of silt, organic matter and phaeopigment levels. The distribution of the major feeding groups was related to the amounts of terrestrial inputs and distance from the shore. The stations on the barrier reef and those in zones adjacent to low population areas were not impacted by these terrestrial inputs. Deposit-feeding communities of capitellid polychaetes were dominant in the channel parts of the lagoon, which acted as decanting ponds. Chaetopterid polychaetes played an important role in recycling sediments of terrigenous origin in the fringing ecosystem. The patterns of diversity, density and biomass of the benthos around the lagoon revealed that some areas were impacted by moderate terrigenous inputs. It appears that the intermediate disturbance hypothesis explains the functioning of the parts of the benthic lagoonal ecosystem which are subjected to human impact. The organic enrichment by terrestrial inputs contrasts with food limitation that appears to occur in the non-disturbed 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.

KEY WORDS: Lagoon · Macrobenthic communities · Polychaetes · Deposit-feeders · Terrestrial inputs · Tropical