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Summary Carbon uptake in terrestrial plants functions under near-constant source carbon dioxide (CO2) concentrations and isotopic ratios, but aquatic macrophytes operate in a more complex system where environmental fluxes and biotic interactions undermine assumptions of constant CO2 concentration and 13C/12C. Many marine macrophytes not only passively access CO2 for photosynthesis, but also actively concentrate CO2 and bicarbonate (HCO3−) using carbon concentration mechanisms (CCMs). These processes change macrophyte carbon fractionation signatures (13C/12C) and elevate seawater pH as high as 10.4 in mesocosm pH assays, in which the pH value reached is termed pH*. I assembled a global data set of 2027 marine macrophyte δ13C and pH assay values for 664 species to assess (i) how macrophyte δ13C varies with the abiotic parameters such as sea surface temperature (SST), latitude and habitat, and the organismal traits of taxonomic and functional group membership and (ii) how species δ13C is related to CCM presence or absence, as determined by a pH drift assay. Across 613 macrophyte species, collection site distance from the equator was negatively related to species δ13C, while collection site variance in SST was positively related to species δ13C. Macrophyte tissue δ13C differed among oceans as well as in intertidal versus subtidal habitats. Species from phylum Rhodophyta had the lowest δ13C, and functional group was related to δ13C, largely due to higher δ13C in calcifying species. Analysis of 141 species with paired pH*–δ13C data found that these metrics of CCM presence are not independent. As species δ13C increases so does the probability of species pH* > 9.0, a threshold value of CCM presence in pH assays. Synthesis. CCMs revealed species patterns in communities at every scale investigated, from local emersion gradients to oceanic and global gradients. Trends in macrophyte δ13C values indicate that macrophytes rely more on CO2 further from the equator, but have increased use of HCO3− at sites with high temperature variance, patterns that may be driven by species turnover rather than intraspecific variation. Patterns in species CCMs will be crucial to understanding how macrophyte communities respond to ocean acidification-induced changes to SST and variability. |
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