delta C-13 variability of macroalgae: Effects of water motion via baffling by seagrasses and mangroves

It has been argued that the C-13-enrichment of benthic compared to pelagic consumers is a function of differential diffusion resistance in the boundary layer that surrounds algae in these 2 environments. We pursued a field test and confirmed the hypothesis that differences in water motion may ultimately translate into differing delta(13)C values by sampling benthic algae both inside and outside the flow-shielded habitats of seagrass beds and mangrove forests. Macroalgae collected within seagrass beds were C-13-enriched relative to macroalgae at the edge or outside the grass canopy. However, the inverse held in the mangrove system where presumably photoassimilation by sheltered macroalgae of respiratory CO2 from the mineralization of mangrove detritus more than compensated for the effect of reduced water motion. Our findings suggest the application of stable isotope analysis for the fine-scale study of foodweb structure on the basis of local differences in water motion, but also caution against the potential for such processes to confound conclusions drawn from data collected on a larger scale.