Analysis of the process of retention of organic matter in stream ecosystems

Lotic ecosystems are substantially different than most other ecosystems because the continuous unidirectional flow through the system tends to transport matter to downstream reaches. This gives rise to the common erroneous view of streams as conduits or pipes. The process of retention removes matter from transport and makes it available for utilization by stream biota, thus providing a critical link between input and storage. The retentive properties of a stream are functions of both hydrologic and substrate-related features. Heterogeneity of channel structure and current velocity creates obstructions and slack water areas that enhance retention efficiencies of streams. The retention of a particle in transport is a function of the probability of entrainment of the particle on an obstacle and the frequency of obstacles in the channel (YOUNG et al. 1978). A particle also will be retained in areas where the current velocity is less than the velocity required to keep the particle in suspension. When one of these conditions is met, the particle will be retained until flow conditions change and it is transported downstream. The influence of riparian vegetation on biological processes in streams extends beyond food resources and habitat. The effects of riparian vegetation on channel structure and organic inputs in streams determine the efficiency with which matter is retained. In this paper, we describe a method for the quantitative assessment of retention of particulate organic matter in streams and identify several major stream retention features that can be influenced by the structure and composition of riparian zones.