Olowalu coral core links Maui land use to reef decline

Chemical signatures trace Maui’s sugar era in Olowalu coral

The U.S. Geological Survey collected the 6-foot-long Olowalu coral core in 2013 and performed initial analysis. In-depth research began in 2024, when a team led by Sam Kekuewa used the 253-year record to reconstruct the reef’s environmental history. The team found that coral calcification increased by about 30% before about 1970 and then declined.

The record covered 1760 to 2013, placing the precolonial era, the rise and fall of Maui’s sugar-plantation economy and the island’s development as a tourist center in the same environmental timeline. The findings appeared in the September issue of Coral Reefs. The scientists found that the reef’s growth closely mirrored conditions on land and said they had pinpointed for the first time that its growth rate began a steep decline in the late 1960s as the plantation economy boomed. The U.S. Geological Survey called the result the “first long-term baseline for understanding ridge-to-reef connectivity on Maui.”

The Olowalu reef spans nearly 1,000 acres off Maui’s southwestern coast, about a 10-minute drive from Lahaina. It is often called a mother reef because it sprays copious amounts of coral larvae into the ocean that currents carry to neighboring Molokaʻi and Lānaʻi for reef building. The reef also has an international reputation as a super reef and turtle reef and is a Mission Blue Hope Spot for its importance to ocean health. According to scientists and reef advocates, healthy reefs protect coastal communities from powerfully destructive waves, provide nursery habitat for fish prized by anglers and snorkelers, and offer cultural significance and spiritual grounding to local residents.

Sam Kekuewa of Hawaii Sea Grant College Program led the research team, which included scientists from the University of Hawaiʻi at Manoa’s School of Ocean and Earth Science and Technology and the U.S. Geological Survey.

Once the researchers gained access to the core, they studied chemical signatures in the coral skeleton. They analyzed the presence of 18 trace elements and connected them to land conditions at the time. From the 18th through the late 20th century, the ratios of barium-to-calcium, phosphorus-to-calcium and iron-to-calcium jumped between 17% and 55%. The largest increases occurred in the mid-1900s, when Maui’s sugar plantations were operating at full scale. By following the elements from watersheds to the coastline and onto reefs, the team used ridge-to-reef connectivity to reconstruct the reef’s environmental history.

Maui’s sugar plantations required massive amounts of water and fertilizer and altered the landscape and natural hydrology through land privatization, native plant removal, the introduction of an industrial-scale monoculture and construction of a water-diversion ditch system. Sediment and nutrient runoff, often associated with agricultural fertilizers, can reduce the sunlight filtering through seawater, bury the reef or cause coral abrasions.

The researchers measured coral density and how much the coral grew each year to calculate calcification rates, which show how much skeleton it builds annually. Higher calcification rates mean the coral is building more skeleton and are one sign the coral is healthy. Adding historical land-use records and large-scale weather patterns such as El Niño helped the researchers understand how the reef responded to environmental stress. The dramatic decline after about 1970 coincided with peak sugar production and slowed as plantations were phased out.

Study co-author Andrea Kealoha said the research addressed a gap in earlier work. “There’s been several studies showing a connection between historical sugarcane production and poor water quality,” Kealoha said. “But nobody has been able to connect that to the decline of reef health.” The research linked historical land conditions to reef health and supplied managers with a long-term context for ridge-to-shore decisions.

Kekuewa is a chemical oceanographer with a doctorate from Scripps Institution of Oceanography. After graduating with his PhD in 2024, he initially came to Maui to work as a postdoctoral fellow with Kealoha, an assistant professor of oceanography at UH Manoa. Kekuewa said he grew up amid the Hawaiian diaspora of the San Francisco Bay Area. He had previously helped Kealoha research how West Maui’s water quality and nearshore environment were affected by the 2023 Lahaina wildfire. While looking for a new research project, Kekuewa found the Olowalu core and its data set through a basic Google search. His idea to focus on better understanding the reef, given the threats it faces, sparked Kealoha’s interest, and a team was assembled.

Kekuewa called the research timely amid the pressures facing Maui’s nearshore environment. “It’s very timely given all the things that are happening now: climate change, the Lahaina fire, more fires becoming more prevalent, increasing storm frequencies, you know three hurricanes and tropical storms that made landfall in Hawaiʻi this year so far,” Kekuewa said.

He said severe storms affecting Hawaiʻi now and into the future and driven by climate change will cause “more runoff, more sedimentation,” which can suffocate the reef. Future fires, new subdivisions and coastal roadways can also affect coral health. Honoapiʻilani Highway, which connects West Maui with the rest of the island, is buckling under chronic erosion, storm surges and king tides. Kekuewa said learning from the reef’s past was imperative for guiding future management actions.

Multiple projects are underway to keep sediment out of Olowalu’s nearshore waters, reforest and replant native species, mitigate wildfires and control the negative effects of axis deer, feral pigs and goats on vegetation mauka of the coastline. The new research provides the long-term context to support and guide that work.

Groups working to study and enhance the reef and the Olowalu ahupuaʻa include the National Oceanic and Atmospheric Administration, the Hawaiʻi Department of Land and Natural Resources, Hui O Ka Wai Ola, Kipuka Olowalu and The Nature Conservancy. “We don’t really do science for the sake of science. We always want to be improving our coastal ecosystem health in Hawaiʻi,” Kealoha said. “We went into this study with that intention.”

Tamara Farnsworth, a coastal resilience project manager for The Nature Conservancy in Hawaiʻi and Palmyra, said the groups are coordinating their work. “We are all working together, working mauka to makai, addressing sedimentation in particular, and just the health of our forests and our coastal areas and ecosystems there,” Farnsworth said. As part of its work, the conservancy draws on ʻike kupuna, the wisdom of elders or traditional ancestral knowledge. “They help remind us of how conditions used to be, how it used to look, the plants that used to be there, the activities they used to be able to do,” Farnsworth said.

Farnsworth tied the core record to the conservancy’s practice of working with people of the place and listening to them. “We’ve seen so much damage over the years as evidenced by these core samples,” Farnsworth said. “This is why it’s so important to talk to the people of the place and to work with them and listen to them. We are working with science as well as ʻike kupuna. One is not more important than the other. They’re both important to inform how we work.”