From Peatlands to Boreal Lakes: Fill-and-Spill Hydrology and Dissolved Organic Carbon (DOC) Transport in the Headwaters of the Hudson Bay Lowlands, Canada

The boreal peatlands of the northern Canadian Shield in Ontario, Canada, serve as headwater systems for the Hudson Bay Lowlands (HBL), the third-largest peatland complex globally and a critical carbon reservoir. This landscape—shaped by a heterogeneous mix of exposed bedrock outcrops and low-conductivity glacio-marine sediments—comprises a mosaic of treed peatlands, post-glacial lakes, and river systems, which play a key role in regulating water and carbon fluxes to downstream ecosystems. Despite their importance, the hydrological connectivity of these peatlands and their role in dissolved organic carbon (DOC) transport remain poorly understood, especially in the context of changing hydrological conditions.

This study investigates the hydrological and DOC dynamics along two 400 m flowpaths that originate in peatlands and terminate at Tomorrow Lake (49°55'2"N, 80°41'59"W), a 2.5 km² post-glacial lake draining into the North French River watershed. In June 2024, five monitoring nests were installed along each transect, equipped with porewater sippers (30 and 50 cm below ground surface) and screened pipes at depths of 75, 100, 150, and 200 cm. Continuous water table data were logged, and DOC concentrations were measured during June, August, and September 2024. A meteorological station, installed in August, captured local hydrological inputs and outputs, providing a detailed view of seasonal variability.

Results reveal a complex “fill-and-spill” hydrological connectivity at the flowpath outlets, driven by variations in topography. In the steeper transect, water tables dropped sharply from <30 cm below ground surface (bgs) at the peatland center to >150 cm bgs at the lake interface, entering the underlying low-conductivity mineral soil. This suggests slow, diffuse subsurface flow as the dominant transport mechanism. Average DOC concentrations correspondingly declined from 33 mg/L in the peatland center to 19 mg/L at the lake edge, aligning closely with average lake outflow concentrations (16 mg/L) and indicating potential carbon filtration through the mineral soil. By contrast, in the flatter transect, water tables remained elevated near the lake interface (<30 cm bgs), and a pipe-like surficial flow point was observed at the outlet in June transporting disproportionately large volumes of water—up to five orders of magnitude greater than subsurface flow—while maintaining elevated DOC concentrations (35–40 mg/L). DOC concentrations at the outflow remained high throughout the summer. However, the discharge rate progressively declined as the water table levels receded, almost ceasing entirely by September.

DOC concentrations in Tomorrow Lake are comparable the median annual concentration in downgradient North French River (~19 mg/L) the larger Moose River that this watershed supports (~16 mg/L), suggesting high connectivity within this landscape. These findings underscore the need to evaluate hydrological and biogeochemical processes holistically, integrating headwater and downstream dynamics, while considering seasonal and interannual variability to better understand contemporary carbon transport, transformation, and the anticipated responses of these systems to climate warming.