Abstract
In many regions, chemical recovery in lakes from acidic deposition has been generally slower than expected due to a variety of factors, including continued soil acidification, climate-induced sulphate (SO4) loading to lakes and increases in organic acidity. In central Ontario, Canada, atmospheric sulphur (S) deposition decreased by approximately two-thirds between 1982 and 2015, with half of this reduction occurring between 2005 and 2015. Chemical recovery in the seven lakes was limited prior to 2005, with only small increases in pH, Gran alkalinity and charge-balance ANC (acid-neutralizing capacity). This was because lake SO4 concentrations closely followed changes in S deposition, and decreases in base cation concentration closely matched declines in SO4. However, decreases in S deposition and lake SO4 were more pronounced post-2005, and much smaller decreases in lake base cation concentrations relative to SO4 resulted in large and rapid increases in pH, alkalinity and ANC. Dissolved organic carbon concentrations in lakes increased over the study period, but had a limited effect on lake recovery. Clear chemical recovery of these lakes only occurred after 2005, coinciding with a period of dramatic declines in S deposition.
Highlights
Chemical recovery in lakes from the large reductions in acidic deposition that occurred in easternNorth America and Europe over the past 40 years or so has been mixed [1,2]
While sulphate (SO4 ) and nitrate (NO3 ) are the main components of acidic deposition, SO4 has traditionally been the primary contributor to aquatic acidification because NO3 inputs in deposition are retained in terrestrial ecosystems [3]
Chemical recovery from acidification in lakes has been affected by increases in organic acidity caused by higher dissolved organic carbon (DOC) export from catchments [12]
Summary
Chemical recovery in lakes from the large reductions in acidic deposition that occurred in easternNorth America and Europe over the past 40 years or so has been mixed [1,2]. While sulphate (SO4 ) and nitrate (NO3 ) are the main components of acidic deposition, SO4 has traditionally been the primary contributor to aquatic acidification because NO3 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]. It is important to recognize that three different metrics typically are used to describe chemical recovery: pH, owing to its biological relevance, charge-balance ANC
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