The world's coral reefs are being damaged by harmful runoff, which new research shows is being caused by the clearing of forests. Preventing soil erosion may help protect the reefs, which shield coastlines from ocean waves, draw tourists with their brilliant colors and provide a habitat for fish.

For centuries, humans around the world have been clearing away forests to make room for livestock, farming, and other uses. Scientists say that when the forest cover is lost, soil washes away into rivers that flow into the oceans and, they believe, onto coral reefs.

That runoff adds nutrients to the ocean water. It's essentially too much of a good thing, and scientists have long suspected it can kill the coral. Runoff also clouds the water, blocking light from reaching the reef.

But researcher Paul Sammarco, at the Louisiana Universities Marine Consortium, says it's been hard to prove that clearing land increased how much runoff reaches the reefs because nobody was looking until recently. "We can talk until we're blue in the face about what we think happened, and what we think the environmental impacts were," he said. "But to get hard data on it is really very much a different thing because nobody was taking this kind of data 130 years ago."

But a group of researchers at the Australian National University, and the Australian Institute of Marine Science, has found a way to get some of that data. In a study published in a recent issue of the journal Nature, Tim Wyndham and his colleagues took a sample, or core, from a centuries-old coral in the Great Barrier Reef off the Australian coast.

"When you take a coral core, what you get is a core which has annual layers which look much like tree rings," explained Mr. Wyndham. "So using those layers we're able to count back to any particular year over the last 250 years."

Mr. Wyndham and his group looked at a chemical element in those annual layers delivered to the coral in runoff. When European settlers arrived in Australia about 130 years ago, they began cutting down forests to raise sheep and cattle, and to grow crops. Mr. Wyndham says according to the coral record, the amount of runoff delivered to the reef jumped by five- to ten-fold beginning shortly after the settlers came.

Reef researcher Julia Cole at the University of Arizona says the research is evidence of what researchers have suspected. "What we see is a definite trend toward the worsening of conditions for reefs associated with things as simple as cultivation," she said.

According to Ms. Cole and others, soil erosion threatens reefs in the Caribbean, off Southeast Asia and Africa, as well as Australia.

That runoff can begin a downward spiral for coastal communities who depend on the reefs for food and income. Paul Sammarco says as the runoff makes it harder for some fish living in the reefs to survive, people may have to fish the reef harder.

"When they fish harder, they may use techniques which are destructive, even more destructive to the environment," he said. "In other words, ball-and-chain techniques which are used to gather fish which destroy the three-dimensional structure of the reef. Dynamite fishing. A number of poison techniques which are used."

Mr. Sammarco said these extreme methods, which cause irreparable harm to coral reefs, would not be necessary if soil conservation methods were put in place to help maintain the health of the ocean ecosystems.

Study author Tim Wyndham says soil erosion is not just bad for reefs, its also bad for farmers, and soil conservation efforts may help in two ways. "What we're hoping is that land degradation can be treated, and that should also, as well as treating the land degradation, that should treat the runoff to the reef," he said.

So farming with less impact on the land would also have less impact on the sea.