Sediment pollution from land has severe consequences for fragile coral reef ecosystems, and Hawai‘i is no exception: Soil erosion from development and agriculture creates excess sediment that chronically muddies reefs across the state, making it difficult for corals and fish to thrive.
But what are the most effective means to prevent sediments from reaching the reefs? And once sediments reach the reefs, how quickly are they flushed out by wind and tide?
A new study answers some of these questions. The paper, published in the open-access journal Collabra, shows that even along the same coastline, reefs can experience drastically different water flow conditions affecting water quality, particularly exposure to reef-killing sediment pollution. Led by researchers from Conservation International’s (CI) Hawai‘i program, the study is the first to examine the sediment dynamics of Lānaʻi Island in Hawai’i.
The research team monitored sediment from two streambeds on the island of Lānaʻi, each within a mile of each other, and their corresponding adjacent coral reefs to test whether the combined physical conditions of water movement, and volume of sediment flowing into the ocean at each location, could provide useful information to guide where watershed restoration and better land management was needed. Of the two reef sites, one experienced quick-moving currents that efficiently flushed sediment from the site, keeping the water clear, while the other site was subjected to currents and wave conditions that do not remove sediment from the water, allowing for sediment to be continuously re-suspended in the water.
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“This research tells us that even without rain events, reefs can be continually exposed to sediment stress through the effects of wind, waves and tide,” said Jack Kittinger, director of CI Hawai‘i and a co-author of the study. “Sediment pollution is a major threat here in Hawai‘i and globally, and prolonged exposure can kill corals. This ultimately impacts the quality of the habitat for reef fish populations, which in turn affects food security and livelihoods of our communities.”
With this study, researchers now have information that could help improve terrestrial and reef habitat health, and develop a detailed plan to monitor restoration measures and adapt restoration strategies. Knowing the natural rate of sediment removal in a reef environment can guide planning for how much to limit the sediment input from land, which in steep watersheds on montane tropical islands in the Hawaiian archipelago could be done by restoring vegetation and controlling invasive species.
For example, the streambed adjacent to the reef that experienced heavy water and sediment flow had gabions — check dams — constructed out of invasive kiawe trees by a local community group. These dams stopped 77 tons of sediment, the equivalent of nearly 10 dump trucks, from flowing into the ocean. Without the check dams, the adjacent reef area would have needed about five weeks of natural water flow around the reefs to flush that amount of sediment.
While there is always more that can be learned about how land management affects reef health, this study will enable further improvement of reef-to-ridge management projects to continue the restoration of Hawai‘i’s reefs, Kittinger said.
Researchers from the University of Hawaii at Manoa and community leaders from the Maunalei Community Managed Makai Area, as well the Pacific Islands Ocean Observing System (PacIOOS), contributed to the study.
Eric Walton is a communications intern at Conservation International.