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Exploring the relationship between colored dissolved organic matter and dissolved organic carbon at the Lower Amazon River

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Novel studies relate the colored dissolved organic matter (CDOM) to dissolved organic carbon (DOC) in coastal areas and emphasizes their high regional and seasonal dependency. The aim of this study is to explore the potential use of CDOM absorption coefficient (acdom) to estimate DOC concentration at the Lower Amazon River region. The study area gathers contrasted water types as very turbid water of the Amazon River and clear waters of the Tapajos, Xingu and Paru rivers. Surface water samples were taken at 45 stations for CDOM absorption as well for DOC concentration measurements, during high, low and falling water seasons. The data were fitted by linear regression and the results demonstrate that a direct relationship between acdom and DOC is not possible in this study area (N= 45; R2=0.24; RMSE = 55.95), but an alternative approach that relates the variation in the acdom(412) to DOC ratio (a*cdom (412)) had a good assessment (N=45, R2=0.65, RMSE=0.0024). We conclude that acdom represents a valuable proxy to retrieve DOC concentration in the study area. It is clear that seasonal discharges have a great influence on the results as well as the water color of the rivers. Considering that acdom can be estimated by orbital sensors, the possibility of exploring CDOM-DOC relationships empowers a synoptic monitoring of aquatic environments through remote sensing observations, expanding our comprehension of the global carbon cycle at the Lower Amazon River region.