The use of fresh and saline water sources by the mangrove Avicennia marina

Mangroves are distributed along tropical and subtropical riverine and coastal shores. Although mangroves are highly adapted to saline environments, maintaining water uptake under saline conditions is energetically expensive. Therefore, salinity is a limiting factor for mangrove growth and productivity, and access to fresh water sources, such as rainwater and groundwater, which reduce water salinity, increase mangrove ecosystem productivity. Here, we investigated the extent of fresh water utilization by mangroves to better predict current and future mangrove productivity. We used the abundance of ¹⁸O isotope in stem water to assess: (1) the extent of fresh water utilization by Avicennia marina (Forssk.) Vierh across hydrological settings; and (2) whether growth, measured as increments in stem circumference, is sensitive to variation in rainfall availability. The δ¹⁸O isotopic composition of stem water indicated mangroves use both fresh and saline water sources for metabolic processes. However, our results suggest that the proportion of fresh water used by mangroves increases with the availability of fresh water. Growth of the main stems of trees was correlated with rainfall (r² = 0.34 and r² = 0.37, P = 0.001). Our results indicate that access to fresh water is important for mangrove productivity because it enhances their growth rates.