Blue Planet Prize Story

Around 1981 he began collaborating in experiments on the trophic cascades with Jim Hodgson and James Kitchell, two other scientists interested in the dynamics of lake ecosystems. The purpose of the experiments was to understand the dynamics of food webs through the manipulation of lake ecosystems.

For example, in the early stages, they conducted an experiment to compare three lakes in which they manipulated the number of large predatory fish, which occupy the top of the food chain (in the lake). The results were intriguing, as the researchers found that increasing the number of larger fish led to the reduction of phytoplankton. This was puzzling because larger fish do not eat phytoplankton. How did such a thing happen?

Larger fish eat smaller fish; therefore, if we increase the number of larger fish, the number of smaller fish in the lakes should decrease. If the number of smaller fish decreases, zooplankton that feed smaller fish should increase, resulting in an increase of zooplankton, leading to a decrease of phytoplankton. As shown above, changes in the number of larger fish at the top of the food chain can affect the number of organisms at the lower end of the food chain, even though they are not being directly consumed by the apex predators. This pattern is called a "trophic cascade" because it is transmitted from top to bottom in a web-like pattern of connections.

Paul, Peter and Tuesday Lakes.
In 1984, most of the largemouth bass of Peter Lake were moved to Tuesday Lake, and most of the minnows from Tuesday lake were moved to Peter Lake in 1985. Paul Lake served as the unmanipulated reference lake.

The trophic cascades experiments conducted by Stephen at the lake directly led to the improvement in the lake management methods. For this reason, the State of Wisconsin provided funds for new food web experiments at Lake Mendota, with the hope of finding a potentially simultaneous solution for both fish and water quality management using what had been learned about trophic cascades. Based on the experiments, Stephen was promoted to associate professor at the University of Wisconsin-Madison, continued working on aquatic plant management to improve fishing grounds, used artificial manipulation to control the number of big and small fish, and managed experiments to restore the original food web after removing harmful species that had been introduced. Although the experiments were conducted at Lake Mendota; however, they could also be applied to other lakes.

Between 1997 and 2011, Professor Stephen Carpenter examined the influence of aquatic and terrestrial organisms on the lake food web and investigated the signs of food web destruction and regeneration in cooperation with Prof. Kitchell and other researchers.

3. Excessive nutrients