Periodic defaunation and recovery in a subtropical epibenthic community, in relation to organic pollution

Abstract

A monthly survey was carried out for two years on the epifaunal community in Tolo Harbour and Tolo Channel, Hong Kong, a subtropical embayment subjected to a gradient of organic pollution. The number of animals and species, biomass and species diversity (Shannon's function H′ and evenness J), were higher at less polluted stations in the outer Harbour and Channel than at the polluted stations in the inner Harbour. The Channel community was dominated by the crab Portunus hastatoides Fabricius and the gastropod Nassarius crematus (Hinds). Along an increasing gradient of organic pollution, N. crematus was gradually replaced by the crab Charybdis vadorum (Alcock), and a community dominated by Portunus hastatoides and Charybdis vadorum was found in the polluted inner Harbour. The abundance and dominance of predatory gastropods also showed a decrease from the Channel to the inner Harbour, reflecting changes in the trophic structure of the community in relation to pollution. No significant change in the percentage of deposit-feeders was found along the pollution gradient. Summer mortality of benthos appeared to occur regularly in inner Tolo Harbour, and was attributed to summer oxygen depletion resulting from eutrophication. The benthic community was, however, soon restored to its original state by rapid winter recolonization. It is postulated that the cyclic phenomenon of summer mortality followed by winter recovery may be a common characteristic in subtropical benthic communities subjected to a high level of organic pollution. © 1982.