A series of paint-can-sized pails are bobbing in the Straits of Florida where the Gulf of Mexico mingles with the Atlantic Ocean, ready to chronicle what impacts, if any, will descend on Dry Tortugas, Everglades, or Biscayne national parks from the Deepwater Horizon disaster.

Will a sheen of oil coat the beaches of the seven islands that comprise Dry Tortugas, the dozens of keys in the waters of Florida Bay that Everglades claims, and the coral reefs and shipwrecks that lie within Biscayne's waters? Will the oil arrive as relatively innocuous tar balls, or will the fluky currents that travel through the straits protect the parks by carrying the oil elsewhere?

Not only are white-sand beaches at risk from the oil, which as of Thursday was estimated to be about 200 miles west of Dry Tortugas, but perhaps more at risk are the rich marine resources of the parks. At Dry Tortugas, the Research Natural Area that surrounds the park's islands is a valuable preserve that is hoped to help fish populations rebound and to provide a sanctuary if you will for coral reefs, which face threats from anchor damage, degraded water quality, and warming oceans.

The natural area covers 46 square miles and complements the Tortugas Ecological Reserve, which covers 151 square miles. While the ecological reserve is in deeper water, the natural reserve is more shallow and valuable as a nursery grounds for many fish species. It is hoped that if these nurseries prove to be more productive, the benefits could one day be felt as far away as the Florida Keys.

Oil from the Deepwater Horizon catastrophe could jeopardize those benefits.

“We don’t know how it’s going to impact the RNA," said Larry Perez, the National Park Service's media point person in south Florida. "There are too many variables at the moment to even begin to guess. Most notably how, when, and if it’s going to hit, and if it does hit, what sort of form is that oil actually going to be in by the time it gets here? With all that up in the air, it’s kind of hard to say.”

To better understand any impacts that might develop in the weeks ahead, the Park Service has turned to the paint-can-like passive sentries formally called "semi-permeable membrane devices." These devices, known as SPMDs in shorthand, continuously sample the ocean waters in a bid to, essentially, catch a whiff of hydrocarbons. By deploying 20 sensors now around the three parks, the Park Service is tracing a baseline of conditions in the parks' waters that can be revisited when, and if, oil from the Deepwater Horizon incident reaches the parks.

"The membranes that are held within the SPMDs have the potential to accumulate hydrocarbon residues on them, and can be analyzed at a future date to determine whether or not there is some evidence of oil contamination in the water," says Dave Hallac, the Park Service's supervisory biologist at both Everglades and Dry Tortugas national parks. “What we’re doing with those is we put them out before the spill impacts us, then we take those sensors and bring them back in, the membranes, and if and when we have a spill we will put out a second set and we will measure any hydrocarbons in the water at that time.”

Of particular interest to the scientists will be how the RNA around Dry Tortugas responds if it is polluted by oil. While the Loop Current that runs through the Gulf of Mexico and out into the Atlantic does not pass through the park's waters, according to Mr. Hallac, that doesn't mean that other currents or storms couldn't push the oil into them. And if oil swirls into the RNA, it likely would impact a preserve that, like other marine reserves, is managed for wide-ranging benefits.

"A common thing that we hope for with the Research Natural Area and other marine reserves is called the 'spillover effect.' And that is, for fish for example, is you produce more and bigger fish, and those fish that are bigger or more numerous actually provide a benefit to areas outside the Research Natural Area," explained the biologist. “Also, the hope is that we have female fish that are more fecund, meaning that they produce more eggs, and a lot of them have young that have a larval phase that can be planktonic, meaning that they are at the mercy of the currents.

"And the idea is that if you have big fish, big breeder fish that are in these Research Natural Areas that you are protecting, then the eggs and the young fish that they produce could populate areas outside of the Research Natural Area."

Following through on that line of thinking, if the RNA is contaminated, those impacts could have downstream ramifications on fish in, for instance, Everglades National Park.

"If we have substantial impacts on different levels of the ecosystem there, from the sea grasses and plankton all the way up to the fisheries, there’s no question that the impacts that we could have in the RNA could extend beyond," Mr. Hallac said.