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.