Monday, March 3, 2008

Mud kicks up daily on Moloka'i reef

A muddy shoreline can starve marine life of light—the equivalent of trying to grow a garden in a dark room.

(Image: South Moloka'i coastline, showing Kaunakakai Pier at right. NOAA/USGS aerial photo.)

Research on the south shore of Moloka'i suggests that the light deprivation can in some areas continue long after the rain has stopped, since the sediments remain on the reef, and are lifted into the water column each time the winds and currents come up.

The process is described in a new paper, “Diurnal variability in turbidity and coral fluorescence on a fringing reef flat: Southern Molokai, Hawaii,” by researchers Gregory Piniak of NOAA's Center for Coastal Fisheries and Habitat Research in North Carolina, and Curt Storlazzi, of the U.S. Geological Survey's Pacific Science Center in California.

The work is published in the Elsevier journal, Estuarine Coastal and Shelf Science.

Piniak, in an email, said that many people worry about runoff from the land into the sea only when it's happening, not recognizing that the damage can continue for an extended period of time.

“Generally runoff is thought of as a single event, like storms that deliver big pulses of sediment to the reef,” he said.

But on some leeward shores, where big onshore surf doesn't quickly dilute the muddy water and flush it out to sea, things can be different.

“On the Moloka'i reef flat sediment builds up over time and is constantly stirred up, so sediment events become more of a chronic issue.

“The reef flat on the southern coast of Moloka'i can be really turbid—the water can be calm and fairly clear in the morning, but when the trade winds and waves kick up it can be like swimming through coffee,” Piniak said.

And it's not just Moloka'i's wide, protected southern side. Citing previous research by others, Piniak and Storlazzi note that a 2002 runoff event at Honolua Bay on Maui left sediment on the reef for six months.

“Periodic sediment damage decreased coral cover in the bay by 33 percent between 1992 and 2002,” they write.

In the Maui situation, agricultural runoff is the prime culprit. On Moloka'i, it is generations of overgrazing on the slopes above the shore. Moloka'i's situation, at least near Kaunakakai, is made worse by the long Kaunakakai Pier, which blocks the movement of mud down the coast.

“The wharf at Kaunakakai further inhibits longshore transport and removal of sediment from the Molokai reef flat, resulting in some of the lowest coral cover in the state of Hawaii,” the paper says.

The researchers studied the impacts of the muddy water on the coral Montipora capitata.

Not surprisingly, perhaps, they found these corals were somewhat adapted to handle the pulses of sedment, and a daily regiment of being pounded by bright sunlight in the morning when the weather was calm, and then getting shaded out each afternoon as the tradewinds came up and disturbed the sediment.

“The coral species we studied tends to handle sediment pretty well, but our data suggest it would probably do better physiologically if the water were clearer,” Piniak wrote.

It may be that some corals are able to develop a tolerance for the sediment conditions, but the authors gave two examples of things the community could do to address the sedimentation issue.

One is to revegetate the Moloka'i hillsides to reduce sediment-filled, and another is to make changes to the Kaunakakai Pier, to promote water low from one side to the other.

© 2007 Jan W. TenBruggencate