Oil pollution and polychaeta in an estuarine mangrove community

The structure of benthic biocenosis of the estuarine mangroves of the Mahakam delta (East Kalimantan, Indonesia), is relatively uniform and dominated by polychaeta worms and crabs. The most important environmental variables determining dominant benthic species are salinity, emersion degree (intertidal zone), granulometry and redox potential. Generally speaking, a community's reaction to pollution variesfrom decrease in diversity and total numbers to complete azoic conditions. The demographic structure of Nereis sp. has been shown to evolve in this way along the oil pollution gradient in an experimental oil pollution location and in areas of chronic pollution. After 3 months the largest size categories are first eliminated, while the number of small individuals increases significantly. Then total numbers suddenly drop. The examination of redox profiles enables effects of chronic oil nuisances to be distinguished from effects due to experimental spills. In experimentally polluted zones profiles show low redox values just below the surface of sediment, while in chronically polluted zones the profile is uniformly low. After 1 year, re-colonization is achieved for the total number of individuals but destabilization within the population structure still persists. Puliella sp. is also very abundant. Its average biomass per station is very much weaker in a polluted than in a non-polluted area. Thus this organism also appears to be a suitable pollution indicator. Only very strong pollution can eliminate this species.