Effects of Acid pH Shock on Phosphate Concentrations and Microbial Phosphate Uptake in an Acidifying and a Circumneutral Lake

Abstract

Water samples from Plastic Lake and St. Nora Lake, Dorset, Ontario, were subjected to a rapid decrease in pH (acid pH shock). In both lakes, phosphate concentrations changed following pH shock. Minimal phosphate concentrations were observed in the pH 5.2–5.8 range. Maximal concentrations in both lakes occurred at pH 4.0: in Plastic Lake the mean increase at pH 4 was 3.0 times ambient and in St. Nora Lake was 2.5 times ambient. Particles > 0.45 μm were the major source of the phosphate released on acid pH shock. Rate constants of ³²PO₄ uptake by microorganisms decreased in response to acid pH shock, and bacteria were affected more than algae; the decrease in rate constants was more severe in Plastic than in St. Nora Lake but we could not distinguish the relative contributions of H⁺ toxicity and isotope dilution (by the extra phosphate released) to the observed decline.