Export of terrigenous dissolved organic matter in a broad continental shelf

Export of terrigenous dissolved organic matter (DOM) from rivers to the ocean plays an important role in the carbon cycle. Observations from six research cruises in 2014 were used to characterize the seasonal evolution of terrigenous DOM in the shallow and broad South Atlantic Bight (SAB) shelf. While DOM with a strong terrigenous molecular, optical and isotopic signature was restricted to a coastal band early in the year, a plume with terrigenous DOM extended further to the shelf break in late spring. The offshore transport of this terrigenous DOM was consistent with wind-driven advection in a surface Ekman layer. On time scales spanning about 1 month, the traceable riverine DOM compounds were mostly resistant to bio- and photo-degradation, and the decrease in their relative abundance over the shelf following peak river discharge during spring was consistent with dilution of the river plume due to entrainment of oceanic water associated with wind-driven mixing. Comparisons between optical absorbance measurements and ultrahigh resolution mass spectrometry data revealed that the fraction of the DOM pool with a riverine signature in the SAB can be estimated using the spectral slope coefficient of chromophoric DOM in the 275-295 nm range. This finding opens up the possibility of observing the distribution of riverine DOM on the SAB shelf in high spatial resolution and by using remote sensing methods, a crucial step for quantifying shelf-slope exchange and the fate of terrigenous DOM in shelf seas.