Factors controlling the geographical distribution of fluorescent dissolved organic matter in the surface waters of the Pacific Ocean

Abstract

Dissolved organic matter (DOM) in the epipelagic ocean is produced by marine organisms and consumed by microbes. Thus, the distributional patterns of DOM quantity and quality in surface waters are possibly related to marine ecological provinces. In this study, surface waters collected throughout the Pacific Ocean were used to investigate the geographical distributions of fluorescent DOM (FDOM) quantity and quality. An excitation‐emission matrix and parallel factor analysis revealed two humic‐like and two protein‐like components. The levels of humic‐like components showed clear meridional trends with some zonal variability and were highest and lowest in the northern high‐latitude and the subtropical provinces, respectively. The photochemical reactivity, determined by the ratio of two humic‐like components, was found to be lowest in the subtropical provinces, implying that the major factor controlling the geographical distribution of humic‐like components is the degree of photobleaching. The distributional patterns differed between levels of two protein‐like components, i.e., tryptophan‐like and tyrosine‐like. The ratio of tyrosine‐like to tryptophan‐like components was established as a possible indicator of microbial degradability, and the highest ratio occurred in subtropical provinces. A negative correlation was found between this ratio and the chlorophyll a concentration. Such geographical distributions of protein‐like components imply that relatively recalcitrant protein‐like components are distributed uniformly throughout the surface waters, but substantial contributions of reactive fractions occur in regions characterized by high biological production. Cluster analysis with the FDOM composition clarified that the diagenetic states of DOM were similar and variable in the northern high‐latitude and the subtropical provinces, respectively.

Interannual Variability in the Absorption and Fluorescence Characteristics of Dissolved Organic Matter in the Canada Basin Polar Mixed Waters

“…However, no significant correlation was found between protein-like C3 and Chl a (p > 0.01), suggesting that the protein-like signal was not primarily due to primary production. This contrasts with Yamashita et al (2017) and Makarewicz et al (2018), who found a positive correlation between tryptophan-like and the extracted Chl a concentration. Although protein-like FDOM is mainly derived from aromatic amino acids, such as tryptophan and tyrosine molecules in marine environments (Yamashita et al, 2015(Yamashita et al, , 2017Yamashita & Tanoue, 2003), it may also contain low molecular weight aromatics (Timko et al, 2015) such as tannins and lignins (Hernes et al, 2009;Maie et al, 2007;Yamashita et al, 2017).…”

Distribution and Sources of Dissolved Black Carbon in Surface Waters of the Chukchi Sea, Bering Sea, and the North Pacific Ocean

“…The lowest level of CDOM was evident in surface waters of the subtropical North Pacific (0.8-1.5 m −1 ); CDOM in surface waters of the subarctic North Pacific (1.1-1.7 m −1 ) was in the middle range. Overall, the distribution pattern of CDOM levels in surface waters was similar to that of colored dissolved and detrital material (CDM) observed by ocean color imagery (Siegel et al, 2005) and that of humic-like fluorophores determined by excitation emission matrices combined with parallel factor analysis (Yamashita et al, 2017).…”

Response to the Review of Anonymous Referee#1

“…In situ quantitative correlation between chlorophyll a concentrations and fluorescence intensity of the protein-like FDOM fraction has been observed and documented recently. Yamashita et al (2017) reported significant positive correlation between the tryptophan-like component and Chla (r = 0.53, p < 0.001) in the surface waters of the Pacific Ocean. Yamashita et al (2017) also found spatial coupling between the tryptophan-like component and chlorophyll a concentration, which was strongest in the Bering Sea.…”

“…Yamashita et al (2017) reported significant positive correlation between the tryptophan-like component and Chla (r = 0.53, p < 0.001) in the surface waters of the Pacific Ocean. Yamashita et al (2017) also found spatial coupling between the tryptophan-like component and chlorophyll a concentration, which was strongest in the Bering Sea. A study by Loginova et al (2016) from a Peruvian upwelling system also reported a positively correlated chlorophyll a concentration and protein-like component (R 2 = 0.40, p < 0.001).…”

“…After cleaning with ultrapure water, stability instrument readings were inspected with in-air measurements. The required correction of absorption signal for scattering was performed with the so-called proportional method by which zero absorption is estimated at 715 nm (Zaneveld et al, 1994). Subtraction of absorption coefficients from attenuation coefficients determined volume scattering coefficient, b(λ).…”