P Soup: It's green, but it's not good for you. That benign-looking pond scum signifies a far-reaching shift in the global phosphorus cycle.

• Author: Bennett, Elena

Think of global environmental change, and you'll probably think most immediately of such sweeping atmospheric phenomena as global warming or ozone depletion. Many of the other environmental disruptions we're familiar with--toxic dumps, decimated forests, eroded fields--seem largely confined to particular localities. Yet there are some environmental changes that, while appearing to be locally confined, are in fact manifestations of worrisome global patterns. Look at the algae forming on a local farm pond, for example, and you're seeing the result of a process--the phosphorus cycle--that extends far beyond that farm.

Algae thrives (literally "blooms") on runoff of waste fertilizer or other materials containing phosphorus. While human-caused changes in the closely related nitrogen cycle have been widely publicized (see "Toxic Fertility" in the March/April 2001 WORLD WATCH), impacts on the phosphorus cycle are less well known. Our research suggests, however, that the movement of phosphorus is indeed a global phenomenon--and that that patch of algae you see in the pond at your feet may be affected by changes in the soil hundreds or thousands of miles away.

Both nitrogen and phosphorus are essential nutrients for plants and are therefore present in most fertilizers in addition to being present in agricultural and municipal waste products. As a result, the movement of large amounts of fertilizers around the planet can also mean the movement of excessive nutrients from one place to another. Typically, some of the fertilizer used on a farm does not stay there but moves downhill where it can get into a downstream aquatic ecosystem--a river, lake, or bay. Concentrations of excess nutrients in these bodies of water cause the patches of algae to expand prolifically. Such "eutrophication," as the green blanketing of the water is called, can be a crippling process: it suffocates the life under the slime--killing fish, diminishing biodiversity, and emitting noxious odors. It reduces the value of the water for most human uses--whether for drinking, fishing, swimming, or even boating.

Lake Mendota, in Madison, Wisconsin is a classic example of a eutrophic lake in an urbanizing, but primarily agricultural setting. The lake has exhibited many of the symptoms associated with eutrophication since agriculture became the primary land cover in the surrounding watershed, in place of the native prairie and oak savannah. Blooms of blue-green algae have been common here since the 1880s. Along with these blooms came dramatic changes in the food web, including loss of some native species and increased populations of non-native species such as Eurasian milfoil and carp. Eutrophication has greatly diminished the lake's recreational value.

Historically, Madison has never developed public swimming pools, because people were always able to swim in the area's plentiful lakes. Now, as eutrophication worsens, there is increasing public pressure to develop pools because many of the lakes are no longer swimmable. For Lake Mendota alone, the cost of eutrophication has been estimated to be about $50 million in lost recreation and property values. Even so, what happened to this lake is seen as a local story--of little interest to someone in North Carolina or South China. If the people in Madison want to deal with this problem, obviously it is their own local farms and sewer pipes they have to deal with. What's not so obvious is that what has happened here results from massive changes in the flow of phosphorus around the globe.

Human Impact on the Global Phosphorus Cycle

Long before humans arrived on the scene, phosphorus was moving around the planet in a natural cycle that probably took millions of years to complete. A phosphorus molecule might be tapped in rock, then released by erosion to start its gravity-driven journey to the ocean. Along the way, it might be taken up by plants and then animals, then returned to the soil or water via dead vegetation or urine, to continue its slow trek downhill until it finally reached the ocean. Once in the ocean--probably after taking more detours through plants and animals along the way--it would sink into the sediment. In time, geological processes would turn this sediment to terrestrial rock--reincorporating the phosphorus molecule. The cycle would begin again.

With the advent of human agriculture and urbanization, the natural cycle was in some respects short-circuited. Technological advances, especially in the past 50 years, enabled us to mine phosphorus on a large scale, make fertilizer and other products from it, and transport these products around the world, dramatically accelerating the now not-so-natural phosphorus cycle. Globally, we estimate that the annual accumulation of phosphorus in the Earth's freshwater and terrestrial ecosystems has almost quadrupled, from around 3.5 terragrams per year before humans began mining and farming on a large scale, to around 13 terragrams per year now.

To understand the cycle as it was before human interventions began, it is useful to think of the cycle as a flow from the earth's crust back to earth's crust, through four main compartments (see figure, page 31). First, phosphorus-containing rock is weathered--worn by wind, rain, freezing and thawing, etc.--until it becomes soil. With the soil, it moves into lakes and rivers, which transport it to the ocean. Some of it is dissolved in water, and some adsorbed to soil particles that are carried by erosion down to the sea and, ultimately, to the ocean bottom. There it awaits the tectonic movement that will lift up the rock and make it part of the land again. The cycle is described by Aldo Leopold in his...