Factors affecting photosynthetic rates of periphyton in shallow streams of southeastern Pennsylvania

Photosynthetic rates during low-flow summer conditions were examined for eight reaches of the Christina Watershed in southeastern Pennsylvania with respect to light and nutrient availability. Photosynthetic rates in these shallow streams ranged from 0.9 to 25.4 g O-2/m(2).d. Soluble orthophosphorus concentrations ranged from 0.004 to 0.63 mg/L, and phosphorus was considered the limiting nutrient for periphyton growth based on a nitrogen/phosphorus ratio exceeding 10 (by weight). Light availability varied among reaches from densely wooded canopies to open pasture. Nutrient availability was examined in terms of concentration and advective flux, which was calculated as the product of stream velocity and nutrient concentration. Cluster analyses were applied to group the nutrient and light availability factors and the photosynthetic rate responses into low, moderate, and high levels. The most consistent predictor of high photosynthetic rates was the joint occurrence of moderate-to-high levels of light and nutrient flux.