| Abstrakt: |
The Amazon River plume (ARP) has been shown to support high rates of nitrogen fixation and primary production. However, nitrogen fixation alone cannot account for total primary production determined in the region, hinting that other nitrogen uptake processes might play a role. For the first time, we measured nitrate uptake rates in the ARP during three cruises in May 2018, June 2019 and April/May 2021, along with primary production rates and an analysis of phytoplankton community composition via high performance liquid chromatography. Based on a classification according to the salt content the region was divided into estuarine (ES), mesohaline (MH) and oceanic (OC) stations. Primary production was light limited near the river mouth at ES stations and was maximal off the coasts of French Guiana and Suriname, where also nitrate uptake was highest with rates of 11.4 mmol m−2 d−1. The role of eddies pinching off a deflecting plume are discussed as possible reason for higher nutrient concentrations at the MH stations. Surprisingly, at most MH stations north of 5°N, nitrate uptake rates were low despite the presence of sufficient substrate concentration (up to 1.44 μM nitrate). Diatoms, dinoflagellates or Synechococcus sp. dominated phytoplankton communities. OC stations showed lowest productivity rates in accordance with oligotrophic conditions. However, rates seem to be sufficient to completely deplete the remaining riverine nitrate, preventing its export to the open ocean. Plain Language Summary: The Amazon River is thought to deliver important nutrients to the Western Tropical North Atlantic, which enable high phytoplankton growth and especially favor nitrogen fixers. Previous studies have focused on nitrogen fixation, but we propose that other nitrogen sources could also be important in the river plume to support the high phytoplankton growth there. In this study, we measured the uptake of nitrate by phytoplankton in the plume for the first time. We found that nitrate uptake is of variable importance for phytoplankton growth in the Amazon River plume; it seems to be more important close to the river mouth, where diatoms and dinoflagellates dominate, compared to the northern plume, where nitrogen fixers are dominant. Investigations like this will help us to better understand, which nitrogen sources are important for the growth of different phytoplankton species in the plume. With this information, we can improve predictions of competition of phytoplankton species for nitrogen and their general growth in the future. Some phytoplankton groups have a higher potential to contribute to oceanic CO2 storage than others, so understanding the future of these phytoplankton groups is critical to any assessment of the amount of CO2 that can be stored by the ocean. Key Points: Nitrate uptake rates were measured in the Amazon River plume for the first time to resolve its role for overall productivity in the plumeNitrate uptake displayed a pronounced gradient and was not only regulated by concentrationsNitrate from the Amazon River is completely consumed on the shelf and not exported to the open ocean [ABSTRACT FROM AUTHOR] |
Plný text