DELTA-C-13 VARIABILITY OF BENTHIC ALGAE - EFFECTS OF WATER COLOR VIA MODULATION BY STREAM CURRENT

1. Increased water motion is expected to reduce boundary layer diffusi on resistance of autotrophs, thereby enabling greater isotopic discrim ination against C-13 such that lower delta(13)C values (ratio of C-13: C-12) should ensue. A field test of this hypothesis was undertaken by sampling benthic algae in streams of differing current speed. 2. Contr ary to the expected negative relationship between delta(13)C, and wate r motion, filamentous benthic algae were found to exhibit higher delta (13)C values in rapid water. 3. Under conditions of low current in the streams studied, concentrations of dissolved organic carbon as measur ed by water colour are elevated through the microbial decomposition of largely terrestrial organic matter. Photoassimilation of this respire d carbon by benthic filamentous algae generates C-13-depletion and low er delta(13)C values, and appears to be substantial enough in the stre ams used in the present study to override the competing influence of w ater motion on boundary layer thickness.