Distribution and photo-reactivity of chromophoric and fluorescent dissolved organic matter in the Northeastern North Pacific Ocean

Abstract

Deep waters in the Northeastern North Pacific Ocean store the most aged, largely biologically refractory dissolved organic matter (DOM) in the global ocean. We conducted high resolution vertical profiles to survey the optical properties of this DOM in the Gulf of Alaska (GoA) onboard the R/V Melville using ultraviolet/visible (UV/Vis) absorbance spectra and excitation-emission matrix spectra (EEMs) along with parallel factor analysis (PARAFAC). Chromophoric DOM (CDOM) absorption coefficients in the GoA were similar to the global open ocean waters reported elsewhere outside the Northeastern North Pacific. Three humic-like and two protein-like fluorescent components were identified. Profiles of fluorescent DOM (FDOM) showed a low signal for the humic-like organic matter in the surface that increased in the subsurface and remained fairly constant at depth. In contrast, the protein-like components had elevated levels in surface samples that decreased with depth. Both CDOM and FDOM optical proxies remained spatially uniform within the deep ocean water masses. Photochemical irradiations performed onboard showed that photochemistry of the FDOM from deep seawater transformed the deep ocean fluorescent organic matter signals into a fluorescent component distribution that resembled surface water with a dominance of protein-like fluorescent material. This work contributes to the global ocean survey database of chromophoric and fluorescent DOM and provides evidence of a role for the photochemical alteration of deep, biologically refractory DOM once it is mixed back into the sunlit surface ocean.