Western Tropical North Atlantic Ocean Research Articles

Mercury methylation linked to nitrification in the tropical North Atlantic Ocean

Methylmercury (MeHg) is a toxin that poses health risks to humans and wildlife, primarily through consumption of seafood (Sunderland and Mason, 2007). Most MeHg bioaccumulation by marine fish likely occurs in the upper 200 m of the ocean, which is mostly oxygenated (Mason et al., 2012). However, mechanisms of Hg methylation in oxic seawater remain unknown, since the hgcAB gene cluster, which encodes proteins for MeHg production, had been found exclusively in anaerobic microbes (Gilmour et al., 2013; Parks et al., 2013). Recent work, however, has shown that hgc genes are widespread in oxic seawater, including in the microaerophilic nitrifier, Nitrospina (Tada et al., 2020; Tada et al., 2021; Gionfriddo et al., 2016; Bowman et al., 2020). Here, we show that potential MeHg production rates in Western Tropical North Atlantic Ocean surface waters, within and near the Amazon River plume, were correlated positively and strongly to nitrification rates and Nitrospina-specific 16S gene expression. Potential Hg methylation and nitrification rates were highest at the most saline and least turbid stations, indicating that sediment particles and nutrient-rich, riverine discharges were not the primary factors promoting either process. These novel results in oxic seawater provide further evidence that Hg methylation is linked to abundant, nitrifying microbes and may help explain marine MeHg distributions.

Temporal Variability of Air‐Sea CO2 flux in the Western Tropical North Atlantic Influenced by the Amazon River Plume

AbstractThe partial pressure of carbon dioxide (pCO2) was surveyed across the Amazon River plume and the surrounding western tropical North Atlantic Ocean (15–0°N, 43–60°W) during three oceanic expeditions (May–June 2010, September–October 2011, and July 2012). The survey timing was chosen according to previously described temporal variability in plume behavior due to changing river discharge and winds. In situ sea surface pCO2 and air‐sea CO2 flux exhibited robust linear relationships with sea surface salinity (SSS; 15 < SSS < 35), although the relationships differed among the surveys. Regional distributions of pCO2 and CO2 flux were estimated using SSS maps from high‐resolution ocean color satellite‐derived (MODIS‐Aqua) diffuse attenuation coefficient at 490 nm (Kd490) during the periods of study. Results confirmed that the plume is a net CO2 sink with distinctive temporal variability: the strongest drawdown occurred during the spring flood (−2.39 ± 1.29 mmol m−2 d−1 in June 2010), while moderate drawdown with relatively greater spatial variability was observed during the transitional stages of declining river discharge (−0.42 ± 0.76 mmol m−2 d−1 in September–October 2011). The region turned into a weak source in July 2012 (0.26 ± 0.62 mmol m−2 d−1) when strong CO2 uptake in the mid‐plume was overwhelmed by weak CO2 outgassing over a larger area in the outer plume. Outgassing near the mouth of the river was observed in July 2012. Our observations draw attention to the importance of assessing the variable impacts of biological activity, export, and air‐sea gas exchange before estimating regional CO2 fluxes from salinity distributions alone.

A quest for the biological sources of long chain alkyl diols in the western tropical North Atlantic Ocean

Abstract. Long chain alkyl diols (LCDs) are widespread in the marine water column and sediments, but their biological sources are mostly unknown. Here we combine lipid analyses with 18S rRNA gene amplicon sequencing on suspended particulate matter (SPM) collected in the photic zone of the western tropical North Atlantic Ocean at 24 stations to infer relationships between LCDs and potential LCD producers. The C30 1,15-diol was detected in all SPM samples and accounted for >95 % of the total LCDs, while minor proportions of C28 and C30 1,13-diols, C28 and C30 1,14-diols, as well as C32 1,15-diol were found. The concentration of the C30 and C32 diols was higher in the mixed layer of the water column compared to the deep chlorophyll maximum (DCM), whereas concentrations of C28 diols were comparable. Sequencing analyses revealed extremely low contributions (≈0.1 % of the 18S rRNA gene reads) of known LCD producers, but the contributions from two taxonomic classes with which known producers are affiliated, i.e. Dictyochophyceae and Chrysophyceae, followed a trend similar to that of the concentrations of C30 and C32 diols. Statistical analyses indicated that the abundance of 4 operational taxonomic units (OTUs) of the Chrysophyceae and Dictyochophyceae, along with 23 OTUs falling into other phylogenetic groups, were weakly (r≤0.6) but significantly (p value <0.01) correlated with C30 diol concentrations. It is not clear whether some of these OTUs might indeed correspond to C28−32 diol producers or whether these correlations are just indirect and the occurrence of C30 diols and specific OTUs in the same samples might be driven by other environmental conditions. Moreover, primer mismatches were unlikely, but cannot be excluded, and the variable number of rRNA gene copies within eukaryotes might have affected the analyses leading to LCD producers being undetected or undersampled. Furthermore, based on the average LCD content measured in cultivated LCD-producing algae, the detected concentrations of LCDs in SPM are too high to be explained by the abundances of the suspected LCD-producing OTUs. This is likely explained by the slower degradation of LCDs compared to DNA in the oxic water column and suggests that some of the LCDs found here were likely to be associated with suspended debris, while the DNA from the related LCD producers had been already fully degraded. This suggests that care should be taken in constraining biological sources of relatively stable biomarker lipids by quantitative comparisons of DNA and lipid abundances.

Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

AbstractLargely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso‐ and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume‐influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso‐ and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.

Metagenomic and metatranscriptomic inventories of the lower Amazon River, May 2011.

BackgroundThe Amazon River runs nearly 6500 km across the South American continent before emptying into the western tropical North Atlantic Ocean. In terms of both volume and watershed area, it is the world’s largest riverine system, affecting elemental cycling on a global scale.ResultsA quantitative inventory of genes and transcripts benchmarked with internal standards was obtained at five stations in the lower Amazon River during May 2011. At each station, metagenomes and metatranscriptomes were obtained in duplicate for two microbial size fractions (free-living, 0.2 to 2.0 μm; particle-associated, 2.0 to 297 μm) using 150 × 150 paired-end Illumina sequencing. Forty eight sample datasets were obtained, averaging 15 × 106 potential protein-encoding reads each (730 × 106 total). Prokaryotic metagenomes and metatranscriptomes were dominated by members of the phyla Actinobacteria, Planctomycetes, Betaproteobacteria, Verrucomicrobia, Nitrospirae, and Acidobacteria. The actinobacterium SCGC AAA027-L06 reference genome recruited the greatest number of reads overall, with this single bin contributing an average of 50 billion genes and 500 million transcripts per liter of river water. Several dominant taxa were unevenly distributed between the free-living and particle-associated size fractions, such as a particle-associated bias for reads binning to planctomycete Schlesneria paludicola and a free-living bias for actinobacterium SCGC AAA027-L06. Gene expression ratios (transcripts to gene copy ratio) increased downstream from Óbidos to Macapá and Belém, indicating higher per cell activity of Amazon River bacteria and archaea as river water approached the ocean.ConclusionThis inventory of riverine microbial genes and transcripts, benchmarked with internal standards for full quantitation, provides an unparalleled window into microbial taxa and functions in the globally important Amazon River ecosystem.Electronic supplementary materialThe online version of this article (doi:10.1186/s40168-015-0099-0) contains supplementary material, which is available to authorized users.

Phosphorus cycling. Major role of planktonic phosphate reduction in the marine phosphorus redox cycle.

Phosphorus in the +5 oxidation state (i.e., phosphate) is the most abundant form of phosphorus in the global ocean. An enigmatic pool of dissolved phosphonate molecules, with phosphorus in the +3 oxidation state, is also ubiquitous; however, cycling of phosphorus between oxidation states has remained poorly constrained. Using simple incubation and chromatography approaches, we measured the rate of the chemical reduction of phosphate to P(III) compounds in the western tropical North Atlantic Ocean. Colonial nitrogen-fixing cyanobacteria in surface waters played a critical role in phosphate reduction, but other classes of plankton, including potentially deep-water archaea, were also involved. These data are consistent with marine geochemical evidence and microbial genomic information, which together suggest the existence of a vast oceanic phosphorus redox cycle.

The phosphorus redox cycle

Phosphorus Cycling Phosphorus in the oceans cycles between +5 and +3 oxidation states. Most of the oceans' phosphorus is present as oxidized bioavailable phosphate (+5) compounds. Reduced organophosphorus compounds are also present but at much lower concentrations. Through field measurements in the western tropical North Atlantic Ocean and a series of laboratory incubations, Van Mooy et al. measured fast reduction rates of a small but appreciable amount of phosphates by plankton communities, forming phosphites and phosphonates (see the Perspective by Benitez-Nelson). On a global scale, this phosphorus redox cycle adds as much reduced phosphorus to the oceans as all pre-anthropogenic land runoff. Science , this issue p. [783][1]; see also p. [759][2] [1]: /lookup/doi/10.1126/science.aaa8181 [2]: /lookup/doi/10.1126/science.aab2801

The Amazon continuum dataset: quantitative metagenomic and metatranscriptomic inventories of the Amazon River plume, June 2010.

BackgroundThe Amazon River is by far the world’s largest in terms of volume and area, generating a fluvial export that accounts for about a fifth of riverine input into the world’s oceans. Marine microbial communities of the Western Tropical North Atlantic Ocean are strongly affected by the terrestrial materials carried by the Amazon plume, including dissolved (DOC) and particulate organic carbon (POC) and inorganic nutrients, with impacts on primary productivity and carbon sequestration.ResultsWe inventoried genes and transcripts at six stations in the Amazon River plume during June 2010. At each station, internal standard-spiked metagenomes, non-selective metatranscriptomes, and poly(A)-selective metatranscriptomes were obtained in duplicate for two discrete size fractions (0.2 to 2.0 μm and 2.0 to 156 μm) using 150 × 150 paired-end Illumina sequencing. Following quality control, the dataset contained 360 million reads of approximately 200 bp average size from Bacteria, Archaea, Eukarya, and viruses. Bacterial metagenomes and metatranscriptomes were dominated by Synechococcus, Prochlorococcus, SAR11, SAR116, and SAR86, with high contributions from SAR324 and Verrucomicrobia at some stations. Diatoms, green picophytoplankton, dinoflagellates, haptophytes, and copepods dominated the eukaryotic genes and transcripts. Gene expression ratios differed by station, size fraction, and microbial group, with transcription levels varying over three orders of magnitude across taxa and environments.ConclusionsThis first comprehensive inventory of microbial genes and transcripts, benchmarked with internal standards for full quantitation, is generating novel insights into biogeochemical processes of the Amazon plume and improving prediction of climate change impacts on the marine biosphere.

The distinct nitrogen isotopic compositions of low and high molecular weight marine DON

To constrain the sources and cycling of bulk and size-fractionated marine dissolved organic nitrogen (DON), samples were collected for concentration and isotopic (δ15N) analysis from the western tropical North Atlantic Ocean as well as over the North Australian shelf. Bulk DON concentrations are typically between 4 and 6μM, with low molecular weight (LMW, i.e., <1000Da) DON accounting for 60 to 70% of total DON. The δ15N of both bulk and high molecular weight (HMW, i.e., >1000Da to <0.2μm) DON from the western tropical North Atlantic is similar to previous measurements, as well as to bulk and HMW DON collected off the North Australian Shelf, ~4‰. Here we report the first measurement of marine LMW DON δ15N, and a coherent pattern emerges where LMW DON δ15N<HMW DON δ15N. An analytical concern is that the bulk DON δ15N as calculated from our coupled HMW and LMW measurements is often lower than that measured directly for the bulk DON. Despite this discrepancy, the self-consistency of the data in other regards argues for the robustness of the basic observation of lower δ15N in LMW relative to HMW DON. One explanation for this isotopic difference is considered most likely, based on the model that DON δ15N elevation relative to PON is fundamentally due to DON production from PON without isotopic fractionation, coupled with DON destruction occurring with fractionation. In this model, HMW DON loss must occur predominantly through the breakage of N-containing bonds, for which isotope fractionation should be substantial, whereas LMW DON loss is partly due to direct assimilation by phytoplankton and other microbes, which appears to occur with only minor isotopic fractionation. However, an alternative hypothesis exists: LMW and HMW DON may have distinct sources, for example, prokaryotic and eukaryotic plankton, respectively. In either case, it appears that most LMW DON has not passed through the HMW DON pool, as this would require an unrealistically large isotope fractionation at the HMW to LMW conversion. Finally, neither the concentration nor the δ15N of the bulk or either size fraction of DON varies with in situ N2 fixation rates measured at the time of sample collection. However, the δ15N of both bulk and LMW DON from the western tropical North Atlantic shows significant differences between samples collected six months apart, which may be due to seasonally variable stimulation of primary production by the Amazon River.

Limitation of One-Dimensional Ocean Models for Coupled Hurricane–Ocean Model Forecasts

Abstract Wind stress imposed on the upper ocean by a hurricane can limit the hurricane’s intensity primarily through shear-induced mixing of the upper ocean and subsequent cooling of the sea surface. Since shear-induced mixing is a one-dimensional process, some recent studies suggest that coupling a one-dimensional ocean model to a hurricane model may be sufficient for capturing the storm-induced sea surface temperature cooling in the region providing heat energy to the hurricane. Using both a one-dimensional and a three-dimensional version of the same ocean model, it is shown here that the neglect of upwelling, which can only be captured by a three-dimensional ocean model, underestimates the storm-core sea surface cooling for hurricanes translating at &amp;lt;∼5 m s−1. For hurricanes translating at &amp;lt;2 m s−1, more than half of the storm-core sea surface cooling is neglected by the one-dimensional ocean model. Since the majority of hurricanes in the western tropical North Atlantic Ocean translate at &amp;lt;5 m s−1, the idealized experiments presented here suggest that one-dimensional ocean models may be inadequate for coupled hurricane–ocean model forecasting.

Influence of the Amazon River plume on distributions of free‐living and symbiotic cyanobacteria in the western tropical north Atlantic Ocean

The vertical and horizontal distributions of seven diazotrophic populations in the western tropical north Atlantic (WTNA) Ocean were examined using a nifH DNA quantitative polymerase chain reaction (QPCR) approach. The nifH phylotype abundances were highest near the surface and decreased with depth, with the exception of the cyanobacterial symbiont Calothrix, which was not detected at any station. Richelia associated with the diatoms Rhizosolenia clevei and Hemiaulus hauckii were distributed within the freshwater lens of the Amazon plume. Abundances of H. hauckii‐Richelia nifH genes dominated all depths in 6 of 10 vertical profiles and 10 of 20 surface samples. In addition, estimates of Richelia associated with H. hauckii increased northwest (8‐ 12°N, 56‐54°W) from the river mouth, where significantly ( p &lt; 0.001) higher abundances (&gt;105 copies L−1) were found in mesohaline waters (31‐34.9). nifH copy abundance for surface populations of the H. hauckii‐Richelia symbioses were positively correlated (r2 = 0.59) with salinity. Three unicellular cyanobacterial groups and Trichodesmium had similar horizontal distributions, where the highest nifH copy estimates were at stations with salinity ≥35 and northeast (6‐10°N 50°W) of the freshwater lens. The abundance of Trichodesmium spp. and unicellular Group B nifH gene copies co‐varied (r2 = 0.60). The QPCR study showed the dominance of H. hauckii‐Richelia symbioses in the Amazon plume waters, implying that these associations had an ecological advantage over the other diazotrophs. Outside of the plume nutrients were below detection, abundances of freeliving unicellular cyanobacterial phylotypes, including a novel group designated Group C, were abundant (&gt;105 copies L−1) and comparable to the abundances of Trichodesmium spp. Thus, there appeared to be a cascade of diazotrophic communities along gradients of salinity and nutrients in the WTNA.

Physical and biological contributions to the western tropical North Atlantic Ocean carbon sink formed by the Amazon River plume

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were measured in the upper 1000 m of the western tropical North Atlantic Ocean (WTNA; study area 3–15°N, 40–59°W) in January–February and July–August 2001. Concentrations of DIC and TA in surface samples (0–10 m) influenced by the Amazon River plume were up to 400 μmol C kg−1 (∼20%) lower than oceanic surface samples. In this region, physical dilution by river water dominates DIC and TA inventories, driving CO2 partial pressure (pCO2) well below atmospheric levels. Nevertheless, DIC concentrations at most plume‐influenced stations were 10–90 μmol C kg−1 below levels expected from conservative mixing of seawater with low‐salinity, low‐CO2 Amazon River water. In this otherwise oligotrophic region, the diazotrophs Trichodesmium spp. and Richelia intracellularis were often abundant, supporting a link between increased carbon drawdown and nitrogen fixation in the outer plume. Net community production in the plume must surpass the fluxes of inorganic carbon from below and air‐sea CO2 replacement to leave biologically mediated DIC deficits, which is possible under observed conditions. Biological activity lowers plume pCO2 30–120 μatm below the conservative mixing line, and contributes to a CO2 deficit in the northern WTNA that outlasts the plume's physical structure.

Influence of the Amazon River on the surface optical properties of the western tropical North Atlantic Ocean

The distribution of chromophoric‐dissolved‐organic‐matter (CDOM) was investigated in the western tropical North Atlantic Ocean (WTNA) during the low and high flow periods of the Amazon River. A strong correlation was observed between in situ CDOM absorption at 490 nm and in situ diffuse attenuation coefficient at 490 nm (K490) during the high flow period (ratio of 0.7). SeaWiFS monthly composites of K490 showed (1) very low values over most of the WTNA, with higher values restricted to the continental shelf during the low flow season and (2) higher K490 over much of the WTNA extending offshore to &gt;1000 km from the river mouth during the high flow season. The relative contribution to light absorption by particulate (phytoplankton and detritus) and dissolved (CDOM) materials was investigated at selected wavelengths. At 440 nm during the high flow period (1) CDOM dominated (∼70%) total light absorption closer to the river mouth; (2) phytoplankton and detritus dominated (∼40% each) absorption farther offshore but still within the Amazon plume; and (3) CDOM contributed ∼20%, phytoplankton &gt;60%, and detritus ∼10% outside the plume. In situ data suggests a terrestrial source for the CDOM off the coast of South America during the high flow period with possible offshore local sinks (photodegradation) as well as sources (in situ production). The Amazon River strongly influences the optical properties of the WTNA at distances over 1000 km from the river mouth. Retrievals of phytoplankton biomass based on satellite ocean color data can be largely overestimated in the WTNA, if the contribution to light absorption by CDOM and detritus is not taken into account.

Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation

Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records1,2,3,4. Rapid reorganization of ocean circulation is considered to exert some control over these changes5, as are shifts in the concentrations of atmospheric greenhouse gases6. The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic7,8, whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes9. So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900–15,400 calendar years bp) and the Younger Dryas event (12,900–11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

Upper layer circulation in the western tropical North Atlantic Ocean during August 1989

Shipboard acoustic Doppler current profiler velocity measurements and profiles of temperature, salinity, and dissolved oxygen measurements are used to map the flow field above the σθ = 26.8 isopycnal (approximately the upper 300 m of the water column) in the North Brazil Current (NBC) retroflection region (0° to 14°N, 60° to 40°W) during August of 1989. The water column is divided into a near‐surface, upper thermocline layer (above σθ = 24.5) and a main to subthermocline layer (σθ = 24.5 to σ0 = 26.8). In the upper layer the eastward flowing North Equatorial Countercurrent (NECC) is composed of 16×106 m3/s of NBC transport that has retroflected from the coast between 6°N and 8°N and 8×106 m3/s of North Equatorial Current (NEC) transport returning eastward. An anticyclonic eddy with a 10×106 m3/s transport lies northwest of the NBC retroflection. No throughflow is observed along the boundary, and only a small portion of the observed NEC transport (1.5×106 m3/s) enters the Caribbean Sea. In the lower layer the NBC transports 8×106 m3/s into the subthermocline North Equatorial Undercurrent (NEUC). In addition, 15×106 m3/s joins the NEUC from the north. At 44°W the subsurface core of the NEUC lies south of the near‐surface core of the NECC. Nearly half of the NEUC transport is made up of a mixed water type with salinity‐oxygen (S‐O2) characteristics intermediate to the characteristics of the original component transports. There is no evidence of continuous NBC flow into the Caribbean Sea in the lower level. The closed eddy to the north of the retroflection, however, contains water masses with South Atlantic S‐O2 properties.

Topographic effects on the bottom water circulation of the western tropical North Atlantic Ocean

A simple numerical model of the abyssal circulation of the ocean is used to explore the possibility, first suggested by Warren (MIT Press, pp. 6–41, 1981), that the northward deepening of the sea floor in the western basin of the tropical North Atlantic Ocean is responsible for the observed eastward confinement of the Antartic Bottom Water current, a result contrary to what the Stommel-Arons model predicts. It is shown that (a) a topographic slope of magnitude observed in this region of the Atlantic forces separation of the northward-flowing deep western boundary current at a low latitude, (b) north of this latitude the circulation is dominated by a large-scale cyclonic gyre extending up to where the sea floor becomes flat again, with a northward flow along the eastern boundary as a part of it and a westward flow at about 25°N. Simulated Lagrangian particle trajectories show that new abyssal water from the south is confined to a band next to the eastern boundary in the gyre region as observed. These results are robust under variation of topographic, forcing and frictional parameters, although quantitative aspects and details are sensitive to them. Eastward flow through the Vema Fracture Zone, recently estimated by McCartney et al. ( Journal of Physical Oceanography, 21, 1089–1121, 1991) to be about 2 Sv, diverts some of the northward-flowing eastern boundry current, but otherwise does not affect the circulation. These results compare favorably with recent observations that report southeastward flow along the western boundary at least down to 8°N and suggest a recirculation of abyssal water in this region, as well as with those that suggest a northern limit of the Antarctic Bottom Water flow along the eastern boundary at about 25°N and a possible westward separation there. An analytical solution of the planetary geostrophic equation applied to an abyssal ocean with topography, which captures some features of the numerical model flow, is presented in the Appendix.