For 17 years, scientists deliberately added sulfuric acid to a remote Ontario lake to recreate the effects of acid rain.
The experiment was designed to answer a straightforward question: What happens to an entire freshwater ecosystem when its water becomes increasingly acidic?
The answer turned out to be more complicated than watching the lake's chemistry change. Fish struggled to reproduce, some species disappeared, and changes spread through the food web. Even after the acid was removed and the water began returning toward normal, parts of the ecosystem did not simply bounce back.
Decades later, researchers are still studying Lake 223 to understand why.
How Scientists Turned A Lake Acidic
Lake 223 is part of the Experimental Lakes Area in northwestern Ontario, where scientists conduct experiments on whole lakes rather than just studying organisms in a laboratory.
In 1976, researchers began adding sulfuric acid to the lake in carefully controlled amounts. The aim was to mimic acid rain, which had become a major environmental problem in parts of North America and Europe because sulfur emissions from industry could make rainfall more acidic.
The lake's pH gradually fell from about 6.8 to roughly 5.2. Because the change happened over years rather than all at once, researchers could watch how different parts of the ecosystem responded as the water became more acidic.
The effects soon extended far beyond the water itself.
The Damage Spread Through The Food Web
As acidity increased, some of the small organisms living in the lake began to disappear.
Among them was Mysis Diluviana, a small freshwater shrimp that plays an important role in the food web and provides food for larger fish. Other organisms also declined as conditions became increasingly difficult for them.
The effects eventually reached the fish.
Fish reproduction stopped across the lake's major fish species during the acidification experiment. Fathead minnows and slimy sculpin disappeared, while lake trout experienced declining growth and poorer physical condition.
Some trout became severely emaciated as changes lower down the food chain reduced the prey available to them.
The fish were therefore only part of the problem. Once organisms at the bottom and middle of the food web began disappearing, the effects moved upward to species that depended on them.
The Water Started To Recover
After years of acidification, researchers stopped adding sulfuric acid and allowed the lake to begin recovering.
As the water became less acidic, some encouraging changes appeared. Fish that had survived the experiment began reproducing again, while the fathead minnow eventually returned to the lake.
Lake trout also showed signs of improvement. Their condition and survival became better as the effects of acid stress eased and some of their food sources returned.
But the recovery was uneven.
The lake's chemistry could move back toward its earlier state, yet that did not mean every species would immediately return.
A Healthier Lake Did Not Mean A Restored Ecosystem
Lake trout remained a particular concern.
Their recovery was slowed by the lasting changes to the food web, including the loss of important prey. Even after the water became less acidic, the ecological relationships that existed before the experiment had not simply reassembled themselves.
That difference is important because water chemistry and biological communities do not necessarily recover at the same speed.
A lake can have a healthier pH while still missing species that once played important roles in keeping the ecosystem functioning.
In Lake 223, scientists were able to see that process unfold over decades.
Scientists Are Trying To Restore A Missing Species
The story did not end when the original acidification experiment ended.
In 2019, researchers began trying to restore Mysis diluviana, the freshwater shrimp that had disappeared during the acidification period. The idea was to see whether bringing the species back could help repair part of the food web and, ultimately, improve conditions for lake trout.
By 2021, researchers had evidence that a new Mysis population had become established in the lake. Scientists are continuing to monitor what happens as the shrimp population develops and whether its return affects lake trout growth and survival.
Decades after scientists first altered the lake to study acid rain, they are now changing it again — this time to see whether restoring a missing species can help repair some of the ecological damage.
Why Lake 223 Still Matters
The original experiment helped demonstrate the damage acid rain could cause to freshwater ecosystems and contributed to the scientific evidence behind efforts to reduce sulfur emissions in Canada and the United States.
But Lake 223 revealed another problem that is harder to solve: stopping an environmental threat does not necessarily undo everything it has changed.
Once species disappear and food webs are disrupted, restoring the original chemical conditions may be only the first step.
That is why the lake remains valuable to scientists today. Its water may look much healthier on a chemical test, but beneath the surface, the species and relationships that make up the ecosystem can take far longer to catch up.
