Importance of surface‐subsurface exchange in stream ecosystems: The hyporheic zone

In many streams, significant amounts of water are exchanged between saturated sediments surrounding the open channel (the hyporheic zone) and the channel itself. Such exchanges with the hyporheic zone have the potential to cause large changes in streamwater chemistry because the rates of biogeochemical processes and the actual types of processes (e.g. anaerobic vs. aerobic metabolism) may be fundamentally different. I propose an organizational scheme for grouping stream systems into clusters of minimal, moderate, and maximal contribution of hyporheic metabolism to the overall ecosystem. Lack of information on quantitative hydrology for different stream systems prohibits actual testing of this framework. An alternative scheme is presented that organizes several well‐studied streams as a function of subsurface residence time and differences in oxygen content. This second scheme is intended to stimulate discussion of conceptual frameworks for cross‐system comparisons of the importance of hyporheic sediments in stream ecosystems.