A comparison of MAGIC and paleolimnological predictions of preindustrial pH for 55 Swedish lakes

Two fundamentally different approaches to define reference conditions for acidification assessments are hydrogeochemical modeling and paleolimnological reconstructions. Both methods have been applied to calculate the preindustrial chemistry for 55 Swedish lakes in two independent studies. This paper investigates whether these methods give similar reconstructions of the preindustrial pH for these lakes. Special focus has been attached to the importance of total organic carbon concentrations and CO2 partial pressure (pCO(2)) in the conversion from ANC to pH in the hydrogeochemical modeling. With a uniform pCO(2) value for all the lakes of 0.63 matm, the mean absolute difference between pH from the hydrogeochernical model and the paleolimnological pH was +0.23 units (mean absolute difference 0.36 units). If instead a lake specific preindustrial pCO(2) is assumed, equal to contemporary pCO(2), the mean difference in the predicted preindustrial pH between the two methods was reduced to +0.03 units (mean absolute difference 0.22 units). Statistical analyses indicated that with a lake specific pCO(2), the difference between the reconstructions is smaller than 0.13 pH-units at a 95% level of significance. The results of this study build confidence in the reliability of both methods, providing that lake-specific estimates of pCO(2) are used.