Lake/watershed sulfur budgets and their response to decreases in atmospheric sulfur deposition: watershed and climate controls

<jats:title>Abstract</jats:title><jats:p>Atmospheric sulfur (S) emissions peaked in North America in the early 1970s followed by declines in S deposition and sulfate (SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup>) concentrations in surface waters. Changes in S biogeochemistry affect the mobilization of toxic (Al<jats:sup>+3</jats:sup>, H<jats:sup>+</jats:sup>) and nutrient (Ca<jats:sup>2+</jats:sup>, Mg<jats:sup>2+</jats:sup>, K<jats:sup>+</jats:sup>) cations, and the acid–base status of ecosystems. We focused on lake/watersheds in the Adirondack Mountains of New York, USA, one of the most acid‐sensitive and acid‐impacted regions in North America. We used 16 of the 17 original Adirondack Long‐Term Monitoring Lakes from 1984 through 2010 and found significant declines (−2.14 µmol<jats:sub>c</jats:sub> l<jats:sup>−1</jats:sup> year<jats:sup>−1</jats:sup>) in SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> concentrations. There were significant declines (−0.28 kg S ha<jats:sup>−1</jats:sup> year<jats:sup>−1</jats:sup>) in total S deposition for all lake/watersheds. We constructed S mass balances for 14 lakes/watersheds from wet and dry S deposition and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> loss from drainage and found a comparable decline (−0.26 kg S ha<jats:sup>−1</jats:sup> year<jats:sup>−1</jats:sup>) in lake SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> export. There was a discrepancy (mean 2.34 kg S ha<jats:sup>−1</jats:sup> year<jats:sup>−1</jats:sup>) between atmospheric S deposition and watershed S loss due to internal S sources. Using major solute chemistry including dissolved silica and watershed characteristics, it was evident that the watershed S budget discrepancy increased with thickness of surficial deposits. The annual discrepancies in S mass balances were strongly linked with annual watershed discharge. These results suggest that internal S sources are becoming increasingly important as atmospheric S inputs have declined. The internal SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> supply of watersheds decreased concomitantly with lake acid neutralizing capacity (ANC). These findings suggest that the limited contributions from internal sources of SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> will facilitate the recovery of ANC from those lake/watersheds with the lowest ANC. With long‐term decreases in atmospheric S deposition, the effects of climate, especially increases in precipitation, will play an increasingly important role in regulating S budgets and the amount of SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> mobilized from internal watershed sources. Copyright © 2012 John Wiley & Sons, Ltd.</jats:p>