The ecology of four species of Psephenidae (water penny beetles) was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two-year period. Quantitative benthic samples were taken from different microhabitats in the stream on 47 occasions and, in combination with light-trap catches of adult beetles, were used as a basis for investigating life-history patterns and estimating secondary production. Total psephenid annual production over the period 1977–79 was 930–5 mg AFDW (ash-free dry weight) 0.5 m-2, but was more than twice as high in 1977–78 (1381–6mg AFDW 0.5m-2) than in 1978–79 (562.6mg AFDW 0.5m-2). Sinopsephenus chinensis and Eubrianax sp. were the most abundant psephenids in TPKFS, but the larger S. chinensis contributed most to total psephenid secondary production. Eubrianax sp. made the second largest contribution to production, while the contribution of the two remaining species (Psephenoides sp. and Mataeopsephus sp.) was minor. Eubrianax sp. production was generally constant between years, but production (and population densities) of the other three psephenids declined in 1978–79. Sinopsephenus chinensis was clearly bivoltine in TPKFS, but Eubrianax sp. and Psephenoides sp. showed asynchronous growth and almost continuous recruitment. Accurate estimates of production: biomass ratio (P: B) ratio were possible for S. chinensis only, and varied between 12.8 and 19.4 over the two years of the study.
Eubrianax sp. and (especially) S. chinensis were most abundant in midstream microhabitats but Psephenoides sp. was more numerous close to the stream banks. Mataeopsephus sp. was relatively rare and lacked an obvious microdistribution pattern. Multivariate statistics (principal components analysis followed by multiple regression) were used to investigate the relationship between psephenid densities in each benthic sample and the sediment characteristics, detrital standing stocks and algal biomass in the environment where the sample was taken. The analysis revealed that psephenid microdistribution was influenced by sediment grain-size characteristics rather than by algae or detritus, but the proportion of variation in the abundance of psephenid species accounted for by sediment characteristics was rather low.