The effects of water quality, epilithic algae and organic matter on communities of stone-dwelling invertebrates in a Hong Kong river

Abstract

Samples of water, periphyton and benthic animals were collected over a 9-month period from two sites on the Lam Tsuen River, New Territories, Hong Kong, to assess the effects of water quality, epilithic algae and organic matter on the structure of stone-dwelling invertebrate communities. The study sites were approximately 2.5 km apart; the lower one received domestic and agricultural wastes while the upstream site was relatively unaffected by such wastes.

Temperature, pH, electrical conductivity and dissolved oxygen were similar at the two study sites. However, Biological Oxygen Demand (BOD₅), dissolved phosphates, epilithic organic matter and epilithic algal biomass were higher at the lower site. The quantities of epilithic organic matter and epilithic algal biomass at the lower site varied seasonally in relation to the monsoonal rains; values were low and constant in the wet season but increased up to 5-fold in the dry season. The abundance of stone-dwelling animals at the lower site was correlated positively with algae and organic matter, and peaked during the dry season. The species richness and diversity during the study period were lower at the downstream site compared with the upper stream site, although population densities at the downstream were five times higher on average than the upper stream site. This difference was especially obvious during the dry season. Faunal community structure (i.e. relative abundance of major taxonomic groups) also differed between the two sites, and Ephemeroptera and Trichoptera were relatively abundant upstream, whereas Chironomidae dominated the downstream site. This study demonstrates that the influence of organic enrichment on the stone-surface invertebrate community of a stream in the seasonal tropics parallels such effects in temperate regions.