Transport and retention of detritus in upland streams: a comparison of an open stream and an adjacent wooded site

Abstract

Retention of detritus on the bed of a river is crucial to the effective functioning of biological breakdown processes. It is now well understood that increasing retention of detritus within a stream channel leads to a higher standing stock of detritus and greater numbers of detritivores. What is less clear, however, is the proportion of leaf litter that is retained, particularly in non-wooded catchments, although anecdotal evidence suggests that, in open country and in channels managed for flood control, the vast majority of detritus entering the stream is washed downstream without being retained. This was investigated in the upper catchment of the River Goyt in northwest England. Detritus settling on the bed was compared with that in transit in the water column in an open moorland site (River Goyt) and an enclosed, forested site (Stake Clough). The total mass in transit in both rivers was similar but retention was higher in Stake Clough. Furthermore, indirect evidence suggests that detritus was retained for shorter periods in the River Goyt. Mass of detritus in transit was significantly correlated with rainfall in the River Goyt, though not in Stake Clough. The differences can be related to channel structure and the type of detritus entering each river: the River Goyt is a turbulent river. Its catchment is devoid of trees and the main detrital inputs are grasses. Stake Clough is low gradient, with an input of leaves from broadleaved and coniferous trees. The results presented have implications for community processes because the dominant detrital inputs to the River Goyt are refractory forms, and their breakdown will be hampered by their short retention times. There are also implications for water quality in reservoirs further down in the catchment, as most of their inputs are from low retention streams carrying large volumes of incompletely processed detritus.