Eutrophication, the over-enrichment of lakes and rivers with nutrients, causes toxic algae blooms, deoxygenation, foul odors, fish kills, and heavy economic losses to communities that depend on clean water for drinking, industrial use, or recreation. The fix-up sounds simple: stop adding nutrients to the water. However, lakes and rivers are embedded in complex systems of people and nature, affected by changing social trends, economics, land use, and climate. The many interacting changes can lead to surprises, as seen in the history of water quality management for the Yahara Watershed around Madison, Wisconsin. This urbanizing agricultural watershed of about 1000 km2 is home to nearly 400,000 people and contains five major lakes and extensive wetlands along the Yahara River system.
The first surprise was eutrophication itself. The lakes were clear and clean when Madison was settled around 1840. By 1880, reeking algae blooms and fish kills were common in summer. Massive erosion of topsoil from newly-broken prairie and untreated human waste caused the blooms. People coped by spraying algae with toxins such as copper sulfate, or by avoiding the lakes during summer.
Then in the 1950s things got worse. Industrial fertilizers became cheap and abundant, and farmers over-fertilized their fields leading to more nutrient runoff. Sewage from a growing human population added even more nutrients to the lakes. During the 1950s a political debate began over sewage treatment. Finally, in 1971, the last raw sewage was diverted from the lakes.
To the surprise of scientists and managers, there was no change in water quality. Growing dairy herds were adding more manure to the lakes, and over-application of commercial fertilizers continued to increase nutrient runoff. Urban runoff also increased. These sources of nutrients, collectively called "nonpoint pollution", more than compensated for the elimination of human waste. Of course, the condition of the lakes might have been even worse had sewage input not been eliminated.
In 1981, managers attempted to decrease nonpoint pollution by changing the practices of farmers. The program failed due to lack of participation by landowners.
A completely different approach, biomanipulation, began in 1987. A collaboration of fish managers, angling clubs, and UW-Madison scientists planned to increase stocks of game fish. The idea was that game fish would eat the smaller fishes, allowing Daphnia (a highly effective grazer of algae) to increase and control the algae by grazing. This innovative program worked. Water clarity improved.
However, massive floods in 1993-1994 showed that grazers could be overwhelmed by extraordinary runoff. Urbanization increased the area of hard surfaces in the watershed, thereby increasing risk of flooding. Farm soils were rich in nutrients and susceptible to flooding. In spite of Daphnia, water quality was poor during the flood years.
In 1998, a new program to decrease nonpoint pollution worked directly with farmers, bringing new incentives, regulations and technology. Urban runoff was cleaned up too. Despite a number of successes, by 2008 nutrient inputs to the lakes had not changed. Rainfall came in heavier storms (though total precipitation was unchanged). Even more hard surfaces were built. Fertilizer use declined, while animal numbers stayed the same. Nonetheless soil nutrient levels remained high. Of course, the situation could have been worse without the new management program.
A new invasive species, the spiny water flea, appeared in 2010. Spiny water fleas eat Daphnia. Loss of Daphniais leading to further declines of water quality.
Management of Yahara lakes water quality requires resilience to changing drivers. Fixed targets will not work when land use, farming practices, invasive species and climate are all changing. Instead we need to build the resilience of the watershed, by increasing area of wetlands and vegetated buffers, working with farmers to decrease soil nutrient levels and runoff, preventing introductions of invasives, and maintaining healthy food webs. Can such actions reverse the degradation of the lakes despite urban development and climate change? NTL-LTER researchers have initiated new research to address this question. Ultimately, the answer will come from ongoing observations by NTL-LTER scientists.