Bottom-up Ecological Processes Better For Clear Waters, Study Finds

ell and taste foul.

Unnaturally murky lakes are more commonly found in the middle and eastern United States and Europe. Eutrophication was responsible for the dramatic decline in water quality seen in several of the Great Lakes during the 1960s and 1970s and is responsible for the often abysmal water clarity of urban lakes and ponds. Lake Tahoe in the Sierra Nevada, for example, is losing about one foot of its famous clarity a year due to algae.

But algae also are a critical part of a lake's food web. They are eaten by slightly larger shrimp-like creatures called zooplankton. In turn these tiny crustaceans are preyed upon by small fish, such as minnows, which in turn are often devoured by bigger fish, such as pike or bass.

In the mid-1980s, ecologists had gathered enough evidence to suggest that the clarity of a lake could be regulated by manipulating this structure of the food web. For example, putting more large fish in the lake would mean more smaller fish would be eaten. With fewer smaller fish swimming around looking for dinner, the zooplankton would increase and eat more algae. The algae, in turn, would decrease and water clarity would increase.

This idea of "top-down" control of the food web is known as the "trophic cascade hypothesis," named and popularized by Steven Carpenter, with the Center for Limnology at the University of Wisconsin, Madison. The professor of limnology is considered among of the world's most influential lake ecologists.

Yet, his top-down hypothesis has not been without its detractors. The trophic cascade hypothesis was sharply criticized several years ago as being "unsoundly based on many half-truths and much hand-waving and overextrapolation of the data," according to a 1992 paper in the journal Limnology and Oceanography by Rita DeMelo and colleagues.

DeMelo used a statistical technique called "vote counting," a common method of summarizing information from large bodies of resear

Contact: Carol Morton
University of California - Davis

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