Abstract
Coloured dissolved organic matter (CDOM) is one of the major contributors to the absorption budget of most freshwaters and can be used as a proxy to assess non-optical carbon fractions such as dissolved organic carbon (DOC) and the partial pressure of carbon dioxide (pCO2). Nevertheless, riverine studies that explore the former relationships are still relatively scarce, especially within tropical regions. Here we document the spatial-seasonal variability of CDOM, DOC and pCO2, and assess the potential of CDOM absorption coefficient (aCDOM(412)) for estimating DOC concentration and pCO2 along the Lower Amazon River. Our results revealed differences in the dissolved organic matter (DOM) quality between clearwater (CW) tributaries and the Amazon River mainstream. A linear relationship between DOC and CDOM was observed when tributaries and mainstream are evaluated separately (Amazon River: N = 42, R2 = 0.74, p<0.05; CW: N = 13, R2 = 0.57, p<0.05). However, this linear relationship was not observed during periods of higher rainfall and river discharge, requiring a specific model for these time periods to be developed (N = 25, R2 = 0.58, p<0.05). A strong linear positive relation was found between aCDOM(412) and pCO2(N = 69, R2 = 0.65, p<0.05) along the lower river. pCO2 was less affected by the optical difference between tributaries and mainstream waters or by the discharge conditions when compared to CDOM to DOC relationships. Including the river water temperature in the model improves our ability to estimate pCO2 (N = 69; R2 = 0.80, p<0.05). The ability to assess both DOC and pCO2 from CDOM optical properties opens further perspectives on the use of ocean colour remote sensing data for monitoring carbon dynamics in large running water systems worldwide.
Highlights
Terrestrial humic substances are the dominant contributor to the dissolved organic matter pool (DOM) in freshwaters [1]
The higher average S275-295 and SR for the CW samples (Table 2) tend to indicate the presence of DOM of lower molecular weight than for the Amazon samples. Such differences might be attributed to variation in the DOM origin as larger contribution of the phytoplankton derived DOM in CW [8] as well as variation in the intensity of the photo-degradation processes. These results suggest that the estimation of Dissolved organic carbon (DOC) from coloured dissolved organic matter (CDOM) absorption coefficient using a simple linear model might be possible taking into account specific information about the optical water type, and biogeochemical quality (Fig. 3(b), (c))
Our results further indicate that a unique S275-295 vs a*CDOM(412) model is not sufficient to take into account the occurrence of very specific conditions such as the ones identified for the samples collected during T4 and T6, for which a specific model is required (Fig. 4(a))
Summary
Terrestrial humic substances are the dominant contributor to the dissolved organic matter pool (DOM) in freshwaters [1]. The fraction of DOM that absorbs ultraviolet (UV) and visible light, coloured dissolved organic matter (CDOM), is one of the major contributors to the absorption budget of most freshwaters. The importance of freshwater systems on controlling the export and transformation of terrestrial DOM and the intimate relationship between these dynamics and the partial pressure of carbon dioxide (pCO2) is well recognized [3,7,8,9]. The dynamics of CDOM, DOC and pCO2 in the Amazon River are largely influenced by the seasonal changes in the river discharge [8,9,12,13,14,15]. Several recent studies have emphasized trends towards the occurrence of extreme climatic events in some regions of the Amazon basin, illustrating the potential impact of such events on rain and river discharge patterns in the area [18,19,20,21,22]
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