Using CDOM optical properties for estimating DOC concentrations and pCO2 in the Lower Amazon River

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 (pCO(2)). 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 pCO(2), and assess the potential of CDOM absorption coefficient (a(CDOM)(412)) for estimating DOC concentration and pCO(2) 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, R-2 = 0.74, p<0.05; CW: N = 13, R-2 = 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, R-2 = 0.58, p<0.05). A strong linear positive relation was found between a(CDOM)(412) and pCO(2)(N = 69, R-2 = 0.65, p<0.05) along the lower river. pCO(2) 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 pCO(2) (N = 69; R-2 = 0.80, p<0.05). The ability to assess both DOC and pCO(2) 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. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement