DRY TORTUGAS — Out here, far from the Keys' sewage outfall and hordes of divers, the reefs look great — at first glance.

Tall fish-covered pinnacles rising from the sea floor seem painted with healthy coral.

Tortugas reefs have the most coral cover in the Florida Keys National Marine Sanctuary — coral cover is the percent of a reef that has coral growing on it, and, while much of the sanctuary has 10 percent coral cover, parts of the Tortugas have 40 percent.

Even out here, though, diseases and bleaching are killing corals.

"You look at a map and say, 'That's remote: No problem,' but the data disagrees with that," said coral expert Walt Jaap, of the Florida Fish and Wildlife Research Institute, who has been studying coral in the Tortugas since 1975. "Some people have the hypothesis that because it's remote, there's a buffer, and you won't see declines in coral. We started seeing declines the second year we were sampling.

"Generally, the environmental quality on this reef is superior to the Keys, but it does have its problems."

Jaap is a member of the multi-agency Coral Reef Evaluation and Monitoring Project team, which has been studying reefs in the 2,800-square-nautical-mile sanctuary since 1996.

He and a team of state scientists were in the Tortugas earlier this month, as were a team of federal scientists.

Some of the team's findings are disturbing:

• In 1996, coral disease was observed at 26 of 140 sampling stations. By 2001, disease was observed at 131 stations. During the same time, the number of coral species affected increased from 11 to 36.

• Between 1996 and 2002, the team documented a 38 percent decline in coral cover.

Much of the trouble along the Keys reef tract from Key Largo to Key West is the result of poor water quality.

Viruses and bacteria from 6,000 cesspits and 24,000 septic tanks pour onto the reef tract and spawn a host of coral diseases.

Elements in human fecal matter also feed the red boring sponge, which bores into corals then slowly envelops and kills the entire colony.

Although the nearest septic tank or cesspit is more than 70 miles away, water quality is a source of coral mortality in the Dry Tortugas.

"It's a complex hydrological gig out here," Jaap said.

In other words: Lots of things that are bad for coral, including bacteria and viruses from human waste, nutrients, pesticides and herbicides, can ride a long way on ocean currents to reach the Dry Tortugas.

"The Gulf of Mexico is a pit that carries water from the Mississippi River, Tampa Bay, Texas, Louisiana, and Atlanta down the Chatahoochee River," said Lauri MacLaughlin, a Florida Keys National Marine Sanctuary resource manager. "When the Mississippi flooding was happening in 1998 and '99, we got muddy water and critters — eels, snakes, weird stuff that we don't see — down here. What happens in the Mississippi ends up in our back yard."

MacLaughlin and Debbie Santavy, a research ecologist for the United States Environmental Protection Agency, have been evaluating coral health at 45 stations in the Keys, including 17 in the Dry Tortugas, since 1998.

Trouble comes not only from the United States.

"We get water masses from Central and South America that are probably not as pristine as we'd like," Jaap said.

Probably the most dramatic coral disappearance in the Dry Tortugas took place sometime between 1933 and 1975.

In the 1880s, marine scientist Alexander Agassiz documented 108.7 acres of elkhorn coral in the Tortugas; the corals were still alive and healthy when John Wells surveyed the area in 1933.

Nobody studied the Tortugas elkhorn system from the 1930s until 1975, and in the interim, elkhorn colonies had shrunk to 1.5 acres.

"There's a 40-year gap in knowledge," Jaap said. "Between 1933 and 1975, either a hurricane or cold winter front must have come through."

Hurricanes and cold fronts are natural, but a disease called white pox that infects only elkhorn coral was first documented in 1996 and is caused by the human fecal bacterium Serratia marcescens.

White pox has killed 85 percent of all remaining elkhorn coral in the Keys.

Another troubled coral species is the elkhorn's cousin, staghorn coral, a victim of disease and an extremely cold winter in 1977-78.

Since the 1970s, elkhorn and staghorn corals have suffered from 85 percent to 98 percent declines in the Keys and Caribbean — the two species and the hybrid fused staghorn coral have been nominated as candidates for the Endangered Species List.

"Staghorn coral used to be everywhere," Jaap said. "It was like a weed. We used to curse the damn staghorns because they were so ubiquitous, and it was difficult to work in a lot of places because there was so much of it."

Obviously, nothing can be done about hurricanes and cold fronts to help Tortugas coral, but something can and must be done about the human impact, Jaap said.

"Let's say if you got advanced water treatment systems for waste water and runoff, and you got a few more management techniques for carrying capacity — there are just too many divers and boaters — that would benefit the reefs," Jaap said. "In 10 years, if management improves, you'll see a general improvement. If not, things will continue to decline."

The Keys government already has started to address water quality — by 2010, the Keys will have a new $430 million central sewage system.

But that's not enough, MacLaughlin said.

"Water quality needs more attention and focus," she said. "It's being done in the Keys, but it needs to be done regionally and globally.

"It may take our lifetime to understand that. By then, who knows what will be left?"