Acid toxicity levels in Nova Scotian rivers have not declined in synchronywith the decline in sulfate levels

The Atlantic salmon (Salmo salar) resource of eastern Canada is impacted by acid rain in the Southern Upland (Atlantic Coast) area of Nova Scotia. Sal mon runs in this area have become extinct in 14 rivers, are severely impact ed in 20 rivers, and lightly impacted in 15 rivers. Water chemistry and fis h communities in nine Southern Upland salmon rivers were studied from 1982 to 1996 as part of the effort to monitor the effects of the emission contro l programs in Canada and the United States. There has been no statistically significant change in total ion content of Southern Upland river water, bu t there was a significant decline in sulfate levels that was balanced by an increase in organic anions, and declines in calcium and magnesium that wer e balanced by increases in sodium and potassium. A geochemical scenario is proposed to account for these chemical changes. River water pH levels showe d no overall linear trend, but at borderline toxicity sites the year-to-yea r variations in pH were correlated with changes in juvenile salmon populati on densities. Ten fish species were collected, but none showed any signific ant overall time trend in population density. Fish species diversity was po sitively correlated with pH.