Abstract
Abstract. Downstream mineralization and sedimentation of terrestrial dissolved organic carbon (DOC) render lakes important for landscape carbon cycling in the boreal region. The chemical composition of terrestrial DOC, the downstream delivery of terrestrial DOC and its processing within aquatic ecosystem may all be influenced by climate change, including increased occurrence of wildfire. Here, we assessed composition and lability (during both dark- and UV incubations) of DOC from peatland groundwater and mineral soil groundwater, and from shallow lakes within a peatland-rich region on the Boreal Plains of western Canada that was recently affected by wildfire. Wildfire was found to increase aromaticity of DOC in peat pore water above the water table, but had no effect on the concentrations or composition of peatland groundwater DOC or mineral soil DOC. Using a mixing model we estimated that on average 98 and 78% of terrestrial DOC inputs to the lakes in fine- and coarse-textured settings, respectively, originated from peatland groundwater sources. Accordingly, lake DOC composition reflected primarily a mixing of peatland and mineral soil groundwater sources, with no detectable influence of the recent wildfire. Lake and peatland DOC had low biodegradability, lower than that of mineral soil DOC. However, both mineralization and sedimentation of peatland DOC increased substantially during UV incubations through selective removal of aromatic humic and fulvic acids. Similar shifts in DOC composition as observed during the UV incubations were also observed across lakes with longer water residence times. The mixing model estimated that on average 54% (95% confidence interval: 36–64%) of terrestrial DOC had been removed in lakes as a result of mineralization and sedimentation. Meanwhile, the reduction in absorbance at 254 nm was 71% (58–76%), which suggests selective removal of aromatic DOC. Hence, incubation results, patterns of DOC composition among lakes and mixing model results were consistent with significant within-lake removal of terrestrial DOC through UV-mediated processes. Selective removal of highly aromatic DOC through UV-mediated processes implies that organic sources that are considered stabile in terrestrial ecosystems can be readily mineralized once entering aquatic ecosystems. Together, our results suggest that regional characteristics (climate, surface geology and lake morphometry) can prevent wildfire from causing pulse perturbations to the linkages between terrestrial and aquatic C cycling and also regulate the processes that dominate within-lake removal of terrestrial DOC.
Highlights
Lakes act as reactors where dissolved organic carbon (DOC) leached from terrestrial ecosystems is mineralized, transformed and sedimented (Tranvik et al, 2009)
Our study suggests that wildfire alters DOC composition in surficial pore water in peatlands by increasing its aromaticity, but it suggests that this effect is not apparent in deeper peat layers at or below the water table
Our results suggest that deeper peat layers were the dominant source of terrestrial DOC export to lakes, likely due to regional characteristics such as sub-humid climate, thick overburden and widespread peatlands
Summary
Lakes act as reactors where dissolved organic carbon (DOC) leached from terrestrial ecosystems is mineralized, transformed and sedimented (Tranvik et al, 2009). Lakes accumulate carbon (C) in sediments while simultaneously being net sources of CO2 to the atmosphere (Sobek et al, 2005; von Wachenfeldt et al, 2008). Lakes are believed to play a significant role for the C balance of the boreal region, since they cover a large fraction of the landscape and receive DOC inputs from terrestrial ecosystems that often are equal in magnitude to long term carbon accumulation rates in soils (Prairie, 2008; Olefeldt et al, 2013a). D. Olefeldt et al.: Sources and fate of terrestrial dissolved organic carbon
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