Dissolved organic matter (DOM) and barium in James Bay: Distribution, sources, and climate change implications

The properties of dissolved organic matter (DOM) were examined in a dataset representative of the James Bay (Canada) marine system following two summer expeditions in 2021 and 2022. The absorption coefficients at 275 nm (a₂₇₅) and 295 nm (a₂₉₅) of the chromophoric DOM (CDOM) were used to establish a region-specific, usable in-depth, predictive model of dissolved organic carbon (DOC) concentration in summer, with a mean absolute percentage error of only 11 %, comparable to other models in polar environments. The DOC concentration, a₃₅₀, the spectral slope between 275 and 295 nm (S_275–295), the dissolved barium concentration, and four PARAFAC components (three humic-like and one protein-like) were employed as freshwater tracers. At the surface, we observed a very low DOM region in the north-west of the bay associated with the inflow of marine waters from Hudson Bay, a highly DOM-rich region in the south and east in James Bay caused by strong riverine inputs and, in the north-east, a very fresh region, but with lower DOM concentrations than to the south, induced by the discharge of the La Grande River. At depth, a homogeneous, low DOM distribution is observed in the north of James Bay, extending from east to west. In contrast, the deep water to the south is much richer in DOM and appears to circulate along the east coast. The lack of exchange at depth between the northern and southern regions is attributed to the presence of a sill that creates a physical barrier. The analysis of lignin-phenols in surface samples revealed a more “woody gymnosperm” vegetation compared to the Hudson Bay Rivers further north, which is in line with the known vegetation surrounding the Bay. The portrait of the DOM in offshore James Bay in summer thus provides a benchmark for studying the evolution of the carbon pool under climate change in this region, which is likely to undergo significant upheaval due to permafrost thaw, increased riverine inputs, and alteration of the vegetal ecosystem.