Research Document 1997/28

The Atlantic Region Acid Rain Monitoring Program in Acidified Atlantic Salmon Rivers: Trends and Present Status

By Walton D. Watt

Abstract

The Atlantic salmon resource of the Maritimes is impacted by acid rain in the Southern Upland (Atlantic Coast) area of Nova Scotia. Salmon runs in this area are extinct in 14 rivers, severely impacted in 20 rivers, and lightly impacted in 16 rivers. There are 13 other salmon rivers, with higher ANC generation capacity, that do not have acid toxicity. Over the period 1981-1990, water temperatures rose and dissolved organic carbon levels declined in Nova Scotian rivers. The pH-levels in the rivers were controlled by organic acid levels, and pH rose as the organic acid levels fell. The 1991 Pinatubo eruption produced two years of lower water temperature, and subsequently lower pH levels and higher levels of DOC. This result is evidence of the role of water temperature in controlling DOC/pH levels. Levels of sulphate and hardness (magnesium plus calcium) in river water declined by equivalent amounts over the period 1981-1995. The most likely explanation is that SO₄⁼ levels are declining in response to the declining deposition, but instead of the hoped for decline in H⁺ there have been declines in deposition and leaching of Ca⁺⁺ and Mg⁺⁺. More base cation retention in the soils and biota will increase the capacity for ANC generation and will eventually result in rising pH levels in the rivers, but this has not yet become noticeable. The decline in sulphate emission/deposition should become more pronounced over the next 5-10 years, as the Canada/USA Air Quality Accord takes effect, and water temperatures may continue to rise; so we anticipate a return of the trend toward less toxic salmon habitat. At sites which are borderline for acid toxicity, the population densities of Atlantic salmon juveniles rose in response to the lower river toxicities in the eighties and early nineties, and then fell again when the pH levels declined. There were no population trends at sites with higher (non-toxic) pH levels, or for any of the other nine fish species encountered in the acidified rivers

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