Effect of Stream Acidification on Periphyton Composition, Chlorophyll, and Productivity

We determined periphyton community composition, cell number and biovolume, chlorophyll a density, and areal and chlorophyll-specific primary productivity along a pH (4.5–6.4) and total monomeric aluminum (0.018–0.242 mg∙L−1) gradient in high-elevation streams in the Great Smoky Mountains National Park (eastern Tennessee). The periphyton community was generally dominated by small chrysophytes and blue-green algae at sites with baseflow pH ≥ 5.5 and by diatoms and, to a lesser extent, green algae at sites with pH ≤ 5. Total cell biovolume, chlorophyll a density, and areal primary productivity were greatest at the most acidic sites. Chlorophyll-specific rates of primary production were not significantly different among sites, although the most acidic site was lowest. Experiments involving short-term manipulation of pH, inorganic carbon, PO4, and aluminum concentrations resulted in some statistically significant changes in chlorophyll-specific productivity, indicating that inorganic carbon concentrations at sites with pH < 6 and increased aluminum concentrations at sites with pH ≤ 5 may limit productivity at times. Studies of phosphatase activity and PO4 turnover indicated that the least acidic sites were the most phosphorus limited, perhaps the result of coprecipitation of aluminum and PO4 as pH increased downstream from acidified reaches. However, the low cell biovolume, Chlorophyll a density, and areal primary productivity at the second-order sites with pH ≥ 5.7, compared with the highly acidic second- and third-order sites (pH ≤ 5), appear to be the result of increased density of scraper/grazer macro-invertebrates at the higher pH sites. Low chlorophyll a density and areal primary productivity at the much larger fourth-order site (pH 6.4) may be the result of physical factors, such as increased scouring of the streambed, rather than chemical or biological factors.