Alkalinities of Maine lakes: Are they really changing?

Abstract

Alkalinity changes between 1938–1944 and 1985 were evaluated for Maine lakes (zmax ≳ 15 m) in granitic watersheds. Modern (late summer 1985) Gran alkalinities for the hypolimnion were compared with historical alkalinities computed using: urn:x-wiley:00243590:media:lno19903561238:lno19903561238-math-0001 where pH(T) is the historic reported colorimetric pH at each hypolimnetic sampling depth and temperature, pK'1(T) the temperature‐dependent negative logarithm of the first dissociation constant for carbonic acid, and H2CO3* represents initial CO2 at last spring overturn, plus the change in CO2 associated with the hypolimnetic oxygen deficit developed over summer (estimated from the historic oxygen data). With modern data for representative Maine lakes and historical (1932) data on dissolved gases reported for northern Wisconsin lakes, the apparent respiratory quotient relating ΔCO2 to ΔO2 = 0.92. The above model yields alkalinity estimates with a relative root‐mean‐square (RMS) error of ∼15%.The alkalinities of the Maine lakes have increased by 15–50 µeq liter−1 (>30%) since the original surveys. Lake alkalization is not the result of differences in antecedent weather, since abnormally dry weather predating 1942 would have acted to elevate alkalinities.