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Keywords:

  • acidification;
  • DOC;
  • droughts;
  • hydrologic processes;
  • sulphate;
  • wetlands

Abstract

Rising dissolved organic carbon (DOC) concentrations observed at a number of sites in the northern hemisphere over recent decades are the subject of much debate, and recent reports suggest a link between DOC patterns in surface waters and changes in sulphate (SO4) related to droughts or deposition. In order to investigate the potential influence of changes in SO4 concentration on DOC patterns in south-central Ontario, we used long-term (1980–2001) stream monitoring data from a wetland-dominated catchment (Plastic Lake-1 subcatchment, PC1) that has been the focus of intensive investigations of both SO4 and DOC dynamics. Annual average volume-weighted DOC concentration increased significantly between 1980 and 2001, whereas SO4 concentration declined, but the decrease was not significant due to large increases in SO4 that occurred during drought years. There was no relationship between SO4 and DOC in annual data series; however, seasonal analyses indicated significant negative correlations between SO4 and DOC concentrations in spring (March–April–May), summer (June–July–August) and fall (September–October–November). In spring, DOC concentration was negatively correlated with flow whereas SO4 concentrations increased with flow, and their opposing relationships with discharge explain the negative correlation between SO4 and DOC in this season. In summer and fall, low SO4 concentrations occur during periods of low flow as a result of microbial SO4 reduction, whereas correspondingly high DOC concentrations in the summer and fall can be attributed to optimal conditions (i.e. stagnant flow, warm temperatures) for DOC production in the wetland. Increases in SO4 (and acidity) following droughts were not associated with declines in DOC; instead the primary impact of droughts on DOC was to limit DOC export due to diminished stream flow. Rather than an acidification effect, we suggest that negative relationships between SO4 and DOC were either directly (spring) or indirectly (summer/fall) caused by underlying relationships with hydrology.