Patterns and trends in lake concentrations of dissolved organic carbon in a landscape recovering from environmental degradation and widespread acidification

Concentrations of dissolved organic carbon (DOC) have increased in lakes throughout North America and Europe over the last three decades. Recovery from acid deposition and climate change have both been postulated as the primary mechanisms for the increase in DOC. To provide a clearer insight into the mechanisms responsible for increasing DOC we evaluated changes in lake and peat porewater chemistry collected in an area of approximately 33,000 km(2) surrounding Sudbury, Ontario, a region undergoing dramatic recovery from acidic deposition. DOC concentrations varied considerably among the 44 lakes and over time (samples annually from 1981 to 2018), but the Sens Slope value showed a strong increase in lake DOC concentration over time, at 0.05 mg/L y-1 (p < 0.001) that was related to increasing pH [0.03 units y(-1), p < 0.001] and decreasing lake SO4 concentration [-0.24 mg/L y(-1); p< 0.001], but showed no relationshipwith temperature or precipitation. Similar strong relationships between DOC and pH(positive) and SO4 (negative) were observed in 18 peatlands sampled in the region. In a spatial analysis of 82 lakes sampled in 2018, concentrations of DOC in lakes were highest in flatter catchments with a greater wetland area, suggesting thatwetlands are amajor source of DOC in lakes. Optical properties of DOC obtained from extracts of wetland and upland soils at 6 catchments could be distinguished, primarily due to upland litter extracts having distinct optical properties from mineral soils or wetland soils. Optical properties of DOC in lakes however were inconsistent with those measured in soil extracts indicating that they are not useful for distinguishing DOC sources in these lakes. A predictivemodel was developed to explain DOC trends within Sudbury lakes using a stepwise linear regression combined with hierarchical partitioning to confirm themost influential processes on DOC. Almost 50% of the variability in DOC change in the 44 lakes was explained by the magnitude in lake pH change, catchment size and catchment sparse tree cover showing that recovery from acidic deposition is overwhelmingly responsible for increasing DOC in Sudbury lakes. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study evaluated changes in lake and peat porewater chemistry in the Sudbury area and found that DOC concentrations increased over time, primarily affected by increasing pH and decreasing lake SO4 concentration. Strong relationships between DOC and pH (positive) and SO4 (negative) were observed in lakes, indicating that recovery from acidic deposition is the main factor contributing to the increase in DOC in Sudbury lakes.