Long-term C accumulation and total C stocks in boreal lakes in northern Quebec

Here we assess total sediment organic C stocks and long-term C accumulation rates in 13 boreal lakes in northern Quebec 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(-2) yr(-1), 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 (root 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(-2) (per unit of lake area), or 3.8 kg m(-2) (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.