Using CDOM spectral shape information to improve the estimation of DOC concentration in inland waters: A case study of Andean Patagonian Lakes.

For the last two decades different scientific disciplines have focused on lacustrine dissolved organic matter (DOM) given its importance in the biogeochemistry of carbon and in ecosystem functioning. New satellites supply the appropriate resolutions to evaluate chromophoric dissolved organic matter (CDOM) in inland waters, opening the possibility to estimate DOM at appropriate spatiotemporal scales. This requires, however, a robust relationship between CDOM and dissolved organic carbon (DOC). In this work, we evaluated the use of CDOM as a proxy of DOC in 7 Andean Patagonian lakes. Considering the entire data set, CDOM absorption coefficients (a355 and a440) were linearly related with DOC. Shallow lakes, however, drove this relationship showing a moderate relationship, whereas, deep lakes with lower colour presented a weaker relationship. Therefore, we assessed the use of CDOM spectral shape information to improve DOC estimates regardless of observed DOM differences due to climatic seasonality and lakes' morphometry. The use of well-known CDOM spectral shape metrics (i.e., S275-295 and a250:a365 ratio) significantly improved DOC estimation. Particularly, using a Gaussian decomposition approach we found that much of the variation in the spectral shape, associated with the variability of CDOM:DOC ratio, was explained by differences in two dynamic regions centred at 270 and 320 nm. A strong nonlinear relationship was found between the a270:a320 ratio and the DOC-specific absorption coefficients a*355 and a*440. This was translated into a further improvement in DOC estimation yielding the higher R2 and lower mean absolute differences (MAPD < 16%), either considering the entire data set or shallow and deep lakes separately. Our results highlight that incorporating the CDOM spectral shape information improves the characterization of the DOC pool of inland waters, which is particularly relevant for remote and/or inaccessible sites and has significant implications for the environmental management, biogeochemical studies and future remote sensing applications.

Dynamics of the dissolved organic matter in a stream-lake system within an extremely acid to neutral pH range: Agrio-Caviahue watershed.

The Agrio-Caviahue is a natural stream-lake system composed of the Upper Agrio River (UAR) -Lake Caviahue-Lower Agrio River (LAR). The system has a broad pH gradient from extremely acid in the hot spring to almost neutral pH only 60 km downstream the source, and varies as a function of the distance. The aim of this study was to analyze the dynamics of the dissolved organic matter (DOM) of this extreme system. For that matter, an absorbance and fluorescence-based characterization was performed on natural water samples and hydrophobic resin DOM extracts during different seasons in nine sampling stations. Between August and December, the hydrological connectivity is maximum due to precipitation and snowmelt, respectively. During these months, the stream that exits the lake governs the chemical characteristics downstream mainly in the period of high connectivity. In contrast, in the dry season when the flow of LAR is minimum, the two major affluents of this river influence its pH and DOM features, and deliver inputs of humified DOM from the wetlands where the tributaries are born. DOM was characterized by having low absorbance. The PARAFAC analysis of the fluorescent DOM (FDOM) validated three fluorescent components in the natural samples (with humic and non-humic features), two of which were also recorded in the extracts, meaning that no additional hydrophobic components were missed under the limit of detection of the fluorometer. The unique features of Agrio-Caviahue DOM resemble those found in DOM from Yellowstone hot springs, both acid and alkaline.

Fluorescence components of natural dissolved organic matter (DOM) from aquatic systems of an Andean Patagonian catchment: Applying different data restriction criteria for PARAFAC modelling.

Fluorescence spectroscopy is a widely applied technique to characterize the composition of the fluorescent fraction of dissolved organic matter (DOM), allowing to infer sources and diagenetic state of soil, marine and freshwater DOM samples. The analysis of fluorescent DOM (FDOM) is often carried out by multi-way models such as parallel factor analysis (PARAFAC), which allows decomposing excitation-emission matrices (EEMs) obtained from DOM samples into their underlying chemical components. Some aspects of the performance/accuracy of the EEM-PARAFAC technique regarding the use of selective vs. non-selective EEM data are still in discussion. In this investigation, we evaluated the outcome of two different approaches (non-selective and selective) applied to study the composition of DOM from four headwater streams (Case 1) and from two neighboring shallow lakes (Case 2), all belonging to the same Andean watershed within Nahuel Huapi National Park in North Patagonia (Argentina). In both cases, the outcome of the PARAFAC performed to non-selective data (EEM datasets from all the streams and the two lakes) vs. selective datasets (EEMs from each stream and each lake treated separately) is compared on the basis of modelled fluorescent components. Regardless of the restriction criteria applied for the analyses, the results obtained indicated similar component loadings in the four streams and in the two lakes. The similarity of the outcomes likely relates to the low internal variation of the EEMs, since these are located in the same catchment, influenced by similar soils and vegetation which are the main DOM sources. Therefore, we conclude that the use of a small selective EEM dataset may not condition the validation of the FDOM components and their temporal dynamics.