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The nutrient-rich waters of the Amazon River plume (ARP) support dense bloomsof diatom-diazotroph assemblages (DDAs) that introduce large quantities ofnew nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers,are likely to directly fuel vertical carbon flux. To investigate the factorscontrolling DDA blooms, we develop a five phytoplankton (cyanobacteria,diatoms, unicellular microbial diazotrophs, DDAs, and<i>Trichodesmium</i>), two zooplankton model and embed it within a1/6° resolution physical model of the tropical and subtropicalAtlantic. The model generates realistic DDA blooms in the ARP and alsoexhibits basin-wide primary production, nitrogen fixation, and grazing ratesconsistent with observed values. By following ARP water parcels withsynthetic Lagrangian drifters released at the river mouth we are able toassess the relative impacts of grazing, nutrient supply, and physical forcingon DDA bloom formation. DDA bloom formation is stimulated in thenitrogen-poor and silica-rich water of the ARP by decreases in grazingpressure when mesozooplankton (which co-occur in high densities with coastaldiatom blooms) concentrations decrease. Bloom termination is driven primarilyby silica limitation of the DDAs. In agreement with in situ data, this netgrowth niche for DDAs exists in a salinity range from ∼20–34 PSU,although this co-occurrence is coincidental rather than causative. Becausenet growth rates are relatively modest, bloom formation in ARP water parcelsdepends critically on the time spent in this ideal habitat, with high DDAbiomass only occurring when water parcels spent >23 days in the optimalhabitat niche

Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River plume