Submarine groundwater discharge along the reef flat of Santiago Island, Bolinao
Groundwater from Santiago Island seeps into the Bolinao Reef flat (M King)

Submarine groundwater discharge (SGD) has been suggested as an important natural source of nutrients for coral reefs, seagrass beds, benthic microorganisms and fauna and other coastal communities.

As the quality of groundwater is influenced by past and on-going human activities, an increased discharge of nutrients via SGD to the coast can be expected. In coral reef ecosystems, elevated nutrient inputs via SGD have been linked to loss of coral cover, decline of species diversity, increasing growth of algal colonies, algal blooms in the water column, seagrass epiphytization and die-off, and population explosions for species that rapidly expand using the high nutrient status of coastal waters.

SGD could be particularly important in the Philippines because of the wet humid climate, steep coastal gradients and common limestone terrains that are conducive to rapid underground flow. Oceanographic data from the Gulf shows a distinct tongue of low salinity water on the eastern side of the bay during drier periods of the year (when surface runoff is low). The most likely interpretation of this observation is that there is substantial SGD occurring into the deep waters of Lingayen Gulf from a confined aquifer.

This project, led by Dr Gil Jacinto, is examining SGD and its influence on the coastal waters and coral reef ecosystems of Bolinao through a regional and detailed mapping of incidence and origin of SGD, characterization of the quality and measurement of fluxes, tracking of mixing and dispersal, and correlation with the reef ecosystem.

Initial research is showing the submarine springs are associated with faults or fractures which act as conduits of freshwater. Resistivity profiles are indicating that the springs are connected to a widespread reservoir of relatively fresher water lens 5 to 7 meters below sea level under the reef flat.

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