Acid Rain

What is acid rain?

Acid rain forms when nitrogen oxide (NOx) or sulfur dioxide (SO2) gasses are released into the atmosphere, primarily from the burning of fossil fuels.

These gases react with oxygen and water in the clouds to form nitric and sulfuric acid, which can then be transported long distances within the clouds before falling on the landscape in raindrops, snow, and other forms of precipitation, like fog or even dry particulates.

Why is acid rain a problem?

Decades of acid precipitation falling on the landscape have acidified soils and decreased the pH of lakes across the globe. This can have a variety of impacts on the natural balance of ecosystems and, thus, the health of our freshwater resources.

For example, it can hinder the ability of fish to detect predators. Atlantic salmon, an important fish stock on the east coast of North America, releases a chemical called an alarm cue when they are physically damaged by predators, which alerts other salmon in the area of the predator’s presence. The ability of salmon to detect this alarm cue is dramatically impaired when the pH of the river or stream they live in falls below a certain level.

What has IISD-ELA taught us about acid rain?

In order to mimic the acidity of the rain that was falling on freshwater ecosystems at the time (in the mid-1970s), researchers intentionally acidified lakes at the site to determine acid rain’s impact on the flora and fauna of freshwater lakes.

Among the many effects that Schindler and his team found were reduced body condition (how “fat” a fish is) and low breeding success in white suckers and lake trout, as well as the near extinction of fathead minnows.

How did Canada deal with acid rain?

Acid rain caused by industrial activity was identified in the 1970s and 1980s as a global problem for freshwater ecosystems. The effects that acid rain was having on freshwater ecosystems needed to be comprehensively researched in order to mitigate any potential threats to the health of our freshwater systems and populations. This was especially important for Canada, which has access to almost 20% of the world’s freshwater resources.

Based on evidence provided by this study, as well as other studies conducted throughout the 1970s and 1980s, legislation was put in place to curtail industrial emissions. These measures led to significant decreases in NOx and SO2 emissions from industrial point sources, such as factories, in the decades that followed. However, while legislation has indeed worked to make car exhausts cleaner, the number of vehicles on the road continues to rise with the increasing human population, so vehicle emissions remain a significant source of primarily NOx, but also SO2.

What does IISD-ELA still have to teach us about acid rain?

Lake 223 continues to be monitored decades after the acid rain experiment officially wrapped up. Interestingly, water chemistry results suggest that while the chemical makeup of the lake has since returned to pre-experimental conditions, the biology of the lake (including the Mysis population) has not fully recovered. For example, the lake trout population in the lake has dropped to less than half of what it was before the experiment, and their growth rates are much slower than were observed previously.

These findings likely reflect a broader trend of freshwater populations still feeling the effects of acid rain, despite legislation being enacted decades ago to reduce how much acid rain enters these systems.

During IISD-ELA’s 50th field season, researchers from IISD-ELA and Lakehead University in Thunder Bay, Canada, repatriated Mysis to Lake 223 as part of an ecological restoration project to see if recolonizing the lake with Mysis would restore the lake trout population.