Data collected in 1988–1989, as part of the South Atlantic Ventilation Experiment, have been combined with the historical database to study the circulation and water mass variability of the abyssal water in the Argentine Basin. A map of potential temperature at 4000 m used as an indication of geostrophic shear defines a south and western intensified crescent‐shaped abyssal recirculation. Within this recirculation, and its northward extension to the Brazil Basin, Antarctic Bottom Water (AABW) properties have undergone two modifications during the 1980s: (1) The water mass cooled (0.05°C) and freshened (0.008 in salinity ratio) on surfaces of constant density. (2) The densest layer of AABW was altered to less dense water through mixing or advection out of the study area. This water mass change does not appear to have affected the flow pattern. Data collected in 1983 and 1988 to the north in the Brazil Basin show penetration of the freshwater mass in the deep western boundary current to between 18°S and 10°S, indicating very rapid propagation of the anomaly from the Argentine Basin into the Brazil Basin as a deep western boundary current. It is suggested that open ocean convective events within the Weddell Sea contributed to the change in AABW documented here.

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 61 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.

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 (NO 3 + NO 2) availability whose concentrations were low throughout the plume. Towards the southern, low-salinity region of the plume, NO 3 + NO 2 supplied by the onshore flow of subsurface ($80 m depth) water, ensured the continuous sustenance of the mixed phytoplankton bloom. The large drawdown of SiO 3 and PO 4 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.

[1] In this paper we use a coupled, 3-dimensional, biological-physical model, which includes an explicit, dynamic representation of Trichodesmium, to predict the distribution of Trichodesmium and rates of N 2 -fixation in the tropical and subtropical Atlantic Ocean. It is shown that the model reproduces the approximate observed meridional distribution of Trichodesmium in the Atlantic and elevated concentrations in specific coastal and open ocean regions where this organism is known to occur. The model also appears to reproduce the observed seasonality of Trichodesmium populations at higher latitudes (highest concentrations in summer and fall), but this seasonal cycle may be too pronounced at low latitudes. High and persistent Trichodesmium concentrations and rates of N 2 -fixation are generated by the model in the Gulf of Guinea off of Africa. This unexpected finding appears to be confirmed by historical measurements. In general, increased Trichodesmium concentrations develop in regions where the mixed layer is relatively thin (resulting in high mean light levels) and dissolved inorganic nitrogen (DIN) concentrations and phytoplankton biomass are low for extended periods of time. The model-predicted Trichodesmium distributions are therefore very sensitive to the fidelity of the physical model's representation of mixed layer depth variability, and upwelling intensity, and the biological model's estimated DIN and phytoplankton concentrations. The model generates a three-step successional sequence where (1) high DIN concentrations due to upwelling and/or mixing stimulate phytoplankton growth, followed by (2) Trichodesmium growth after DIN depletion and phytoplankton decline, followed by (3) enhanced phytoplankton growth due to new nitrogen inputs from N 2 -fixation. This sequence develops in response to seasonal variations in mixing in the southwestern North Atlantic and in response to upwelling along the coast of Africa and the equator. We interpret this sequence as representing a diatom-Trichodesmium-flagellate succession, which is consistent with observed species successions off of northwest Africa and in the Gulf of Mexico. The results presented in this paper lead us to conclude that our model includes the primary factors that dictate when and where Trichodesmium and N 2 -fixation occurs in the Atlantic. Moreover, it appears that our model reproduces some of the major effects that diazotrophically-derived inputs of new nitrogen have on the pelagic ecosystem.

[1] The Amazon River plume is a highly seasonal feature that can reach more than 3000 km across the tropical Atlantic Ocean, and cover $2 million km 2 . Ship observations show that its seasonal presence significantly reduces sea surface salinity and inorganic carbon. In the western tropical North Atlantic during April-May 2003, plume-influenced stations exhibited surface DIC concentrations lowered by as much as 563 mmol C kg À1 ($28%) and pCO 2 as low as 201 matm. We combine our data with other data sets to understand the annual uptake and seasonal variability of the plume-related CO 2 sink. Using flux estimates from all seasons with monthly plume areas determined by satellite, we calculate the annual carbon uptake by the outer plume alone (28 < S < 35) to be 15 ± 6 TgC yr À1 . Diazotroph-supported net community production enhanced the airsea CO 2 disequilibrium by 100x and reversed the typical CO 2 outgassing from the tropical North Atlantic. The carbon sink in the Amazon plume depends on climatesensitive conditions that control river hydrology, CO 2 solubility, and gas exchange.

Significance The microbial community of the Amazon River Plume determines the fate of the world’s largest input of terrestrial carbon and nutrients to the ocean. By benchmarking with internal standards during sample collection, we determined that each liter of plume seawater contains 1 trillion genes and 50 billion transcripts from thousands of bacterial, archaeal, and eukaryotic taxa. Gene regulation by taxa inhabiting distinct microenvironments provides insights into micron-scale patterns of transformations in the marine carbon, nitrogen, phosphorus, and sulfur cycles in this globally important ecosystem.

Marine ecosystem models have advanced to incorporate metabolic pathways discovered with genomic sequencing, but direct comparisons between models and "omics" data are lacking. We developed a model that directly simulates metagenomes and metatranscriptomes for comparison with observations. Model microbes were randomly assigned genes for specialized functions, and communities of 68 species were simulated in the Atlantic Ocean. Unfit organisms were replaced, and the model self-organized to develop community genomes and transcriptomes. Emergent communities from simulations that were initialized with different cohorts of randomly generated microbes all produced realistic vertical and horizontal ocean nutrient, genome, and transcriptome gradients. Thus, the library of gene functions available to the community, rather than the distribution of functions among specific organisms, drove community assembly and biogeochemical gradients in the model ocean.

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.