AUTHOR=Sabbaghzadeh Bita , Uher Guenther , Upstill-Goddard Robert 

TITLE=“Dynamics of chromophoric dissolved organic matter in the Atlantic Ocean: unravelling province-dependent relationships, optical complexity, and environmental influences”

JOURNAL=Frontiers in Marine Science

VOLUME=11

YEAR=2024

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2024.1432133

DOI=10.3389/fmars.2024.1432133

ISSN=2296-7745

ABSTRACT=<p>We report on the spatial distributions and optical characteristics of chromophoric dissolved organic matter (CDOM) in the sea surface microlayer (SML), subsurface seawater (SSW), and water column profiles down to 500 m across a range of Atlantic Ocean biogeochemical provinces during two cruises of the UK Atlantic Meridional Transect program (AMT24 and AMT25). We measured the CDOM absorption coefficient at 300 nm, <italic>a</italic><sub>CDOM</sub>(300), and determined CDOM spectral slopes across two UV wavelength ranges:  <italic>S</italic><sub>1</sub> (275-295 nm) and <italic>S</italic><sub>2</sub> (350-400 nm). We used spectral slope ratios (<italic>S</italic><sub>R</sub>: <italic>S</italic><sub>1</sub>/<italic>S</italic><sub>2</sub>) to infer CDOM source characteristics and transformation history. During both cruises, SML <italic>a</italic><sub>CDOM</sub>(300) was highest in the Northern Hemisphere, particularly in the North Atlantic Drift Region (NADR). CDOM was always enriched in the SML, with enrichment factors (SML <italic>a</italic><sub>CDOM</sub>(300) / SSW <italic>a</italic><sub>CDOM</sub>(300)) ranging from 1.03 to 2.00, reflecting preferential accumulation of CDOM in the SML. We also found a significant inverse correlation between <italic>a</italic><sub>CDOM</sub>(300) and <italic>S</italic><sub>1</sub> in both the SML (Spearman’s rank correlation coefficient, <italic>r</italic><sup>2</sup> = -0.75, <italic>p</italic> &lt; 0.001, <italic>n</italic> = 114) and water column profiles (<italic>r</italic><sup>2</sup> = -0.74, <italic>p</italic> &lt; 0.001, <italic>n</italic> = 845). Biogeochemical province-dependent variations in the relationships between CDOM and chlorophyll <italic>a</italic> were also observed. In high-latitude regions, elevated <italic>a</italic><sub>CDOM</sub>(300) and low <italic>S</italic><sub>R</sub> values indicated a dominance of terrestrially-derived CDOM, whereas oligotrophic subtropical areas showed lower <italic>a</italic><sub>CDOM</sub>(300) and higher <italic>S</italic><sub>R</sub> values, suggestive of aged, refractory, and photodegraded biologically-derived CDOM. Taken together, these findings reveal a complexity of drivers affecting CDOM distributions and spectral properties, which may limit the use of CDOM in predictive relationships in the oceans. However, the potential use of chlorophyll <italic>a</italic> as a CDOM proxy may prove most successful in open ocean regions devoid of terrestrial inputs, where biological production predominates.</p>