Watershed vs in-lake alkalinity generation: A comparison of rates using input-output studies

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As a means of assessing the relative contributions of watershed (terrestrial) and in-lake processes to overall lake/watershed alkalinity budgets, alkalinity production rates for watersheds and low alkalinity lakes were compiled from the literature and compared. Based on net alkalinity production data, derived using wet or bulk deposition data, mean and median alkalinity production for 20 watersheds in North America and Europe were 89 and 69 meq m−2 yr−1 (range 20 to 235 meq m−2 yr−1). For a subset of 10 watersheds with dry deposition data, terrestrial alkalinity production neutralized an additional 35 meq m−2 yr−1 of acidic deposition. For 11 lakes, mean and median in-lake alkalinity generation were 99 and 88 meq m−2 yr−1 (range 22 to 240 meq m−2 yr−1). Analysis of data indicates that for the low alkalinity systems described here, areal alkalinity production rates for watersheds and lakes are approximately equal. This relationship suggests that watershed area to lake area ratio can be used as a convenient estimator of the relative importance of watershed and in-lake sources of alkalinity for drainage lake systems. For precipitation-dominated seepage lakes and other systems where hydrology limits soil-water contact, hydrologic flow paths and residence times can be of overriding importance in determining alkalinity sources. For regions dominated by drainage lakes with high watershed area to lake area ratios (such as the Northeastern U.S.), however, alkalinity budgets are dominated by watershed processes. Omission of in-lake alkalinity consideration for most lakes in such regions would have little impact on computed alkalinity budgets or on predicted response to changes in acidic deposition loadings.