We examined ice-nucleating particles (INPs) in the plumes of the Tocantins and Amazon rivers, which drain watersheds with different proportions of degraded land. The concentration of INPs active at −15°C (INP −15 ) was an order of magnitude lower in the Tocantins (mean = 13.2 ml −1 ; s.d. = 7.8 ml −1 ), draining the more degraded watershed, compared with the Amazon (mean = 175.8 ml −1 ; s.d. = 11.2 ml −1 ), where the concentration was also significantly higher than in Atlantic surface waters (mean = 3.2 ml −1 ; s.d. = 2.3 ml −1 ). Differences in heat tolerance suggest that INPs emitted by the Amazon rainforest to the atmosphere or washed into the river might originate from contrasting sources on top of and below the rainforest canopy, respectively. For the Amazon River, we estimate a daily discharge of 10 18 INP −15 to Atlantic waters. Rivers in cooler climate zones tend to have much higher concentrations of INPs and could, despite a smaller water volume discharged, transfer even larger absolute numbers of INP −15 to shelf waters than does the Amazon. To what extent these terrestrial INPs become aerosolized by breaking waves and bubble-bursting remains an open question.

Large river plumes such as the Amazon River Plume are known as regions of intense biological, chemical, and physical forcing affecting the delivery and speciation of nutrients. On the shelf, sediments may act as a net source or sink of dissolved inorganic nitrogen (DIN) for the water column depending on the preservation of organic matter (OM) and its remineralization pathways in benthic environments. Previous studies on sediments of the Amazon shelf suggested that the seabed mainly acted as a sink of DIN, with denitrification as the main loss process. However, the Amazon Basin faced drastic changes over the past decades, with climate change, deforestation, and damming potentially impacting the DIN and OM loads to the estuary. These perturbations might have led to changes in benthic remineralization, calling for updates of previously studied benthic remineralization processes on the Amazon shelf. Our analysis of eight short cores confirmed the occurrence of intense recycling of mostly terrestrial material (> 60%) in the river mouth, and little organic carbon was remaining in shelf sediments. We measured a sediment oxygen demand of ~ 23 mmol m−2 d−1, which can explain the ammonification and nitrification of all the organic nitrogen (N) reaching the sediment, underlining the importance of the seabed as a sink for organic N species. Therefore, in spite of the changes in the Amazon Basin, nitrification still seems to be balanced by denitrification, as, similarly to previous results, no DIN diffused out of the sediment.