Periphytic algae nutrient limitation in streams draining the Beaver Reservoir basin, northwest Arkansas, USA 2005--2006

The objectives of this study were to (1) 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, USA; (2) measure periphytic growth responses using in situ bioassays to available ambient photosynthetic active radiation (PAR); (3) test the Lotic Ecosystem Trophic Status Index (LETSI) as a simplifying metric to compare effects of the limiting variables of N, P, and light across the basin; and (4) quantify standing periphytic biomass at each study location to evaluate the Autotrophic Index relationship with LETSI. P limitation was observed across the basin. A Michaelis-Menten growth equation describes LETSI as a function of ambient PO4-P concentrations (P<0.05); a concentration of approximately 0.003 mg/L PO4-P would produce a LETSI of 0.50. PAR was negatively correlated to LETSI values at light-saturation (R2=0.90, P<0.05), illustrating that when ambient conditions produce algal growth responses approaching maximum potential productivity, periphyton chl a production increases to a point of light limitation.