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

Large differences in d2 H 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 d2 H 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: DH = (d2 Hom - d2 Hwater) 7 (1 ? d2 Hwater 7 1,000). We summarized DH values from published literature and we measured the d2 H of water and primary producers in order to compare DH 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 DH values (i.e. greater isotopic discrimination) than both aquatic macrophytes and terrestrial vegetation. The more positive d2 Hom and hence lower DH of terrestrial vegetation was expected due to relative increases in the heavier isotope, deuterium, during transpiration. However, the more positive values of d2 Hom and relatively low DH in aquatic macrophytes, even submerged species, was unexpected. Marine macroalgae had high variability in d2 Hom as a group, but low variability within distinct species. Variability among types of primary producers in d2 Hom and in DH should be assessed when hydrogen is used in isotopic studies of food webs.