Influence of organic acids on model projections of lake acidification

We employed three mathematical models to make quantitative estimates of the pH of 33 statistically-selected lakes in the Adirondack mountains, New York (USA) prior to the Industrial Revolution (1840). The models included 1) the MAGIC watershed acidification model, 2) a paleolimnological model of diatom-inferred pH, and 3) the MAGIC model modified to incorporate an empirically-based model of natural organic acidity. Application of approaches 2) and 3) yielded consistent estimates of pre-industrial Adirondack lakewater pH. However, when the organic acid model was not included, MAGIC calculations and diatom-inferred values showed poor agreement. MAGIC projections of lakewater pH 50 years into the future, under differing atmospheric deposition scenarios, were also sensitive to inclusion of the organic acid model. MAGIC predicted greater recovery in response to reduced deposition when organic acids were not considered. These results suggest that failure to consider the pH buffering of naturally-occurring organic acidity will often result in biased projections which overemphasize the response of lakewater pH to changes in atmospheric inputs of strong acid.