New research shows how submarine groundwater affects coral reef growth

Groundwater that seeps into the coastal zone beneath the ocean’s surface—termed submarine groundwater discharge (SGD)—is an important source of fresh water and nutrients to nearshore coral reefs throughout the globe. A recently published study, led by researchers at tSOEST, sheds light on the ways SGD affects coral reef growth.

“SGD is common on nearshore coral reefs, especially in Hawai‘i, so we set out to test how SGD affects coral reef growth in Maunalua Bay, O’ahu.” said Megan Donahue, associate researcher at the Hawai‘i Institute of Marine Biology (HIMB) in SOEST and senior author of the study.

Two processes contribute to the overall growth of coral reefs: coral growth and bioerosion, the natural breakdown of coral reefs by reef organisms. In areas with high levels of SGD, it was a double whammy for coral reefs. Corals that were right next to SGD seeps performed poorly, likely due to the stress of too much fresh water.

“Additionally, we found that marine organisms responsible for bioerosion broke down the skeletal reef framework very quickly when exposed to high amounts of SGD,” said lead author Katie Lubarsky, who completed this research as part of her graduate degree in Marine Biology at UH Mānoa.

To the researchers’ surprise, SGD actually enhanced coral growth when the nutrient enrichment and freshwater from the groundwater was at low levels.

Their results indicate that corals can thrive on SGD-impacted reefs if isolated from secondary stressors such as competition from seaweeds and sedimentation.

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