Mechanisms of hydrogen ion neutralization in an experimentally acidified lake

The experimental acidification of Lake 223 (Experimental Lakes Area, northwestern Ontario) with sulfuric acid in 1976-1983 allowed a detailed examination of the capacity of the lake to neutralize hydrogen ion. A whole-lake alkalinity and ion budget for Lake 223 showed that 6681% of the added sulfuric acid was neutralized by alkalinity production in the lake. Nearly 85% of in situ alkalinity production was accounted for by net loss of sulfate through bacterial sulfate reduction, coupled with iron reduction and iron sulfide formation, in littoral sediments (60%) and in the hypolimnion (25%). Exchange of hydrogen ion for calcium and manganese in the sediments accounted for 19% of the alkalinity generated, while other cations were net sinks for alkalinity. Alkalinity input from the watershed of Lake 223 was very small, averaging about 5% of that produced in the lake. The seasonal production of 1,000 peq liter-* alkalinity in the anoxic hypolimnion of this softwater lake could be attributed to bacterial sulfate reduction coupled with iron sulfide formation, ammonium production, and iron (II) production. Only the alkalinity produced from bacterial sulfate reduction coupled with iron sulfide formation remained throughout the annual cycle. Although the acidification of lakes in Europe and North America has been well documented (e.g. Drablos and Tollan 1980; Natl. Res. Count. Can. 198 1; Natl. Res. Count. 1981; U.S. EPA 1983), the biogeochemical mechanisms involved are still incompletely understood. To understand how lakes become acidic and to predict the rate of future acidification or recovery of lakes under known atmospheric inputs of acids, we need to improve our knowledge of the capacity of watersheds and lakes to neutralize hydrogen ion. Recent work has shown the significance of various mechanisms in neutralizing acidity in terrestrial and aquatic ecosystems (e.g. Sollins et al. 1980; Hemond and Eshleman 1984; Schindler 198 5) and wetlands (Hemond 1980). Schindler et al. (1986) have shown that in situ biogeochemical processes, rather than processes in the terrestrial drainage, are the predominant sources of alkalinity for lakes in poorly buffered geological strata. Most studies of lake acidification have been carried out after the fact. In very few studies have the preacidification conditions of the lakes been well enough known to describe the progress of lake acidification so that a detailed understanding of the acidification process could be gained. The wholelake, experimental acidification of Lake 223, Experimental Lakes Area (ELA), northwestern Ontario, provides an ideal opportunity for examining how biogeochemical processes respond to lake acidification. The background conditions for this soft-water, low alkalinity (100 peq liter-l; pH 6.6) lake were determined in 1974 and 1975. Additions of electrolyte-grade sulfuric acid were made for the next 7 years at a hydrogen ion loading rate about four times that of the northeastern U.S., causing the pH to drop to between 5.2 and 5.0. In contrast to the situation in lakes acidified by atmospheric deposition, the input of acid to Lake 223 is known precisely. Sulfur budgets and various chemical and biological changes resulting from the acidification of Lake 223 have been reported elsewhere (e.g. Schindler 1980; Nero and Schindler 1983; Mills 1984). Schindler et al. (1980) reported that dur-