Anthropogenic Nutrient Loading and Coral Health at Ofu, American Samoa

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Coral reefs in the National Park of American Samoa's Ofu Island unit are facing a threat from an outbreak of Valonia fastigiata algae. The Ofu reef hosts a rich biodiversity with nearly 300 fish species and 80 coral species, some of which are protected under the U.S. Endangered Species Act. These reefs play a crucial role in protecting coastal communities from flooding and mitigating erosion of the island's beaches, which are important nesting sites for endangered sea turtles. Additionally, the corals in Ofu's reef flat exhibit remarkable resilience to thermal stress, enduring daily temperature fluctuations exceeding 6°C and often surpassing the bleaching threshold of 30°C. Consequently, they serve as valuable subjects for studying coral adaptation to climate change. However, the presence of Valonia fastigiata indicates a potential threat from human-induced eutrophication, jeopardizing this unique ecosystem and its thermally-tolerant coral genotypes.

Identifying the specific impacts of water quality stressors is challenging due to the complex interplay of various physical and biological factors affecting coral reef health. While efforts have historically focused on mitigating nutrient pollution from surface runoff, it is increasingly recognized that SGD plays a crucial role in transporting nutrients into coastal waters, particularly in short, steep coastal areas lacking rivers. Coral reefs influenced by SGD are particularly vulnerable to human activities altering groundwater quality and quantity. SGD can alter marine biotic communities by delivering high nutrient loads, leading to eutrophication and harmful algal blooms, thereby affecting coral reef health by increased susceptibility to disease and decreased coral abundance and diversity.

This study aims to reconstruct decadal to centennial trends in local hydrology, nutrient inputs, and thermal variability using coral cores. Cores will undergo high-resolution X-ray CT scanning and precise sampling for geochemical analysis. These analyses will help trace the history of SGD inputs to Ofu's pools and assess changes in nutrient enrichment over time. Additionally, coral health metrics derived from CT imaging will be combined with geochemical data to monitor the health and function of the ecosystem. Similar approaches have been successful in detecting changes in other coral reef systems, providing valuable insights into ecosystem health trends.

Scientists predict that coral bleaching will occur annually within decades due to rising sea-surface temperatures, posing a threat of extinction to over one-third of reef-building corals. However, the natural acclimatization of the Endangered Species Act-listed corals on Ofu's coral reef to high water temperatures offers hope for future coral resilience. While the modern sea-surface temperatures on the Ofu reef are known, it's uncertain how long corals were exposed to stress due to high temperatures in the past. Understanding this is crucial for determining if Ofu's corals' resilience is unique or common and assessing SGD's contribution to their acclimatation to high temperatures. Decadal-scale reconstructions from coral cores will establish pre-human baselines, identify the impact of human activities, and potentially reveal the timespan of local coral acclimatization.