Arrhythmia in the Amazon: La Niña influence on riverine organic molecular signatures

Abstract As climate-driven El Niño Southern Oscillation (ENSO) events are projected to increase in frequency and severity, much attention has focused on impacts regarding ecosystem productivity and carbon balance in Amazonian rainforests, with little attention given to carbon dynamics in fluvial ecosystems. We compared the wet 2012 La Niña period to the following normal hydrologic period in the Amazon River. Elevated water flux during the La Niña was accompanied by dilution of inorganic ion concentrations. Furthermore, the La Niña period exported 2.77 TgCyr-1 more dissolved organic carbon (DOC) than the normal period, an increase greater than annual Mississippi River DOC export. Using ultra-high-resolution mass spectrometry, we detected both seasonal and inter-annual variations in dissolved organic matter (DOM) compositions, revealing that DOM exported during the dry season and the normal period was more aliphatic, whereas compounds in the wet season and following the La Niña event were more aromatic, with ramifications for its environmental role. Furthermore, compounds were highly correlated to a 6-month lag in Pacific temperature and pressure anomalies, demonstrating that ENSO events impact DOM compositions exported to the Atlantic Ocean.