A practical method for measuring integrated solar radiation reaching streambeds using photodegrading dyes

Abstract

AbstractIncoming solar radiation is an important driver of aquatic ecosystem processes, such as gross primary production and photodegradation of organic matter. Rates of incoming solar radiation can be estimated in several ways, but their utility is limited for collecting a large number of replicate samples needed to quantify variation in light availability within and among streams. We evaluated the utility of 2 photodegrading organic dyes (rhodamine WT [RWT] and fluorescein) for measuring light exposure, especially at the level of the stream bed. We attached vials with known concentrations of the RWT or fluorescein to the stream bed and used regressions of concentration vs accumulated light to estimate photodegradation rates. Initial concentrations of RWT (20–100 µg/L) did not affect rate of photodegradation, but RWT decay rates were 93% slower in the dark than in the light. We also tested fluorescein, which degrades faster than RWT when exposed to light and is stable when kept in the dark. On average, R...