Increasing trends of dissolved organic carbon (DOC) in surface waters have been documented in northern Europe and North America where these waters are being influenced by acid deposition recovery.   Lower S deposition and associated acidity can increase the solubility of organic carbon by decreasing the charge density of soluble organic matter and lowering the ionic strength of soil solutions as atmospheric deposition becomes more dilute.   With several decades of declining S deposition, the focus has been on the effect of decreased acidity in moving DOC concentrations toward levels that naturally occurred before acid deposition.  However, despite S deposition approaching levels estimated for the early 1900s, increasing DOC has continued.  These ongoing increases in DOC are of ecological significance because (1) the associated organic acidity suppresses pH and ANC increases from lower acid deposition, (2) increased DOC alters the thermal properties and mixing of lakes, and (3) nutrient relationships of surface waters are likely to be altered.

To improve our understanding of the mechanisms controlling trends in surface water DOC concentrations, stream chemistry data were collected from over 50 streams (stratified random selection of streams from severe to no acidification) in the western Adirondack region of New York.   Data collected in 2004/05 were compared by season (spring snowmelt, August base flow) with data collected in the same manner from the same streams in 2014/15.   Data collected from high elevation streams (> 860 m sampling elevation) were also collected during late summer base flow in 2010 and spring snowmelt in 2011.  From 2004/05 to 2014/15, pH averaged for all streams increased in August, but decreased during snowmelt, whereas concentrations of DOC decreased during the summer and increased during snowmelt.  These results are the opposite of what would be expected if acidity was the primary factor controlling DOC trends.  However, comparison of snowmelt data from these two sampling periods did show that higher DOC coincided with lower ionic strength in 2014/15, which suggests that ionic strength was controlling DOC concentrations.   High elevation stream chemistry also showed that even though concentrations of strong acid anions (SO₄, NO₃, and Cl) were lower during snowmelt than summer, pH was also lower during snowmelt because of acute dilution of base cations.  The driver of increasing trends in DOC concentrations appear to be shifting from decreases in acid inputs to decreases in ionic strength, which is dependent on the base saturation of soils as well as S inputs.