Molecular Transformations of Dissolved Organic Matter in the Lower Amazon River

Vast amounts of carbon are processed in the Amazonian ecosystem with the Amazon River plume transporting substantial amounts of terrigenous dissolved organic matter (DOM) to the tropical North Atlantic Ocean. Yet, the DOM composition in the river and its connection to processes, especially in the highly dynamic and productive reaches close to the river mouth, are still not well understood. Using ultrahigh-resolution mass spectrometry, we show that a large pool of riverine DOM in the Amazon mainstem consists of similar molecular formulae suggesting that DOM is continuously being added by the input of degraded terrestrial organic matter during its 800 km journey from Óbidos to the river mouth. However, depending on hydrological conditions, a part of the riverine DOM changes, probably due to variable inputs of organic compounds from flood plains and tributaries resulting in a significantly different DOM composition close to the river mouth at Macapá compared to the upstream station at Óbidos. In addition, a considerable pool of riverine DOM compounds is transformed by photo- and bio-degradation. Our data suggest that seasonal changes in the hydrology affect the molecular DOM composition in the Amazon River and the quality of DOM that is being transported to the coastal ocean.