Hydrogen isotope response to changing salinity and rainfall in Australian mangroves

Abstract

Hydrogen isotope ratios (2H/1H, δ2H) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their δ2H values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid δ2H and leaf and xylem water δ2H and δ18O values from three mangrove species over 9.5 months in a subtropical Australian estuary. Net 2H/1H 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 2H depleted relative to surface water, reflecting 2H discrimination of 4-10‰ during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water 2H enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water δ2H values cannot explain the salinity-dependent 2H depletion in leaf lipids, nor the 30‰ range in leaf lipid δ2H 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.