Unraveling dissolved organic matter sources and their link to land use along the Rapel River Continuum, Chile

Inland aquatic ecosystems play a crucial role in the carbon cycle, acting as an interface for carbon exchange between the atmosphere, terrestrial ecosystems, and the oceans. Due to their importance, dissolved organic matter (DOM) dynamics in continental water bodies have been widely studied. However, most studies are limited to specific sections of river basins, such as headwaters or estuaries, leaving a significant gap in understanding continuous fluvial systems encompassing the entire watershed, particularly regarding the behavior of DOM at the basin scale. To address this gap, we investigated the DOM dynamics within a watershed of 14,000 km² with diverse geomorphological features, following its entire course from the Andes to its only outlet into the Pacific Ocean. This watershed is highly diverse, combining high mountain areas impacted by mining activities with intensively farmed agricultural zones, livestock production in the central region, residential areas, and various recreational activities. The study aims to analyze variations in DOM characteristics along a fluvial continuum and their relationship with land use in different basin sections. A total of 25 sampling points were selected across the basin, including locations within the three sub-basins and the most significant tributaries. At each station, water physicochemical properties were measured by using a portable multiparametric probe, and water samples were collected for measurements of dissolved organic carbon (DOC) concentration, DOM optical properties (fluorescence and absorbance spectroscopy), isotopic analyses, as well as metalloids. A portable sensor was also used to measure nitrate concentrations directly on site. The results allowed us to (i) identify the sources of DOM, (ii) characterize DOM dynamics along the continuous river, and (iii) establish the relationship between DOM sources and different land use types across the basin's sections. This study provides the first regional-scale investigation of DOM dynamics along a river continuum in Chile and offers valuable insights into DOM responses across such systems, raising questions about existing theories of the river continuum concept. Finally, this study represents the first step of a more comprehensive and multidisciplinary study that will also cover seasonality and interannual variability of DOM dynamics and aquatic microbial community diversity in this region.