[1] Here we assess total sediment organic C stocks and long-term C accumulation rates in 13 boreal lakes in northern Québec spanning a wide range of morphometric shapes. The lake basins were mapped using a sub-bottom profiler to obtain total sediment volume, which we combined with organic carbon profiles from Holocene cores to obtain total C mass. The estimated long-term areal C accumulation rates averaged 3.8 g C m À2 yr À1 , lower than previous reports for other boreal and temperate regions. The difference relative to previous studies may have resulted from our use of the detailed echosounding mapping approach, which yields more realistic estimates of total sediment volume. Total sediment C stocks were not related to lake trophic status or to DOC concentration, but rather to lake area and to the lake dynamic ratio (√lake area/mean water depth). We hypothesize that scaling of C accumulation to lake morphometry is more a reflection of the intrinsic capacity of lakes to retain carbon. We show that C loading does in fact play a significant role in the patterns of C accumulation in lakes, but that this role is strongly modulated by both lake size and shape, which in turn determine the ability of lakes to retain the carbon that has been loaded. Upscaling to the regional level using the empirical lake size relationships developed here results in an areal-weighted average C stock of 23 kg C m À2 (per unit of lake area), or 3.8 kg m À2 (per unit landscape), which represents around 25% of the total landscape C storage in this boreal region. Because of the lake-size scaling of C accumulation, the total lake C stocks at the regional level depend not only on the total lake area, but more importantly on the local lake size distribution.Citation: Ferland, M.-E., P. A. del Giorgio, C. R. Teodoru, and Y. T. Prairie (2012), Long-term C accumulation and total C stocks in boreal lakes in northern Québec, Global Biogeochem. Cycles, 26, GB0E04,

We investigated the role of lake sediments as carbon (C) source and sink in the annual C budget of a small (0.07 km 2 ) and shallow (mean depth, 3.4 m), humic lake in boreal Sweden. Organic carbon (OC) burial and mineralization in the sediments were quantified from 210 Pb-dated sediment and laboratory sediment incubation experiments, respectively. Burial and mineralization rates were then upscaled to the entire basin and to one whole year using sediment thickness derived from sub-bottom profiling, basin morphometry, and water column monitoring data of temperature and oxygen concentration. Furthermore, catchment C import, open water metabolism, photochemical mineralization as well as carbon dioxide (CO 2 ) and methane (CH 4 ) emissions to the atmosphere were quantified to relate sediment processes to other lake C fluxes. We found that on a whole-basin and annual scale, sediment OC mineralization was three times larger than OC burial, and contributed about 16% to the annual CO 2 emission. Other contributions to CO 2 emission were water column metabolism (31%), photochemical mineralization (6%), and catchment imports via inlet streams and inflow of shallow groundwater (22%). The remainder (25%) could not be explained by our flux calculations, but was most likely attributed to an underestimation in groundwater inflow. We conclude that on an annual and whole-basin scale (1) sediment OC mineralization dominated over OC burial, (2) water column OC mineralization contributed more to lake CO 2 emission than sediment OC mineralization, and (3) catchment import of C to the lake was greater than lake-internal C cycling.

Carbon (C) storage in lakes is now recognized as a significant sink of C at a global scale, but the pathways that lead to this storage remain poorly understood. In this study, we attempt to reconstruct and connect the processes that lead to long-term C accumulation in boreal lakes. These include the rate of particulate organic C (POC) sedimentation in the water column and sediment metabolism operating at a temporal scale of weeks to months, organic C accumulation in the top sediment layers integrated over scales of tens of years, and long-term organic C burial in lake sediment integrated over hundreds to thousands of years. The sinking POC flux was tenfold higher than the short-term sediment C accumulation rates in all systems, and we found no direct relationship between this downward C flux and either the short-term or long-term C accumulation rates. However, the resulting C burial efficiency (which ranged from 5 to 62%) was strongly related to lake shape, which ultimately constrains the time freshly deposited material that is exposed to oxygen and thereby regulates the fraction of the carbon sinking flux that is mineralized back to the atmosphere or permanently buried in the sediments. Small and deep lakes act as more efficient C sinks than large and flat lakes. We also show that long-term burial rates are nearly identical to current centennial-scale accumulation rates and that therefore, little degradation occurs after a few decades. Sediment C storage tends to be small (<5%) relative to lake C emissions, but that this balance is also strongly related to lake morphometry.

Summary1. Fire frequency is expected to increase in boreal forests over the next century owing to climate change. In Quebec (Canada), the location of the northern limit of commercial forests (c. 51°N) was established in 2000 taking into account mainly forest productivity and fire risk. The location of the limit is currently under debate and is being re-evaluated based on a more extensive survey of the territory. We characterized the natural variability of fire occurrence (FO) in the area surrounding the northern limit, and these results are a useful contribution to discussions on the re-evaluation of its location. 2. Regional FO over the last 7000 years was reconstructed from sedimentary charcoal records from 11 lakes located in three regions surrounding the northern limit (i.e. south, north and near the limit). Holocene simulated precipitation and temperature from a general circulation model (GCM) were used to identify the long-term interactions between climate and fire. 3. Fire histories displayed similar trends in all three regions, with FO increasing from 7000 calibrated years before present (cal. years BP) to reach a maximum at 4000-3000 cal. years BP, before decreasing during the late-Holocene. This trend matches the simulated changes in climate, characterized by drier and warmer conditions between 7000 and 3500 cal. years BP and cooler and moister conditions between 3500 and 0 cal. years BP. 4. Northern ecosystems displayed higher sensitivity to climate change. The natural variability of FO was narrower in the southern region compared with the limit and northern regions. An abrupt decrease in FO was recorded close to and north of the limit at 3000 cal. years BP, whereas the decrease was more gradual in the south. 5. Synthesis and applications. We reconstructed the natural variability in fire activity over the last 7000 years near the current location of the northern limit of commercial forests in Quebec. Fire occurrences were more sensitive to climate change near to and north of the limit of commercial forestry. In the context of predicted increase in fire activity, the lower resilience of northern forests advocates against a northern repositioning of the limit of commercial forests.

Boreal lake sediments are important sites of organic carbon (OC) storage, which have accumulated substantial amounts of OC over the Holocene epoch; the temporal evolution and the strength of this Holocene carbon (C) sink is, however, not well constrained. In this study we investigated the temporal record of carbon mass accumulation rates (CMARs) and assessed qualitative changes of terrestrially derived OC in the sediment profiles of seven Swedish boreal lakes, in order to evaluate the variability of boreal lake sediments as a C sink over time. CMARs were resolved on a short-term (centennial) and long-term (i.e., over millennia of the Holocene) timescale, using radioactive lead ( 210 Pb) and carbon ( 14 C) isotope dating. Sources and degradation state of terrestrially derived OC were identified and characterized by molecular analyses of lignin phenols. We found that CMARs varied substantially on both short-term and long-term scales and that the variability was mostly attributed to sedimentation rates and uncoupled from the OC content in the sediment profiles. The lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant OC source to the sediments over the Holocene. Furthermore, lignin-based degradation indices, such as acid-to-aldehyde ratios, indicated that postdepositional degradation in the sediments was very limited on longer timescales, implying that terrestrial OC is stabilized in the sediments on a permanent basis.