Stephen R. Carpenter of the University of Wisconsin-Madison modeled the levels of phosphorus in and around Lake Mendota, a body of fresh water located in Madison, Wisc. with a watershed that is still about 80 percent farmland. "The global pattern is the same," Carpenter remarks, suggesting that the results may be applicable to many other similar lakes around the world. "We are releasing far more phosphorus to the soil than would be released by weathering [alone]." When too many nutrients are present in lake water algae growth can be stimulated to the point of choking off the oxygen supplies for other lake inhabitants, including fish. This process, known as eutrophication, can continue for years. Indeed, Carpenter's simple model, described in a report published online this week by the Proceedings of the National Academy of Sciences, suggests that a eutrophic state could persist for hundreds of years.
Intensive fertlization began around the middle of last century making widespread eutrophication a relatively new environmental problem, Carpenter writes. In addition, he notes that "There's a huge amount of phosphorus in the watershed that hasn't washed into the lake yet" and steps--such as changes in soil management and reducing rates of erosion--should be taken now to improve water quality.