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The Atlantic Meridional Overturning Circulation (AMOC) entails vigorous thermohaline transformations in the subpolar North Atlantic (SPNA). There, warm and saline waters originating in the subtropics are converted into cooler and fresher waters by a combination of surface fluxes and sub-surface thermohaline mixing. Using microstructure measurements and a small-scale variance conservation framework, we quantify the diapycnal and isopycnal contributions to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for 65% of thermal and 84% of haline variance dissipation in the upper 400 m of the eastern SPNA, suggesting an important role of isopycnal stirring in regional water-mass transformations. By applying the tracer variance method to two tracers, we underscore the special significance of isopycnal stirring for tracers weakly coupled to density, such as biologically-active tracers. Our findings thus highlight the central role of isopycnal stirring in both the AMOC and biogeochemical dynamics within the SPNA.

Bays are often ecological hotspots within highly-productive eastern boundary upwelling systems. Though the physics of such bays are well understood, there is no consensus about the factors underlying their high productivity. Three weeks of high-temporal-resolution observations in two long, narrow bays (Rías Baixas, NW-Iberia), showed that during an upwelling pulse, deep, nutrient-rich isopycnals rose into the euphotic zone inside the rías in a few hours. The response of the isopycnals to changes in wind forcing is approximately three times faster inside the rías than the Ekman spin-up time, triggering rapid nutrient uptake and subsequent formation of a subsurface chlorophyll and production maximum. The tight coupling and rapid response of phytoplankton growth to wind forcing could explain the higher productivity of the rías, and also be at play in other upwelling bays with similar morphologies and orientations. Resolving short-term variability of physical–biological coupling is crucial to discern the future evolution of upwelling bays

Marine diazotrophs convert dinitrogen (N2) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N2 fixation rates. The measurements of N2 fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43–57 versus 45–63 Tg N yr−1 ; ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223±30 Tg N yr−1(mean ± standard error; same hereafter) compared to version 1 (74 ± 7 Tg N yr−1). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88±23 versus 20±2 Tg N yr−1), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40 ± 9 versus 10 ±2 Tg N yr−1). Moreover, version 2 estimates the N2 fixation rate in the Indian Ocean to be 35 ± 14 Tg N yr−1,which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional 15N2 bubble method yields lower rates in 69 % cases compared to the new 15N2 dissolution method. This updated ve

The CVFZ is a highly dynamic ocean margin characterised by two well-defined oceanographic features: the thermohaline Cape Verde Front (CVF), that separates the subtropical from the tropical waters of the Northeast Atlantic, and the Cape Blanc Giant Filament (CBGF), which exports the organic matter produced in the Mauritanian upwelling to the open ocean. During an oceanographic cruise in the summer of 2017, we sampled a hydrographic box encompassing the CVFZ. Full-depth profiles of absorption spectra of coloured (CDOM) and excitation-emission matrices of fluorescent (FDOM) dissolved organic matter were collected and examined to produce an assortment of CDOM indices and 5-FDOM PARAFAC components. In epipelagic waters, we identified distinctive optical signatures in open ocean waters north and south of the CVF and in coastal waters transported by the CBGF that provide new insights on the origin (terrestrial vs. marine), photochemical and microbial reactivity (lability vs. recalcitrance), and molecular properties (molecular weight, aromaticity) of dissolved organic matter (DOM). In mesopelagic waters, we optically characterised the water masses of opposing origin that meet in the CVFZ: North and South Atlantic Central Water, Subpolar Mode Water, Mediterranean Water, Antarctic Intermediate Water and Labrador Seawater. In bathypelagic waters, we recognised the bottom nepheloid layer close to the Mauritanian coast as a massive source of optically active DOM that is not noticeable in dissolved organic carbon (DOC) distibutions

Dataset used for analyses in the publication "Rapid Fluctuations of the Subsurface Chlorophyll Maximum in Response to Wind Forcing in a Long, Narrow Bay", ESS Open Achive, 2021

The Cape Verde Frontal Zone (CVFZ) is a highly dynamic region located in the southern boundary of the Canary Current Eastern Boundary Upwelling Ecosystem. Due to the interaction of the Cape Verde Front with the Mauritanian coastal upwelling, the area features large vertical and horizontal export fluxes of organic matter. Full-depth profiles were recorded during FLUXES I cruise, with four consecutive transects defining a box embracing the giant filament of Cape Blanc and the Cape Verde front. Fifteen levels were sampled in medium and long stations (down to 4000 dbar) and 10 levels in short stations (down to 2000 dbar) where inorganic nutrients (NO3, NO2, Si(OH)4 and PO4), dissolved organic carbon/total dissolved nitrogen (DOC/TDN), and suspended particulate organic carbon and nitrogen (POC and PON) were sampled. Micromolar concentrations of nutrient salts were determined simultaneously by segmented flow analysis in an Alliance Futura autoanalyser. The determination of suspended POC and PON was carried out by high temperature catalytic oxidation at 900 °C in a Perkin Elmer 2400 elemental analyser. DOC and TDN were analysed by high temperature catalytic oxidation at 680 °C with a Shimadzu TOC-V analyser connected in line with a TNM1 measuring unit. Alongside with water samples conductivity, temperature and depth (CTD; SeaBird SBE911 plus), and dissolved oxygen (SeaBird SBE43), fluorescence of chlorophyll (SeaPoint SCF), and turbidity (SeaPoint STM) were measured. CTD conductivity was calibrated with water samples taken from the rosette and analysed on board with a Guildline 8410-A Portasal salinometer. Samples for dissolved oxygen (O2) determination were analysed on board by the Winkler potentiometric method. The chlorophyll (Chl) fluorescence sensor was calibrated with water samples taken at 4 depths in the photic layer which were estimated fluorometrically by means of a Turner Designs bench fluorometer 10-AU

Bays within eastern boundary upwelling systems (EBUS) are ecological hot-spots featuring a diverse range of spatio-temporal dynamics. At the EBUSs’ poleward limit, upwelling occurs in short-lived (<1 week) pulses modulated by synoptic wind variability. The circulations in long, narrow bays can respond to these fluctuations within few hours. The short-term biological response to these pulses was investigated in two of these bays (Rias Baixas, NW-Iberia) with a two-week quasi-synoptic spatio-temporal survey in the summer 2018. A four-day-long upwelling pulse caused deep, nutrient-rich isopycnals to rise into the euphotic zone inside the bays, triggering a rapid (~1.7 days) nutrient uptake and formation of a subsurface chlorophyll maximum (~3.8 days). The phytoplankton biomass was transported rapidly toward deep, offshore waters when the winds weakened. These results suggest that high productivity in narrow bays is controlled by the transient exposure of deep, nutrient-rich waters to light during upwelling pulses

This dataset includes data collected during the cruise REMEDIOS-TL in the Ría de Pontevedra (NW Iberia) at station P2 (42.357°N, 8.773°W) from 29 June to 18 July 2018 onboard of the Research Vessel Ramón Margalef belonging to the Spanish Institude of Oceanography. The archived data are described in a manuscript entitled "Intense upper ocean mixing due to large aggregations of spawning fish" by Fernández Castro et al. published in Nature Geoscience: Fernández Castro, B., Peña, M., Nogueira, E. et al. Intense upper ocean mixing due to large aggregations of spawning fish. Nat. Geosci. 15, 287–292 (2022). https://doi.org/10.1038/s41561-022-00916-3. The manuscript presents evidence that night-time aggregations of anchovies produce intense ocean turbulence and mixing. All the data needed to support the conclusions of the article are included in this dataset

Small-scale turbulent mixing plays a pivotal role in shaping ocean circulation and a broad range of physical and biogeochemical processes. Despite advances in our understanding of the geophysical processes responsible for this mixing, the nature and importance of biomixing—turbulent mixing caused by marine biota—are still debated. A major source of uncertainty pertains to the efficiency of biomixing (the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically), which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in an area of coastal upwelling. We show that turbulent dissipation is elevated 10- to 100-fold (reaching 10−6–10−5 W kg−1) every night of the survey due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently with dissipation, and occurs with an efficiency comparable to that of geophysical turbulence. Our results demonstrate that biologically driven turbulence can be a highly effective mixing agent, and call for a re-examination of its impacts on productive upper ocean regions. Large groups of spawning fish can induce upper ocean mixing on the same scale as geophysical processes, according to observations of small-scale turbulence caused by anchovy spawning aggregations in a coastal upwelling area.

Small-scale turbulent mixing plays a pivotal role in shaping ocean circulation and a broad range of physical and biogeochemical processes. Despite advances in our understanding of the geophysical processes responsible for this mixing, the nature and importance of biomixing—turbulent mixing caused by marine biota—are still debated. A major source of uncertainty pertains to the efficiency of biomixing (the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically), which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in an area of coastal upwelling. We show that turbulent dissipation is elevated 10- to 100-fold (reaching 10−6–10−5 W kg−1) every night of the survey due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently with dissipation, and occurs with an efficiency comparable to that of geophysical turbulence. Our results demonstrate that biologically driven turbulence can be a highly effective mixing agent, and call for a re-examination of its impacts on productive upper ocean regions

Distributions of dissolved (DOM) and suspended (POM) organic matter, and their chromophoric (CDOM) and fluorescent (FDOM) fractions, are investigated at high resolution (< 10 km) in the Cape Verde Frontal Zone (CVFZ) during fall 2017. In the epipelagic layer (< 200 m), meso- and submesoscale structures (meanders, eddies) captured by the high resolution sampling dictate the tight coupling between physical and biogeochemical parameters at the front. Remarkably, fluorescent humic-like substances show relatively high fluorescence intensities between 50 and 150 m, apparently not related to local mineralization processes. We hypothesize that it is due to the input of Sahara dust, which transports highly re-worked DOM with distinctive optical properties. In the mesopelagic layer (200-1500 m), our results suggest that DOM and POM mineralization occurs mainly during the transit of the water masses from the formation sites to the CVFZ. Therefore, most of the local mineralization seems to be due to fast-sinking POM produced in situ or imported from the Mauritanian upwelling. These local mineralization processes lead to the production of refractory CDOM, an empirical evidence of the microbial carbon pump mechanism. DOM released from these fast-sinking POM is the likely reason behind the observed columns of relatively high DOC surrounded by areas of lower concentration. DOM and POM dynamics in the CVFZ has turned out to be very complex, in parallel to the complexity of meso- and submesoscale structures present in the area. On top of this high resolution variability, the input of Sahara dust or the release of DOM from sinking particles have been hypothesized to explain the observed distributions

The Cape Verde Frontal Zone (CVFZ) is a highly dynamic region located in the southern boundary of the Canary Current Eastern Boundary Upwelling Ecosystem. Due to the interaction of the Cape Verde Front with the Mauritanian coastal upwelling, the area features large vertical and horizontal export fluxes of organic matter. While the flux, composition and biogeochemical role of sinking organic matter have been thoroughly studied, much less attention has been paid to the dissolved (DOM) and suspended particulate (POM) organic matter fractions. Full-depth profiles of DOM and POM were recorded during an oceanographic cruise in the CVFZ, with four consecutive transects defining a box embracing the giant filament of Cape Blanc and the Cape Verde front. The distributions of DOM and POM and their C:N stoichiometric ratios in the epipelagic layer were strongly influenced by the position of the transects relative to the giant filament and the front. Geographical heterogeneity in POM and DOM distributions and elemental composition was also observed within each of the different water masses of contrasting origin present in the area (North and South Atlantic Central Water, Subpolar Mode Water, Mediterranean Water, Antarctic Intermediate Water, Labrador Sea Water and North East Atlantic Deep Water). These facts suggest that water masses properties are re-shaped by biogeochemical processes occurring within the CVFZ. Nevertheless, our analysis indicates that DOM and POM mineralisation represents only 8.1% of the inorganic carbon and 17.8% of the inorganic nitrogen produced by the local mineralisation of organic matter. Intense lateral export of POM and DOM out of the boundaries of the CVFZ is the likely reason behind these low contributions, which confirm the prominent role of sinking fluxes of organic matter for mineralisation processes in this region. The DOM distribution in the CVFZ interior is apparently affected by the dissolution of fast sinking particles

Dissolved organic matter (DOM) is produced in the surface and exported towards the deep ocean, adding ∼ 2 PgC/year to the global carbon export. Due to its central role in the Meridional Overturning Circulation, the eastern subpolar North Atlantic (eSPNA) contributes largely to this export. Here we quantify the transport and budget of dissolved organic nitrogen (DON) in the eSPNA, in a box delimited by the OVIDE 2002 section and the Greenland-Iceland-Scotland sills. The Meridional Overturning Circulation exports >15.9 TgN/year of DON downward and, contrary to the extended view that these are materials of subtropical origin, up to 33% of the vertical flux derives from a net local DON production of 7.1 ± 2.6 TgN/year. The low C:N molar ratio of DOM production (7.4 ± 4.1) and the relatively short transit times in the eSPNA (3 ± 1 year) suggest that local biogeochemical transformations result in the injection of fresh bioavailable DOM to the deep ocean

Today we know that diazotrophs are common and active in nitrogen (N) replete regions, however the factors controlling their distribution remain elusive. Previous studies in upwelling regions revealed that the composition of diazotrophs responded to changes in hydrodynamic forcing over seasonal scales. Here we used high-frequency observations collected during a 3-week cruise in the upwelling region off NW Iberia to describe changes in the activity and composition of diazotrophs over shorter temporal scales. The cruise started after a strong upwelling event followed by a few days of relaxation-downwelling, and soon after another upwelling pulse. Higher N2 fixation rates (2.2 ± 0.7 µmol m-3 d-1) were measured during relaxation-downwelling, when surface nitrate concentration was low. During the fertilization associated with the upwelling, N2 fixation dramatically decreased to 0.10 ± 0.09 µmol m-3 d-1. The comparison with nitrate consumption and diffusion confirmed the minor role of N2 fixation (<1%) as a source of new N for primary production. The unicellular cyanobacterium UCYN-A2 was the dominant diazotroph during the cruise. UCYN-A2 abundance was four times higher during relaxation-downwelling (4x104 copies L-1) compared to upwelling conditions (0.2x104 copies L-1), when the unusual Epsilonproteobacteria increased their relative abundance. These results indicate that diazotrophs can respond rapidly to changes in the environment, and point out to the availability of N as a key factor controlling the activity, composition and distribution of diazotrophs in eutrophic regions.

Dissolved organic matter (DOM) plays a major role in the recycling, export and sequestration of biogenic organic carbon, being a key component of ocean biogeochemical cycles and of the biological and microbial carbon pumps. Microbial degradation of DOM not only produces CO 2 but also generates dissolved molecules of decreasing bioavailability that can accumulate in the oceans for hundreds to thousands of years. The size-reactivity continuum (SRC) model is the conceptual framework to explain the DOM reactivity on a size basis, although field tests are still scarce and some of the pieces of this puzzle remain unclear. Taking advantage of the FLUXES-I cruise in the Cape Verde Frontal Zone (CVFZ), we have studied the size fractionated reactivity of the high (HMW; >1 KDa) and low (LMW; <1 KDa) molecular weight fractions of the DOM from surface down to 4000 m, using a high-efficiency and low-concentration-factor ultrafiltration cell. The wide ageing range covered by the water masses of the CVFZ makes it an excellent site to test the SRC model. Regarding the bulk C and N pools, the water masses with higher oxygen utilization were more depleted in HMW molecules, with a significant preference for the degradation of large N-containing compounds. Accordingly, preferential degradation of HMW fluorescent protein-like compounds was observed. In parallel, fluorescent humic-like compounds of both HMW and LMW were generated as by-product of the degradation of HMW organic compounds, and the remineralization of the DOM increases the aromaticy of both fractions, but especially the LMW one.

Ocean water masses have been traditionally characterized by the thermohaline and conservative chemical properties (e.g. preformed nutrients) at their respective source regions. However, water masses also can exhibit characteristic levels of other individual compounds or emerging properties associated to compound classes. In this regard, the objective of this contribution is to characterize the dissolved organic matter (DOM) molecular fingerprint of the water masses present in the Cape Vert Frontal Zone (CVFZ). For this purpose, a set of 133 samples was collected from the surface to 4000 m depth in the CVFZ during the FLUXES I cruise (12 July - 11 August 2017) and isolated by solid-phase extraction (SPE), using styrene divinyl benzene polymer cartridges (PPL). The molecular analysis of these SPE-PPL extracts was performed using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), a method capable of identifying thousands of molecular formulae in DOM. These analyses have been combined with an optimum multiparameter (OMP) water mass analysis to obtain characteristic molecular indices for the eleven water masses present in the CVFZ, stemming from the subtropical and subpolar North and South Atlantic as well as from the Arctic and Antarctic Oceans. In particular, emerging properties such as the molecular diversity (D), mean molecular mass (MW), mean C:N ratio, aromaticity index (AI), double bond equivalent (DBE), and main molecular groups, as well as different compounds (e.g. Lignin) and individual heteroatoms were quantified.

The Cape Verde Frontal Zone (CVFZ) in the southern boundary of the Canary Current Upwelling Ecosystem, is a highly dynamic area, featuring large vertical and horizontal export fluxes of organic matter (OM) due to the interaction of the Cape Verde Front (CVF) with the Mauritanian upwelling. To study the interplay between transport and biogeochemical processes driving the distribution of OM in the CVFZ, full-depth profiles of dissolved (DOM) and suspended particulate (POM) OM were obtained during the FLUXES I cruise in August 2017. Distributions of surface DOM and POM and their stoichiometry were influenced by the mesoscale variability at the frontal region, showing significant differences north and south of the CVF and between stations close and distant to the Mauritanian coast. The C&colon;N molar ratio of DOM and POM showed average vertical gradients, increasing from 12.1 and 8.0 in surface to 15.6 and 17.0 respectively in deeps waters, deviating from the traditional Redfield ratio. In the meso- and bathypelagic zones, meridional and cross-shore gradients were detected within samples belonging to the same water mass, indicating that their properties were re-shaped by biogeochemical processes within the CVFZ. Correlations between apparent oxygen utilization and OM indicate that DOM&plus;POM contributed only to 8.1&percnt; of the carbon and 17.8&percnt; of the nitrogen mineralisation in the water column, suggesting that the local carbon demand is mainly supported by sinking POM and N containing compounds are mineralised to a larger extend than C containing compounds

In long, narrow bays, the response to wind forcing is not mediated by Coriolis, allowing the depth of the pycnocline to respond more rapidly to changes in wind stress than offshore. The rapid, wind-driven Fuctuations of isopycnals -which expose deep, nutrient-rich waters to light-should lead to similarly rapid biological responses. However, due a lack of high-resolution observations, the dynamics and implications of these responses have not been explored. Here we investigated the response of phytoplankton to synoptic wind pulses in two bays (Rias Baixas, NW-Iberia) and offshore waters. We measured hydrographic properties, inorganic nutrients and chlorophyll-a during 3 weeks of spatial surveys and time-series stations in summer of 2018. The cruise began after an upwelling event, and sampled relaxation-downwelling conditions for a few days, followed by another upwelling pulse. The subsurface chlorophyll maximum was located on the same nutrient-rich isopycnal range (26.4-27 kg m3) in all surveys. During upwelling events, these isopycnals rapidly shoaled in the bays, but less so offshore. The transient exposure to light drove rapid increases in chlorophyll concentration in the bays. As the winds weakened, the isopycnals deepened, and phytoplankton biomass slowly decayed. These results show that phytoplankton respond rapidly to the transient exposure of deep, nutrient-rich isopycnals to light during upwelling pulses in bays. Short-lived phytoplankton blooms are thus controlled by the adiabatic, reversible upwelling of deep isopycnals rather than the fertilization of surface waters by diapycnal mixing

The circulation patterns in the confluence of the North Atlantic subtropical and tropical gyres delimited by the Cape Verde Front (CVF) were examined during a field cruise in summer 2017. We collected hydrographic data, dissolved oxygen (O2) and inorganic nutrients along the perimeter of a closed box embracing the Cape Verde Frontal Zone (CVFZ). The detailed spatial (horizontal and vertical) distribution of water masses, O2 and inorganic nutrients in the CVF was analyzed, allowing for the independent estimation of the transports of these properties in the subtropical and tropical domains down to 2000 m. Overall, at surface and central levels, a net westward transport of 3.76 Sv was observed, whereas at intermediate levels, a net 3 Sv transport northward was obtained. We observed O2 and inorganic nutrient imbalances in the domain consistent with O2 consumption and inorganic nutrient production by organic matter remineralization, resulting in a net transport of inorganic nutrients to the ocean interior by the circulation patterns

Small-scale turbulent mixing plays a pivotal role in shaping the circulation and properties of the ocean. Despite recent advances in understanding most aspects of this role, the nature and significance of biomixing — turbulent mixing effected by marine biota — remains controversial. The main point of contention pertains to the efficiency of biomixing, which the few in situ observations available suggest to be much lower than that of geophysical turbulence or than expected from bulk assessments of biomixing's large-scale influence. Here, we shed new light on this problem by analysing high-resolution measurements of small-scale turbulence in a coastal upwelling area. We show that turbulent dissipation is elevated 10—100-fold (up to 10-6-10-5 W kg-1) every night during the 14-day measurement record due to the swimming activity of anchovy spawning aggregations. Turbulent mixing is invigorated concurrently to dissipation, and occurs with an efficiency comparable to that of geophysical turbulence. Our results demonstrate that biologically-driven turbulence can be a highly effective mixing agent, and call for a re-examination of its impacts on productive upper-ocean regions