EFFECTS OF LIGHT-INTENSITY, WATER VELOCITY, AND SPECIES COMPOSITION ON CARBON AND NITROGEN STABLE-ISOTOPE RATIOS IN PERIPHYTON

Periphyton was allowed to grow on glass plates suspended in the water column of a small stream under two conditions of light and water veloc ity, over two seasons, to assess the influence of the thickness of ben thic boundary layers on stable isotope fractionation. Isotopic signatu res for both carbon and nitrogen in samples of periphyton varied with light intensity and season, but not with current velocity. In summer, periphyton grown under low-light conditions had depleted delta(13)C an d delta(15)N values relative to periphyton grown under high light. In autumn, isotopic signatures were generally more depleted than in summe r, but did not vary systematically with light intensity or water veloc ity. These results suggest that isotopic fractionation in periphyton w as more strongly influenced by the intensity of metabolic activity tha n by variations in the thickness of the benthic boundary layer.