PHOTOCHEMISTRY OF DISSOLVED ORGANIC MATERIAL AND EFFECTS ON STREAM ECOSYSTEMS

Approaching questions of aquatic biogeochemistry from the perspective of “hotspots” and/or “hot moments” allows for integration of these results within the broad concepts of aquatic ecosystem function. This approach also allows for forging connections to the dynamic chemistry of the reactive constituents of interest to limnologists. One example is the role of sunlight in determining the structure of stream ecosystems, not only by controlling photosynthesis, a theme of the River Continuum Concept (RCC), but also by driving photochemical processes. These photochemical processes can transform dissolved organic material (DOM), nutrients and many trace constituents. 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. From a biogeochemical perspective, the noonday sun irradiating a stream or river reach represents a “hot moment” that can have a sustained ecological effect through the rest of the diel cycle. The hydrologic transport of these reactive photochemical constituents to downstream reaches and to adjacent reducing environments, such as hyporheic zones, may influence ecosystem function. These changes are especially important in the context of the bioavailability of toxic metals, such as copper, to invertebrates and fish in streams receiving acid mine dranage in their headwaters, as will be illustrated based on studies in the Colorado Rocky Mountains. Further, the changes occurring over a day-night cycle are superimposed upon seasonal changes in light regime and in hydrologic regime. Incorporating these dynamic processes into the broad scale of the RCC and into monitoring approaches may be useful in understanding the current effects of the substantial changes in water quality, land use and hydrologic regime.