Amazon Plume Research Articles

Role of the Tide on the Structure of the Amazon Plume: A Numerical Modeling Approach

AbstractThe dynamical balance on the Amazon shelf and its implication on the properties of the Amazon River plume is not fully understood and poorly represented in global‐ and basin‐scale ocean models. In this study, the sensitivity of the Amazon shelf dynamics to tidal forcing is explored with a set of high‐resolution numerical simulations (1/36°) with and without the tide. A comparison of the simulations with sea surface salinity in situ measurements at 5°N (a location where the plume seasonally detaches from the coast and retroflects toward the east) revealed that the explicit resolution of the tide significantly improves the representation of the offshore spread of the river plume. This study further highlights the finding that tidal currents affect the properties of the whole Amazon plume. This sensitivity is explained by a near total collapse of the northwestward alongshore mean flow located near the river mouth, once the tidal forcing is included. This weakening of the ambient flow reduces (i) the dilution ratio between the ambient salty shelf waters and the riverine freshwaters and (ii) the constraint on the cross‐shore extension of the low‐salinity bulge. With tides, the plume is fresher near the river mouth (by up to 5 units), more extended in the cross‐shore direction, and more easily exported offshore by the North Brazil Current at the shelf break.

Water mass analysis along 22 °N in the subtropical North Atlantic for the JC150 cruise (GEOTRACES, GApr08)

This study presents a water mass analysis along the JC150 section in the subtropical North Atlantic, based on hydrographic and nutrient data, by combining an extended optimum multiparameter analysis (OMPA) with a Lagrangian particle tracking experiment (LPTE). This combination, which was proposed for the first time, aided in better constraining the OMPA end-member choice and providing information about their trajectories. It also enabled tracing the water mass origins in surface layers, which cannot be achieved with an OMPA. The surface layers were occupied by a shallow type of Eastern South Atlantic Central Water (ESACW) with traces of the Amazon plume in the west. Western North Atlantic Central Water dominates from 100 to 500 m, while the 13 °C-ESACW contribution occurs marginally deeper (500–900 m). At approximately 700 m, Antarctic Intermediate Water (AAIW) dominates the west of the Mid-Atlantic Ridge (MAR), while Mediterranean Water dominates the east with a small but non-negligible contribution down to 3500 m. Below AAIW, Upper Circumpolar Deep Water (UCDW) is observed throughout section (900–1250 m). Labrador Sea Water (LSW) is found centered at 1500 m, where the LPTE highlights an eastern LSW route from the eastern North Atlantic to the eastern subtropical Atlantic, which was not previously reported. North East Atlantic Deep Water (encompassing a contribution of Iceland-Scotland Overflow Water) is centered at ~2500 m, while North West Atlantic Bottom Water (NWABW, encompassing a contribution of Denmark Strait Overflow Water) is principally localized in the west of the MAR in the range of 3500–5000 m. NWABW is also present in significant proportions (>25%) in the east of the MAR, suggesting a crossing of the MAR possibly through the Kane fracture zone. This feature has not been investigated so far. Finally, Antarctic Bottom Water is present in deep waters throughout the section, mainly in the west of the MAR. Source waters have been characterized from GEOTRACES sections, which enables estimations of trace elements and isotope transport within water masses in the subtropical North Atlantic.

Amazon River influence on nitrogen fixation in the western tropical North Atlantic

We measured rates of N- and C-fixation with a direct tracer method in regions of the western tropical North Atlantic influenced by the Amazon River plume during the high flow period of 2010 (May–June 2010). We found distinct regional variations in N-fixation activity, with the lowest rates in the plume proper and the highest rates in the plume margins and in offshore waters. A comparison of our N- and C-fixation measurements showed that the relative contribution of N-fixation to total primary production increased from the plume core toward oceanic waters, and that most of the C-fixation in this system was supported by sources of nitrogen other than those derived from biological N-fixation, or diazotrophy. We complemented these rate experiments with measurements of the δ15N of suspended particles (δ15PN), which documented the important and often dominant role of diazotrophs in supplying nitrogen to particulate organic matter in the water column. These coupled measurements revealed that small phytoplankton contributed more new nitrogen to the particulate nitrogen pool than larger phytoplankton. We used a habitat classification method to assess the fac- tors that control diazotrophic activity and contribution to the suspended particle pool, both of which increased from the plume toward oceanic waters. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.

Manatee genomics supports a special conservation area along the Guianas coastline under the influence of the Amazon River plume

The West Indian manatee (Trichechus manatus) occurs along the Atlantic coastline and the adjacent freshwater systems of South, Central and North America, from Alagoas (Brazil) to Florida (USA) and the Greater Antilles. The Amazonian manatee (Trichechus inunguis) is the only sirenian adapted exclusively to freshwater and is endemic to the Amazon River basin. Previous studies have reported hybrids between T. inunguis and T. manatus close to the Amazon River mouth, likely composing an extensive hybrid zone under the influence of the Amazon River plume along the Guianas coast in South America. We have generated ddRAD SNP data, and sequences of nuclear and mtDNA loci to characterize the manatees’ genomic composition along the French Guiana coastline. Using analyses and simulations to examine the process of hybridization, we found this population to be formed by introgressed or later generation interspecific hybrids. We also describe the first pure T. inunguis found outside the Amazon River basin. Our results indicate that T. inunguis can survive in the Amazon River plume and have colonized independent water streams of the Guianas coastline where they likely hybridize with T. manatus. This hypothesis offers a plausible explanation for the known extension of the hybrid zone between the two species along the Guianas coastline. It also reinforces the importance of the Amazon plume, which flows westwards to the Guianas coastline and favors the dispersion of the freshwater species. The Amazon plume functions as a large estuary-like system that provides an ecological continuum from the Amazon River mouth to the disconnected waterflows of the Guianas, which deserves status as a special conservation area.

Optimum satellite remote sensing of the marine carbonate system using empirical algorithms in the global ocean, the Greater Caribbean, the Amazon Plume and the Bay of Bengal

Improving our ability to monitor ocean carbonate chemistry has become a priority as the ocean continues to absorb carbon dioxide from the atmosphere. This long-term uptake is reducing the ocean pH; a process commonly known as ocean acidification. The use of satellite Earth Observation has not yet been thoroughly explored as an option for routinely observing surface ocean carbonate chemistry, although its potential has been highlighted. We demonstrate the suitability of using empirical algorithms to calculate total alkalinity (AT) and total dissolved inorganic carbon (CT), assessing the relative performance of satellite, interpolated in situ, and climatology datasets in reproducing the wider spatial patterns of these two variables. Both AT and CTin situ data are reproducible, both regionally and globally, using salinity and temperature datasets, with satellite observed salinity from Aquarius and SMOS providing performance comparable to other datasets for the majority of case studies. Global root mean squared difference (RMSD) between in situ validation data and satellite estimates is 17 μmol kg−1 with bias < 5 μmol kg−1 for AT and 30 μmol kg−1 with bias < 10 μmol kg−1 for CT. This analysis demonstrates that satellite sensors provide a credible solution for monitoring surface synoptic scale AT and CT. It also enables the first demonstration of observation-based synoptic scale AT and CT temporal mixing in the Amazon plume for 2010–2016, complete with a robust estimation of their uncertainty.

Manatee genomics supports a special conservation area along the Guianas coastline under the influence of the Amazon River plume

The West Indian manatee (Trichechus manatus) occurs along the Atlantic coastline and the adjacent freshwater systems of South, Central and North America, from Alagoas (Brazil) to Florida (USA) and the Greater Antilles. The Amazonian manatee (Trichechus inunguis) is the only sirenian adapted exclusively to freshwater and is endemic to the Amazon River basin. Previous studies have reported hybrids between T. inunguis and T. manatus close to the Amazon River mouth, likely composing an extensive hybrid zone under the influence of the Amazon River plume along the Guianas coast in South America. We have generated ddRAD SNP data, and sequences of nuclear and mtDNA loci to characterize the manatees’ genomic composition along the French Guiana coastline. Using analyses and simulations to examine the process of hybridization, we found this population to be formed by introgressed or later generation interspecific hybrids. We also describe the first pure T. inunguis found outside the Amazon River basin. Our results indicate that T. inunguis can survive in the Amazon River plume and have colonized independent water streams of the Guianas coastline where they likely hybridize with T. manatus. This hypothesis offers a plausible explanation for the known extension of the hybrid zone between the two species along the Guianas coastline. It also reinforces the importance of the Amazon plume, which flows westwards to the Guianas coastline and favors the dispersion of the freshwater species. The Amazon plume functions as a large estuary-like system that provides an ecological continuum from the Amazon River mouth to the disconnected waterflows of the Guianas, which deserves status as a special conservation area.

Amazon River plume influence on Western Tropical Atlantic dynamic variability

This study focuses on analysing the potential impact of the Amazon and Pará Rivers on the salinity, temperature and hydrodynamics of the Western Tropical North Atlantic (WTNA) region between 60.5°–24 °W and 5 °S–16 °N. The Regional Ocean Model System (ROMS) was used to simulate ocean circulation with 0.25° horizontal resolution and 32 vertical levels. Two numerical experiments were performed considering river discharge and river input. Temperature and salinity distributions obtained numerically were compared with Simple Ocean Data Assimilation (SODA) and in situ observations from the Prediction Research Moored Array in the Tropical Atlantic (PIRATA) buoys located at 38 °W8 °N and 38 °W12 °N. Surface currents were compared with Surface Currents from Diagnostic model (SCUD). Once we verified that model results agreed with observations, scenarios with and without river discharges were compared. The difference between both simulations in the Sea Surface Temperature distribution was smaller than 2 °C, whereas the Sea Surface Salinity (SSS) changed by approximately 8 psu in the plume area close to the coast from August to December and reaching SSS differences of approximately 4 psu in the region of the North Equatorial Counter Current (NECC). The surface current velocities are stronger in the experiment with river discharge, mainly in the NECC area from September to December and close to the coast in June to August. The results show that river discharges also cause a phase shift in the zonal currents, anticipating the retroflection of the North Brazil Current by two months and enhancing eastward NECC transport, which is in agreement with observations. The Mixed Layer Depth and Isothermal Layer Depth in the presence of river discharge is 20–50 m shallower over the entire extension of the Amazon plume compared with the situation without continental inflows. As a consequence, stronger Barrier Layers develop in the river plumes, reducing the Oceanic Heat Content in the WTNA.

Delayed and Quasi‐Synchronous Response of Tropical Atlantic Surface Salinity to Rainfall

AbstractZonally elongated patterns of tropical Atlantic rainfall variability associated with the Atlantic meridional (dipole) and the El Niño‐Southern Oscillation (ENSO) modes extend over the ocean and adjacent land and lead to a complex sea surface salinity (SSS) response. SSS response to local ocean rainfall is quasi‐instantaneous, while SSS response to land rainfall is delayed through river hydrology. Amazon rainfall associated with the Atlantic meridional mode concentrates over coastal north‐east Brazil and results in a fast Amazon response. In contrast, ENSO rainfall occupies vast inland areas and results in 3‐ to 7‐month delayed Amazon discharge response. Although ocean profile analyses represent well interannual SSS forced by open ocean rainfall, they miss interannual SSS in the plume. Including thermosalinograph transect data improves the representation of plume SSS, but its dynamic is better captured by ocean reanalyses. In Simple Ocean Data Assimilation, the plume SSS anomaly persists several months following the peak of land rainfall and is mixed out by seasonally accelerating winds in boreal winter. Its discharge‐correlated magnitude is only a modest few tenths of PSU. Perhaps, such weak projection of continental discharge variations on simulated SSS is not surprising due to other factors contributing in this dynamically active western boundary area. Significant transient variability in local currents not associated with rainfall/discharge variability is a factor explaining why ocean analyses based on occasional profile observations fail in resolving the rain‐induced SSS in the Amazon plume.

Meter-Scale Early Diagenesis of Organic Matter Buried Within Deep-Sea Sediments Beneath the Amazon River Plume

Gravity cores and multi-cores were collected from the Demerara Abyssal plain to examine meter-scale downcore features of early diagenesis in the sediments and relate them to the location of the Amazon River plume in the North Atlantic Ocean. At all sites, the oxygen penetration depth, inferred from nitrate and dissolved manganese profiles, was ~10-20 cm and nitrate was depleted within ~50 cm. However, most of the cores also had a secondary nitrate maximum (4-13µM) at ~50 cm, at a location where we observed changes in gradients of dissolved manganese, iron, and ammonium. Although there is spatial heterogeneity in the profile behavior across the study, we do find subtle diagenetic profile patterns that occur in sediments in relation to their position below the Amazon plume. Dissolved silica profiles show an initial increase downcore, but then all show a decrease to depths of 30-100 cm, thereafter concentrations increase. We suggest this zone of silica uptake is due to reverse weathering processes, possibly involving iron oxidation. A semi-lithified iron crust appeared at nearly all sites, and its position is relict, likely an indicator of the transition from glacial to interglacial sediments.

Holocene sedimentation on the distal Amazon Fan/Demerara Abyssal Plain

We have studied the rock magnetic characteristics of 31 multicores and 7 gravity cores from deep-sea sediments of the distal Amazon Fan and Demerara Abyssal Plain. This study has permitted us to delineate the space/time pattern of Holocene physical sedimentation along the northeastern margin of South America associated with the Amazon River sediment plume. The rock magnetic variability is correlatable over the entire study region. This degree of correlation has permitted us to develop a composite picture of Holocene rock magnetic (and overall physical clastic) sedimentation. 26 radiocarbon dates have been combined with the rock magnetic correlations to provide a composite Holocene chronostratigraphy for that variability. We have built isopach maps of overall sedimentation for three windows in the Holocene (3000, 4500, and 7000 calendar years BP). The overall sedimentation in the center of our region is ~5 cm/kyr. But, sedimentation rates are faster closer to the coastline and associated with the distal clastic flux from the Amazon Plume. There is a secondary high in sediment accumulation under the North Brazil Current retroflection. Our maps suggest that the retroflection has been active at the millennial scale for at least the last 7000 years. We estimate that the mid-Holocene (~4.5–9000 calendar years BP) pattern of sedimentation was distinctly different from that of the last few thousand years. The clastic sediments were ~25% reduced in concentration (with the biogenic sediments similarly increased by ~25%), they were finer–grained, and significantly less rich in ferric iron phases (hematite/goethite). We think these three covarying clastic parameters are due to an interval of reduced clastic flux from the Amazon River Plume with a reduced proportion of Amazon Basin sediments (as opposed to Andes-derived sediments). We hypothesize this was due to significantly drier Amazon Basin environmental conditions during that time interval.

C&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; glycolipids of heterocystous cyanobacteria track symbiont abundance in the diatom &amp;lt;i&amp;gt;Hemiaulus hauckii&amp;lt;/i&amp;gt; across the tropical North Atlantic

Abstract. Diatom–diazotroph associations (DDAs) include marine heterocystous cyanobacteria found as exosymbionts and endosymbionts in multiple diatom species. Heterocysts are the site of N2 fixation and have thickened cell walls containing unique heterocyst glycolipids which maintain a low oxygen environment within the heterocyst. The endosymbiotic cyanobacterium Richelia intracellularis found in species of the diatom genus Hemiaulus and Rhizosolenia makes heterocyst glycolipids (HGs) which are composed of C30 and C32 diols and triols with pentose (C5) moieties that are distinct from limnetic cyanobacterial HGs with predominantly hexose (C6) moieties. Here we applied a method for analysis of intact polar lipids to the study of HGs in suspended particulate matter (SPM) and surface sediment from across the tropical North Atlantic. The study focused on the Amazon plume region, where DDAs are documented to form extensive surface blooms, in order to examine the utility of C5 HGs as markers for DDAs as well as their transportation to underlying sediments. C30 and C32 triols with C5 pentose moieties were detected in both marine SPM and surface sediments. We found a significant correlation between the water column concentration of these long-chain C5 HGs and DDA symbiont counts. In particular, the concentrations of both the C5 HGs (1-(O-ribose)-3,27,29-triacontanetriol (C5 HG30 triol) and 1-(O-ribose)-3,29,31-dotriacontanetriol (C5 HG32 triol)) in SPM exhibited a significant correlation with the number of Hemiaulus hauckii symbionts. This result strengthens the idea that long-chain C5 HGs can be applied as biomarkers for marine endosymbiotic heterocystous cyanobacteria. The presence of the same C5 HGs in surface sediment provides evidence that they are effectively transported to the sediment and hence have potential as biomarkers for studies of the contribution of DDAs to the paleo-marine N cycle.

Dynamics of Primary Productivity and Oceanographic Parameters under Influence of the Amazon River Plume

The watershed of the Amazon River discharges about 120,000 m3·s-1 of freshwater into the adjacent platform and oceanic region. The aim of this work was to analyze the distribution of oceanographic parameters, chlorophyll a and primary productivity under the influence of the Amazon River plume, during the period of greatest extension of the Amazon plume. Collections were carried out in September 14 in 16 stations including continental platform and oceanic region. It was possible to observe superficial currents along the coast in the northwest direction, but with less intensity and currents with greater speeds towards the east due to the North Brazil Current retroflexion at this time of the year, transporting the plume to the central Atlantic Ocean. The spatial influence of the plume was observed in the salinity, which ranged from 28 to 36.75, although the high precipitation in the region may also have contributed as a source of freshwater. However, the sampled region showed strong negative linear correlation of silicate, chlorophyll a and primary productivity with salinity. The primary productivity values ranged from 0.04 to 18.81 mg C m-3 day-1, whereas chlorophyll a concentrations ranged from 0.15 mg·m-3 to 1.83 mg·m-3, decreasing their values as they move away from the coast. The Amazon River plume can reach and influence the oceanographic and biological parameters in a large area of this oligotrophic region. However, the results also suggest that the export of material from the adjacent coastal region is another determinant of the region’s productivity.

Net Heterotrophy in the Amazon Continental Shelf Changes Rapidly to a Sink of CO2 in the Outer Amazon Plume

The Amazon continental shelf and adjacent oceanic area were sampled for inorganic and organic carbon parameters in order to improve data coverage and understanding of carbon cycling dynamics within this important region. Seasonal coverage of the Amazon plume on the French Guiana continental shelf further north, was provided by CO2 monitoring using a merchant ship sailing from France to French Guiana (2006-2016). Salinity ranged from 1 to 36 (transects in April 2013, and May 2014). At salinity below 10, strong outgassing was observed with fugacity of CO2 (fCO2) over 2000 atm. This region displayed net heterotrophy, fueled by organic matter with terrestrial origin, as shown by 13C and 15N values of suspended particles. A 13C cross shelf average of -31‰ was measured during May 2014, contrasting with oceanic values in excess of -20‰. The reactivity of this terrestrial material resulted in the local production of dissolved inorganic and organic carbon as well as fluorescent humic compounds. Further offshore, the dilution of freshwater by ocean waters created a sink for CO2, enhanced by biological activity. The strongest CO2 drawdowns, associated with high chlorophyll a concentrations, were observed on the French Guiana continental shelf in the outer Amazon plume, with fCO2 values below 150 uatm. Here, a CO2 sink was present almost throughout the year, with a seasonal maximum of -9.2 mmol CO2 m-2d-1 observed in June 2015. However, both the CO2 and salinity distributions could vary significantly within a few days, confirming the presence of many eddies in this region. The Amazon continental shelf hence behaved as a transition zone between an inshore source of CO2 to the atmosphere and an offshore sink. Some marine phytoplankton production was detected but occurred mainly close to the French Guiana shelf. A mean net CO2 outgassing of 44 ± 43.6 mmol m-2d-1 was estimated for the area. Quantifying the CO2 flux for the entire Amazon shelf area led to a revised annual estimate of the net ocean carbon export of 2.86 1012 mol C from the region.

Amazon Plume Salinity Response to Ocean Teleconnections

Pacific and Atlantic sea surface temperature (SST) variability strongly influences rainfall changes in the Amazon River basin, which impacts on the river discharge and consequently the sea surface salinity (SSS) in the Amazon plume. An Empirical Orthogonal Function (EOF) analysis was performed using 46 years of SST, rainfall, and SSS datasets, in order to establish the relationship between these variables. The first three modes of SST/rainfall explained 87.83% of the total covariance. Pacific and Atlantic SSTs led Amazon basin rainfall events by four months. The resultant SSS in the western tropical North Atlantic (WTNA) lagged behind basin rainfall by three months, with 75.04% of the total covariance corresponding to the first four EOF modes. The first EOF mode indicated a strong SSS pattern along the coast that was connected to negative rainfall anomalies covering the Amazon basin, linked to El Nino events. A second pattern also presented positive SSS anomalies, when the rainfall was predominantly over the northwestern part of the Amazon basin, with low rainfall around the Amazon River mouth. The pattern with negative SSS anomalies in the WTNA was associated with the fourth mode, when positive rainfall anomalies were concentrated in the northwest part of South America. The spatial rainfall structure of this fourth mode was associated with the spatial rainfall distribution found in the third EOF mode of SST vs rainfall, which was a response to La Nina Modoki events. A statistical analysis for the 46 year period and monthly anomaly composites for 2008 and 2009 indicated that La Nina Modoki events can be used for the prediction of low SSS patterns in the WNTA.

Origin and processing of terrestrial organic carbon in the Amazon system: lignin phenols in river, shelf, and fan sediments

Abstract. The Amazon River transports large amounts of terrestrial organic carbon (OCterr) from the Andean and Amazon neotropical forests to the Atlantic Ocean. In order to compare the biogeochemical characteristics of OCterr in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments, we analysed riverbed sediments from the Amazon mainstream and its main tributaries as well as marine surface sediments from the Amazon shelf and fan for total organic carbon (TOC) content, organic carbon isotopic composition (δ13CTOC), and lignin phenol compositions. TOC and lignin content exhibit positive correlations with Al ∕ Si ratios (indicative of the sediment grain size) implying that the grain size of sediment discharged by the Amazon River plays an important role in the preservation of TOC and leads to preferential preservation of lignin phenols in fine particles. Depleted δ13CTOC values (−26.1 to −29.9 ‰) in the main tributaries consistently correspond with the dominance of C3 vegetation. Ratios of syringyl to vanillyl (S ∕ V) and cinnamyl to vanillyl (C ∕ V) lignin phenols suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Although the Amazon basin hosts a rich diversity of vascular plant types, distinct regional lignin compositions are not observed. In the marine sediments, the distribution of δ13CTOC and Λ8 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100 mg OC) values implies that OCterr discharged by the Amazon River is transported north-westward by the North Brazil Current and mostly deposited on the inner shelf. The lignin compositions in offshore sediments under the influence of the Amazon plume are consistent with the riverbed samples suggesting that processing of OCterr during offshore transport does not change the encoded source information. Therefore, the lignin compositions preserved in these offshore sediments can reliably reflect the vegetation in the Amazon River catchment. In sediments from the Amazon fan, low lignin content, relatively depleted δ13CTOC values and high (Ad ∕ Al)V ratios indicating highly degraded lignin imply that a significant fraction of the deposited OCterr is derived from petrogenic (sourced from ancient rocks) sources.

Sea surface salinity observed from the HY-2A satellite

Motivated by the shortcomings of radio frequency interferences (RFI) associated with the spaceborne L-band radiometers near the Northwest Pacific and previous study near the Amazon plume, this study presents a sea surface salinity (SSS) retrieval algorithm from the microwave radiometer onboard the HY-2A satellite. The SSS signal is improved by differentiating the reflectance between the C and X band. A reflectance calibration method is proposed by using a combination of radiative transfer model (RTM) and the Klein-Swift emissivity model. Evaluations of the retrieved SSS from the HY-2A satellite indicate that the root mean square error (RMSE) is about 0.35 psu on 0. 5 degree grid spacing and monthly time scale which is comparable to the accuracy of SMOS and Aquarius-SAC/D satellites.

Sea surface salinity observed from the HY-2A satellite

Motivated by the shortcomings of radio frequency interferences (RFI) associated with the spaceborne L-band radiometers near the Northwest Pacific and previous study near the Amazon plume, this study presents a sea surface salinity (SSS) retrieval algorithm from the microwave radiometer onboard the HY-2A satellite. The SSS signal is improved by differentiating the reflectance between the C and X band. A reflectance calibration method is proposed by using a combination of radiative transfer model (RTM) and the Klein-Swift emissivity model. Evaluations of the retrieved SSS from the HY-2A satellite indicate that the root mean square error (RMSE) is about 0.35 psu on 0.5 degree grid spacing and monthly time scale which is comparable to the accuracy of SMOS and Aquarius-SAC/D satellites.

Amazon River influence on nitrogen fixation and export production in the western tropical North Atlantic

AbstractAs part of the ANACONDAS program, we investigated the role of the Amazon plume in stimulating offshore nitrogen fixation and export production during the river's high‐discharge period (May–June 2010). Using the shipboard underway system, we performed high‐resolution sampling of over 450,000 km2 of surface waters, characterizing the distribution of nutrients, phytoplankton, particulate organic matter (POM), and stable carbon and nitrogen isotopes of POM in the offshore plume. We found distinct regional variations in diazotroph communities, with the Diatom‐Diazotroph Associations (DDA) Hemiaulus hauckii – Richelia intracellularis dominating the low N : P mesohaline waters to the northwest of the plume axis and Trichodesmium spp. primarily occupying oceanic waters to the east. Nutrient availability broadly shaped diazotroph distributions along the salinity gradient, but habitat longevity may also play a role in the finer‐scale distributions of communities, particularly of DDAs. H. hauckii and Trichodesmium spp. affected the nitrogen and carbon budgets in fundamentally different ways within the plume‐influenced regions, with H. hauckii making much greater contributions to the particulate nitrogen pool and to CO2 drawdown than Trichodesmium spp., leading to much higher export fluxes. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.

The overlooked tropical oceanic CO2 sink

AbstractThe intense rainfall in the tropical Atlantic spatially overlaps with the spread of the Amazon plume. Based on remote‐sensed sea surface salinity and rainfall, we removed the contribution of rainfall to the apparent Amazon plume area, thus refining the quantification of its extension (0.84 ± 0.06 × 106 km2 to 0.89 ± 0.06 × 106 km2). Despite the previous overestimation of the Amazon plume area due to the influence of rainfall (&gt;16%), our calculated annual CO2 flux based on rainfall‐corrected sea surface CO2 fugacity confirms that the Amazon River plume is an atmospheric CO2 sink of global importance (−7.61 ± 1.01 to −7.85 ± 1.02 Tg C yr−1). Yet we show that current sea‐air CO2 flux assessments for the tropical Atlantic could be overestimated in about 10% by neglecting the CO2 sink associated to the Amazon plume. Thus, including the Amazon plume, the sea‐air CO2 exchange for the tropical Atlantic is estimated to be 81.1 ± 1.1 to 81.5 ± 1.1 Tg C yr−1.

Distribution of CO2 parameters in the Western Tropical Atlantic Ocean

The variability of sea surface Total Alkalinity (TA) and sea surface Total Inorganic Carbon (CT) is examined using all available data in the western tropical Atlantic (WTA: 20°S-20°N, 60°W-20°W). Lowest TA and CT are observed for the region located between 0°N-15°N/60°W-50°W and are explained by the influence of the Amazon plume during boreal summer. In the southern part of the area, 20°S-10°S/40°W-60°W, the highest values of TA and CT are linked to the CO2–rich waters due to the equatorial upwelling, which are transported by the South Equatorial Current (SEC) flowing from the African coast to the Brazilian shore. An increase of CT of 0.9±0.3μmolkg−1yr−1 has been observed in the SEC region and is consistent with previous published estimates. A revised CT-Sea Surface Salinity (SSS) relationship is proposed for the WTA to take into account the variability of CT at low salinities. This new CT-SSS relationship together with a published TA-SSS relationship allow to calculate pCO2 values that compare well with observed pCO2 (R2=0.90).