PERIPHYTON NUTRIENT LIMITATION AND MAXIMUM POTENTIAL PRODUCTIVITY IN THE BEAVER LAKE BASIN, UNITED STATES

The objectives of this study were to measure periphytic growth responses to enrichment with nitrogen (N), phosphorus (P), and simultaneous N and P using in situ bioassays in streams draining Beaver Reservoir Basin, Northwest Arkansas; compare periphytic growth responses measured with in situ bioassays with a range of land use and point sources; and test the lotic ecosystem trophic status index (LETSI) as a simplifying metric to compare effects of nonpoint-source pollutant-limiting variables of N, P, and sediment across the basin. P limitation was observed at sites across a transect of stream orders throughout the basin; however, at the two sites with highest ambient nitrogen concentrations, limitation was often coupled with nitrogen limitation. Nutrients were at nonlimiting levels at both of two sites below wastewater treatment plants in all seasonal deployments. A Michaelis-Menten growth equation described LETSI as a function of ambient PO4-P concentrations (p < 0.05); the midpoint (LETSI of 0.50) corresponded with a PO4-P concentration of approximately 3 mu g/l. Change-point analysis indicated a threshold point at LETSI of 0.80 and 15 mu g/l PO4-P. These low values show that the periphytic community has a high affinity for available P, and that the watershed as a whole is sensitive to available nutrient inputs.