A whole stream approach to metabolism measurement with implications for nutrient utilization and light limitation

The relationships among gross community primary production (GPP), community respiration (CR[sub 24]) and nutrient uptake were investigated in a 62 m reach of Walker Branch, a first order woodland stream. GPP and CR[sub 24] were measured utilizing the upstream/downstream dissolved oxygen method under varying conditions of forest canopy development and day length. Dissolved oxygen and temperature were measured at 1 minute intervals for approximately 40 hours. Changes in DO were corrected for oxygen reaeration using a reaeration coefficient calculated with data from conservative and volatile tracer injections done within several days of DO measurements. Gross phosphorus uptake was measured during 90 minute injections of carrier-free [sup 33]PO[sub 4] and [sup 3]H[sub 2]O to the steam reach. Benthic GPP varied with light conditions as controlled by forest canopy development and day length. GPP was highest (25 [mu]g O[sub 2] m[sup [minus]2] sec[sup [minus]1]) during the spring just prior to canopy closure and dropped rapidly to [approximately]1.7 [mu]g O[sub 2] m[sup [minus]2]sec[sup [minus]1] as the forest watershed. Ambient stream nutrient concentrations and phosphorus uptake length varied inversely with total metabolic activity, suggesting an important biotic linkage between water quality and benthic metabolism.