Water sources of plant uptake along a salt marsh flooding gradient.

Salt marsh plants are affected by regular tidal inundation exposing them to saline water as a potential water source. This study aimed at quantifying the water uptake of plants depending on their distance from the sea and exploring plant responses to changing inundation regimes. We used stable isotope ratios (δ18O) to determine the proportions of seawater and precipitation water used by three salt marsh species (Spartina anglica, Atriplex portulacoides and Elytrigia atherica) from a German North Sea coast salt marsh. Additionally, A. portulacoides was transplanted to experimental islands at three elevation levels to investigate its plasticity in water use in the course of future sea level rise. We found a marked gradient in plant seawater use from the lowermost pioneer zone (79-98% seawater uptake by S. anglica) to the lower marsh (61-95% by A. portulacoides) and the upper marsh (25-39% by E. atherica). Seasonal differences in water use were not pronounced, likely due to the absence of longer dry periods during summer in these temperate salt marshes. Contradicting our expectation, roots in deeper soil showed higher water uptake rates per fine root mass than topsoil roots suggesting effective root adaptation to the anoxic subsoil. Transplanted A. portulacoides plants significantly increased the uptake of seawater with increasing inundation indicating flexibility in the use of water sources by this species which may facilitate acclimation to rising sea levels. We conclude that the zonation of salt marsh vegetation reflects the availability of water sources along the inundation gradient.