Carbon inputs sourced to the lake during different periods of winter and spring. Melt events play dual roles in carbon export, both mobilizing (black arrows) and diluting (gray arrows) terrestrial carbon inputs to the lake, with dissolved organic carbon mainly derived from surface soil leaching (white dots) and greenhouse gases carbon and methane more closely associated with groundwater and mineral layer leaching (black stripes). Carbon input sources are differentiated between mire‐dominated (brown arrows) and forest‐dominated (green arrows) basins, where the arrow size represents the relative concentration difference for the specific carbon input observed. (a) During early winter melt, the surrounding soils freeze downward, creating a solute freeze‐out zone. (b) During winter base flow, external inputs are minimized, although the surrounding mire is still active. (c) During spring onset melt, overland flow from snowmelt has a dilution effect on C inputs. (d) During spring peak melt, all flow paths are activated. Credit: Denfeld et al., 2018, Figure 6
Source: Journal of Geophysical Research: Biogeosciences

Winter is coming, but for limnologists, winter, until recently, has often been ignored when it comes to lake biogeochemistry. Most mid and high latitude lakes are covered seasonally by ice, and what appears to be a quiet and dormant world to the ice skaters and snowmobiles atop is actually a complex world of decomposers and chemistry in the cold water below. These processes are suspected to have an important effect on the net annual budgets of lake carbon emission, but current theories are based on simple assumptions.

Denfeld et al. [2018] made rare measurements of under-ice carbon dioxide and methane concentrations in a small boreal lake in Sweden. Their measurements and mapping of lake carbon after ice off were coupled together to reveal a strong role of winter melt event driving external carbon inputs leading to under-ice carbon build-up. Further, the ratio of methane to carbon dioxide accumulation shifted through the winter.

With climate projections showing warmer winters and less consistent ice cover, the future may make winter lake dynamics an even more prominent component of lake carbon cycling, influencing dynamics regarding lake nutrients and water quality, and also the contribution of freshwaters to the global carbon budget and climate change feedbacks.

Citation: Denfeld, B. A., Klaus, M., Laudon, H., Sponseller, R. A., & Karlsson, J. [2018]. Carbon dioxide and methane dynamics in a small boreal lake during winter and spring melt events. Journal of Geophysical Research: Biogeosciences, 123.

—Ankur Rashmikant Desai, Editor, JGR: Biogeosciences

Text © 2018. The authors. CC BY-NC-ND 3.0
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