Airborne laser fluorosensor survey of the Columbia and Snake rivers: simultaneous measurements of chlorophyll, dissolved organics and optical attenuation

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Abstract
An airborne laser fluorosensor has been used to make concurrent, near-continuous (microscale) profiles of chlorophyll-a, dissolved organic carbon (DOC) and the beam attenuation coefficient along the Columbia and Snake rivers. Cross-reference between profiles is used to expose anomalies and explain ambiguities encountered in their interpretation. The unique potential for remote sensing of DOC concentrations is based on a strong correlation with the fluorescence of dissolved organic matter (DOM). Similarly, the reciprocal of the remotely sensed water Raman emission is shown to be a reliable predictor of the beam attenuation coefficient. Profiles of a 734 km segment of the lower Snake and Columbia rivers obtained during peak discharge revealed a number of interesting features. In particular, the chlorophyll-a fluorescence data suggested the existence of subsurface chlorophyll-a maxima in the reservoirs formed by the eight dams of the Columbia-Snake Inland Waterway, and the absence of variability in the remotely measured DOM fluorescence data for the lower Columbia River was in close agreement with an independent DOC survey. In addition, large-amplitude long-timescale fluctuations in the chlorophyll-a fluorescence profile observed during one flight appear to be due to solar-induced photoinhibition of the in vivo chlorophyll-a fluorescence.