New USGS, NPS Study: Submarine Groundwater Discharge Fuels Algal Growth, Threatening Coral Reefs in American Samoa
New USGS-led research finds that submarine groundwater discharge (SGD)—the unseen flow of freshwater and nutrients from land to sea—is a key driver of water quality and nutrient loading on the island of Ofu, American Samoa, with implications for reef resilience in a changing climate.
The corals of Ofu are vibrant, diverse, and unusually tolerant of above-average water temperatures; other researchers have noted that these corals may provide critical insights into the potential of coral acclimatization and adaptation to climate change. However, excess nutrients from groundwater can fuel increased algal growth—which may compete with corals for space and light—potentially undermining the resilience of even thermally tolerant coral species.
Using an advanced suite of tools—including drone-based thermal imaging, radon tracing, electrical resistivity surveys, and direct water sampling—USGS scientists quantified how groundwater seeps into two shallow reef pools, Papaloloa and Fattuana, within the National Park of American Samoa on Ofu Island.
Their findings show that SGD rates and nutrient flows vary significantly between sites, with Papaloloa exhibiting higher and faster groundwater discharge due to its permeable sand and gravel substrate. At Fattuana, groundwater flow was slower and delayed, suggesting inputs from a deeper, less permeable aquifer.
Importantly, nutrient concentrations tracked closely with groundwater influx, confirming SGD as the main nutrient delivery pathway to these reefs. Yet, despite lower groundwater inputs, Fattuana had higher algal cover, suggesting that other factors such as wave energy, substrate type, and water circulation may also fuel algal proliferation.
This research highlights the complex role of groundwater in reef ecosystems, where land-based nutrient sources, hydrology, and ocean processes interact in unexpected ways. With climate change and coastal development intensifying pressures on coral reefs worldwide, this research underscores the need for integrated land–sea management, including characterizing and quantifying the effects of SGD. Understanding groundwater inputs could help managers mitigate nutrient pollution and protect coral habitats critical to biodiversity and local livelihoods.
Read the study, Land-based nutrient flux to a fringing reef: insights from Ofu Island, American Samoa, in Frontiers in Marine Science.
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Nutrient Loading and Coral Health at Ofu, American Samoa
Coral Reef Project
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Nutrient Loading and Coral Health at Ofu, American Samoa