The light availability is a major habitat characteristic that influences stream ecosystems, as initially proposed in the River Continuum Concept (RCC) put forward in 1980. A specific hypothesis of the RCC was that the chemical diversity of dissolved organic material (DOM) increases downstream as the production of DOM by algae and macrophytes contributes to DOM coming from runoff of soils and vegetation in forested catchments. Studies have shown that direct photochemical processes influence the chemistry of the DOM pool, changing its bioavailability and its reactivity with trace metal and organic contaminants. Recent research has shown that a major DOM fraction, fulvic acid, can be used as electron acceptors by heterotrophic microorganisms in reducing environments and that reduced fulvic acid can then be used as an energy source by other microorganisms. The hydrologic transport of reduced DOM from reducing environments, such as hyporheic zones, to oxidizing environments in streams may influence ecosystem function. Thus, the chemical diversity and biogeochemical reactivity of DOM in stream ecosystems are dynamic, with changes occurring over a day-night cycle being superimposed upon seasonal changes in light regime and in hydrologic regime. Incorporating these dynamic processes into the broad scale of the RCC may be useful in understanding the effects of changes in water quality, land use and hydrologic regime.