Monroe Harbor

Lakes and rivers have built-in defenses that help them stay healthy even when water quality degrades, food becomes scarce, or habitats are lost. But some changes, even small ones, can push an ecosystem past a brink and rapidly change how it functions. These limits are known as tipping points.

Tipping points can be hard to predict, but scientists believe the Great Lakes are in the midst of a major ecological change thanks to the rapid spread of invasive zebra and quagga mussels. Each of these invaders – totaling in the billions in Lake Michigan alone – filters about a quart of water a day. The result is a clear lake stripped of algae that form the base of the food chain, forcing other species to compete for less food. The overwhelming number of invasive mussels in the lakes may have pushed the ecosystem so far that the food web will never look the same. To learn more about the spread and impact of zebra mussels, watch this video.

Decisions made by city planners and resource managers can also drive aquatic ecosystems past tipping points. The growth of urban populations along lakes and rivers, and the pollution that comes with it, can impair water quality, reduce oxygen levels, and alter water temperatures so much that an ecosystem cannot function like it should. To prevent this, Chicago is replacing some roads, roofs, and other watertight surfaces with vegetation, green roofs, and porous pavement that can absorb water and filter out harmful contaminants and nutrients.