The magnitude and seasonality of aquatic insect subsidies to tropical stream riparia in Hong Kong

Abstract

Seasonal changes in emergence rates of aquatic insects and the relative abundance of volant aquatic and terrestrial insects were monitored using emergence, UV-light and Malaise traps over a 14-month period along two forested streams in Hong Kong. Although the mean annual emergence rates in terms of aquatic insect numbers, ranged from 9399 to 17,127 individuals m−2 and were comparable to studies elsewhere, rates in biomass terms (0.713–1.168 g DW m−2 year−1) were at the lower end of reported values. The absolute magnitude of the annual aquatic subsidy to terrestrial habitats around the two study streams thus appeared to be rather low. Seasonal patterns of emergence trap catches generally matched yields of aquatic insects from light and Malaise traps, with high emergence and abundance from March to October including the entire wet season, and relatively low dry-season emergence. Inter-stream differences in abundance were detected during this period, when local spate events appeared to cause short-term reductions in the aquatic insect subsidy, although data from multiple years would be needed to confirm this. Seasonal patterns of volant aquatic and terrestrial insects were similar in light- and Malaise-trap samples at both sites, although aquatic insects accounted for 74–87 % of light-trap captures but only 13–14 % of insects in Malaise traps. The relative abundance of aquatic versus terrestrial insects was similar between seasons in both trap types, suggesting that aquatic subsidy may remain important for riparian insectivores during the dry season when all insects are scarce.