Long‐term patterns of dissolved organic carbon in lakes across eastern Canada: Evidence of a pronounced climate effect

Abstract

We analyzed the 21-yr dynamics of dissolved organic carbon (DOC) in 55 lakes during ice-free periods in five regions across eastern Canada in relation to total solar radiation (TSR), precipitation, air temperature, sulfate deposition (SO 4 ), Southern Oscillation Index (SOI), North Atlantic Oscillation, and Pacific Decadal Oscillation (PDO). A synchronous pattern in DOC was found among lakes within each region; however, a synchronous pattern in DOC was not found among regions, except for Kejimkujik and Yarmouth. Long-term trends of increasing or decreasing DOC concentration were not evident except at the Experimental Lakes Area (ELA), where an increase in DOC correlated with a decrease in summer TSR and an increase in summer precipitation. Annual mean temperature increased at the Nova Scotia and Turkey Lakes Watershed regions (TLW) over the study period, but there was no corresponding change in DOC. TSR and precipitation were important explanatory variables across all regions, except for the TLW. Summer TSR, or annual TSR, had a negative relationship, while summer precipitation had a positive relationship with the temporal DOC pattern in all regions except TLW. TSR and precipitation explained 78%, 49%, and 84% of the variation in the long-term DOC patterns at Dorset, ELA, and Nova Scotia (NS) regions, respectively. In contrast, the long-term pattern in DOC at TLW was only weakly related to SOI and PDO.

Long-term Trends of Organic Carbon Concentrations in Freshwaters: Strengths and Weaknesses of Existing Evidence

“…In practice, monotonicity as such has rarely been statistically tested and data are generally assumed to be monotonic. However, cyclical patterns have been observed by some authors [34,35,46,56,65,77] either by looking at the graphical representation of the data or when applying smoothing methods such as the calculation of moving averages (also called running averages). Although not exempt from problems [105], calculation of moving averages provides a robust description of a data pattern and, although it has been seldom applied to OC data series, where it has been, cyclical patterns have appeared.…”

“…Data have sometimes been normalized using Z-scores [14,34,35,43,46]. Z-scores are calculated by subtracting from all values the mean over the period under study and dividing by the standard deviation.…”

Increases in terrestrially derived carbon stimulate organic carbon processing and CO2 emissions in boreal aquatic ecosystems

“…There is now mounting evidence that shifts in climate and other environmental factors are driving increasing aquatic concentrations of terrestrially derived DOC in many landscapes of the world [7][8][9] , in what has been termed the 'browning' 10 of inland waters. Whereas the magnitude and regulation of the browning phenomenon are the focus of intense research and debate 7,[11][12][13][14][15][16][17] , the potential implications of such a shift in terrestrial carbon (C) concentrations for the functioning of the receiving aquatic systems have been less explored, particularly in terms of the impacts on aquatic CO 2 emissions.…”

Long-term trends of chemical and modelled photochemical parameters in four Alpine lakes

“…Variations in exposure of surface waters to sunlight may be due to changes in the duration of the ice-cover period that, however, do not apply to Garda which never freezes over. Moreover, a decrease of the ice-cover duration due to climate warming should increase sunlight exposure and might cause a DOC decrease through photomineralisation of organic matter (Hudson et al, 2003;Zhang et al, 2010). In contrast, the observed increase in DOC concentration in both lakes could be linked to higher runoff.…”

“…The DOC content of lake water is largely influenced by runoff, which in turn may be affected by climate change (Hudson et al, 2003;Zhang et al, 2010). Changes in DOC concentration are important because they affect the penetration of sunlight (and most notably of UV radiation) into the water column (Laurion et al, 2000), as well as several other processes including photochemical reactions (Schindler, 1997).…”