Lake 223 acid rain experiment: Scientists added sulfuric acid to an Ontario lake for 17 years. The water recovered, but the fish didn’t

For 17 years, scientists deliberately made a remote Canadian lake more acidic to understand what acid rain was doing to freshwater ecosystems. The experiment ended more than three decades ago, and Lake 223’s water chemistry has since moved back toward its original state.

But the living system inside the lake tells a different story. The lake trout population remains below half of what it was before the experiment, while surviving fish are growing more slowly. Scientists are now studying whether rebuilding a damaged food web can help the ecosystem recover.

A lake became a giant laboratory

Lake 223 sits within the Experimental Lakes Area in northwestern Ontario, Canada. The research site contains 58 lakes and their surrounding watersheds, allowing scientists to study environmental changes at the scale of complete ecosystems rather than isolated laboratory samples.


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In 1976, researchers began adding sulfuric acid to Lake 223 every week. The goal was to reproduce, in a controlled way, the increasing acidity associated with acid rain at the time. The changes were gradual. Over the 17-year experiment, the lake's pH dropped from 6.8 to 5.2.

That shift had consequences across the ecosystem. Some fish and invertebrate species disappeared, while others experienced problems with reproduction, growth and survival. Fathead minnows, for example, nearly vanished.

Lake trout were also hit as changes in the lake altered the organisms they depended on for food.

Why acid rain was such a big environmental threat

The Lake 223 experiment took place during a period when acid rain was one of the major environmental concerns in North America and Europe.

The problem was closely linked to air pollution. Sulfur dioxide and nitrogen oxides released by activities such as burning fossil fuels can undergo chemical reactions in the atmosphere, producing acidic compounds. These can eventually reach the ground through rain, snow and other forms of precipitation.

Lakes in regions exposed to high levels of acid deposition were particularly vulnerable. Scientists already had evidence that acid rain was harming freshwater ecosystems. But experiments such as the one at Lake 223 provided something different: an opportunity to watch an entire ecosystem respond as its chemistry changed.

The results helped demonstrate that acidification was not simply a water-quality issue. It could disrupt the food web itself. Research from the Experimental Lakes Area contributed to the wider scientific evidence used in efforts to reduce industrial emissions in Canada and the United States.

The surprising part came after the experiment ended

The researchers stopped adding acid to Lake 223 in 1993. Over time, the lake's chemistry recovered. Its water eventually returned to conditions broadly similar to those that existed before the experiment. That might sound like the end of the story. It wasn't.

The biological recovery has been much slower. According to the International Institute for Sustainable Development's Experimental Lakes Area, lake trout numbers remain at less than half their pre-experiment level. The fish are also growing more slowly.

The difference between the recovery of the water and the recovery of the ecosystem is one of the most important lessons from Lake 223. A lake can regain its chemical balance without immediately regaining the biological relationships it once had.

One missing species may hold a clue

Researchers have been looking closely at what happened lower down the food chain. One important species is Mysis diluviana, a small freshwater shrimp-like invertebrate. It disappeared from Lake 223 during the acidification experiment.

Its loss mattered because species at the lower levels of a food web can have consequences much farther up the chain. With the original food-web relationships disrupted, lake trout did not simply return to their former numbers once the acidity declined. Their environment had changed in more complicated ways.

This is why scientists increasingly view ecological recovery as more than restoring a single environmental measurement such as pH.

Scientists are trying to rebuild the food web

Lake 223 has now become a second experiment in ecological restoration.Beginning in 2019, researchers started reintroducing Mysis diluviana into the lake. By 2021, they had evidence that a new population had become established.

The work is continuing. Scientists are monitoring what happens to the wider ecosystem, including whether the return of Mysis affects lake trout growth and survival. They are also examining changes in mercury concentrations in the fish.

The aim is not simply to bring back one missing organism. Researchers want to find out whether restoring an important link in the food web can help trigger broader recovery.

A warning that lasts beyond the pollution

Lake 223 offers a rare long-term look at what environmental damage can leave behind. The experiment showed that acidification could alter an entire freshwater ecosystem. But the decades that followed revealed something equally important: stopping the original pollution does not necessarily mean an ecosystem instantly returns to normal.

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Water chemistry can recover first. Species interactions may take much longer.For scientists studying damaged ecosystems, that makes Lake 223 an unusually valuable natural laboratory. It provides a decades-long record of disturbance, recovery and attempted restoration.

And for the wider public, its story carries a simple lesson. Environmental damage can outlast the pollution that caused it.