Changes in spectral slopes of colored dissolved organic matter absorption with mixing and removal in a terrestrially dominated marine system (Hudson Bay, Canada)

We investigated the changes in absorption and spectral slopes of colored dissolved organic matter (CDOM) using a data set of salinity, δ18O and CDOM absorption in Hudson Bay. Following the fraction of river water (determined with salinity and δ18O tracers) one can track the changes in terrestrial CDOM optical properties with mixing and removal, as salinity cannot alone be used in waters with significant influence from sea-ice melt. CDOM in Hudson Bay was controlled by terrestrial inputs, in contrast to adjacent Hudson Strait. CDOM absorption was removed significantly, likely due to photobleaching. There was no or negligible indication of absorption removal during initial estuarine mixing, in agreement with earlier studies. Of the many absorption spectral slope (S) parameters that have been used as proxies for CDOM dynamics, the ones at shorter wavelengths proved the best indicators for absorption removal by photobleaching. Increase in absorption spectral slopes at 275 to 295 (S275–295) and 290 to 350 nm (S290–350) are strongly correlated with the apparent removal of CDOM absorption. S275–295 and S290–350 in combination with spectral slopes and ratios at other wavelength intervals, which are sensitive to other processes and sources, can potentially reveal more information about CDOM origin and dynamics than a single slope alone.