Dead zones occur wherever oceanic oxygen is depleted below the level necessary to sustain marine life, a result of eutrophication, or the release of excess nutrients into the sea, usually from agricultural fertilizers. Fifty years ago no one imagined that the Green Revolution would prove so lethal to the world ocean. But now we know that chemical fertilizers cause plants to bloom in the sea as miraculously as they do on land, with deadly consequence. It’s no coincidence that almost all of the nearly 150 (and counting) dead zones on earth lie at the mouths of rivers.
The Gulf of Mexico suffers the downstream effects of the mighty Mississippi, which drains 41 percent of the contiguous United States, including all the intensively farmed breadbasket. This outflow delivers enough nitrogen to stimulate explosions of plankton and microalgae, some of which form the red tides that produce major fish kills and dolphin or manatee die-offs. At even higher densities, as these plankton die en masse and settle to the bottom, they fuel a bloom of bacterial decomposers, which consume all the available oxygen in the water. The resulting condition, known as hypoxia, strikes the Gulf whenever oxygen levels fall below two milligrams per liter—an annual summertime event in the warming waters of the Gulf since the 1970s. For sea life, it’s as if all the air were suddenly sucked out of the world. Those creatures that can swim or walk away fast enough may survive. Those that can’t, die.
Nancy Rabalais shows me around Pelican’s home in Cocodrie, in far southern Louisiana. Three months ago, as the newly appointed executive director of Louisiana Universities Marine Consortium (LUMCON), she took the helm of this 75,000-square-foot complex of laboratories, teaching facilities, apartments, offices, and seagoing vessels. So far her tenure has been largely spent digging out of the mud, repairing the wind damage, and casting an eye to the weather. “This used to be a beautiful place,” she says of the striking waterfront facility built on stilts. Now it’s boarded up with storm shutters and surrounded by bulldozers, piles of garbage, stacks of dismantled roofing, stripped palm trees, and muck. Only the estuarine wetlands all around seem untouched, lovely, given that hurricanes are a familiar part of their evolutionary world.
Rabalais is weary. It’s late. She still has a two-hour drive ahead of her to Baton Rouge, where she teaches at Louisiana State University— though I suspect she would rather board Pelican for a couple of days and leave her worries behind. Instead, she’s relying on her research associates and graduate students to conduct the scientific cruise she normally looks forward to each month. A Texan by birth and schooling, she has been diving these waters since it was a fun thing to do; nowadays, it requires a certain courage. A week earlier, while diving in zero visibility on a research station 26 miles offshore, Rabalais encountered an alligator at the surface blown out to sea by one or both of the hurricanes. Diving to the bottom, she “felt something bump against my ankle. But I figured a gator wasn’t diving 65 feet deep, so it must have been something else.”
Rabalais calls the Gulf of Mexico hypoxic zone the poster child of dead zones because it’s been so well documented by herself and others over the past 20 years. Oddly, it acts like a living thing: growing in spring, thriving in summer, decaying in fall, gaining in size almost every year. Core sediment samples and computer hindcasting pinpoint its birth date to the aftermath of World War II, when a surplus of nitrogen destined for TNT was redeployed as agricultural fertilizer.
By one o’clock the next afternoon, we’ve already visited four of the seven stations on the day’s transect, launching and retrieving the CTD in quick time because water depths here are rarely more than 180 feet. Along with collecting conductivity, temperature, and depth data, Rabalais’ crew aboard the Pelican is also conducting HPLC (high performance liquid chromatography) analysis: quantifying and separating pigments, which indicate chlorophyll and hence phytoplankton abundance. The six young men and women work efficiently, hurrying back to the mess deck between workstations, where the satellite TV plays back-to-back college football games.
But for a first-time visitor to the northern Gulf of Mexico, this is far too fascinating a world, in a futuristic kind of way, to ignore. The horizon in all directions is dotted with what from a distance look like small mangrove islands. Only these are oil and liquid natural gas rigs, with all their attendant satellites. At any given time, at least 50 structures punctuate the horizon, and often more than 100. When we draw close, they prove enormous. Servicing them are countless powerful and speedy crew boats, most bigger and faster than Pelican, along with a constant fleet of helicopters in flight between rigs. Although we’re out of sight of land, there is no silence and no hint of wilderness anywhere. This is an urban ocean, the first I’ve ever seen.
Even more strange is the lack of visible sea life. Generally, in waters this far from shore yet still atop the productive continental shelf, we’d be seeing feeding aggregations of seabirds, fish, billfish, sharks, and marine mammals. But here there is only emptiness and the occasional bobbing flight of a laughing gull. It’s the same underwater, apparently, only there’s not enough visibility to actually see it; sometimes, according to Rabalais, when the water is clear and the hypoxia is in full swing, the bottom is full of decaying sea life.
And this is only one of many dead zones. Robert Diaz, a hypoxia expert from the Virginia Institute of Marine Science, calculates the global number is doubling every decade. Furthermore, he suggests that at least in some areas hypoxia is rapidly becoming a greater threat to fish stocks than overfishing, since it disperses them off their feeding, spawning, and maturation grounds. And he predicts that hypoxic zones will only increase as the ocean warms further, citing a modeling study predicting that a doubling of atmospheric carbon dioxide will double rainfall across the Mississippi River Basin, increasing runoff by 20 percent and decreasing dissolved oxygen in the northern Gulf by up to 60 percent.
Close to 50 hypoxic zones fester on the coasts of the continental United States, affecting half of all our estuaries. The situation is worse in Europe, with 14 persistent dead zones that never go away, and almost 40 others occurring annually, the biggest and worst being the 27,000-square-mile persistent dead zone in the Baltic Sea, which is nearly the size of South Carolina. Not all of these are caused by riverborne nitrogen. Fossil fuel-burning plants along the Ohio River loft airborne emissions that help create hypoxic conditions in the Chesapeake Bay and Long Island Sound. Excess phosphorus from human sewage, as well as nitrogen emissions from automobile exhaust, impact Tampa Bay. Other dead zones suffer from the nitrogen fixation produced by leguminous crops.
Interestingly, we know how to solve these problems. Rabalais and others have engineered an action plan that calls for the reduction of the Gulf hypoxic zone to just under 2,000 square miles by 2015. “There are modeling studies that show if you reduce nitrogen fertil-izer applications by 12 to 14 percent, you can reach the target without losing crop production. And there are lots of ways to reduce,” she says, listing best management practices such as a reduction in fossil fuel use, cleaner municipal wastewater discharge, restoring wetlands, regulating pen-feed operations, and banning wintertime fertilizer applications.
The problem is, most of these changes need to take place 600 or more miles upstream and be agreed upon by dozens of headstrong states. “We’re moving slowly,” Rabalais admits. “Five years into the process, we’re finding that we haven’t really done a whole lot, and there’s a lot of resistance from the large agricultural and fertilizer corporations.” At best, it will take years to revitalize the dead zone. Meanwhile, as we dither, the target drifts further away; European studies of fallow fields show that leaching of nitrogen continues decades after cropping and fertilizing have ceased.
IN THE LIQUID REALM offshore, change is more fluid than here on the land. I got a sense of this years ago, while diving the pristine reefs along the edge of the Gulf Stream in the Bahamas, where I began to notice the corals strangling under the spread of gauzy marine plants. With each passing year, the reefs became more populated with filamentous algae and contained fewer live corals, fish, and invertebrates. Today I can date the film footage in my library by the obvious decline of biodiversity on those reefs.
These changes coincided with the unprecedented die-off of the once-populous sea urchin Diadema antillarum. Beginning in 1983 in Panama, these pincushionlike creatures began to succumb to an unidentified pathogen, dying within days of exposure. Over the next 13 months, following surface currents, the mortality spread eastward and northward, encompassing the entire Gulf of Mexico, the Caribbean, and the tropical Atlantic to Bermuda, 2,500 miles from onset. No known New World population was left intact, and up to 99 percent of these sea urchins died in the worst marine invertebrate epidemic ever seen—possibly due to infection by spore-bearing bacteria traveling through the Panama Canal from the Pacific.