Effects of light on NO₃^- uptake in small forested streams: Diurnal and day-to-day variations

We investigated the effects of autotrophy on short-term variations in nutrient dynamics by measuring diurnal and day-to-day variations in light level, primary productivity, and NO₃^- uptake during early and late spring in 2 forested streams, the East and West Forks of Walker Branch in eastern Tennessee, USA. We predicted that diurnal and day-to-day variations in NO₃^- uptake rate would be larger in the West Fork than in the East Fork in early spring because of higher rates of primary productivity resulting from a more stable substratum in the West Fork. We also predicted minimal diurnal variations in both streams in late spring after forest leaf emergence when light levels and primary productivity are uniformly low. Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O₂ change technique, and reach-scale rates of NO₃^- uptake were determined by tracer ¹⁵N- NO₃^- additions. In the West Fork, significant diurnal and day-to-day variations in NO₃^- uptake were related to variations in light level and primary productivity in early spring but not in late spring, consistent with our predictions. In early spring, West Fork NO ₃^- uptake rates were 2 to 3× higher at midday than during predawn hours and 50% higher on 2 clear days than on an overcast day several days earlier. In the East Fork, early spring rates of GPP were 4 to 5× lower than in the West Fork and diurnal and day-to-day variations in NO₃^- uptake rates were <30%, considerably lower than in the West Fork. However, diurnal variations in NO₃^- uptake rates were greater in late spring in the East Fork, possibly because of diurnal variation in water temperature. Our results indicate the important role of autotrophs in nutrient uptake in some forested streams, particularly during seasons when forest vegetation is dormant and light levels are relatively high. Our results also have important implications for longer-term assessments of N cycling in streams that rely on daytime measurements or measurements only under limited weather conditions (i.e., clear days).