Abstract
Abstract. The nutrient-rich waters of the Amazon River plume (ARP) support dense blooms of diatom-diazotroph assemblages (DDAs) that introduce large quantities of new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers, are likely to directly fuel vertical carbon flux. To investigate the factors controlling DDA blooms, we develop a five phytoplankton (cyanobacteria, diatoms, unicellular microbial diazotrophs, DDAs, and Trichodesmium), two zooplankton model and embed it within a 1/6° resolution physical model of the tropical and subtropical Atlantic. The model generates realistic DDA blooms in the ARP and also exhibits basin-wide primary production, nitrogen fixation, and grazing rates consistent with observed values. By following ARP water parcels with synthetic Lagrangian drifters released at the river mouth we are able to assess the relative impacts of grazing, nutrient supply, and physical forcing on DDA bloom formation. DDA bloom formation is stimulated in the nitrogen-poor and silica-rich water of the ARP by decreases in grazing pressure when mesozooplankton (which co-occur in high densities with coastal diatom blooms) concentrations decrease. Bloom termination is driven primarily by silica limitation of the DDAs. In agreement with in situ data, this net growth niche for DDAs exists in a salinity range from ∼20–34 PSU, although this co-occurrence is coincidental rather than causative. Because net growth rates are relatively modest, bloom formation in ARP water parcels depends critically on the time spent in this ideal habitat, with high DDA biomass only occurring when water parcels spent >23 days in the optimal habitat niche.
Highlights
The Amazon River discharges 1.93 × 105 m3 s−1 (Perry et al, 1996) of nutrient-rich freshwater into the western tropical North Atlantic
Our model predicted an annual mean surface value of 2.8 nmol N L−1 d−1 in the MUS, and a weak latitudinal trend. It appears that our model may underestimate the mean spatial variability in Nfix, this may reflect high seasonal variability rectified in the model mean
The similar sensitivity of the model to phytoplankton growth and zooplankton grazing parameters supports our conclusion that both bottom-up and top-down forcing play a role in diatom-diazotroph assemblages (DDAs) blooms. While it did not affect DDA bloom formation, while tuning the model we discovered that the co-existence of Cya and unicellular microbial diazotrophs (UMDs) was extremely sensitive to θS
Summary
The Amazon River discharges 1.93 × 105 m3 s−1 (Perry et al, 1996) of nutrient-rich freshwater into the western tropical North Atlantic. This low-salinity water forms a surface plume of up to 1.2 × 106 km that can extend greater than 3000 km from the river mouth (Hu et al, 2004) and supports a successional series of phytoplankton communities within the gradient from rich coastal waters to oligotrophic offshore regions where the plume dissipates into the tropical and subtropical Atlantic. Subramaniam et al (2008) defined three regions within the plume (oligohaline, mesohaline, and oceanic) each supporting distinct phytoplankton assemblages. In the oceanic regions, the DDAs were replaced by a typical oceanic phytoplankton distribution with Trichodesmium dominating the N2-fixing assemblage and significantly reduced Nfix rates. Subramaniam et al (2008) attribute this successional cycle to a series of bottom-up controls of the phytoplankton community
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
