Incoming 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, RWT degraded at a slower rate (3.5 µg L−1 d−1) than fluorescein (40.8 µg L−1 d−1) when exposed to similar levels of light accumulation. Water temperature did not affect the decay rate of RWT at 10 or 20°C, but RWT did not decay significantly at 30°C, a result suggesting that high temperatures might affect decay rates differently than lower temperatures. Water temperature did not affect the decay rate of fluorescein. The strength of this method is that it enables researchers to integrate light measurements into a single value. Researchers can deploy multiple arrays within a reach to develop a relative measure of incoming light that has the potential to cover large spatial and temporal scales.
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