A Method Comparison and Stressor-Response Experimental Study of Nitrogen and Phosphorus Impacts to Periphyton in Ozark Streams

Stream bioassessment is important for understanding algal-nutrient relationships and the development of scientifically defensible numeric nutrient criteria. However, multiple methods of periphyton data collection are currently used, and little is known about the comparability of resulting datasets. Literature also suggests other factors besides nutrients (i.e. variable grazing, light, and flow) can confound algal-nutrient relationships. A one-year method comparison study and 31-day algal biomass-nutrient manipulative experiment were conducted in the southern Ozarks of Arkansas. The method comparison study was implemented using two common bioassessment procedures (whole-surface and delimiter-reduced periphyton removal) to assess the potential for combining datasets. During the manipulative experiment, cobbles from the Buffalo River Watershed were exposed to a range of phosphorus (P) and nitrogen (N) concentrations during P-only and N + P enrichment periods to evaluate algal biomass responses using recirculating streamside mesocosms. Results of the method comparison study showed no statistical difference between bioassessment procedures for chlorophyll-a (chl-a) and ash-free dry mass (AFDM) (p = 0.123 and p = 0.550, respectively) or any interaction between method and season. Differences in chl-a and AFDM from both methods were detected (p < 0.001 and p = 0.012, respectively) when comparing warmer versus cooler seasons. Temperature-dependent grazing pressures were a potential explanation for the observed seasonal variability in biomass. The experiment revealed a positive linear relationship between benthic chl-a and increasing P and N addition up to 0.2 mg/L P (p < 0.001), with apparent N-limitation observed during the Ponly enrichment period. After 17 days of P-only enrichment, chl-a increased with increasing P concentrations (p < 0.001), ranging from 4.4 to 57.9 mg/m. After 14 additional days of N + P enrichment, mean chl-a had almost tripled across respective treatments, ranging from 13.3 to 171.1 mg/m. Results support the need for controlling N and P in freshwater systems to avoid excessive algal biomass accrual and provide insight into how possible increases in nutrient loading may influence the Buffalo River Watershed, disregarding confounding factors. Overall, both studies further scientific understanding of algal-nutrient relationships and verify the combining of both bioassessment methods for developing regional nutrient criteria and protecting stream designated uses.