Sewage pollution and the infaunal benthos of Victoria Harbour, Hong Kong

Abstract

Victoria Harbour, Hong Kong, receives 800 million litres · day −1 of untreated sewage from 3.6 million people, but is flushed by tidal currents at velocities up to 1.1 m · s −1. Two replicate Smith-McIntyre grab samples were collected at 66 stations in August 1979, yielding a total of 5797 animals belonging to 68 species. Agglomerative hierarchical classification was used to delineate five station groups; a sixth group contained four stations where no fauna were found. All but one of the groups inhabited fine sediments (> 70% silt-clay) and could be placed along a gradient conforming to the typical pattern of organic pollution effects found in boreal and temperate waters. Abiotic areas, found in enclosed typhoon shelters, were successively replaced by a very poor fauna, a peak of opportunists, an ecotone point and a transition to normal fauna. The peak of opportunists was dominated by the oligochaete Thalassodrilides gurwitschi (Hrabe); the polychaetes Capitella capitata (Fabricius) and Minuspio cirrifera (Wiren) were also numerous. Overall abundance was low (784 individuals · m −2) compared with opportunists in boreal and temperate waters. The remaining station group was found in coarser sediments (<40% silt-clay) and an inflexion was observed in the log-normal distribution of individuals per species. Dominant species included the bivalve Tapes philippinarum (Adams & Reeve) and the polychaete Minuspio cirrifera; overall abundance was high (1103 individuals · m −2). In general, pollution was assessed by the pattern of changes in number of species, abundance, biomass, diversity and evenness, by log-normal distributions of individuals per species, and by the indicator species M. cirrifera. When compared with earlier (1976) data, community conditions were generally similar but the abundance of individual species varied greatly and M. cirrifera was absent. The severely polluted area was ≈ 15 km 2; this is similar in extent to offshore discharge sites and indicates the importance of tidal flushing in this inshore channel.