Acidification at Plastic Lake, Ontario: Has 20 Years Made a Difference?

Abstract

In response to reduced sulphur emissions, there has been a large decrease in sulphate (${\text{SO}}^{{2 - }}_{4}$; −0.97 μeq l⁻¹ year⁻¹) and hydrogen (−1.18 μeq l⁻¹ year⁻¹) ion concentration in bulk precipitation between 1980 and 2000 at Plastic Lake in central Ontario. The benefit of this large reduction in ${\text{SO}}^{{2 - }}_{4}$ deposition on stream water chemistry was assessed using the gauged outflow from a conifer-forested catchment (PC1; 23.3 ha), which is influenced by a small wetland located immediately upstream of the outflow. Sulphate concentrations declined, but not significantly due to large inter-annual variation in ${\text{SO}}^{{2 - }}_{4}$ concentration. Between 1980 and 2000, there were significant increases in dissolved organic carbon, ammonium and potassium concentration likely reflecting increased mineralisation in the wetland. Calcium concentrations in PC1 decreased during the two decade period (−2.24 μeq l⁻¹ year⁻¹), as a consequence there was no improvement in stream pH and the Ca:Al ratio in PC1 continued to decline. A similar response was noted in an upland-draining sub-catchment of PC1-08 that has been monitored since 1987. Despite large reductions in ${\text{SO}}^{{2 - }}_{4}$ deposition and almost complete retention of nitrogen in soil, there has been no improvement (in terms of pH) in stream water at PC1 due to a combination of soil acidification and climatic (droughts, increased mineralisation) perturbations.