Knowledge of the effects of nutrient concentration on the composition and structure of photoautotrophic periphyton is essential to understand the impact of eutrophication on shallow lotic systems. Reaches of Sulphur Fork Creek upstream and downstream of effluent from Springfield Wastewater Treatment Plant in Middle Tennessee were sampled to assess the effects of trophic state on characteristics of photoautotrophic periphyton, including composition of diatom and soft-algae assemblages. Pearson's correlation coefficient (r) for log(10)-transformed concentrations of soluble reactive phosphorus (log(10) [SRP]) to percent composition was significant for 4 of 63 soft-algae taxa sampled from cobbles. Five algae trophic indices (ATIs) to assess the effects of trophic state on soft-algae assemblages were developed using different taxon-trophic indicators which included: (1) r values for log(10) [SRP] to percent composition (ATI(r)), (2) abundance-weighted averages of [SRP] (ATI(A-WA [SRP])), (3) abundance-weighted averages of log(10) [SRP] (ATI(A-WA log [SRP])), (4) weighted averages of log(10) [SRP] where the taxa occur, and (5) abundance-weighted ranks to phosphorus tolerance listed by the National Water-Quality Assessment Program. Eutrophication-induced impairment of Sulphur Fork Creek downstream of the effluent from the wastewater treatment plant was indicated by: (1) high concentrations of photoautotrophic periphyton, (2) low values for the pollution tolerance index of diatom assemblages, (3) positive values for the ATI(r), and (4) high values for both the ATIA-WA [SRP] and ATIA-WA log [SRP]. Of the indices using soft-algae taxa evaluated, the ATI(r) exhibits the strongest and significant correlations to [SRP], [NO2 + NO3 nitrogen], and the pollution tolerance index of diatom assemblages. The ATI(r) accurately reflects the trophic state of the sites studied and provides a novel additional tool to evaluate the effects of nutrient concentration on the structure of photoautotrophic-periphyton assemblages.
