The problem

One of the important species that predominates in the ecosystem is the Dragon's Blood Tree (Pterocarpus officinalis) whose adaptations include shallow roots that spread laterally which makes them particularly vulnerable to sea level rise. Also, climate change is expected to alter natural hydrological patterns in the Caribbean by altering rainfall patterns and accelerating sea level rise. For this reason, coastal freshwater wetlands are especially vulnerable due to the threat of rising sea level  and alternate flood patterns represent. Due to the reasons established , the largest coastal wetland in Puerto Rico is at risk of disappearing and that is why we created our research group. 

Our project

In order to detect the presence of saltwater intrusion among other hydrological sources, we use the stable isotopes of hydrogen (deuterium) and oxygen 18. They are a useful tool to environmental samples such as ours becasuse during the natural hydrogen cycle, isotopic fractionationation occurs where its value becomes positive (corresponding to heavy) or negative (corresponding to light). 

The objectives of this project were:

• • 1. Study how the isotopic values ​​of the different waters that reach the Humacao Nature Reserve vary

    2. Create a database of these water sources (in English “end points”), which would help to carry out studies of wetland hydrology. This database is part of a worldwide effort to gather information about these values ​​around the world.

Findings

We successfully collected data 15 months of data, including water from tropical storms Irene and Eric. Surface waters are the main water source for the DragonBlood Tree forest and it is the one being used by those species. These trees have a low tolerance to salinity and has been causing the forest decay. Our statistical analyzes indicate that the salt water intrusion into the wetland varies according to the rainy and dry season, since a sand barrier is formed during the dry season that interrupts the flow of salt water to the wetland. This project was the first to use this technique to study the hydrology of a wetland and the second to apply it to ecosystems in Puerto Rico, the first being Dr. Elvira Cuevas and her laboratory at UPR Río Piedras. Finally, we observe an amount effect in the rainfall samples (where the low values ​​of the stable oxygen isotope 18 are generally associated with large rain events and vice versa), a characteristic of tropical areas .

Acknowledgements

This project was funded by the Hispanic Leaders in Agriculture and the Environment (HLAE) Program at Texas A&M University and the Alfred P. Sloan Foundation. Also, we thank Dr. Ben Wu and Manuel Piña of Texas A&M University for their support. Finally, we thank Alejandro Santana, Carlos Zayas, Marangely Torres and Francisco Hernández (QDEP) for their assistance in the field during  sampling.

This research was presented both in poster and oral forms at: Ocean Sciences  and Ecological Society of America Annual Meetings 2012,  Louis Stokes Alliance for Minority Participation Puerto Rico (LSAMP-PR) Junior Technical Meeting  and Aquatic Sciences Meeting 2013.

Article published June 2017: Hydrological modification, saltwater intrusion, and tree water use of a Pterocarpus officinalis swamp in Puerto Rico

Awards under this project: