The link between the exceedance of acidity critical loads for freshwaters, current chemical status and biological damage: a re-interpretation

Abstract

Critical loads have for several years been employed bypolicymakers to aid in the development of strategies for aciddeposition abatement. They provide an effects-based approachwhereby an acid deposition flux greater than the critical load(known as critical load exceedance) implies that long-termharmful effects on a selected target organism will occur.Implicit in this approach are two assumptions: first, theexceedance of a critical load will harm the target organism,and second, the severity of biological impact is related to themagnitude of exceedance. However, static models give noindication of when the predicted damage might occur. One suchmodel, the Steady-State Water Chemistry (SSWC) model, employs aseries of empirical relationships to derive the pre-industrial,baseline leaching rate of base cations from measured waterchemistry using the so-called `F-factor'. The SSWC model setsthe critical load relative to pre-industrial base cationleaching (a permanent buffer of acid deposition) and a selectedacid neutralizing capacity (ANC) value which corresponds with aknown likelihood of damage to a biological target organism.Here we interpret the meaning of critical load exceedance as aprediction of steady-state ANC, and explore the relationshipbetween exceedance of the critical load and current chemistry. We demonstrate that a critical loadexceedance with the SSWC model does not necessarily indicatethat the critical chemical threshold (zero ANC) has alreadybeen crossed, and there may be no correlation betweenexceedance and biological status. A reformulation of the SSWCmodel is proposed which provides a direct link between currentdeposition and current chemical conditions, and is thereforemore likely to indicate current biological damage. Thereformulation illustrates the discrepancy between currentchemical status and that predicted by the SSWC model atsteady-state, which is a function of the `F-factor'.