Importance of Light and Nutrients in Structuring an Algal Community in a Rocky Mountain Stream

Abstract

Abstract The effects of light regime and nutrient enrichment on periphytic algae in a second-order Rocky Mountain stream were determined in a field experiment. We established a light gradient along a 400 m stretch of West St. Louis Creek using the following four light treatments: artificially shaded in the open reach, closed-canopy, artificially illuminated in the closed reach, and open-canopy. Nitrate and phosphorus were manipulated with nutrient-diffusing substrata. Despite order-of-magnitude differences in light levels, metrics of periphytic and algal abundance were very similar under the open and closed canopy treatments. Under the artificially illuminated treatment, however, all parameters were two to four times greater, indicating that light was limiting under both of the natural light regimes, (i.e., insufficient light under the closed canopy and photoinhibition in the open section). Addition of phosphorus caused significant increases in Chl-a, periphytic ash-free-dry-mass, and algal cell density, ...