Electrode pH error, seasonal epilimnetic pCO₂, and the recent acidification of the maine lakes

Abstract

Based on laboratory experiments involving standard pH protocols, on statistical analysis of data from the Eastern Lake survey, and on independent field evidence (1985–86), recent (post 1975) electrometric pH measurements for Maine lakes are reevaluated for accuracy. A standard pH protocol used in Maine up until September 1985 can introduce bias of ca. −0.3 to −0.4 pH units for representative epilimnetic samples with low CO₂ concentrations (near atmospheric equilibrium). The bias arises out of the very long electrode equilibration times for such samples. A new protocol using these same Ross combination electrodes provides pH estimates accurate within 0.03 pH units for a wide range of northeastern environmental samples, and independent of sample DOC and conductance. The new pH measurements, when paired with the earliest colorimetric measurements from the same Maine lakes, indicate that epilimnetic pH's may have increased significantly (by 0.1 to 0.2 units) since the pioneering lake surveys of G. P. Cooper (1938–44). Earlier, often-cited, reports documenting historical pH reductions in Maine lakes are apparently vitiated by analytical error in many modern electrode pH measurements. The error also affects the calibration of paleo-ecological models of recent lake acidification in New England.