Lingering Phosphorus Means Slow Recovery for Polluted Lakes
Lakes can flip-flop between two stable states-one with clear water and abundant rooted vegetation, the other murky and clouded by algal blooms. Now a study in this week's online edition of
Proceedings of the National Academy of Sciences provides new insights into the dynamics behind these flips.
Lakes overenriched with nutrients, usually phosphorus, can switch into a "eutrophic" state dominated by algae. This excess phosphorus comes from sewage and industrial discharges, along with agricultural fertilizers and manure that accumulate in soils and then wash into lakes during rain and storm events. Once in lakes, phosphorus tends to accumulate in sediments and in living organisms. From there, it recycles into the water column. As scientists have known for a long time, an already overenriched lake can continue to recycle phosphorus from the sediments long after other inputs of phosphorus decline. This means that a lake could remain eutrophic, even after nutrient flows from the watershed decline.
To explore the role of soil and sediment phosphorus in these transitions between a lake's clear water and eutrophic states, limnologist Steve Carpenter at the University of Wisconsin in Madison developed a general model to describe how phosphorus moves through the watershed and outfitted it with measured parameters for Lake Mendota, a popular site for water-based recreation in Madison. The water quality of Lake Mendota has deteriorated in the past 100 years, but it is not yet considered a degraded lake — which means that things could still get a lot worse, Carpenter says.
Carpenter used the model to test Lake Mendota's response to two management scenarios: 1) Balance the phosphorus budget of the watershed so that there is no longer any net increase in phosphorus (an improvement over the current course); and 2) Drop phosphorus inputs to levels to reflect pre-industrial times (a more aggressive improvement that would require more intensive management).
Merely balancing the phosphorus budget is not enough, according to Carpenter's model. In the balanced budget simulation, Lake Mendota continued to move into a severely degraded state due to runoff and the pool of phosphorus already in the sediments. In the second scenario, in which phosphorus levels returned to pre-disturbance conditions, Lake Mendota began a slow course toward recovery. Phosphorus levels in the sediment declined, but it took close to 1,000 years to reverse the effects of agricultural overenrichment of the watershed. So "viewed from the perspective of a human lifetime, eutrophication is often a one-way trip," Carpenter writes in his article.
Carpenter says that his model demonstrates how essential it is to reduce phosphorus in the lake's watershed now. Lake Mendota is eutrophic but not yet in the severely degraded or "turbid" state. "We still have time, but we need to use the time effectively because the penalty for failure is so severe," he says.
"The 1,000-year time frame really opens your eyes to the fundamental issues and management scenarios," says ecosystem ecologist Michael Kemp from the University of Maryland Center for Environmental Science. It also underscores the importance of coupling the management of the watershed to the water body, he says. Kemp studies the dynamics of nutrient loading and their impacts on living organisms in Chesapeake Bay. In the Bay, both phosphorus and nitrogen loads in the watershed contribute heavily to eutrophication, leading to algal blooms and oxygen-starved conditions in summer months.
Although the interactions between phosphorus and nitrogen in the Bay would generate different nutrient cycling dynamics than in a freshwater system like Lake Mendota, the approach taken in this paper could have general applicability, says chemical ecologist Thomas Jordan from the Smithsonian Environmental Research Center in Edgewater, Maryland.
"The idea that recovery time could be on the order of centuries is disturbing but intriguing," says Jordan. This paper suggests that it wouldn't be enough to just balance the nutrient inputs from agriculture, but that it is necessary to actively "bail out" excess phosphorus from the watershed. The thought that this might also be true for Chesapeake Bay is worrisome, he says.
But the need to bail out phosphorus from overenriched watersheds and transport it to phosphorus-poor areas may also create an opportunity for management and encourage the use of new technologies, according to Carpenter. Manure digesters, for example, use anaerobic bacteria to transform the nutrients in manure to methane (which can be used for electric power) and a high quality fertilizer sludge that can be used in phosphorus-deficient watersheds. "The technology is operational, but it is not cheap," he says.
- Erica Goldman