Light

The quantity of light reaching the streambed is one of the most fundamental environmental factors that affect stream ecosystems. Solar radiation drives photosynthesis and has strong indirect effects on the many lotic processes connected to primary production. The quantity of light incident to streambeds is highly variable on both temporal and spatial scales, and an accurate assessment of the lotic light regime is a highly relevant activity for many research projects. Light measurements that span at least one dawn-to-dusk period (and preferably more than one) are essential for characterizing the daily light regime at any stream location. These measurements can be made with quantum sensors that directly measure photosynthetically active radiation (PAR) (400–700 nm) or with sensors that can be calibrated to PAR. Data loggers connected to the sensors are essential to recording light measurements made at intervals during the day. Photosensitive dye-based sensors that integrate light exposure over time may be a useful and relatively inexpensive alternative to electronic sensors. For any study involving multiple sites or stretches of streams with potentially variable spatial light environments, multiple sensor-logger combinations should be employed.