Island Rivers Play Crucial Role in Shaping Coral Reef Passages

Volcanic islands, such as those in Hawaii and the Caribbean, are often fringed by intricate coral reefs that are not uniform but rather contain natural gaps known as reef passes. These reef passes serve as vital channels connecting the sheltered lagoons with the open ocean, facilitating the movement of water and nutrients essential for the survival of coral ecosystems. Recent research conducted by the Massachusetts Institute of Technology (MIT) and the Woods Hole Oceanographic Institution has unveiled an unexpected relationship between island rivers and the formation of these coral reef passages.

The study, led by Megan Gillen, a graduate student in the MIT-WHOI Joint Program in Oceanography, highlights that island rivers are not merely passive entities but actively contribute to the shaping of coral reefs. "A lot of discussion around rivers and their impact on reefs today has been negative due to human impact and agricultural practices," Gillen stated. "This study shows the potential long-term benefits rivers can have on reefs, which I hope reshapes the paradigm and highlights the natural state of rivers interacting with reefs."

The research team focused their investigation on the Society Islands, a group of volcanic islands in the South Pacific that includes Tahiti and Bora Bora. Unable to conduct fieldwork due to the pandemic, the researchers utilized satellite imagery from Google Earth. They observed an intriguing correlation between the locations of reef passes and the mouth of rivers draining into the sea. Gillen remarked, "The islands in this chain have these iconic, beautiful reefs, and we kept noticing these reef passes that seemed to align with deeply embayed portions of the coastline. We started asking ourselves, is there a correlation here?"

To further explore this relationship, the researchers employed radar data from NASA's Shuttle Radar Topography Mission, which mapped approximately 80% of the Earth's surface in 1999. By analyzing the topographic data, they traced the river basins of the islands and identified the locations of reef passes. The analysis revealed a statistically significant correlation between large river basins and the positioning of reef passes, suggesting that the presence of rivers plays a crucial role in their formation.

The researchers proposed two primary mechanisms through which rivers could influence coral formation: reef incision and reef encroachment. Reef incision occurs when sea levels are low, allowing rivers to flow over exposed reef and gradually erode the coral, while reef encroachment happens during rising sea levels when coral reefs grow upward into old riverbeds, potentially leading to gaps when the coral cannot thrive in deeper waters.

Gillen explained, "Both mechanisms, occurring over dozens of cycles of sea-level rise and island evolution, are likely responsible for the formation and maintenance of reef passes over time." The study also indicated that younger islands exhibit a greater density of reef passes compared to older islands, which tend to have fewer, more widely spaced reef passes due to the gradual sinking of volcanic islands and reduced river flow.

This research holds significant implications for coral reef conservation strategies. With an understanding of how rivers shape coral ecosystems, Gillen is now investigating the feasibility of engineered flows—man-made systems that mimic natural river systems—to create reef passes in areas lacking natural river interactions. "Part of me wonders: If you had a more persistent flow, in places where you don’t naturally have rivers interacting with the reef, could that potentially be a way to increase health by incorporating that river component back into the reef system?" she pondered.

The findings of this study have been published in the journal *Geophysical Research Letters* and contribute to a growing body of literature emphasizing the interconnectedness of terrestrial and marine ecosystems. As researchers continue to unravel the complexities of coral reef ecosystems, the role of island rivers may offer new avenues for enhancing coral resilience in an era of climate change and environmental degradation. Understanding these relationships not only deepens our ecological knowledge but also informs more effective conservation practices aimed at preserving these vital marine habitats.