Impact of taxonomy, geography and depth on δ13C and δ15N variation in a large collection of macroalgae.

The natural abundance of carbon stable isotopes (delta13C) of marine macrophytes has been measured in previous studies and used to analyze differences in Ci assimilation among the three macroalgal phyla, Chlorophyta, Ochrophyta, and Rhodophyta, and seagrasses, distinguishing diffusive CO2 entry from the operation of a CO2-concentrating mechanisms (CCM). The work reported here further resolves the patterns of delta13C variation in aquatic macrophytes related to their taxonomy, geographic location (and consequently climatic conditions), and vertical zonation. Analyses of delta13C for 87 species are reported, including eight that have not been previously examined, belonging to taxa in the three macroalgal phyla, plus two species of seagrasses, collected at different latitudes. For one species of each phylum, analyses were also conducted through a vertical depth gradient. Representative species were used in a pH drift experiment, in order to compare the mechanism of Ci acquisition for photosynthesis with the delta13C subsequently determined on the same specimen. Our results suggest that the delta13C values were mostly determined by taxonomy. Depth effects on C stable isotope composition differed among taxa. The parallel measurements of delta15N are more difficult to interpret mechanistically; + there are no robust phylogenetic and large-scale biogeographic correlations; local factors of natural (e.g., upwellings) and anthropogenic (e.g., sewage outfall) inputs predominate in determining the macrophyte delta15N.