Limnocorral Studies of Chemical and Biological Acid Neutralization in Two Freshwater Lakes

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Abstract
Twenty-nine 1 m diameter limnocorrals were installed in two Canadian Shield lakes to examine the relative importance of individual acid neutralization processes. Once the water column alkalinity was depleted, the within-lake alkalinity production processes were confined to the lake sediments. The flocculent organic-rich sediments of a shallow dystrophic lake (Lake 114) neutralized acid at a significantly greater rate than the sandy littoral sediments of an oligotrophy lake (Lake 302). Since the water column alkalinity of Lake 114 is much lower than Lake 302, ambient alkalinity is not a good predictor of acid neutralization capacity in the sediments. In both lakes, HNO3 was neutralized at a substantially higher rate than HCl or H2SO4. The rates of nitrate consumption, sulfate consumption, and abiotic acid neutralizing processes are independent of each other. Denitrification to an uncharged end-product is the primary mechanism of NO3− removal. The principal source of non-anion-specific buffering in the enclosure experiments was Ca2+ release from the sediments in Lake 302 and Ca2+ and NH4+ release in Lake 114. There is no significant difference between the organic-rich sediments of Lake 114 and the sandy sediments of Lake 302 in the concentration of exchangeable cations (μeq∙cm−3) for all cations except NH4+ and Mn2+. Alkalinity production in epilimnetic sediments is an important component of the resistance of softwater lakes to anthropogenic acidification.