Identification and evaluation of nutrient limitation on periphyton growth in headwater streams in the Pawnee Nation, Oklahoma

The objective of the study was to identify nutrient impacts, if any, on stream periphyton growth in Black Bear Creek (north central Oklahoma) and its tributaries. Passive diffusion periphytometers were deployed at ten study sites within the Black Bear Creek basin to evaluate periphyton growth in response to nutrient enrichment. These sites were selected to represent a gradient of land uses, from predominantly agricultural to predominantly urban. Periphytometer treatments included phosphorus (P) (1.0 mg/L PO4-P, n = 10), nitrogen (N) (10.0 mg/L NO3-N, n = 10), N plus P (n = 10) and control (reverse osmosis-treated water, n = 10). Results indicated that average dissolved inorganic N (DIN, PQL = 0.04 mg/L) concentrations were significantly correlated (R2 = 0.63, p < 0.01) with chlorophyll a production on the periphytometer control treatments in the Black Bear Creek basin. Periphytic growth was nutrient-limited (increased chlorophyll a was measured on nutrient-enriched growth media) at four of the ten sites sampled; two sites were limited by N and two sites were co-limited by both N and P. The lotic ecosystem trophic status index (LETSI), the ratio of C to N + P chlorophyll a, was calculated to compare treatment responses across sites. At nutrient-limited sites, LETSI was positively correlated to ambient DIN values (R2 = 0.97, p < 0.01). However, some sites that were not nutrient-limited had ambient nutrient concentrations similar to sites with observed nutrient limitation, indicating other factors were limiting periphyton growth at those sites.