Organic matter accumulation and salinity change in open water areas within a saline boreal fen in the Athabasca Oil Sands Region, Canada

Saline boreal fens represent potential models for post-mining landscape reclamation in the Athabasca Oil Sands Region (Canada) where wetland construction is challenged by salinization. One of the key indicators of reclamation success is the accumulation of organic matter within constructed fens. Thus, a better understanding of the linkages between salt content and organic matter storage in saline boreal fens can be useful for advancing fen construction in this region. As such, this study investigates how salinity fluctuations estimated by diatom-based transfer functions, coupled with changes in hydrological conditions and vegetation inferred from macrofossils have influenced organic matter accumulation rates (OMAR) over the last ~100 years in open-water areas (ponds) within a saline boreal fen near Fort McMurray, Alberta. Median OMAR (181 g m−2 yr−1) of the site suggests that the ponds situated within saline boreal fens can accumulate organic matter at a rate comparable to freshwater boreal and subarctic ponds, and the estimated salinity levels (3-21 ppt) did not severely affect organic matter accumulation. Strong significant positive (Lager Pond), strong significant negative (South Pond), and weak insignificant (Pilsner Pond) correlations between OMAR and diatom-inferred salinity were observed, suggesting that relations between organic matter accumulation and salt content are not straightforward, and salinity was not the main control on OMAR. Macrofossil data showed that organic matter accumulation has been mainly driven by water level, type of primary producers and pond regime. OMAR was the highest during the transition from peatland to ponds due to low decomposition rates resulting from high inputs of relatively resistant plant litter, and anoxic conditions. A macrophyte-dominated pond regime was associated with higher OMAR relative to algae-dominated regime.