This study quantified the bioavailable nitrogen contribution from riverine plumes to Great Barrier Reef (GBR) coastal environments. The potential bioavailable nitrogen from two Dry Tropics riverine plumes was considerable [9 - 30% added to the end-of-catchment dissolved inorganic nitrogen (DIN) load]. Particulate inorganic nitrogen conversion to DIN was an important process in short timeframes (25% to 100% of the generated load). The remaining load was contributed by microbial mineralisation of organic nitrogen. Flood plume sediment has potential to generate nitrogen once deposited and/or resuspended. Nitrogen generation was insignificant in a few plumes where immobilisation of nitrogen in bacteria biomass occurred. The source of organic matter in the plumes and availability of nitrogen relative to organic matter were important determinants of mineralisation/immobilisation. This research demonstrates that riverine plumes have potential to be considerable sources of bioavailable nitrogen to coastal environments of the GBR and that organic matter is a key bioavailability driver.