A PERSPECTIVE ON DRYLAND RIVER ECOSYSTEMS

The ecosystem concept should be reappraised as a basic model for river s, with regard for flow as an organizing variable. This would facilita te comparisons between the large rivers of humid climates, where flow regimes are comparatively regular, and those of arid and semi-arid are as, where river regimes are highly variable. Ecosystem processes might be modelled by combining the river continuum and flood pulse concepts , with refinements to accommodate a complex flood pulse (e.g. variatio ns in stage amplitude, timing, duration, rates of rise and fail). Patc h boundaries (ecotones) such as the riverine littoral zone warrant clo se study because they strongly influence the structure and dynamics of the ecosystem. The general model needs a quantitative basis, perhaps focused on the balance of processes involved in the physical transport and biological transformation of carbon. The ultimate test of such a model will be in its capacity to predict the effects of flow regulatio n. Further development, however, is limited by data. In both research and management monitoring programmes need to be established to provide information and to develop a sustained, comprehensive approach to dry land rivers as ecosystems.