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Abstract:
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 evolu-
tion 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 reso-
lution and by using remote sensing methods, a crucial step for quantifying shelf-slope exchange and the fate
of terrigenous DOM in shelf seas.
