The Way Back: Sudbury Lakes Recovering from Acid Rain

More than a century of metal mining and smelting took a heavy toll on the environment around Sudbury, Ontario. Sulphur dioxide emissions from local smelters acidified an estimated 7 000 lakes in a zone of about 17 000 square kilometres—much of which was previously pristine wilderness.

Pollution control measures applied in the 1970s and 1990s reduced local emissions by about 90 per cent and produced remarkable improvements in the chemistry of some lakes in the area. Others, however, remain severely acidified. Researchers from government, industry, and Laurentian University in Sudbury are working together to learn more about this recovery and to assist in rehabilitating damaged aquatic ecosystems.

Environment Canada's National Water Research Institute (NWRI) is one of the partners in the Aquatic Restoration Group, along with the mining companies Inco Limited and Falconbridge Limited, and the Ontario ministries of the Environment and Natural Resources. Established in 1997 and coordinated by Laurentian’s Cooperative Freshwater Ecology Unit, the group is tracking the chemical and biological changes in lakes that have occurred in response to reduced sulphur and metal emissions. It is also exploring links between the acidification recovery process and impacts of other environmental stresses.

Researchers have observed changes in water chemistry, such as increased pH and decreased concentrations of sulphate, base cations (e.g., calcium, sodium, potassium, and magnesium) and aluminum. These improvements in water quality have, in turn, encouraged a biological recovery for several groups of organisms—including phytoplankton, zooplankton and fish—in some of the area's lakes. Scientists worldwide consider the developing recovery of Sudbury’s lakes one of the most convincing and best documented case studies of ecosystem responses to reduced acid rain. The studies have also highlighted complicating factors that can impede recovery.

In recent decades, extremely clear acidic lakes in the area have become much clearer, likely due to changes in climate and increased ultraviolet (UV) radiation. Even though they continue their slow recovery from acidification, these clear lakes are more transparent and, therefore, allow the sun's rays to penetrate deeper. Greater exposure to UV radiation from this sunlight is potentially harmful to aquatic species and prevents the restoration of healthy aquatic communities.

Drought, another climatic variation, can also play a role in stalling the recovery process. When a dry period occurs, the sulphur stored in lake catchments and sediments from years of high atmospheric deposition can be released. This can delay recovery or even cause re-acidification of the lake, with serious consequences for the biological revival that is still in its early stages.

The members of the Aquatic Restoration Group are aware that this is a critical time for environmental assessment research in the Sudbury area. Their water quality studies will not only assess the effectiveness of emission reductions to date, but also provide the scientific knowledge on which to base decisions about the need for greater pollution controls. The group has begun new work to identify which components of the ecosystem are capable of unassisted recovery and which require active restoration measures, such as introducing species or modifying habitat. It will also investigate further the role played by the long-range atmospheric transport of pollutants—now thought to be the dominant source of acid input to most Sudbury-area lakes—and continue to explore the impacts of climate fluctuations and the storage of contaminants in watersheds.

An initiative such as the Aquatic Restoration Group is one way to share information and join forces in the fight to promote the recovery of ecosystems from acidification. Although still at an early stage, results in the Sudbury lakes hold promise that similar recoveries from acidification can take place elsewhere in the world.

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