We present field data documenting near-surface chlorophyll a enhancement associated with the Orinoco River outflow and experimental evidence identifying photomineralization of riverine fluorescent dissolved organic matter (DOM) as a potential fertilization mechanism capable of sustaining phytoplankton biomass up to several hundred kilometers from the river delta. Low-salinity waters from the Gulf of Paria in the Orinoco River estuary with high content of fluorescent DOM were observed to decrease their fluorescence yield while releasing ammonium upon experimental exposure to natural sunlight. Our field data collected during two cruises across the eastern Caribbean Sea evidence the disappearance of DOM fluorescence beyond that expected from dilution, thus indicating that during the seasonal extension of the Orinoco River plume throughout the eastern Caribbean Sea, ammonium release from DOM constitutes a major source of inorganic nitrogen to phytoplankton communities in the river plume. We propose that this mechanism, along with nitrogen retention through recycling, explains enhanced surface phytoplankton biomass of up to 1000 km from the river delta. ¿ 2001 American Geophysical Union