Abstract
The global distribution of chromophoric dissolved organic matter (CDOM)
in the euphotic layer of the Atlantic, Indian, and Pacific oceans
(between 35° N and 40° S) was analyzed by absorption spectroscopy during
the Malaspina 2010 circumnavigation. Absorption coefficients at 254 nm
(a254) and 325 nm (a325), indices (a254/a365) and spectral slopes (between 275 and 295 nm, S275-295)
were calculated from the dissolved fraction of the UV absorption
spectra to describe the amount and quality of CDOM. Generalized Additive
Models (GAMs) were applied to evaluate the relevance of physical and
biogeochemical drivers for the variability of CDOM. Besides the low CDOM
values, a first division of our data following the Longhurst’s
biogeographic classification showed significant differences in CDOM
levels among provinces. The lowest values of a254 and a325
were found in the oligotrophic gyres, particularly in the Indian Ocean,
and the highest in the upwelling areas, particularly in the Equatorial
Pacific. Opposite distributions were obtained for S275-295 and a254/a365, indicative of higher photobleaching in the gyres. Within each province, whereas a254 was constant through the photic layer, a325
increased significantly with depth as a result of the dominance of
photobleaching over biological production in the surface layer and the
opposite at depth. The Pacific provinces, including the subtropical
gyres, showed, however, significantly higher a325 values, indicative of lower photobleaching/higher biological production. The GAM analysis indicates that a254 and a325 were primarily related to chlorophyll a (Chl a), exhibiting a significant positive linear response. Interestingly, Prochlorococcus and Synechococcus
abundances were related to these absorption coefficients. Apparent
oxygen utilization also contributed to explain the distributions of
these absorption coefficients, being inversely related to a254 and directly related to a325. These results are consistent with the premise that a254 could be a proxy for the concentration of dissolved organic carbon and a325 for the aromatic by-products of biological degradation. The GAM analysis also shows that a254/a365 and S275-295
exhibited inverse relationships with solar radiation, indicating that
the biological production of CDOM counteracts photodegradation as solar
radiation increases. In summary, whereas photobleaching dictates the
vertical distribution of CDOM, Chl a explains the CDOM
differences among the photic layer of the tropical and subtropical ocean
provinces visited during the circumnavigation