The hypothesis that relative water motion and boundary layer diffusion processes affect carbon isotope ratios of aquatic plants was tested in tidal pool and surge zone comparisons of the surfgrass Phyllospadix spp. No evidence was found that submerged plants growing in still upper tidal pools were isotopically different from those growing submerged in lower tidal surge zones. Significant decreases in 13C/12C ratios for plants growing emersed in the intertidal may have been caused by uptake of atmospheric carbon dioxide. Marine algae (Egregia menziesii and Halosaccion americanum) growing at the same location and tidal elevations as the seagrasses showed somewhat different isotopic fractionation patterns, suggesting that causes of isotopic variability in the seagrasses were not necessarily the same as those in the two marine algae.
Keywords: Egregia; Halosaccion; Phyllospadix; Seagrasses; δ13C.