Using isotope hydrology to understand the impacts of climate change to tropical coastal wetlands

Background/Question/Methods

In Puerto Rico, climate change is expected to create a rise in sea level and an alteration of precipitation patterns.  The hydrological implications of both events represent a threat to coastal wetlands in the island.  The Humacao Natural Reserve in Puerto Rico (HNR) is composed of different coastal wetland ecosystems that are home to many threatened species, making it an ideal place to apply isotope hydrology to understand the natural hydrological cycles of the site.  We collected monthly water samples from different water sources reaching the HNR (runoff, lagoons, ocean and rainfall) and analyzed their isotopic ratios content (δ¹⁸O, δ²H).  Our objectives were the following: 1) develop a local database of isotopic data, 2) analyze the temporal and spatial variations of these values, and 3) Determine the contribution of rainfall to the wetland ecosystems. 

Results/Conclusions

Data analysis show a variation of water isotope values in a small spatial scale (10² m). As expected, we observed an “amount effect” on the rainfall samples (where low δ¹⁸O values are usually associated with large rainfall events and vice versa). Lower δ values were found on Las Mulas Creek (runoff) samples taken closer to the reserve, particularly after the outlet channels of nearby farms. This particular result provides an important contribution to a Pterocarpus officinalis mitigation process at the HNR, mainly because it identifies agricultural runoff as an important source of freshwater to the swamp. Surface waters in the forested wetlands and tidal creek area of the lagoon have δ¹⁸O values ranging from -2‰ to 1‰, similar to the trend observed in Las Mulas Creek, which is a possible indication of the influence of runoff in the Northeastern areas of the HNR. Although the coastal lagoons are connected to the ocean, stable isotope analysis show an important influence of runoff in the lagoon's hydrology.  We expect further variation of isotopic values across scales, especially in rainfall samples.  Results from this ongoing research will clarify important aspects of hydrology and climate change impacts on the HNR by providing a database for an ongoing research project regarding the freshwater-saltwater interactions of the coastal lagoons and the Pterocarpus swamp in the Humacao Natural Reserve.