Leaf litter retention in tropical streams in Hong Kong

Leaf litter can be an important food resource in small forest streams, but its availability to detritivorous macroinvertebrates depends on stream retentiveness. Previous work in Hong Kong streams revealed a scarcity of large detritivores, and it is possible that this reflects the availability or instability of litter resources. Using leaf analogues made from waterproof paper, we undertook short-term experiments to measure the retentiveness of eight Hong Kong streams under baseflow conditions during the dry season; four streams had coarse bed sediments (boulder dominated) and four had relatively fine sediments (cobble-gravel dominated). Initially we compared the transport properties of soaked and unsoaked leaf analogues with the leaves of four native trees in one of the streams. Soaked and unsoaked analogues were used to mimic waterlogged and newly-fallen leaves. Mean travel distances (S p ) of leaves and analogues in the study streams ranged from 1-22 m. These are comparable to values recorded from similar-sized streams in temperate latitudes. Unsoaked analogues traveled further (mean S p = 10.2 m versus 3.1 m) than soaked analogues in all study streams, but retention behaviour of analogues generally matched that of real leaves. Mean travel distances of both analogue types were unrelated to streambed sediment size or sediment heterogeneity, but travel distance of soaked analogues was positively related to stream discharge. Litter retention remained high over a six-day period in one of the streams, suggesting that litter would be readily available to detritivores during periods with stable discharge, and that the scarcity of shredders must be caused by other factors.