Abstract
Increasing our knowledge on dissolved organic carbon (DOC) spatio-temporal distribution in the coastal ocean represents a crucial challenge for better understanding the role of these ecosystems in the global oceanic carbon cycle. The assessment of DOC concentration from the absorption properties of the colored part of the dissolved organic matter (a(cdom)) was investigated from an extensive data set covering a variety of coastal environments. Our results confirmed that variation in the a(cdom)(412) to DOC ratio (a*(cdom)(412)) can be depicted from the CDOM spectral slope in the UV domain (S(275-295)). They also evidenced that regional first order variation in both a*(cdom)(412) and S(275-295) are highly correlated to variation in a(cdom)(412). From these observations, generalized relationships for estimating a*(cdom)(412) from S(275-295) or a(cdom)(412) were parameterized from our development sites (N = 158; English Channel, French Guiana, Hai Phong Bay) and tested against an independent data set covering others coastal regions (N = 223; French Polynesia, Rhone River estuary, Gulf of Maine, Chesapeake Bay, Southern Middle Atlantic Bight) demonstrating the possibility to derive DOC estimates from in situ CDOM optical properties with an average accuracy of ~16% over very contrasted coastal environments (with DOC ranging from 50 to 250 µmol.L(-1)). The applicability of these generalized approaches was evaluated in the context of ocean color remote sensing observation emphasizing the limits of S(275-295)-based formulations and the potential for a(cdom)-based approaches to represent a compelling alternative for assessing synoptic DOC distribution.
Highlights
Dissolved organic carbon (DOC) represents the largest reservoir of organic carbon in the ocean [1,2,3,4]
Samples collected in Vietnam and French Guiana coastal regions globally illustrate the range of variability between freshwaters and marine end-members while measurements performed in the estuarine and coastal waters of the eastern English Channel are mostly representative of brackish and marine waters with salinity values higher than 18 psu (Table 2)
This study aimed at characterizing variations in the acdom-DOC relationships in three contrasted coastal sites: the eastern English Channel, the Tonkin Gulf and the French Guiana coastal margin (78% of the data set is directly under the influence of river inputs, the remaining part being more under oceanic influence)
Summary
Dissolved organic carbon (DOC) represents the largest reservoir of organic carbon in the ocean [1,2,3,4]. Considering the large DOC inputs from terrestrial ecosystems and marine production as well as the important fluxes occurring in coastal waters through carbon mineralization or export processes, the understanding of DOC dynamics in continental margins represents a key issue for better constraining the actual role of these ecosystems in the global carbon cycling [5,6,7]. This represents an important challenge considering the wide spatial and temporal variability of DOC loads in these ecosystems where numerous and complex physical and biogeochemical factors regulate the interaction between the diverse source or sink processes controlling this carbon reservoir. The presence of significant linear CDOM-DOC relationships have been documented in various estuarine and coastal domains in the recent years [8,9,10,11,12,13,14,15,16,17,18]
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