Influences of Canopy Cover on Spectral Irradiance and Periphyton Assemblages in a Prairie Stream

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Influences of the transmission properties of the riparian canopy and the water column on spectral irradiance and periphyton composition were examined at four sites in Otter Creek, a small spring-fed stream in the Sand Hills region of western Nebraska. Values for per cent sky in the canopy at the four sites were: 65.1% (open site), 31.9% (partial shade), 19.0% (shade), and 15.4% (heavy shade). Above canopy, below canopy, and underwater irradiahces were measured at 2-nm intervals from 300-800 nm in late morning, mid-day, early afternoon, and late afternoon at each site on a clear day (13 June 1991). Photosynthetically active radiation (PAR) incident at the stream surface did not consistently correspond to per cent sky values at the sites because of temporal changes in the penetration of direct sun through small openings in the canopy (sunflecks). The relationship of solar path to canopy structure and orientation was the primary determinant of sunfleck activity. There was very little absorbance of specific wavelengths by the canopy at the open site, but the proportion of blue light (400-500 nm) increased at all sites by late afternoon. At the three shaded sites, the amount of red light (600-700 nm) was proportionally lower underneath the canopy than above it. Underwater PAR was always greater at the open site than at the more shaded sites, but the relationship between canopy cover and underwater irradiance for the more shaded sites varied temporally. The spectral distribution of PAR at the mean depth in Otter Creek (39.7 cm) was similar to that found at shallow depths in clear, oligotrophic lakes, in that there was only a slight maximum transmission in green wavelengths (-550 nm). The degree of temporal variability in the underwater spectral distribution at a site was often greater than differences between sites because of the effects of sunflecks and surface reflection by ripples. Algal chlorophyll a concentrations at the sites were significantly correlated with per cent sky for gravel substrata but not for sand substrata. The flora on both substrata was dominated by diatoms, and the distributions of several species were related to the degree of canopy cover at the sites. The relative importance of the spectral light field versus total PAR in structuring benthic lotic assemblages warrants further study.