Scientists Come to Molokai to Study Imperiled Reef—Research Can Help Improve Land-Management Practices

[Reprinted, with permission, from the Dec. 10, 2007, issue of the Molokai Times.]

Above: The vessel Alyce C anchored in a sand channel on a coral reef off Moloka‘i. [larger version] Above: USGS scientists drill into a coral head to obtain a core for study of the impacts of sediment in runoff from Moloka‘i. [larger version] Above: After drilling a core from the coral head, researchers seal the hole with a plug made beforehand from ready-mix concrete. The plug keeps out boring organisms that could weaken the structure of the coral head. [larger version] Above: USGS Mendenhall Postdoctoral Research Fellow Nancy Prouty examines a core drilled from a coral head off Moloka‘i. She will analyze this and similar cores to see whether they contain a record of storm-runoff carrying soil from the island out to the coral reef. [larger version]

Last week, after storms poured an astronomical quantity of rain across Molokai, anyone who ventured out of the house could see the brown streams washing across Kamehameha V Highway. These streams can be dangerous for people in cars trying to cross them, but the damage they can cause does not stop where land meets sea.

The dirt carried in these streams comes from higher elevations, where human alterations to the land have left no means of securing the soil. Factors like feral goats and pigs, nonnative animals that rip up vegetation with steadfast voracity, make it easy for rain on the mountain to wash the soil down to the sea. Modifications to residential properties have the same effect. The soil becomes more hazardous when it washes out onto Molokai Reef Flat, the longest fringe reef in all of the United States and its protectorates.

The United States Geological Survey and the University of Hawaii, among others, have been looking into the true impacts of all this dirt, or sediment runoff, on the corals that populate the reef. So far, they have found that corals do not respond well. Sediment, according to recent research, is inhibiting corals' ability to multiply on Molokai Reef Flat.

In late November, USGS researchers Mike Field and Nancy Prouty came to Molokai from the mainland to look at sediment behavior on the reef. Venturing out in Captain Joe Reich's boat Alyce C, the crew braved "snarly" weather and misbehaving equipment to get samples that will help them map the history and future of sediment on the reef. "The goal is to see whether those corals contain a record of runoff through time," Field said.

Field says that while scientists have been taking "plugs" of coral in an attempt to study the animal, doing so to study sediment behavior is a new method of studying the effect of runoff on coral-reef systems. "We're trying to break some ground here," Field said. "This may be the template" for future reef studies.

Field and Prouty said that they took nine core samples of coral in six locations on the reef. The cores are cylindrical samples 2 inches in diameter and range from 24 to 80 centimeters in height, depending on the age of the coral head they sampled. Field said that finding large, or old, enough coral heads was a challenge. "We had to search pretty diligently to find any," he said.

Between Kawela and the Kaunakakai Wharf, he said, finding coral heads old enough to be ideal specimens was especially tough. While 2 meters is an optimal height, he said, the tallest they could find was 11⁄2 meters.

Prouty, a recipient of a USGS Mendenhall Postdoctoral Research Fellowship after recently earning her Ph.D. from Stanford, has taken the reins on this project. The samples arrived at Prouty's office at the USGS Pacific Science Center in Santa Cruz, California, shortly after her return.

Prouty said that she hopes that the cores will reach back 100 years. Field compared their study of the core samples to that of determining a tree's ages and stress periods by looking at its rings. This, however, would still not give them a picture of sediment on the reef before Westerners, the alien animals they brought, and their land-management practices altered the terrain of Molokai, they said.

Prouty will X-ray the corals. By measuring the amount of sediment contained in each coral segment, or annual "time band," they hope to equate periods containing a high amount of sediment with major rainfall events.

Some areas of the reef may have had more sediment than others at any given time, so knowing where the sediment settles after a storm, and where it drifts as time passes, will help Field and Prouty get a picture of where runoff settles, resuspends, and settles again.

This kind of research, Field said, will help managers determine best land-management practices. Putting together the puzzle pieces, he said, will be a lengthy process. "It's not like next week we'll have answers," Field said.

Field said that there was no new or startling evidence of impacts out on the reef, but that everything they observed confirms what his team has already known: coral is under stress in many areas of the reef.

Because of sediment's constant suspension and resettling on different parts of the reef, one area may show stress at a different time than another. Near Kawela it may be worse than it was 100 years ago, Field said, but the reverse may be true for the reef that sits off Pauwalu.

No matter where the dust settles, there is much more of it on its way to the reef. With the rainy season in full and relentless swing, the reef is set to bear its brunt.