Coastal conduit in southwestern Hudson Bay (Canada) in summer:Rapid transit of freshwater and significant loss of coloreddissolved organic matter

摘要

Distributions of freshwater (sea-ice melt and runoff) were investigated along inshoreoffshore sections in southwestern Hudson Bay for fall conditions. Conductivitytemperature- density profiles and bottle samples collected for salinity, oxygen isotope (δ~( 18)0), and colored dissolved organic matter (CDOM) analyses were used to discriminate between contributions of river water (RW) and sea-ice melt (SIM). Stations had a fresh summer surface mixed layer 5-25 m thick overlying a cold subsurface layer indicative of the previous winter's polar mixed layer (PML). The fraction of RW decreased strongly with distance from shore, while the opposite was true for SIM. The majority of RW was constrained in a coastal domain within 100-150 km from shore, which, because of high alongshore velocities, accounts for the majority of freshwater and volume transports. On the basis of freshwater inventories and composition, brine and RW accumulate in the PML over winter because of ice formation and downward mixing. The summer surface circulation results in an annual net export of SIM from the region. Residence times for freshwater components in the southwestern sector of the bay, based on currents derived from a 3-D ocean model for Hudson Bay, are about 1-10 months, implying rapid transit of freshwater. Despite the short residence time for RW (13 months), CDOM is significantly photobleached and provides an unreliable tracer for RW. Photobleaching represents an important sink for dissolved organic carbon entering from rivers and could, in part, explain why Hudson Bay is only a minor sink for atmospheric CO_2 in the open water season.