Cascading impacts of groundwater input to coral reefs

Groundwater input to coral reefs directly affects water chemistry and triggers a cascade of changes in the coastal ecosystem, according to a new study led by University of Hawaiʻi (UH) at Mānoa oceanographers. Freshwater from land that flows into the ocean beneath the sea surface, termed submarine groundwater discharge, was found to increase nutrient availability, change acidity of the seawater, and impact the process by which corals build their skeletons. This research, published recently in Ecological Monographs, provides critical insights into the complex interactions between the land and ocean. 

“Submarine groundwater discharge is a widespread and underappreciated land–sea connection that delivers terrestrial nutrients and carbon to coastal ecosystems,” said Nyssa Silbiger, lead author of the study, associate director of the Uehiro Center for the Advancement of Oceanography, and associate professor in the Department of Oceanography at the UH Mānoa School of Ocean and Earth Science and Technology. “This profoundly influences coral reef health by triggering a cascade of chemical and biological changes that alter the cycling of carbon in these ecosystems.” 

The fundamental connection between land and sea through the flow of freshwater is a universal principle recognized as important for coastal health across all cultures. Porous volcanic islands throughout the tropics deliver much of this water through rivers and streams, but a major fraction emerges unseen directly into the coral reefs that ring these islands. This submarine groundwater discharge has long been recognized by Pacific peoples as important, with seeps frequently named and associated with specific communities of algae and fish relevant to subsistence. The new research has helped define the complex interplay of chemistry and biology that makes these inputs so important to the ecology of coral reefs. 

Cascading impacts on Mo‘orea reefs

Silbiger and collaborators studied two coral reef sites in Moʻorea, French Polynesia that were chosen based on surveys of salinity and isotopes indicating fresh groundwater, as well as discussions with local fishers who have historical knowledge of submarine groundwater discharge seepage points around the island. The research team measured a suite of water quality parameters that can change directly in response to the groundwater input as well as change indirectly in response to altered biological activity of the coral reef community. 

Nutrients from submarine groundwater boosted the productivity of both photosynthetic algae and corals which, in turn, changed seawater acidity ultimately altering calcification of coral skeletons. Defining this complex exchange between organisms and the chemistry of the seawater around them is crucial for developing a more holistic understanding of how reefs work.

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

Informing management of Pacific coral reefs

Concerns over pollution often dominate the conversation surrounding runoff and inputs from land to coral reefs. 

“One idea we hope this research can communicate is the natural role of groundwater in feeding healthy reefs,” offered Craig Nelson, study co-author and faculty in the UH Mānoa Daniel K. Inouye Center for Microbial Oceanography. “Runoff and springs bring subsidies of nutrients and organic matter that can help reefs thrive, and the complex interplay this work illuminates demonstrates that some reefs are adapted to these inputs.” 

The authors hope that their work can help emphasize the importance of keeping 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.”