Long-term development and trajectories of inferred lake-water organic carbon and pH in naturally acidic boreal lakes

Monitoring of surface waters in the boreal region over the last decades shows that waters are becoming browner. This timeframe may not, however, be sufficient to capture underlying trajectories and driving mechanisms of lake-water quality, important for prediction of future trajectories. Here we synthesize data from seven lakes in the Swedish boreal landscape, withcontemporary lake-water total organic carbon (TOC) concentrations of 1.4–14.4 mg L-1, to conceptualize how natural and particularly human-driven processes at the landscape scale have regulated lake-water TOC levels over the Holocene. Sediment-inferred trends in TOC are25 supported by several proxies, including diatom-inferred pH. Before ~700 CE, all lakes were naturally acidic (pH 4.7–5.4) and the concentrations of inferred lake-water TOC were high (10–mg L-1 27 ). The introduction of traditional human land use from ~700 CE led to a decrease in lakewater TOC in all lakes (to 5–14 mg L-1 28 ), and in four poorly buffered lakes, also to an increase in pH by >1 unit. During the 20th 29 century, industrial acid deposition was superimposed on centuries of land use, which resulted in unprecedentedly low lake-water TOC in all lakes (3–11 mg L-1 30 ) and severely reduced pH in the four poorly buffered lakes. The other lakes resisted pH changes, likely due to close connections to peatlands. Our results indicate that an important part of the recent browning of boreal lakes is a recovery from human impacts. Furthermore, on a conceptual levelwe stress that many contemporary environmental changes have occurred within the context of past, long-term disturbances.