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The spatial extent of N2 fixation in the Atlantic Ocean is examined by determining the isotopic composition of N in suspended particulate organic nitrogen (δ15N PONsusp). The samples were collected from zonal and meridional transects of the Atlantic Ocean during a 3-year period. There is a consistent depleted δ15N PONsusp signal extending over the center of the northem subtropical gyre, which partly coincides with a region where the tracer N* increases westward following the gyre circulation. This nonconservative behavior of N* implies that N2 fixation is responsible for the depleted δ15N PONsusp. A mixing model suggests that N2 fixation over parts of the northern gyre provides up to 74% of the N utilized by phytoplankton. However, since the PONsusp represents only a small fraction of the total N pool, N2 fixation probably only plays a minor role in supplying new N to the euphotic zone in the surface waters of the northern subtropical gyre. [ABSTRACT FROM AUTHOR]

Observations of nitrate (NO3-) uptake across the photic zone of the tropical and subtropical Atlantic Ocean are described. High NO3- uptake rates were commonly found at depth (>75 m), coincident with the deep chlorophyll maximum and the nitracline, whereas highest rates of carbon fixation were found in near-surface waters (0-50 m). Thus NO3- based estimates of in situ new production (NO3- uptake x 6.6) at depth are in excess of in situ measurements of carbon fixation implying NO3- uptake exceeds production requirements. Such surplus NO3- uptake may ultimately result in the production of dissolved organic nitrogen with its measurement within the particulate phase representing a transient enrichment. When only upper euphotic zone (to 14% irradiance depth) rates of NO3- uptake and carbon fixation are considered, new production is less than total carbon fixation, suggesting, as expected, that additional nitrogen sources are required to support the observed production levels. Comparison of the deep NO3- uptake rates with literature estimates of seasonal NO3- drawdown within the nitracline suggests that the drawdown observed represents only 1% of the actual NO3- uptake occurring in this region of the water column, and hence most NO3- uptake must be recycled locally rather than exported. [ABSTRACT FROM AUTHOR]

Abstract: The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At between and during June and July 1996, between the UK and Falkland Islands during September and October 1997, and between South Africa and the UK during May and June 1998. Euphotic zone concentrations of varied between in the northern and southern gyres to a maximum of within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where ), surface elevations of were attributed to the photoinhibition of uptake mechanisms. concentrations were much less variable than , and showed the same general trend of minima within the gyres and maximum concentration (of ) in the Benguela upwelling system. The microbial uptake of was significantly correlated to and chlorophyll concentrations, whilst uptake was less dependent on concentration and showed no association with other environmental variables. Variability in the nitrogen uptake:concentration ratio between oceanographic provinces was associated with physical conditions including the mixed-layer depth and the rate of diapycnal mixing; therefore, uptake rates cannot be predicted simply from the nutrient concentration. In contrast, the relationship between f-ratio (where the source is undefined, and may have both new and regenerated components) and concentration is defined robustly, so that the f-ratio can be predicted (within 95%) by either surface or depth integrated concentrations of . [Copyright &y& Elsevier]

Abstract: We discuss nitrous oxide (N2O) and methane (CH4) distributions in 49 vertical profiles covering the upper ∼300m of the water column along two ∼13,500km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N2O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH4 showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N2O concentrations coincident with very low levels of CH4, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N2O derived from nitrification, as identified by an analysis of N2O, apparent oxygen utilization (AOU) and NO3 −, and presumably depleted in CH4 by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68TgN2Oyr−1 and 0.81–1.43TgCH4 yr−1. Based on contemporary estimates of the global ocean source strengths of atmospheric N2O and CH4, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N2O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH4 than previously thought. [Copyright &y& Elsevier]

Abstract: Measurements of nitrate (NO3 −) uptake made using 15N tracers in the surface oligotrophic Atlantic Ocean on three meridional transects are compared. Cruises occurred over the course of 1 year (AMT12 May–June 2003, AMT13 September–October 2003, AMT14 April–May 2004). Significant differences (p <0.001) in mean daily integrated rates of NO3 − uptake are identified between the two boreal spring cruises (AMT12 and AMT14) over the entire tropical and subtropical ocean (~40°S–40°N) which is caused by widespread increases in ambient NO3 − concentrations in the surface ocean. The increase in NO3 − concentration is small, typically between 5–10 nmol L− 1 but promotes a significant uptake response. Carbon fixation rates meanwhile show small increases during AMT14 relative to AMT12 in the equatorial region but not in gyral latitudes suggesting that the increased NO3 − did not generally increase production rates but may instead have invoked a shift in the preferred form of nitrogen used by photosynthetic organisms. A greater proportion of the observed production might therefore have been available for export during 2004 compared to 2003. Observed intercruise differences in the separation distance between the base of the mixed layer and the nitracline suggest that this may be an important physical consideration which allows for greater nutrient transfer during periods of smaller separation, as was the case during 2004, and that this represents a potential control on export production. [Copyright &y& Elsevier]

Abstract: We report over 600 absorption spectra (250–800nm) of unfiltered surface waters, <250m depth, collected at daily stations during three Atlantic Meridional Transect cruises (AMT9, 10, 11) between the UK and Uruguay. AMT cruises 9 and 11 were southbound (15 September 1999–13 October 1999 and 12 September 2000–11 October 2000, respectively), while AMT10 was northbound (12 April 2000–8 May 2000). Absorption coefficients at 300nm, a 300, ranged from 0.13 to 1.32m−1, showed insignificant differences between filtered and unfiltered samples, and were therefore attributed to chromophoric dissolved organic matter (CDOM). A non-linear single exponential regression provided the best fit to our CDOM absorbance spectra and was used to parameterise the spectral slope, S, of the monotonic absorbance decrease with increasing wavelength over the wavelength ranges 290–350nm (S 290–350) and 250–650nm (S 250–650). We observed distinct patterns in the latitudinal and depth distribution of CDOM absorbance characteristics. Distinct subsurface a 300 maxima, characterised by lowest spectral slope values (S 290–350=0.010nm−1 and S 250–650=0.014nm−1) were observed in the vicinity of the deep chlorophyll maximum (DCM) in open ocean and upwelling regions, and indicated in-situ production of CDOM. Converserly, the surface a 300 minima and surface S 290–350 and S 250–650 maxima in these regions were attributed to CDOM photo-oxidation. In order to assess the nature of the observed CDOM variability along our AMT transects, we grouped our data into 12 individual, oceanographic provinces, and further into two seasons (spring: April–May, and Autumn: September–October) and three depth zones (surface mixed layer, pycnocline, and below pycnocline). Comparisons between individual provinces, seasons and depth zones indicated that CDOM variability was dominated by regional factors and depth distribution patterns, while seasonal variability was generally less important in our data. Based on depth distribution patterns together with analyses of inter-relations between a 300, and S 290–350, S 250–650 we propose that our data reflect the presence of two CDOM end-members, each characterised by distinct spectral slope factors, S 290–350 and S250–650. The first CDOM end-member (S 290–350=0.010 and S 250–650=0.014nm−1) was situated in the vicinity of the DCM, and was attributed to CDOM production from phytoplankton-derived organic matter via planktonic foodweb interactions. The second end-member (S 290–350=0.028nm−1 and S 250–650=0.029nm−1) was attributed to microbial CDOM production. We propose that the CDOM distribution along the AMT cruise track is controlled by autochthonous production near the DCM and subsequent photo-oxidation in surface waters. [Copyright &y& Elsevier]

Abstract: Recent experimental data from the subtropical NE Atlantic appear to support the prevalence of net heterotrophy in unproductive pelagic ecosystems. However, the proximity of these studies to the NW African upwelling does not exclude the possibility that remote areas of the oceanic gyres are in metabolic balance. Here we present measurements of plankton gross photosynthesis (GPP) and community respiration (CR) made during a latitudinal transect (Atlantic Meridional Transect (AMT) cruise 11, 49°N–33°S) that traversed five biogeochemical provinces of the Atlantic Ocean, including both the subtropical NE Atlantic and the central part of the South Atlantic subtropical gyre. In these oligotrophic provinces, the euphotic zone average chlorophyll a concentration (0.15±0.01 and 0.16±0.02mgm−3, respectively), the relative contribution of picoplankton (%Chla<2μm 75±1 and 76±1%, respectively), and GPP (0.41±0.12 and 0.47±0.09mmolO2 m−3 d−1, respectively) were almost identical. However, net heterotrophy prevailed in the euphotic zone of the tropical NE Atlantic (−0.28±0.13mmolO2 m−3 d−1, ), while the net metabolism of the plankton community in the central S Atlantic gyre was autotrophic (0.20±0.02mmolO2 m−3 d−1, ). These results therefore suggest the existence of more than one type of unproductive open-ocean situation, that may be characterised by differences in the relative importance of local vs. allochthonous sources of organic matter. [Copyright &y& Elsevier]

Abstract: We have made daily measurements of phytoplankton pigments, size-fractionated (<2 and >2-μm) carbon fixation and chlorophyll-a concentration during four Atlantic Meridional Transect (AMT) cruises in 2003–04. Surface rates of carbon fixation ranged from <0.2-mmolCm−3 d−1 in the subtropical gyres to 0.2–0.5-mmolCm−3 d−1 in the tropical equatorial Atlantic. Significant intercruise variability was restricted to the subtropical gyres, with higher chlorophyll-a concentrations and carbon fixation in the subsurface chlorophyll maximum during spring in either hemisphere. In surface waters, although picoplankton (<2-μm) represented the dominant fraction in terms of both carbon fixation (50–70%) and chlorophyll-a (80–90%), nanoplankton (>2-μm) contributions to total carbon fixation (30–50%) were higher than to total chlorophyll-a (10–20%). However, in the subsurface chlorophyll maximum picoplankton dominated both carbon fixation (70–90%) and chlorophyll-a (70–90%). Thus, in surface waters chlorophyll-normalised carbon fixation was 2–3 times higher for nanoplankton and differences in picoplankton and nanoplankton carbon to chlorophyll-a ratios may lead to either higher or similar growth rates. These low chlorophyll-normalised carbon fixation rates for picoplankton may also reflect losses of fixed carbon (cell leakage or respiration), decreases in photosynthetic efficiency, grazing losses during the incubations, or some combination of all these. Comparison of nitrate concentrations in the subsurface chlorophyll maximum with estimates of those required to support the observed rates of carbon fixation (assuming Redfield stoichiometry) indicate that primary production in the chlorophyll maximum may be light rather than nutrient limited. [Copyright &y& Elsevier]

Abstract: The aims of the Atlantic Meridional Transect (AMT) programme [www.amt-uk.org] are to quantify the nature and causes of ecological and biogeochemical variability in the planktonic ecosystems of the Atlantic Ocean, and to assess the effects of this variability on biological carbon cycling and air–sea exchange of radiatively active gases and aerosols. Marine and atmospheric data have been collected twice a year along a 13,500km transect in the Atlantic Ocean between 50°N and 52°S since 1995. The cruise track enables biogeochemical measurements to be made within the poorly studied North and South Atlantic oligotrophic gyres as well as within equatorial and coastal upwelling regions. The range of ecosystems sampled during AMT has facilitated the calibration and validation of newly developed optical, microbiological, molecular and analytical techniques and provided a testbed for comparative ecology and the development of atmospheric and oceanographic models. This paper describes the rationale and methodology of the programme. Upper-ocean measurements of density, nitrate and chlorophyll a (Chl a) are presented to illustrate seasonal, inter-annual and decadal variability in hydrography. Seasonal distributions of dissolved nitrous oxide (N2O) and methane (CH4) are used to derive estimates of the sea–air flux of these gases in the South Atlantic Gyre. Observations made during AMT and published since 2000 are reviewed, and the key findings are highlighted. The extent to which the programme aims have been achieved is discussed and improvements for the future suggested. [Copyright &y& Elsevier]

Presents a study that investigated the basin scale variability of plankton respiration in relation to plankton community structure and activity in the eastern Atlantic Ocean. Examination of the latitudinal and depth distribution of community respiration; Relationship between gross production and community respiration; Measurement of plankton biomass and turnover rates; Analysis of the dissolved organic carbon production.

Mercury (Hg) in seawater is subject to interconversions via (photo)chemical and (micro)biological processes that determine the extent of dissolved gaseous mercury (DGM) (re)emission and the production of monomethylmercury. We investigated Hg speciation in the South Atlantic Ocean on a GEOTRACES cruise along a 40°S section between December 2011 and January 2012 (354 samples collected at 24 stations from surface to 5250 m maximum depth). Using statistical analysis, concentrations of methylated mercury (MeHg, geometric mean 35.4 fmol L -1 ) were related to seawater temperature, salinity, and fluorescence. DGM concentrations (geometric mean 0.17 pmol L -1 ) were related to water column depth, concentrations of macronutrients and dissolved inorganic carbon (DIC). The first-ever observed linear correlation between DGM and DIC obtained from high-resolution data indicates possible DGM production by organic matter remineralization via biological or dark abiotic reactions. DGM concentrations projected from literature DIC data using the newly discovered DGM-DIC relationship agreed with published DGM observations., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N2). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (∼0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north-south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial-temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales., Competing Interests: The authors declare no conflict of interest.

Ocean acidification has been suggested as a serious threat to the future existence of cold-water corals (CWC). However, there are few fine-scale temporal and spatial datasets of carbonate and nutrients conditions available for these reefs, which can provide a baseline definition of extant conditions. Here we provide observational data from four different sites in the northeast Atlantic that are known habitats for CWC. These habitats differ by depth and by the nature of the coral habitat. At depths where CWC are known to occur across these sites the dissolved inorganic carbon ranged from 2088 to 2186 μmol kg(-1), alkalinity ranged from 2299 to 2346 μmol kg(-1), and aragonite Ω ranged from 1.35 to 2.44. At two sites fine-scale hydrodynamics caused increased variability in the carbonate and nutrient conditions over daily time-scales. The observed high level of variability must be taken into account when assessing CWC sensitivities to future environmental change.

We report a high performance autonomous analytical system based on the vanadomolybdate method for the determination of soluble reactive phosphorus in seawater. The system combines a microfluidic chip manufactured from tinted poly (methyl methacrylate) (PMMA), a custom made syringe pump, embedded control electronics and on-board calibration standards. This "lab-on-a-chip" analytical system was successfully deployed and cross-compared with reference analytical methods in coastal (south west England) and open ocean waters (tropical North Atlantic). The results of the miniaturized system compared well with a reference bench-operated phosphate auto-analyser and showed no significant differences in the analytical results (student's t-test at 95% confidence level). The optical technology used, comprising of tinted PMMA and polished fluidic channels, has allowed an improvement of two orders of magnitude of the limit of detection (52 nM) compared to currently available portable systems based on this method. The system has a wide linear dynamic range 0.1-60 μM, and a good precision (13.6% at 0.4 μM, n=4). The analytical results were corrected for silicate interferences at 0.7 μM, and the measurement frequency was configurable with a sampling throughput of up to 20 samples per hour. This portable micro-analytical system has a low reagent requirement (340 μL per sample) and power consumption (756 J per sample), and has allowed accurate high resolution measurements of soluble reactive phosphorus in seawater., (© 2013 Elsevier B.V. All rights reserved.)

Little is known about the dynamics of dissolved phosphate in oligotrophic areas of the world's oceans, where concentrations are typically in the nanomolar range. Here, we have budgeted phosphate uptake by the dominant microbial groups in order to assess the effect of the microbial control of this depleted nutrient in the North Atlantic gyre. Low concentrations (2.2 +/- 1.2 nM) and rapid microbial uptake (2.1 +/- 2.4 nM day(-1)) of bioavailable phosphate were repeatedly determined in surface waters of the North Atlantic oligotrophic gyre during spring and autumn research cruises, using a radiotracer dilution bioassay technique. Upper estimates of the concentration of bioavailable phosphate were 7-55% of the dissolved mineral phosphate suggesting that a considerable part of the chemically measured nanomolar phosphate was in a form unavailable for direct microbial uptake. A 1:1 relationship (r(2) = 0.96, P < 0.0001) was observed between the bioavailable total phosphate uptake and the phosphate uptake of all the flow sorted bacterioplankton cells, demonstrating that bacterioplankton were the main consumers of phosphate. Within the bacterioplankton a group of heterotrophic bacteria and Prochlorococcus phototrophic cyanobacteria, were the two major competing groups for bioavailable phosphate. These heterotrophic bacteria had low nucleic acid content and 60% of them comprised of SAR11 clade cells based on the results of fluorescence in situ hybridization. Each of the two competing bacterial groups was responsible for an average of 45% of the phosphate uptake, while Synechococcus cyanobacteria (7%) and picoplanktonic algae (0.3%) played minor roles in direct phosphate uptake. We have demonstrated that phosphate uptake in the oligotrophic gyre is rapid and dominated by two bacterial groups rather than by algae.

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are major contributors to oceanic primary production. The genera are genetically diverse, comprising several known ecotypes or lineages. However, little is known of the distribution of these lineages over large geographic areas. Here, we analysed the relative abundance of Prochlorococcus ecotypes and Synechococcus lineages at the ocean basin scale along an Atlantic Meridional Transect (AMT) using dot blot hybridization and fluorescence in situ hybridization (FISH) techniques. The transect covered several contrasting oceanic provinces (gyres, upwelling, temperate regions) as well as environmentally 'equivalent' regions in the northern and southern hemisphere (northern and southern gyres and temperate regions). Flow cytometric data revealed a discrete separation in abundance of major picocyanobacterial genera. Prochlorococcus reached highest abundance in oligotrophic regions, while more mesotrophic waters were dominated by Synechococcus. Individual genetic lineages of both Prochlorococcus and Synechococcus showed highly similar distributions in corresponding regions in the northern and southern hemisphere. In addition, Prochlorococcus showed a distinctive depth distribution, with HLI and HLII ecotypes near the surface and co-occurring LL ecotypes further down in the water column. Conversely, Synechococcus generally revealed no obvious depth preference, but did show highly specific distribution at the horizontal scale, with clades I and IV particularly dominating temperate, mesotrophic waters in both the northern and southern hemispheres. The data clearly reveal that specific picocyanobacterial lineages proliferate in similar oceanic provinces separated by large spatial scales. Furthermore, comparison with an earlier AMT dataset suggests that basin scale distribution patterns for Prochlorococcus ecotypes are remarkably reproducible from year to year.

Prochlorococcus is the numerically dominant phytoplankter in the oligotrophic oceans, accounting for up to half of the photosynthetic biomass and production in some regions. Here, we describe how the abundance of six known ecotypes, which have small subunit ribosomal RNA sequences that differ by less than 3%, changed along local and basin-wide environmental gradients in the Atlantic Ocean. Temperature was significantly correlated with shifts in ecotype abundance, and laboratory experiments confirmed different temperature optima and tolerance ranges for cultured strains. Light, nutrients, and competitor abundances also appeared to play a role in shaping different distributions.