Hydrogen isotope discrimination in aquatic primary producers: implications for aquatic food web studies

Large differences in delta H-2 of primary producers between aquatic and terrestrial ecosystems are used to identify subsidies, discriminate organic matter sources, and reduce uncertainty in food web studies. Previous investigations of hydrogen isotope ratios suggest there may be predictable differences between the delta H-2 of water and organic matter for different types of primary producers. We define the difference in the net isotopic discrimination between water and bulk organic matter (om) as: Delta(H) = (delta H-2(om) - delta H-2(water)) A center dot (1 + delta H-2(water) A center dot 1,000). We summarized Delta(H) values from published literature and we measured the delta H-2 of water and primary producers in order to compare Delta(H) among aquatic and terrestrial primary producers. Measurements were made from three water body types (lake, river, coastal lagoon) and their associated watersheds. Although we predicted a large and equivalent net isotopic discrimination for aquatic primary producers, we found considerable variability among groups of aquatic producers. Macroalgae, benthic microalgae, and phytoplankton had more negative Delta(H) values (i.e. greater isotopic discrimination) than both aquatic macrophytes and terrestrial vegetation. The more positive delta H-2(om) and hence lower Delta(H) of terrestrial vegetation was expected due to relative increases in the heavier isotope, deuterium, during transpiration. However, the more positive values of delta H-2(om) and relatively low Delta(H) in aquatic macrophytes, even submerged species, was unexpected. Marine macroalgae had high variability in delta H-2(om) as a group, but low variability within distinct species. Variability among types of primary producers in delta H-2(om) and in Delta(H) should be assessed when hydrogen is used in isotopic studies of food webs.