Water velocity and light intensity effects on carbon stable isotope ratios (13C) of Cladophora glomerata

Abstract

Intraspecific variability in aquatic autotrophic carbon isotope ratios, 13C, contests the precision and accuracy of current methods of producer-consumer association with stable isotope analyses. In attempt to understand the influence of physical environmental factors on 13C signatures of aquatic producers, the effects of water velocity and irradiance on 13C were examined in Cladophora glomerata with a controlled flume experiment. A highly significant inverse relationship was observed between water velocity and C. glomerata 13C, likely due to velocity effects on boundary layer and carbon dioxide diffusion limitation. Light regime was not found be a significant independent affecter of 13C, but did exert an interactive effect with velocity, accentuating degree/magnitude/difference of signature enrichment between high and low velocity regimes. It is probable that irradiance acts as an interactive affecter on 13C by 1) increase in photosynthetic rate, and thus carbon demand, accentuating overall carbon limitation and decreased isotopic discrimination at low water velocities; and 2) increased utilization of 13C-enriched bicarbonate as a carbon source for photosynthesis with higher irradiance, contributing to overall signature enrichment. A field study examined in situ effects of water velocity and irradiance on 13C of Cladophora spp. and Spirogyra spp.; algal filaments growing in similar microhabitats varying in water velocity and canopy coverage were sampled and analyzed, but no correlations were found between water velocity or irradiance and 13C. Lack of in situ support for trends observed in controlled experiments are likely attributable to very limited field sample sizes, myriad confounding and noisy factors of dynamic environmental systems, and lack of precision (to-species identification) in algal taxonomic identification.