Amazon River Plume Research Articles

Dispersal and Diving Adjustments of the Green Turtle Chelonia mydas in Response to Dynamic Environmental Conditions during Post-Nesting Migration.

In response to seasonality and spatial segregation of resources, sea turtles undertake long journeys between their nesting sites and foraging grounds. While satellite tracking has made it possible to outline their migration routes, we still have little knowledge of how they select their foraging grounds and adapt their migration to dynamic environmental conditions. Here, we analyzed the trajectories and diving behavior of 19 adult green turtles (Chelonia mydas) during their post-nesting migration from French Guiana and Suriname to their foraging grounds off the coast of Brazil. First Passage Time analysis was used to identify foraging areas located off Ceará state of Brazil, where the associated habitat corresponds to favorable conditions for seagrass growth, i.e. clear and shallow waters. The dispersal and diving patterns of the turtles revealed several behavioral adaptations to the strong hydrodynamic processes induced by both the North Brazil current and the Amazon River plume. All green turtles migrated south-eastward after the nesting season, confirming that they coped with the strong counter North Brazil current by using a tight corridor close to the shore. The time spent within the Amazon plume also altered the location of their feeding habitats as the longer individuals stayed within the plume, the sooner they initiated foraging. The green turtles performed deeper and shorter dives while crossing the mouth of the Amazon, a strategy which would help turtles avoid the most turbulent upper surface layers of the plume. These adjustments reveal the remarkable plasticity of this green turtle population when reducing energy costs induced by migration.

The compositional evolution of dissolved and particulate organic matter along the lower Amazon River—Óbidos to the ocean

River systems play a pivotal role in transporting and transforming organic carbon (OC) fixed by terrestrial primary production. However, there is a fundamental gap in our understanding of the connectivity of terrestrial, aquatic, and marine carbon budgets due to a lack of measurements along the lower (i.e. tidally-influenced) reaches of large river systems. For example, all estimates of carbon fluxes from the world's largest river, the Amazon, are based on measurements made at and upstream of Óbidos, roughly 900km from the mouth. Here we examine the evolution of OC concentrations and composition from Óbidos to two discreet channels near the mouth of the Amazon River during five cruises from 2010 to 2012. OC characteristics of the Tapajós River, which enters the Amazon River downstream of Óbidos, and the Tocantins River, which mixes with the Amazon River plume in the Atlantic Ocean, were also assessed.The average concentration of particulate organic carbon (POC) across the two main channels near the mouth was 0.6±0.3mgL−1 during the study period, decreasing from 1.2±1.0mgL−1 at Óbidos. Average dissolved organic carbon (DOC) concentrations, on the other hand, increased from 3.9±0.6mgL−1 at Óbidos to 4.2±0.9mgL−1 across the mouth. The discharge of total OC to the ocean was composed of 89±3% dissolved load, compared to 76±13% at Óbidos. Measurements of bulk OC stable isotopic signatures, chlorophyll a concentrations, and lignin phenol abundance were used to assess unique OC sources along the continuum. For example, the stable isotopic composition of POC and DOC indicated a replacement of highland forest-derived OC with lowland and floodplain-derived OC from Óbidos to the mouth. Likewise, lignin phenol signatures showed an increase in the degradation state of vascular plant-derived OC from Óbidos to the mouth. Results from this study illustrate that the abundance and composition of OC continue to evolve along the lower reaches of large tropical rivers, which has significant implications on estimations of geochemical fluxes to the ocean.

Improvements to a global ocean data assimilation system through the incorporation of Aquarius surface salinity data

The impact of using sea surface salinity (SSS) data derived from the Aquarius satellite within a global ocean data assimilation system is investigated. In the central‐eastern North Pacific, the more realistic SSS structures introduced by the Aquarius data also influence the salinity, temperature and potential vorticity fields obtained in the subsurface layer via enhanced mode‐water formation. Around the Indonesian maritime continent, the Aquarius data assimilation leads to salinity distributions which are closer to buoy observations, while in the region of the Amazon River plume, subsurface temperatures are improved following a better reproduction of the low‐salinity plume in the surface layer. The SSS model biases are also reduced in the eastern equatorial Pacific and in the Arctic Ocean, although our data are limited in number and accuracy at high latitudes. These results indicate the importance of Aquarius data in deriving improved representations of the global ocean from dynamical models.

Monitoring the spreading of the Amazon freshwater plume by MODIS, SMOS, Aquarius, and TOPAZ

AbstractA synergistic tool for studying the Amazon River plume dynamics based on a novel algorithm for deriving sea surface salinity (SSS) from MODIS reflectance data together with SSS data from the SMOS and Aquarius satellites and the TOPAZ data assimilation system is proposed. The new algorithm is based on a neural network to relate spectral remote sensing reflectance measured by MODIS with SSS measured by SMOS in the Amazon River plume. The algorithm is validated against independent in situ data and is found to be valid in the range of SSS from 29 to 35 psu, for the period of highest rates of Amazon River discharge with RMSE = 0.79 psu and r2 = 0.84. Monthly SSS fields were reconstructed from the MODIS data for late summers from 2002 to 2012 at a 10 km resolution and compared to surface currents and SSS derived from the TOPAZ reanalysis system. The two data sets reveal striking agreement, suggesting that the TOPAZ system could be used for a detailed study of the Amazon River plume dynamics. Both the position and speed of the North Brazilian Current as well as the spreading of the Amazon River plume are monitored. In particular, a recurrent mechanism was observed for the spreading of the rivers plumes, notably that the fresh water is usually advected toward the Caribbean Sea by the North Brazilian Current but get diverted into the tropical Atlantic when North Brazilian Current Rings are shed.

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume.

Biological N2 fixation is an important nitrogen source for surface ocean microbial communities. However, nearly all information on the diversity and gene expression of organisms responsible for oceanic N2 fixation in the environment has come from targeted approaches that assay only a small number of genes and organisms. Using genomes of diazotrophic cyanobacteria to extract reads from extensive meta-genomic and -transcriptomic libraries, we examined diazotroph diversity and gene expression from the Amazon River plume, an area characterized by salinity and nutrient gradients. Diazotroph genome and transcript sequences were most abundant in the transitional waters compared with lower salinity or oceanic water masses. We were able to distinguish two genetically divergent phylotypes within the Hemiaulus-associated Richelia sequences, which were the most abundant diazotroph sequences in the data set. Photosystem (PS)-II transcripts in Richelia populations were much less abundant than those in Trichodesmium, and transcripts from several Richelia PS-II genes were absent, indicating a prominent role for cyclic electron transport in Richelia. In addition, there were several abundant regulatory transcripts, including one that targets a gene involved in PS-I cyclic electron transport in Richelia. High sequence coverage of the Richelia transcripts, as well as those from Trichodesmium populations, allowed us to identify expressed regions of the genomes that had been overlooked by genome annotations. High-coverage genomic and transcription analysis enabled the characterization of distinct phylotypes within diazotrophic populations, revealed a distinction in a core process between dominant populations and provided evidence for a prominent role for noncoding RNAs in microbial communities.

Production of dissolved organic carbon enriched in deoxy sugars representing an additional sink for biological C drawdown in the Amazon River plume

AbstractIn North Atlantic waters impacted by discharges from the Amazon and Orinoco Rivers, where planktonic diatom‐diazotroph associations (DDA) were active, we observed that an average (± standard deviation) of 61 ± 12% of the biological drawdown of dissolved inorganic carbon (DIC) was partitioned into the accumulating total organic carbon pool, representing a flux of up to 9 ± 4 Tg C yr−1. This drawdown corresponded with chemical alteration of ultrafiltered dissolved organic matter (UDOM), including increases in stable C isotopic composition (δ13C) and C:N. The dissolved carbohydrate component of UDOM also increased with biological DIC drawdown and diatom‐associated diazotroph (i.e., Richelia) abundance. New carbohydrates could be distinguished by distinctively high relative abundances of deoxy sugars (up to 55% of monosaccharides), which may promote aggregate formation and enhance vertical carbon export. The identified production of non‐Redfieldian, C‐enriched UDOM thus suggests a mechanism to explain enhanced C sequestration associated with DDA N2 fixation, which may be widespread in mesohaline environments.

Microspatial gene expression patterns in the Amazon River Plume.

We investigated expression of genes mediating elemental cycling at the microspatial scale in the ocean's largest river plume using, to our knowledge, the first fully quantitative inventory of genes and transcripts. The bacterial and archaeal communities associated with a phytoplankton bloom in Amazon River Plume waters at the outer continental shelf in June 2010 harbored ∼ 1.0 × 10(13) genes and 4.7 × 10(11) transcripts per liter that mapped to several thousand microbial genomes. Genomes from free-living cells were more abundant than those from particle-associated cells, and they generated more transcripts per liter for carbon fixation, heterotrophy, nitrogen and phosphorus uptake, and iron acquisition, although they had lower expression ratios (transcripts ⋅ gene(-1)) overall. Genomes from particle-associated cells contributed more transcripts for sulfur cycling, aromatic compound degradation, and the synthesis of biologically essential vitamins, with an overall twofold up-regulation of expression compared with free-living cells. Quantitatively, gene regulation differences were more important than genome abundance differences in explaining why microenvironment transcriptomes differed. Taxa contributing genomes to both free-living and particle-associated communities had up to 65% of their expressed genes regulated differently between the two, quantifying the extent of transcriptional plasticity in marine microbes in situ. In response to patchiness in carbon, nutrients, and light at the micrometer scale, Amazon Plume microbes regulated the expression of genes relevant to biogeochemical processes at the ecosystem scale.

Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River plume

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.

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.

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO2 uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO3) and silicate (Si(OH)4) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35mmolm−2d−1, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1millionkm2. We estimate that 0.15TmolCy−1 is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on Corg export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.

The pathways and properties of the Amazon River Plume in the tropical North Atlantic Ocean

The Amazon River Plume spreads across the tropical North Atlantic creating a barrier to vertical mixing. Here using a 1/6° HYCOM model and data from three research cruises in May–June 2010, September–October 2011, and July 2012, we investigate the pathways and properties of the plume. Four plume pathways for export of freshwater from the western tropical North Atlantic are identified. These consist of direct and indirect pathways to the northwest, and eastward pathways toward the subtropical gyre and toward Africa in the North Equatorial Counter Current. Because of the seasonality and cooccurrence of these pathways, plume characteristics are highly variable. Two pathways export water to the Caribbean, however the time scales associated with those direct and indirect pathways (3 versus 6+ months) differ, leading to different salinity characteristics of the plume water. Models results show that the Amazon river and tropical precipitation have similar magnitude impact on the observed seasonal cycle of freshwater within the western tropical Atlantic and at the 8°N, 38°W PIRATA mooring. Freshwater associated with the Amazon also influences surface salinity in winter as far as 20W in the model. The mean plume salinity minimum leads maximum discharge, highlighting the importance of currents and advection rather than discharge in maintaining plume properties. Plume pathways are tied to the underlying current structure, with the North Equatorial Counter Current jet preventing direct freshwater transport into the southern hemisphere. The plume influences underlying currents as well, generating vertical current shear that leads to enhanced eddy stirring and mixing in the model simulations.

Influence of the Amazon River discharge on the biogeography of phytoplankton communities in the western tropical north Atlantic

An Advanced Laser Fluorometer (ALF) capable of discriminating several phytoplankton pigment types was utilized in conjunction with microscopic data to map the distribution of phytoplankton communities in the Amazon River plume in May–June-2010, when discharge from the river was at its peak. Cluster analysis and Non-metric Multi-Dimensional Scaling (NMDS) helped distinguish three distinct biological communities that separated largely on the basis of salinity gradients across the plume. These three communities included an “estuarine type” comprised of a high biomass mixed population of diatoms, cryptophytes and green-water Synechococcus spp. located upstream of the plume, a “mesohaline type” made up largely of communities of Diatom-Diazotroph Associations (DDAs) and located in the northwestern region of the plume and an “oceanic type” in the oligotrophic waters outside of the plume made up of Trichodesmium and Synechococcus spp. Although salinity appeared to have a substantial influence on the distribution of different phytoplankton groups, ALF and microscopic measurements examined in the context of the hydro-chemical environment of the river plume, helped establish that the phytoplankton community structure and distribution were strongly controlled by inorganic nitrate plus nitrite (NO3+NO2) availability whose concentrations were low throughout the plume. Towards the southern, low-salinity region of the plume, NO3+NO2 supplied by the onshore flow of subsurface (∼80m depth) water, ensured the continuous sustenance of the mixed phytoplankton bloom. The large drawdown of SiO3 and PO4 associated with this “estuarine type” mixed bloom at a magnitude comparable to that observed for DDAs in the mesohaline waters, leads us to contend that, diatoms, cryptophytes and Synechococcus spp., fueled by the offshore influx of nutrients also play an important role in the cycling of nutrients in the Amazon River plume.

Multi-regime non-Gaussian data filling for incomplete ocean datasets

A method is introduced for improved estimation of missing data that preserves the multi-regime characteristics of a dataset. The approach analyzes regime change in spatial time series by applying an Expectation–Maximization algorithm (an iterative procedure that finds the Maximum Likelihood Estimate of statistical model parameters) for the determination of a Gaussian Mixture Model (GMM). We estimate the GMM when only a linear noisy measurement of the underlying process is available. We demonstrate the validity of the method using an idealized dataset and also by applying the method to equatorial sea surface salinity observed by the TAO/TRITON array. A percentage of the total observations is systematically extracted and predicted using the method to allow for validation. Finally, the approach is applied to recently available remote sea surface salinity from the SMOS satellite in the Amazon River plume region. Areas of large noise levels (reduced signal-to-noise ratios) are considered as missing data and predicted with the proposed approach. The method interprets regime changes and provides reconstructions of missing information based on the mean and covariability within each regime.

The distribution of thiamin and pyridoxine in the western tropical North Atlantic Amazon River plume

B-vitamins are recognized as essential organic growth factors for many organisms, although little is known about their abundance and distribution in marine ecosystems. Despite their metabolic functions regulating important enzymatic reactions, the methodology to directly measure different B-vitamins in aquatic environments has only recently been developed. Here, we present the first direct measurements of two B-vitamins, thiamin (B1), and pyridoxine (B6), in the Amazon River plume-influenced western tropical North Atlantic (WTNA) Ocean, an area known to have high productivity, carbon (C) and dinitrogen (N2) fixation, and C sequestration. The vitamins B1 and B6 ranged in concentrations from undetectable to 230 and 40 pM, respectively. Significantly higher concentrations were measured in the surface plume water at some stations and variation with salinity was observed, suggesting a possible riverine influence on those B-vitamins. The influences of vitamins B1 and B6 on biogeochemical processes such as C and N2 fixation were investigated using a linear regression model that indicated the availability of those organic factors could affect these rates in the WTNA. In fact, significant increases in C fixation and N2 fixation were observed with increasing vitamin B1 concentrations at some low and mesohaline stations (stations 9.1 and 1; p value <0.017 and <0.03, respectively). N2 fixation was also found to have a significant positive correlation with B1 concentrations at station 1 (p value 0.029), as well as vitamin B6 at station 9.1 (p value <0.017). This work suggests that there can be a dynamic interplay between essential biogeochemical rates (C and N2 fixation) and B-vitamins, drawing attention to potential roles of B-vitamins in ecosystem dynamics, community structure, and global biogeochemistry.

Characteristics of the seasonal cycle of surface layer salinity in the global ocean

Abstract. The seasonal variability of surface layer salinity (SLS), evaporation (E), precipitation (P), E-P, advection and vertical entrainment over the global ocean is examined using in situ salinity data, the National Centers for Environmental Prediction's Climate System Forecast Reanalysis and a number of other ancillary data. Seasonal amplitudes and phases are calculated using harmonic analysis and presented in all areas of the open ocean between 60° S and 60° N. Areas with large amplitude SLS seasonal variations include: the intertropical convergence zone (ITCZ) in the Atlantic, Pacific and Indian Oceans; western marginal seas of the Pacific; and the Arabian Sea. The median amplitude in areas that have statistically significant seasonal cycles of SLS is 0.19. Between about 60° S and 60° N, 37% of the ocean surface has a statistically significant seasonal cycle of SLS and 75% has a seasonal cycle of E-P. Phases of SLS have a bimodal distribution, with most areas in the Northern Hemisphere peaking in SLS in March/April and in the Southern Hemisphere in September/October. The seasonal cycle is also estimated for surface freshwater forcing using a mixed-layer depth climatology. With the exception of areas near the western boundaries of the North Atlantic and North Pacific, seasonal variability is dominated by precipitation. Surface freshwater forcing also has a bimodal distribution, with peaks in January and July, 1–2 months before the peaks of SLS. Seasonal amplitudes and phases calculated for horizontal advection show it to be important in the tropical oceans. Vertical entrainment, estimated from mixed-layer heaving, is largest in mid and high latitudes, with a seasonal cycle that peaks in late winter. The amplitudes and phases of SLS and surface fluxes compare well in a qualitative sense, suggesting that much of the variability in SLS is due to E-P. However, the amplitudes of SLS are somewhat different than would be expected and the peak of SLS comes typically about one month earlier than expected. The differences of the amplitudes of the two quantities is largest in such areas as the Amazon River plume, the Arabian Sea, the ITCZ and the eastern equatorial Pacific and Atlantic.

Impact of diatom‐diazotroph associations on carbon export in the Amazon River plume

Offshore tropical river plumes are associated with areas of high N2 fixation (diazotrophy) and biological carbon drawdown. Episodic blooms of the diatom Hemiaulus hauckii and its diazotrophic cyanobacterial symbiont Richelia intracellularisare believed to dominate that carbon drawdown, but the mechanism is not well understood. We report primary productivity associated with blooms of these diatom‐diazotroph assemblages (DDAs) in the offshore plume of the Amazon River using simultaneous measurements of O2/Ar ratios and the triple‐isotope composition of dissolved O2. In these blooms, we observe peaks in net community productivity, but relatively small changes in gross primary productivity, suggesting that DDA blooms increase the ecosystem carbon export ratio more than twofold. These events of enhanced export efficiency lead to biological uptake of dissolved inorganic carbon and silicate, whose longer mixed‐layer residence times otherwise obscure the differential impact of DDAs. The shorter‐term rate estimates presented here are consistent with the results derived from longer‐term geochemical tracers, confirming that DDAs drive a significant biological CO2 pump in tropical oceans.

Spatial and temporal coherence between Amazon River discharge, salinity, and light absorption by colored organic carbon in western tropical Atlantic surface waters

The temporal evolution and spatial distribution of surface salinity and colored detrital matter (cdm) were evaluated within and adjacent to the Amazon River Plume. Study objectives were as follows: first, to document the spatial coherence between Amazon discharge, salinity, cdm, and the nature of the salinity‐cdm relationship; second, to document the temporal and spatial variability of cdm along the trajectory of the low‐salinity Amazon Plume, and third, to explore the departure of cdm from conservative mixing behavior along the plume trajectory into the open ocean. Time series (2003–2007) of surface salinity estimated using the Advanced Microwave Scanning Radiometer–Earth Observing System and corresponding satellite cdm absorption (acdm) data documented a plume of freshened, colored water emanating from the Amazon. Salinity and acdm were generally coherent, but there were regions in which spatial patterns of salinity and acdm did not coincide. Salinity was oppositely phased with discharge, whereas acdm was in phase but lagged discharge and typically remained high after maximum discharge. Along the river plume trajectory, acdm was inversely correlated with salinity, yet there was considerable deviation from conservative mixing behavior during all seasons. Positive anomalies in a linear relationship between salinity and acdm corresponded to areas of enhanced satellite‐retrieved net primary productivity, suggesting the importance of phytoplankton biomass or its subsequent remineralization as a source of cdm. Negative anomalies tended to predominate at the distal sections of the plume trajectories, an observation consistent with the process of photo‐oxidation of cdm over observed time scales of days to weeks.

Influence of the Amazon River on dissolved and intra-cellular metal concentrations in &amp;lt;i&amp;gt;Trichodesmium&amp;lt;/i&amp;gt; colonies along the western boundary of the sub-tropical North Atlantic Ocean

Abstract. Despite the ecological importance of Trichodesmium spp. for the global oceanic nitrogen budget, there is limited information on their trace metal composition in field samples. We report dissolved (&lt;0.22 μm) metal concentrations measured in surface waters (Ag, Cd, Co, Cu, Fe, Mo, Ni, P, Pb and V) and in the total and the intracellular pool (Ag, Al, Cd, Co, Cu, Fe, Mn, Mo, Ni, P, Pb, V) of Trichodesmium populations collected in the western subtropical North Atlantic Ocean (April–May 2003) within the influence of the Amazon River plume. Dissolved element distributions were strongly influenced by the River discharge, with concentrations of some elements varying directly (i.e. Cd, Mo and V) or inversely (Ag, Co, Cu, Fe, Ni, P and Pb) with surface salinity. Intracellular metal values to phosphorous ratios (mol:mol) for Cd, Co, Cu, Fe, Mn, Mo, Ni and V ranged from 9.0 × 10−6 for Cd to 4.4 × 10−2 for Fe. Although total metal composition was significantly correlated with the intracellular content in the Trichodesmium colonies for some elements (e.g., Co, Cu, V), metal pools in the phytoplankton did not co-vary with the dissolved metal concentrations, suggesting that water column measurements may not be good predictors of the intracellular metal concentrations. The impact of physical parameters and bioactive elements on biological processes such as nitrogen fixation, carbon drawdown and biomass production in Trichodesmium colonies was explored by using a principal component analysis test (PCA). The analysis indicated that the biological drawdown of dissolved inorganic carbon (DIC) by Trichodesmium seems to be influenced by the internal content of Fe, Co, Cd, and Cu, while nitrogen fixation seems more influenced by mixed layer depth and dissolved Fe and Ni concentrations.

Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N2 fixation in the tropical Atlantic Ocean

The abundances of six N₂-fixing cyanobacterial phylotypes were profiled at 22 stations across the tropical Atlantic Ocean during June 2006, and used to model the contribution of the diazotrophs to N₂ fixation. Diazotroph abundances were measured by targeting the nifH gene of Trichodesmium, unicellular groups A, B, C (UCYN-A, UCYN-B and UCYN-C), and diatom-cyanobiont symbioses Hemiaulus-Richelia, Rhizosolenia-Richelia and Chaetoceros-Calothrix. West to east gradients in temperature, salinity and nutrients [NO₃⁻ + NO₂⁻, PO₄³⁻, Si(OH)₄] showed the influence of the Amazon River plume and its effect on the distributions of the diazotrophs. Trichodesmium accounted for more than 93% of all nifH genes detected, dominated the warmer waters of the western Atlantic, and was the only diazotroph detected at the equatorial upwelling station. UCYN-A was the next most abundant (> 5% of all nifH genes) and dominated the cooler waters of the eastern Atlantic near the Cape Verde Islands. UCYN-C was found at a single depth (200 m) of high salinity and low temperature and nutrients, whereas UCYN-B cells were widespread but in very low abundance (6.1 × 10¹ ± 4.6 × 10² gene copies l⁻¹). The diatom-cyanobionts were observed primarily in the western Atlantic within or near the high Si(OH)₄ input of the Amazon River plume. Overall, highest diazotroph abundances were observed at the surface and declined with depth, except for some subsurface peaks in Trichodesmium, UCYN-B and UCYN-A. Modelled contributions of Trichodesmium, UCYN-B and UCYN-A to total N₂ fixation suggested that Trichodesmium had the largest input, except for the potential of UCYN-A at the Cape Verde Islands.

Alkalinity distribution in the western North Atlantic Ocean margins

Total alkalinity (TA) distribution and its relationship with salinity (S) along the western North Atlantic Ocean (wNAO) margins from the Labrador Sea to tropical areas are examined in this study. Based on the observed TA‐S patterns, the mixing processes that control alkalinity distribution in these areas can be categorized into a spectrum of patterns that are bracketed by two extreme mixing types, i.e., alongshore current‐dominated and river‐dominated. Alongshore current‐dominated mixing processes exhibit a segmented mixing line with a shared mid‐salinity end‐member. In such cases (i.e., Labrador Sea, Gulf of Maine, etc.), the y‐intercept of the high salinity segment of the mixing line is generally higher than the local river alkalinity values, and it reflects the mixing history of the alongshore current. In contrast, in river‐dominated mixing (Amazon River, Caribbean Sea, etc.), good linear relationships between alkalinity and salinity are generally observed, and the zero salinity intercepts of the TA‐S regressions roughly match those of the regional river alkalinity values. TA‐S mixing lines can be complicated by rapid changes in the river end‐member value and by another river nearby with a different TA value (e.g., Mississippi‐Atchafalaya/Gulf of Mexico). In the wNAO margins, regression intercepts and river end‐members have a clear latitudinal distribution pattern, increasing from a low of ∼300 μmol kg−1 in the Amazon River plume to a high value between ∼500–1100 μmol kg−1 in the middle and high latitude margins. The highest value of ∼2400 μmol kg−1 is observed in the Mississippi River influenced areas. In addition to mixing control, biological processes such as calcification and benthic alkalinity production may also affect ocean margin alkalinity distribution. Therefore, deriving inorganic carbon system information in coastal oceans using alkalinity‐salinity relationships, in particular, those of generic nature, may lead to significant errors.