Recovery of acidified Sudbury, Ontario, Canada, lakes: a multi-decade synthesis and update

Abstract

The Sudbury region of northeastern Ontario, Canada, provides one of the world’s best examples of the resilience of aquatic ecosystems after reductions in atmospheric contaminant deposition. Thousands of lakes around the Sudbury metal smelters were badly damaged by acid deposition. Lakes closest to the smelters were also contaminated by metal particulates. However, large reductions in atmospheric SO2 and metal emissions starting in the early 1970s have led to widespread chemical improvements in these lakes, and recovery has been observed for various aquatic biota. Studies of Sudbury-area lakes are advancing our understanding of chemical and biological lake recovery; however, recovery is a complicated process and much remains to be learned. Biological recovery has often been slow to follow chemical recovery, and it has become apparent that the recovery of lakes from acidification is closely linked to interactions with other large-scale environmental stressors like climate change and Ca declines. Thus, in our multiple-stressor world, recovery may not bring individual lakes back to their exact former state. However, with time, substantial natural biological recovery toward typical lake communities can be reasonably expected for most but not necessarily all biota. For organisms with limited dispersal ability, particularly fish, human assistance may be necessary to re-establish typical communities. In lakes where food webs have been severely altered, re-establishment of typical diverse fish communities may in fact be an important element aiding the recovery of other important components of aquatic ecosystems including zooplankton and benthic macroinvertebrates. In the lakes closest to the smelters, where historically watersheds as well as lakes were severely damaged, the recovery of aquatic systems will be closely linked to ongoing terrestrial recovery and rehabilitation, particularly through the benefits of increased inputs of terrestrially derived organic matter. The dramatic lake recovery observed in the Sudbury area points to a brighter future for these lakes. However, continued monitoring will be needed to determine future changes and help guide the management and protection of Sudbury-area lakes in this multiple-stressor age.

Rapid Recent Recovery from Acidic Deposition in Central Ontario Lakes

“…Similarly, Strock et al [34] reported more dramatic declines in SO 4 relative to base cations in lakes in the northeastern USA after 2002. The rate of chemical recovery measured post-2005 is greater than has typically been observed in regional studies remote from point emission sources [10,35] and is more consistent with changes observed close to smelters where substantial reductions in S deposition have occurred [4].…”

“…Similarly, climate-induced SO 4 release from organic soils in catchments can lead to higher SO 4 loadings to lakes than would be expected from deposition alone [9]. In other studies, decreases in base cation (primarily calcium (Ca) and magnesium (Mg)) concentrations in lakes have approximately matched decreases in SO 4 concentrations, which also contribute to delayed chemical recovery [10]. The decrease in base cation concentration is largely attributable to continued soil acidification [11].…”

“…Chemical recovery in lakes from the large reductions in acidic deposition that occurred in eastern North America and Europe over the past 40 years or so has been mixed [1,2]. While sulphate (SO 4 ) and nitrate (NO 3 ) are the main components of acidic deposition, SO 4 has traditionally been the primary contributor to aquatic acidification because NO 3 inputs in deposition are retained in terrestrial ecosystems [3]. In some cases, such as in regions near smelters, rapid and large reductions in atmospheric S deposition have occurred, which have resulted in strong recovery responses [4].…”

“…While sulphate (SO 4 ) and nitrate (NO 3 ) are the main components of acidic deposition, SO 4 has traditionally been the primary contributor to aquatic acidification because NO 3 inputs in deposition are retained in terrestrial ecosystems [3]. In some cases, such as in regions near smelters, rapid and large reductions in atmospheric S deposition have occurred, which have resulted in strong recovery responses [4]. In other areas, chemical recovery has been less than expected, owing to a number of confounding factors, including S release from upland and organic soils, depletion of exchangeable base cation pools and increases in organic acidity that have offset declines in mineral acids [5][6][7].…”

“…Reductions in atmospheric sulphur (S) deposition have generally led to decreases in SO 4 concentrations in lakes, however, in some regions, the net release of S from soils from historical S deposition has resulted in a slower response than expected [8]. Similarly, climate-induced SO 4 release from organic soils in catchments can lead to higher SO 4 loadings to lakes than would be expected from deposition alone [9]. In other studies, decreases in base cation (primarily calcium (Ca) and magnesium (Mg)) concentrations in lakes have approximately matched decreases in SO 4 concentrations, which also contribute to delayed chemical recovery [10].…”