Most marine DOC is thought to be biologically-recalcitrant, especially that in the deep ocean pool (N 1000 m). In particular, the deep waters of the North Pacific should contain the most recalcitrant DOC because they do not form locally, with deep DOC having survived long isolation from the surface during global-scale thermohaline circulation. One of the proposed removal pathways involves photochemical reactions when refractory DOC circulates through sunlit surface waters (Mopper et al., 1991). Here, we reevaluate the general photoreactivity of refractory DOC by investigating the photochemical production of two reactive oxygen species (ROS), hydrogen peroxide (H 2 O 2 ) and superoxide (O 2 − ), using controlled irradiations at sea and in the laboratory. The photoproduction of these two ROS were compared between deep and surface water samples collected in the Gulf of Alaska. For irradiated samples, initial superoxide steady-state concentrations ([O 2 − ] ss ) and first-order decay constants (k pseudo ) were similar between surface and deep waters, ranging from~1-4 nM and 4-12 × 10 −3 s −1 , respectively. Initial photoproduction rates were comparable between surface and deep waters, ranging from-173 nM h −1 for O 2 − and from 1-8.3 nM h −1 for H 2 O 2 , indicating that a large portion of photoproduced O 2 − does not lead to H 2 O 2 formation. In fact, the photoproduction ratio of O 2 − to H 2 O 2 averaged 4:1 in paired experiments, instead of the 2:1 stoichiometry expected for O 2 − dismutation to form H 2 O 2 . Continued irradiation for up to 48 h showed photoproduced H 2 O 2 to be much lower in deep samples compared to surface samples with accumulation slowing or stopping in deep samples despite both measurable [O 2 −] ss and photon absorbance by colored dissolved organic matter (CDOM). These results are consistent with a loss of source material (i.e. CDOM) for O 2 − photoproduction and a shift to a predominantly oxidative pathway for O 2 − decay. Low photoproduction rates, loss of continued accumulation with extended radiation, and an apparent loss of O 2 − source material argues that the deep refractory DOC pool is less photochemically reactive than previously suggested.

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.

Approximately 30% of the ocean's surface is subject to phytoplankton iron (Fe) limitation, especially in the Equatorial Pacific and Southern Oceans where upwelling provides a large flux of nitrate (NO 3 − ) and other nutrients (Moore et al., 2001(Moore et al., , 2013. Elsewhere, stratification of the upper ocean leads to depletion of NO 3 − , ammonia, and other bioavailable forms of nitrogen. In stratified oligotrophic gyres, shallow mixed layers also act to concentrate Fe deposited at the ocean's surface by atmospheric sources (Boyle et al., 2005;Sedwick et al., 2005). The large flux of Fe relative to NO 3 − in these ecosystems results in nitrogen limitation of photosynthesis and selects for phytoplankton like the cyanobacterium Prochlorococcus (Ward et al., 2013;Wu et al., 2000), whose small size allows them to outcompete other phytoplankton for recycled nitrogen species found at nanomolar concentrations (Morel et al., 1991).However, the same stratification that leads to Fe-rich conditions in the surface ocean can also impede Fe supply to the subsurface. Shallow mixed layers ensure that Fe derived from dust deposition does not reach the entirety of the euphotic zone, which can extend below 100 m in subtropical gyres. Stratification also limits the supply of regenerated Fe from below the euphotic zone. Indeed, a common feature of dFe profiles within subtropical gyres is a concentration minimum between 75 and 150 m (Bruland et al., 1994;Fitzsimmons et al., 2015;Sedwick et al., 2005). This subsurface dFe minimum often coincides with the deep chlorophyll maximum (DCM), a unique habitat where low irradiance drives phytoplankton photo-acclimation, increasing chlorophyll per cell to improve photosynthetic light capture (Letelier et al., 2004). Theoretical arguments suggest the increases in chlorophyll per cell should be matched by an equivalent increase in the number of Fe-bearing photosynthetic reaction

Approximately 30% of the ocean's surface is subject to phytoplankton iron (Fe) limitation, especially in the Equatorial Pacific and Southern Oceans where upwelling provides a large flux of nitrate (NO 3 − ) and other nutrients (Moore et al., 2001(Moore et al., , 2013. Elsewhere, stratification of the upper ocean leads to depletion of NO 3 − , ammonia, and other bioavailable forms of nitrogen. In stratified oligotrophic gyres, shallow mixed layers also act to concentrate Fe deposited at the ocean's surface by atmospheric sources (Boyle et al., 2005;Sedwick et al., 2005). The large flux of Fe relative to NO 3 − in these ecosystems results in nitrogen limitation of photosynthesis and selects for phytoplankton like the cyanobacterium Prochlorococcus (Ward et al., 2013;Wu et al., 2000), whose small size allows them to outcompete other phytoplankton for recycled nitrogen species found at nanomolar concentrations (Morel et al., 1991).However, the same stratification that leads to Fe-rich conditions in the surface ocean can also impede Fe supply to the subsurface. Shallow mixed layers ensure that Fe derived from dust deposition does not reach the entirety of the euphotic zone, which can extend below 100 m in subtropical gyres. Stratification also limits the supply of regenerated Fe from below the euphotic zone. Indeed, a common feature of dFe profiles within subtropical gyres is a concentration minimum between 75 and 150 m (Bruland et al., 1994;Fitzsimmons et al., 2015;Sedwick et al., 2005). This subsurface dFe minimum often coincides with the deep chlorophyll maximum (DCM), a unique habitat where low irradiance drives phytoplankton photo-acclimation, increasing chlorophyll per cell to improve photosynthetic light capture (Letelier et al., 2004). Theoretical arguments suggest the increases in chlorophyll per cell should be matched by an equivalent increase in the number of Fe-bearing photosynthetic reaction

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