title: Natural variability in lake pH on seasonal, interannual and decadal time scales: implications for assessment of human impact.
abstract: Reference values define the natural state with respect to environmental stressors and are commonly used for assessments of ecological impacts and to set restoration targets. These reference values are often treated as constants, whereas in reality they can be highly variable. Here, we study the significance of this variability for assessments of human impact on the environment, by using almost two decades of observations from 95 acid-sensitive Swedish lakes. Our approach was to first estimate the preindustrial, steady-state reference level of acid neutralization capacity (ANC) for each lake with the hydrogeochemical model MAGIC. Then the variability in pH around this "baseline" was reconstructed from the contemporary, "natural" variability in the ANC, total organic carbon (TOC) and Al-concentrations, and partial CO2 pressure. The variability in reference pH was then examined for the period 1990-2004, on seasonal (single measurements), interannual (1-year median), and decadal (5-year median) scales. On the seasonal scale, the variability in reference pH ranged between 0.40 and 1.7. The range on the interannual time scale was up to 1.3 units and for the decadal scale up to 0.76 units. Since an anthropogenic pH decline of more than 0.4 units is deemed significant according to the Swedish Environmental Quality Criteria, this natural variability clearly needs to be accounted for when assessing acidification.
authors.affiliation: Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Box 7050, 75007 Uppsala, Sweden. Martin.Erlandsson@ma.slu.se
authors.forename: Martin
authors.initials: M
authors.lastname: Erlandsson
authors.forename: Jens
authors.initials: J
authors.lastname: Fölster
authors.forename: Hjalmar
authors.initials: H
authors.lastname: Laudon
authors.forename: Gesa A
authors.initials: GA
authors.lastname: Weyhenmeyer
authors.forename: Kevin
authors.initials: K
authors.lastname: Bishop