Long-Term and Seasonal Drivers of Organic Matter in the Clearwater Tapajós River and Implications for the Amazon River Basin

The Amazon River exports over 10% of the global riverine dissolved organic carbon (DOC) flux to the ocean. However, several downstream clearwater tributaries, such as the Tapajós River, are typically not included in these measurements, omitting a crucial part of the Amazon carbon cycle. This study investigated near-monthly DOC and dissolved organic matter (DOM) composition via optical, fluorescence spectroscopy, and ultra-high resolution mass spectrometry (FT-ICR MS) of the Tapajós River for 8 years (2016–2024) to better understand patterns and drivers of potential organic carbon export to the lower Amazon River. DOM composition and DOC export were driven by the seasonal flood pulse of the Tapajós River, exporting aromatic terrestrial DOM from the watershed during high discharge and internally produced algal or microbial DOM during dry periods. On average, we report that the Tapajós River exports 1.38 Tg DOC annually to the downstream Amazon mixing zone, representing an amount of DOC exported by other major world rivers such as the Yukon or Mekong River. Furthermore, organic carbon export varied interannually with less DOC exported during dry El Niño events and more algal-derived DOM exported during bloom periods. Finally, as grassland and cropland landcover increased over the study period, we observed an average decrease in aromatic DOM and an increase in microbially processed fluorophores. Our study suggests that temperature, precipitation, and anthropogenic land use changes in clearwater rivers will impact carbon export across the lower Amazon River network.