Hydrogen isotope response to changing salinity and rainfall in A ustralian mangroves

Hydrogen isotope ratios ( 2 H / 1 H , δ 2 H ) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their δ 2 H values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid δ 2 H and leaf and xylem water δ 2 H and δ 18 O values from three mangrove species over 9.5 months in a subtropical A ustralian estuary. Net 2 H / 1 H fractionation between surface water and leaf lipids decreased by 0.5–1.0‰ ppt −1 for n ‐alkanes and 0.4–0.8‰ ppt −1 for isoprenoids. Xylem water was 2 H depleted relative to surface water, reflecting 2 H discrimination of 4–10‰ during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water 2 H enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water δ 2 H values cannot explain the salinity‐dependent 2 H depletion in leaf lipids, nor the 30‰ range in leaf lipid δ 2 H values among species. Biochemical changes in direct response to salt stress, such as increased compatible solute production or preferential use of stored carbohydrates, and/or the timing of lipid production and subsequent turnover rates, are more likely causes.