Tracking long-term acidification trends in Pockwock Lake (Halifax, Nova Scotia), the water supply for a major eastern Canadian city

Abstract

Pockwock Lake, the drinking water supply for the city of Halifax (Nova Scotia, Canada), has been impacted by the deposition of strongly acidic anions. Because long-term monitoring data are lacking, we used diatom-based paleolimnological techniques to track changes in water quality variables in this important water source. Similar to other acidified lakes in this region, Pockwock Lake has undergone changes in diatom assemblages starting ~1940 with a corresponding lakewater pH decrease of 1.2 units. Before ~1940, Pockwock Lake had a diatom-inferred pH ~6.3 and a diatom assemblage dominated by Cyclotella stelligera and Asterionella ralfsii var. americana (>45 μm). With the onset of acidification, diatom-inferred lakewater pH decreased to ~5.8 and there was a shift to dominance by A. ralfsii var. americana (>45 μm) and Tabellaria flocculosa. A subsequent shift in diatom assemblages and inferred lakewater pH was recorded ~1990 suggesting a decrease in dissolved organic carbon (DOC) and water colour. Since ~1992 Pockwock Lake has undergone further acidification, as evidenced by a shift in the diatom assemblage to dominance by Fragilaria acidobiontica, Eunotia spp., and Frustulia pseudomagaliesmontana. Both diatom-inferred and measured lakewater pH = 5.1 during this time interval. The first (~1940–1992) acidification period followed the trend observed in other humic (high DOC) Nova Scotia lakes, whereas the second (post ~1992) acidification event resulted in a diatom assemblage more common in acidified clearwater (low DOC) lakes. Thus, the acidification signal observed in Pockwock Lake likely indicates a change in diatom assemblage resulting from a loss of DOC, and the sudden drop in diatom-inferred pH suggests that the weak acid buffering system of humic DOC had been exceeded. Further acidification and loss of DOC in this lake has the potential to increase the availability of metals in this important water source.