COMPLEX INTERACTIONS OF LONG-TERM PATTERNS IN DISSOLVED ORGANIC CARBON WITH REGIONAL AND GLOBAL VARIABLES IN EASTERN CANADIAN LAKES

Abstract

Dissolved organic carbon (DOC) is an important water quality parameter that can affect many physical, chemical and biological processes in aquatic ecosystems. The long-term patterns in ice-free DOC in 49 eastern Canadian lakes from four sites were re-examined with a ~35-year dataset. Long-term patterns in DOC were synchronous in lakes within a site (Brien’s test, p > 0.05), but not across sites (p ≤ 0.04), except between Kejimkujik and Yarmouth. Hence, these two sites were pooled into a single Nova Scotia site (NS). Increases in DOC concentration in Dorset lakes were evident between 1997 and 2015 (increase of 0.12 mg L-1 year-1, r2 = 0.68, p < 0.001). Similar increases were also apparent in NS between 2000 and 2015 (increase of 0.11 mg L-1 year-1, r2 = 0.34, p = 0.02). Although the long-term DOC concentrations at the ELA lakes showed an increasing trend between 1983 and 2000 (increase of 0.10 mg L-1 year-1, r2 = 0.39, p = 0.005), no evidence of an increase or a decrease was detected between 2001 and 2015 (r2 = 0.02, p = 0.59). The long-term relationships in ice-free DOC concentration and four regional and three global variables were examined. Declining SO4 deposition was the most important factor that explained the DOC increases in the last two decades in Dorset and NS. Summer precipitation also explained a significant amount of variation in DOC at these two sites. Interestingly, some global variables (i.e., North Atlantic Oscillation and Pacific Decadal Oscillation) emerged as important explanatory variables for the first time in Dorset and NS. The negative association between DOC and NAO was explained indirectly by the significant correlations between North Atlantic Oscillation and temperature. However, such correlations were not found between Pacific Decadal Oscillation and regional climate variables. Declining SO4 deposition was also found to be the most important explanatory variable at the ELA. However, despite a declining SO4 deposition pattern throughout the study period, a rise in DOC concentration was not evident between 2001 and 2015 at the ELA. Long-term DOC patterns in the ELA lakes may have reached an equilibrium in 2000, when declining SO4 deposition could not cause any further increase in DOC. Absence of an increase or decrease in DOC concentration after 2000 may be attributed to additional acidity from an increase in nitrogen (N) or ammonium (NH4+) deposition at the ELA.