On the Role of the Amazon River for N₂ Fixation in the Western Tropical Atlantic

Abstract

The high rates of N2 fixation observed in the Western Tropical Atlantic Ocean are powered, at least in parts, by large influx of nutrients from the Amazon River. To disentangle the impact of the Amazon on different factors controlling N2 fixation in the region, we use a high‐resolution regional model (Regional Oceanic Modeling System‐Biogeochemical Elemental Cycling model) that includes two diazotrophic phytoplankton classes (Trichodesmium and Diatom‐Diazotroph‐Assemblages, DDAs). In our simulations, the Amazon enhances marine N2 fixation by 74% (3.8 Tg N yr−1), with most of the enhancement driven by DDAs that represent 90% of the diazotrophic community in plume waters. To determine how the Amazon creates the conditions in which DDAs thrive, we analyze bottom‐up and top‐down controls on phytoplankton along the plume pathway and how the phytoplankton competition is disrupted by the delivery of nutrients by the river. Along the entire plume pathway, DDAs out‐compete Trichodesmium as their higher maximum growth makes them more efficient in building biomass. Nevertheless, DDAs never account for more than 25% of the total phytoplankton biomass as they face the competition of other non‐diazotrophic phytoplankton types in the use of the riverine nutrients. Overall, DDAs thrive in the offshore plume region where their nutrient uptake advantages (N‐fixation, DOP uptake) is concomitant with a relaxed grazing pressure.

Interannual Variability of Marine Nitrogen Fixation in the Western Tropical Atlantic

“…Louchard et al. (2023) demonstrated that the N 2 fixation rate in the WTA is primarily controlled by the supply of P (DOP and $${{\text{PO}}_{4}}^{3-}$$) and Si(OH) 4 altering the growth of diazotrophs, but with grazing modulating the net fixation substantially. Regarding these bottom‐up controls, our simulations show a similar result (Figure 9), especially in the northern plume region, where discharge‐attributed N 2 fixation rates vary the most.…”

“…The implementation of several more refined zooplankton groups, similar to the implementation of Stukel et al (2014), might provide more insight into the role of grazing pressure on interannual timescales. Nonetheless, using a very similar model setup as used in our study, Louchard et al (2023) have shown that grazing is essential to create a competitive advantage for diazotrophs in the mean state. Hence, the lack of top-down control on interannual timescales could be rather a result of zooplankton not being directly impacted by discharge and climate variability and/or changes in phytoplankton biomass being too small to induce strong changes in predation.…”

“…In contrast to Subramaniam et al (2008) and Louchard et al (2021), who have shown that the Si(OH) 4 input of the Amazon River supports the growth of DDAs, we do not find variations in DDA growth associated with variations in Si(OH) 4 , because our modeled DDAs are hardly limited by Si(OH) 4 (Figure 9). This difference is striking, as we used the same parameters as the climatological study of Louchard et al (2023). Thus, this difference indicates that the response to interannual variations can deviate from the zeroth-order assumption, that is, that interannual variations can be understood as modulations of the processes that govern the mean state.…”

“…The model simulates the entire Atlantic Ocean (Figure 1), but by shifting the pole of the grid over the South American continent close to the mouth of the Amazon, it resolves the WTA at an eddy-resolving resolution of a few kilometers. Since the model has already been extensively described and evaluated by Louchard et al (2021Louchard et al ( , 2023, we highlight here only the features of the model that are relevant for the present study.…”

“…For example, being generally slow‐growing organisms with high demands for P and iron (Fe), higher levels of mixing that supply higher levels of nutrients from below to the surface of the ocean tend to put diazotrophic organisms at a strong disadvantage relative to other phytoplankton, thus lowering N 2 fixation rates. These bottom‐up‐driven variations may be strongly modified by top‐down‐driven variations, namely, variations in grazing pressure (Louchard et al., 2023; Stukel et al., 2014). For example, high grazing pressure has been shown to prevent diazotrophs from dominating the phytoplankton community close to the outflow of the Amazon River, but further along the plume pathway, the relative grazing pressure on diazotrophs gradually decreases, allowing them to become more abundant (Louchard et al., 2023; Stukel et al., 2014).…”