Fertilizer run-off threatens long-term balance of lakes

TOP OF PAGE: Lake Mendota, seen from the air, looks pristine on a foggy morning. But a new study suggests it could literally take centuries for the lake to regain its natural chemical balance. ABOVE TOP: Limnology graduate students wade through a blue-green algae bloom in this 1994 file photo. Blooms kill fish and create foul-smelling water that plagues Lake Mendota in the summer. The phosphorus that causes these problems comes from excess agricultural and lawn fertilizers and animal waste. ABOVE BOTTOM: Students collect water samples with a seine. A doctoral student of limnology, after much research, has concluded that the problem for Mendota is that what goes into the lake isn’t necessarily coming out: phosphorus accumulates in the watershed’s soil every year.

A new university study of what happens to all the agricultural run-off pouring into Lake Mendota suggests it could literally take centuries for the lake to regain its natural chemical balance.

A research team led by Elena Bennett, a doctoral candidate of limnology, reached that conclusion after developing a detailed “phosphorus budget” for the watershed that drains into Mendota. Phosphorus is the culprit behind many water-quality woes in the lake.

The problem for Mendota, and for most watersheds in agricultural settings, is that what goes into the watershed isn’t necessarily coming out. The budget found that a huge amount of phosphorus accumulates in the watershed’s soil every year.

Phosphorus causes a “laundry list” of problems, including the blue-green algae blooms, fish kills and foul-smelling water that plague Lake Mendota in the summer, Bennett says. Most phosphorus comes from excess agricultural and lawn fertilizers and animal waste, which land managers generally call non-point source pollution.

Bennett says her findings underscore the need to limit fertilizer use only to what crops really need, since excess fertilizer will only contribute to this phosphorus buildup. She calculated in the study that even if over-fertilizing were stopped immediately, it would take more than 260 years for the level of phosphorus in the Mendota watershed soil to drop to 1974 levels.

“We have to find ways to reduce the amount of fertilizer used in the region,” says Bennett. “When you look at the numbers, it’s the major driving force causing the phosphorus inputs to be so large.”

The study is reported in the current issue of Ecosystems, a peer-reviewed journal based at UW–Madison and devoted to the study of ecosystem science.

Bennett’s faculty adviser, Stephen Carpenter, says the study will be very useful for improving the health of Lake Mendota.

“It suggests that our approaches toward trying to keep phosphorus out of the lake are stopgap at best,” says Carpenter, a professor of limnology and zoology. “The problem is that the agricultural use of phosphorus is way out of whack. Until we can balance that budget, our efforts are just a Band-aid.”

Mendota has become a test bed for new management strategies to improve water quality. The current “priority watershed” project, managed by the Wisconsin Department of Natural Resources, is working with county farmers to reduce soil erosion into the streams that feed Mendota, and controlling construction site run-off.

The lake also underwent a major experiment in the late 1980s called biomanipulation, which attempted to improve water quality by stocking millions of predator fish. The resulting change in the food chain left more small animals that filter algae from the water.

Those strategies all make a difference, Bennett says, by slowing down the deluge of phosphorus into the lake. Soil erosion controls are especially important, since the majority of phosphorus inputs occur during heavy spring storms.

“But they don’t produce the long-term fix of lowering phosphorus levels in the soil,” she says.

Strategies that would make the greatest difference include following the recommendations of soil scientists on how much fertilizer to use per acre. Bennett cited documented cases of farms using five times more fertilizer than recommended. The size of livestock herds should also be kept in check, so they don’t exceed the capacity of the land to handle waste.

Bennett compiled the budget by measuring a number of key “inputs” and “outputs” each year. The report compiles available data on all of the fertilizer used on agricultural crops in the watershed, and all of the feed supplements given to dairy cattle. It also includes estimates on fertilizer used on urban lawns, and various forms of natural deposition.

For outputs, it adds the total amount of phosphorus that ends up in harvested crops, livestock, dairy products and other sources. It also adds the amounts of phosphorus exported into the lake.

The annual budget came up with 1.3 million kilograms of inputs, and 732,000 kilograms of outputs. The researchers concluded that roughly 575,000 kilograms of that phosphorus accumulated in the soil.

While the study centered on Mendota, Bennett says research could apply to lakes anywhere that are situated in agricultural lands. Soil studies have indicated that excess phosphorus accumulation is a global problem.