Freshwater flowing directly from land and into the ocean beneath the sea’s surface has been found to affect the water chemistry in coral reefs and trigger “a cascade of changes” in the coastal ecosystem.

These are the latest findings published by researchers at the University of Hawai’i at Mānoa who have since described the effect as a “Goldilocks scenario” – too much groundwater has a negative impact on that ecosystem, but when the input is “just right”, the reef benefits.

Freshwater flowing from the land and into the ocean beneath the sea surface – otherwise known as submarine groundwater discharge – was found to indirectly affect the ecosystem by changing the acidity of the seawater and impacting the process by which corals build their skeleton.

The research – published in Ecological Monographs – provides critical insight into the complex interactions between the land and the ocean, particularly in scenarios where groundwater discharge increased nutrient availability to help reefs thrive.

“One idea we hope this research can communicate is the natural role of groundwater in feeding healthy reefs,” said Craig Nelson, study co-author and faculty in the UN Mānoa Daniel K. Inouye Center for Microbial Oceanography.

“Runoff and springs bring subsidies of nutrients and organic matter that can help reefs thrive, and some complex interplay this work illuminates demonstrates that some reefs are adapted to these inputs.”

It’s according to Nyssa Silbiger, lead author on the study and associate director of the Uehiro Centre for the Advancement of Oceanography, that submarine groundwater discharge is a “widespread and under-appreciated land-sea connection that delivers terrestrial nutrients and carbon to coastal ecosystems.”

“This profoundly influences coral reef health by triggering a cascade of chemical and biological changes that alter the cycling of carbon in these ecosystems.”

To draw their conclusions, Silbiger and the team studied two coral reef sites in Mo’orea in French Polynesia – both chosen based on surveys indicating fresh groundwater, as well as discussions with local fishers with historical knowledge of the island.

They measured the water quality parameters that change in direct response to the groundwater input as well as change indirectly in response to altered biological activity within the coral reefs.

What they found was that nutrients from submarine groundwater boosted the productivity of both photosynthetic algae and corals which, in turn, changed seawater acidity and altered the calcification of coral skeletons. 

“We show that adding new nutrients to the coastal ocean alters the metabolism of coral reefs ecosystems, thereby changing the patterns of uptake and release of carbon,” said Silbiger. “This highlights a chain reaction where enhanced biological activity – fuelled by submarine groundwater discharge – directly affects the acidity of seawater and, subsequently, the ability of reefs to grow.”

The authors hope this work can now help emphasise the need to keep groundwater free of contaminants for the benefit of both land and ocean ecosystems.

“These findings are vital for protecting coral reefs across the Pacific from land-based pollution and informing sustainable land management,” said Megan Donahue, study co-author and director of the Hawai’i Institute of Marine Biology in SOEST.

“Understanding these cascading effects allows us to predict how coral reefs respond to a changing world and provides a framework to connect ecosystem ecology to land-based activities.”

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