Optical water quality in rivers

Optical water quality (OWQ) governs the quantity and quality of light in aquatic ecosystems, and thus spatiotemporal changes in OWQ affect many biotic and abiotic processes. Despite the fundamental role of light in rivers, studies on riverine OWQ have been limited and mostly descriptive. Here we provide a comprehensive, quantitative analysis of the controls and spatiotemporal dynamics of riverine OWQ, focusing on the inherent optical properties (IOPs), which are those that are only affected by water constituents and not by changes in the solar radiation field. First, we briefly review the constituents attenuating light in rivers. Second, we develop a new method for partitioning (light) beam attenuation into its constituent fractions. This method distinguishes between absorption and scattering by dissolved and particulate constituents, and further isolates particulates into mineral and organic components. Third, we compare base flow IOPs between four rivers with vastly different physical characteristics to illustrate intersite variability. Fourth, we analyze the spatial and temporal patterns of IOPs for the four rivers. Fifth, we quantify a longitudinal water clarity budget for one of the rivers. Finally, available data are synthesized to identify general spatial trends robust across broad geographic areas. Temporal trends in IOPs were largely dictated by storm frequency, while spatial trends were largely dictated by channel network configuration. Generally, water clarity decreased with increasing discharge primarily owing to greater scattering by particulates and secondarily to greater absorption by chromophoric dissolved organic matter. Water clarity also generally decreased longitudinally along the river owing to increased particulate inputs from tributaries; however, for pear-shaped, dendritic basins, water clarity reached a minimum at similar to 70% of the channel length and then increased. By illustrating the controls and spatiotemporal variability of riverine OWQ, these findings will be of interest to water resource managers and fluvial ecologists and specifically for remote-sensing of fluvial environments and river plumes in receiving waters.