Natural organic matter (NOM) plays an important role in elemental cycles and ecology. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is an ultrahigh resolution technique used to molecularly resolve the complexity of NOM mixtures. However, even the very high mass resolution of FT-ICR-MS may result in multiple formula assignments to peaks in an NOM spectrum, especially at the high mass-to-charge ratio (m/z). The absorption mode is one option to process raw FT-ICR-MS data that can further increase the resolution of the peaks and has not been widely applied in NOM studies. In this study, we show the advantages of using the absorption mode for the analysis of NOM samples using a reference sample (Suwannee River fulvic acid). The absorption mode increased the precision of peak detection as well as the number (+23%) and accuracy of formula assignment (by 28%) when compared to the magnitude mode, besides achieving three times higher resolution. The results presented here highlight the potential to reduce the error threshold used during molecular formula assignment. In conclusion, the absorption mode shows advantages in the processing of NOM samples and other complex mixtures and should be promoted in the NOM community.

Abstract. Given the importance of dissolved organic matter (DOM) in the carbon cycling of aquatic ecosystems, information on its seasonal variability is crucial. In this study we assess the use of optical absorption indices available in the literature based on in situ data to both characterize the seasonal variability of DOM in a highly complex environment and for application in large-scale studies using remote sensing data. The study area comprises four lakes located in the Mamirauá Sustainable Development Reserve (MSDR). Samples for the determination of colored dissolved organic matter (CDOM) and measurements of remote sensing reflectance (Rrs) were acquired in situ. The Rrs was used to simulate the response of the visible bands of the Sentinel-2 MultiSpectral Instrument (MSI), which was used in the proposed models. Differences between lakes were tested using the CDOM indices. The results highlight the role of the flood pulse in the DOM dynamics at the floodplain lakes. The validation results show that the use of the absorption coefficient of CDOM (aCDOM) as a proxy of the spectral slope between 275 and 295 nm (S275–295) during rising water is worthwhile, demonstrating its potential application to Sentinel-2 MSI imagery data for studying DOM dynamics on the large scale.

 High-frequency monitoring of water absorbance and high-resolution chemical analysis are highly complementary in the study of DOM dynamic. Ultrahigh-resolution mass spectrometry revealed linkage between dissolved organic matter molecular composition in-stream and soil. At comparatively higher DOC concentration at baseflow, DOM composition reflects the contribution of more superficial layers of the soil.

<p><strong>Abstract.</strong> Given the importance of DOM in the carbon cycling of aquatic ecosystems, information on its seasonal variability is crucial. This study assesses the use of available absorption optical indices based on in situ data to both characterize the seasonal variability of the DOM dynamics in a highly complex environment and their viability of being used for satellite remote sensing on large scale studies. The study area comprises four lakes located at the Mamirauá Sustainable Development Reserve (MSDR). Samples for the determination of coloured dissolved organic matter (CDOM) and remote sensing reflectance (Rrs) were acquired in situ. The Rrs was applied to simulate MSI visible bands and used in the proposed models. Differences between lakes were tested regarding CDOM indices. Significant difference in the average of a<sub>CDOM</sub> (440), a<sub>CDOM</sub> spectra and S<sub>275–295</sub> were found between lakes located inside the flood forest and those near the river bank. The proposed model showed that a<sub>CDOM</sub> can be used as proxy of S<sub>275–295</sub> during rising water with good validation results, demonstrating the potential of Sentinel/MSI imagery data in large scale studies on the dynamics of DOM.</p>

Despite freshwater systems covering less than 3% of the Earth's surface, carbon fluxes from terrestrial into freshwater systems play an important role in the global carbon cycle (Battin et al., 2009;Cole et al., 2007;Raymond et al., 2013). A recent study estimated that 5.1 Pg terrestrial carbon are transported into inland waters every year (Drake et al., 2018). However, there are still many uncertainties regarding the export of carbon from terrestrial systems, which includes particular organic carbon, dissolved inorganic carbon and dissolved organic carbon (DOC). DOC is an important component of dissolved organic matter (DOM), which is a complex mixture of heterogeneous compounds. As carbon is the main element of DOM, DOC is often used as a proxy for DOM