Long-term changes in dissolved organic matter quality are unrelated to ecosystem recovery from acidification in the Adirondack region (New York, USA)

Increasing concentrations of dissolved organic carbon (DOC) and changing dissolved organic matter (DOM) quality in surface waters, a phenomenon known as browning, have been observed at global scales with a range of implications for ecosystem structure and function, global carbon cycling and human health. Ecosystem recovery from chronic acidification resulting from rapid declines in acid deposition over recent decades has been the leading explanation for surface water browning. In this study, long-term dynamics of the quantity, quality, and seasonality of DOM in surface waters of an acid-resistant Adirondack lake and its forested watershed were investigated during a period of rapid regional changes in both acidic deposition and climate (1999–2018). Overall, we found that trends in DOM quality have occurred while the overall quantity and seasonality of DOC fluxes changed little during the same time frame. Lack of DOC trends was consistent with expectations for this acid-resistant ecosystem. Model reconstructions of DOM quality during this period indicated shifts towards a greater proportion of terrestrially-sourced DOM from the watershed, but with occasional ‘pulses’ of more microbially-processed DOM associated with periods of heavy rainfall and high discharge. Our findings suggest that ecologically meaningful changes in DOM quality may be occurring in acid-resistant ecosystems, aside from trends in DOC driven by ecosystem recovery from acid impairment.