Four years ago, Cropp filmed a documentary, "The Coral Reefs are Dying," that took him around the world. His conversations are peppered with references to "the viz" -- how far divers can see through coral-studded depths.
In this case, the cloudiness is due to runoff of nitrogen and other pollutants from farming and land clearing along the coast of the Australian state of Queensland.
Since he began diving in the resplendent, rainbow depths of the reef in the early 1950s, Cropp says, he's seen average visibility at some favorite spots cut in half. Most of the changes came since the 1980s, as wetlands were converted to well-fertilized sugar cane farms and clearing of native vegetation for ranching and farming accelerated.
"To get what's happened, you have to go to the older divers, because around the world the plunge in water clarity, the majority of it, occurred 20 years ago or more," Cropp says. "If you knew the corals before the plunge, well, it has been dramatic."
Cropp concedes that the evidence that such pollutants as nitrogen are a significant threat to Australia's corals is "mostly anecdotal" and not backed conclusively by science.
And overall, the Great Barrier Reef is regarded as among the healthier of the world's greatly troubled coral ecosystems. Almost 60 percent of all reefs around the world are considered at high to moderate risk from pollution, global warming of seawater and overfishing -- often with dynamite or cyanide.
Scientists at the Australian Institute of Marine Science (AIMS) in Townsville note that the Great Barrier Reef stretches more than 1,200 miles and ranges up to 120 miles offshore. So making blanket statements about its health is like generalizing about water quality from Maine to South Carolina.
But "we know there is a problem, and we know we can't wait for the science to sort it all out," says Jon Brodie, who manages the vast, 1,400-square-mile Great Barrier Reef Marine Park, which includes 2,800 coral reefs.
Brodie and AIMS scientists say levels of nutrients flowing from rivers onto the reef now are almost four times natural levels, with much of the change in recent decades.
There's "very strong evidence" that runoff of fertilizers and sediment from Queensland's agriculture and land clearing is threatening the 800 reefs closest to shore, says AIMS researcher Katerina Fabricius. And it also could be slowly affecting many of the park's 2,000 offshore reefs -- including some as far as 12 miles out from the coast.
Compared with a naturally turbid and fertile system like the Chesapeake Bay, the levels of sediment and nitrogen on the Great Barrier Reef are quite low. Australia is a dry continent, and its largest rivers run dry for years at a time. Its ancient soils have been weathered of most of their nutrients.
But coral reefs can be exquisitely sensitive to even a little more fertilizer. They have evolved over 225 million years to thrive in some of the most nutrient-scarce, poorly fertilized waters on Earth -- "like oases in the midst of deserts," ecologist Eugene Odum termed them. Few nutrients mean very little growth of floating algae, which is why reef waters are so clear.
Surprisingly, then, when Odum studied coral atolls in the Marshall Islands, he found them to be as productive as heavily fertilized Iowa cornfields.
The secret? Corals use and recycle scarce nutrients with incredible efficiency. Giant clams on the reef, for example, have green lips made of vegetable matter that extracts energy from sunlight streaming through the air-clear water as the clam opens its shell.
A recent three-year Australian experiment demonstrated that adding even small amounts of nitrogen and other nutrients to coral reefs "dramatically impacted every stage of reproduction," says Selina Ward, a researcher with the University of Sydney. The experiment's message, Ward says, is "to do everything possible" to reduce nutrients running off the land.
Such efforts quickly become involved in "reef politics" in Queensland. The federal park in which the reef lies has vague, never-tested authority over state land use proven to harm the corals. Some Australian officials have been pushing to strengthen this power.
But Queensland authorities see it as a usurpation of state's rights, and continue to allow the clearing of forests and filling of wetlands at a furious pace. Last year, the state forfeited about $20 million in federal funds because it's clearing native vegetation at the rate of 750,000 acres a year.
"The first step is to admit the problem is the way we're using our land," says Peter Bell, a University of Queensland environmental engineering professor. "It's more of a political science problem than a science problem."