Impacts of acid deposition and lake browning on long-term organic carbon storage in Canadian northern forest lakes

Atmospheric acid deposition disrupted terrestrial-aquatic carbon cycling by drastically lowering dissolved organic carbon (DOC) loads in many lakes across NE North America and northern Europe during the 20th century. However, little is known about how acid deposition has altered the role of lakes as long-term carbon sinks. We present contemporary (n = 80) organic carbon accumulation rates (OCAR) and OCAR trends over the past ~ 150 years (n = 8), and other supporting infrared spectroscopic, isotopic, and elemental geochemical proxies, for lakes in and near Sudbury, Ontario, Canada – an area heavily affected by acid deposition from smelting activities in the late-19th and 20th centuries. Contemporary OCAR varied between 4.9 and 35.3 g m^–2 yr^–1 among study lakes (mean: 13.5±6.4 g m^–2 yr^–1). Sediment-inferred trends in lake-water DOC showed a strong response in DOC loadings to the effects of acid deposition during the past century, which is corroborated by increasing observed lake-water DOC concentrations (i.e., lake browning) since the 1980s. Despite these changes in DOC, as well as changes in water acidity, only lakes with direct physical watershed disturbances showed short-lived increases in OCAR, whereas OCAR changed little in remote Sudbury-region lakes with minimal direct human disturbances (mean OCAR: 14.3 ± 8.7 g m^–2 yr^–1). This is in stark contrast to many other northern forest lakes with minimal direct catchment disturbances that experienced significant increases in OCAR during the 20th century. Our results caution that lake browning may not be a dominant driver behind the widespread increase in organic C burial in northern lakes during recovery from acid deposition in recent decades.