Form Submission: Participation Entry

In aquatic and coastal ecosystems, dissolved organic matter (DOM) serves diverse, fundamental roles. Riverine DOM exported to coastal waters comes from two major sources – either produced in situ (autochthonous) through primary production, or delivered from terrestrial runoff (allochthonous) by streamflow – each behaving distinctively. In large river networks, which DOM source dominates is often determined by water residence time, reflecting how much time is available for biogeochemical processing to occur before DOM reaches the ocean. Emerging optical methods, such as UV-vis spectroscopy and fluorescence excitation-emission matrices (EEMs), offer powerful tools to reveal DOM composition, sources, and properties. However, a knowledge gap exists between DOM hydrology and biogeochemistry, partly due to the lack of accurate metrics to characterize water residence time in large, heterogeneous watersheds. Through a 7-year water quality monitoring effort under the USGS National Stream Quality Accounting Network, we identified the sources of DOM from 22 major river basins across North America by UV-vis spectroscopy absorbance and EEMs florescence index of colored DOM. Meanwhile, we constructed proxies for water residence time in watersheds from stable isotopic tracers δ18O and δ2H in streamflow. Both absorbance and fluorescence analyses indicated that DOM from watersheds of shorter water residence time is more likely to preserve terrestrial signatures due to less biogeochemical processing. As climate change leads to more intense precipitation events that effectively shorten water residence time in river basins, more terrestrial DOM could end up in our rivers and eventually oceans. This study is the very first to synoptically characterize DOM sources by water residence time on the whole-watershed scale across diverse biomes and landscapes. Since water residence time profoundly controls ecosystem dynamics, our work could also be translated to study other important processes, from nutrient cycling to pollutant transport, thus greatly advancing aquatic science and management.