Freshwater Uptake of Mangrove Growing in an Extremely Arid Area

Abstract

This study evaluated the freshwater use of two mangrove species (R. mucronate: Rhizophora mucronata and A. marina: Avicennia marina) growing in a coastal area of Djibouti City and on Moucha Island in Djibouti. We accomplished this by comparing hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in the body water with those of creek water, soil water, groundwater, and rainwater samples. Line-conditioned excess (lc-excess) was calculated for the δ2H and δ18O values of the groundwater and rainwater samples. Although two regression lines were drawn using the δ2H and δ18O values of the plant body water as well as the environmental water, no significant differences were found, suggesting that the effects of isotope fractionation due to evaporation in all the samples were almost similar. Comparison of lc-excess and δ18O suggested that the body water from the two mangrove species growing on Moucha Is. were strongly influenced by seawater, but contained some freshwater sources other than the seawater, the soil water, and the creek water. It was also found that Avicennia marina growing in the coastal area was strongly influenced by groundwater. The origin of the freshwater absorbed by the mangroves on the island was not clear, but it could be groundwater recharged in a faraway place.