The watershed of Weber Lake in northern lower Michigan has experienced a nearly 20-fold increase in proton loading from acid precipitation in the past 25 years. But, owing to alkalinity production by biologically mediated processes (nitrate uptake by plants and sulfate reduction by bacteria) and increased rates of carbonate weathering, acid precipitation has doubled the alkalinity, raised the equilibrium pH, and made this soft-water seepage lake more eutrophic. An acid-base budget for Weber Lake and its watershed was constructed from the difference between bulk precipitation input and lake water output. Strict application of the first law of thermodynamics and the principle of electroneutrality permits the calculation of the acid-base budget from the input-output budget. The acid-base budget indicates that alkalinity production resulting from nitrate uptake and sulfate reduction is sufficient to completely neutralize the hydrogen ions entering the system from acid precipitation (including biologically mediated hydrogen ion production). The acid-base budget shows that the alkalinity of Weber Lake has doubled primarily as a result of increased rates of weathering in the watershed. Weathering rates increased as an indirect result of acid precipitation.