δ ¹³ C variability of benthic algae: effects of water colour via modulation by stream current

Increased water motion is expected to reduce boundary layer diffusion resistance of autotrophs, thereby enabling greater isotopic discrimination against 13 C such that lower δ 13 C values (ratio of 13 C : 12 C) should ensue. A field test of this hypothesis was undertaken by sampling benthic algae in streams of differing current speed. 2. Contrary to the expected negative relationship between δ 13 C and water motion, filamentous benthic algae were found to exhibit higher δ 13 C values in rapid water. 3. Under conditions of low current in the streams studied, concentrations of dissolved organic carbon as measured by water colour are elevated through the microbial decomposition of largely terrestrial organic matter. Photoassimilation of this respired carbon by benthic filamentous algae generates 13 C‐depletion and lower δ 13 C values, and appears to be substantial enough in the streams used in the present study to override the competing influence of water motion on boundary layer thickness.