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  • Publication . Preprint . Other literature type . Article . 2022
    Open Access English
    Authors: 
    C. Wohl; C. Wohl; C. Wohl; C. Wohl; A. E. Jones; W. T. Sturges; P. D. Nightingale; P. D. Nightingale; P. D. Nightingale; B. Else; +3 more
    Publisher: Copernicus Publications
    Countries: Spain, United Kingdom, Spain

    The marginal sea ice zone has been identified as a source of different climate-active gases to the atmosphere due to its unique biogeochemistry. However, it remains highly undersampled, and the impact of summertime changes in sea ice concentration on the distributions of these gases is poorly understood. To address this, we present measurements of dissolved methanol, acetone, acetaldehyde, dimethyl sulfide, and isoprene in the sea ice zone of the Canadian Arctic from the surface down to 60 m. The measurements were made using a segmented flow coil equilibrator coupled to a proton-transfer-reaction mass spectrometer. These gases varied in concentrations with depth, with the highest concentrations generally observed near the surface. Underway (3–4 m) measurements showed higher concentrations in partial sea ice cover compared to ice-free waters for most compounds. The large number of depth profiles at different sea ice concentrations enables the proposition of the likely dominant production processes of these compounds in this area. Methanol concentrations appear to be controlled by specific biological consumption processes. Acetone and acetaldehyde concentrations are influenced by the penetration depth of light and stratification, implying dominant photochemical sources in this area. Dimethyl sulfide and isoprene both display higher surface concentrations in partial sea ice cover compared to ice-free waters due to ice edge blooms. Differences in underway concentrations based on sampling region suggest that water masses moving away from the ice edge influences dissolved gas concentrations. Dimethyl sulfide concentrations sometimes display a subsurface maximum in ice -free conditions, while isoprene more reliably displays a subsurface maximum. Surface gas concentrations were used to estimate their air–sea fluxes. Despite obvious in situ production, we estimate that the sea ice zone is absorbing methanol and acetone from the atmosphere. In contrast, dimethyl sulfide and isoprene are consistently emitted from the ocean, with marked episodes of high emissions during ice-free conditions, suggesting that these gases are produced in ice-covered areas and emitted once the ice has melted. Our measurements show that the seawater concentrations and air–sea fluxes of these gases are clearly impacted by sea ice concentration. These novel measurements and insights will allow us to better constrain the cycling of these gases in the polar regions and their effect on the oxidative capacity and aerosol budget in the Arctic atmosphere This work was supported by the Natural Environment Research Council through the EnvEast Doctoral Training Partnership (grant no. NE/L002582/1) and by the UK Department for Business, Energy and Industrial Strategy (United Kingdom & Canada Arctic Partnership: 2017 Bursaries Programme awarded to MY). Financial support was provided to Brent Else by the National Sciences and Engineering Research Council of Canada. This work is a contribution to ArcticNet, a Network of Centres of Excellence Canada 25 pages, 11 figures, supplement https://doi.org/10.5194/bg-19-1021-2022-supplement.-- Data availability: Data have been submitted to Polar Data Catalogue (https://www.polardata.ca/pdcsearch/), where the CCIN Reference number is 13249 and the DOI is https://doi.org/10.21963/13249 With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) Peer reviewed

  • Open Access English
    Authors: 
    Ridame, Céline; Dinasquet, Julie; Hallstrøm, Søren; Bigeard, Estelle; Riemann, Lasse; van Wambeke, France; Bressac, Matthieu; Pulido-Villena, Elvira; Taillandier, Vincent; Gazeau, Frédéric; +3 more
    Publisher: HAL CCSD
    Countries: France, Spain, Denmark

    N2 fixation rates were measured in the 0-1000ĝ€¯m layer at 13 stations located in the open western and central Mediterranean Sea (MS) during the PEACETIME cruise (late spring 2017). While the spatial variability in N2 fixation was not related to Fe, P nor N stocks, the surface composition of the diazotrophic community indicated a strong longitudinal gradient increasing eastward for the relative abundance of non-cyanobacterial diazotrophs (NCDs) (mainly 3-Proteobacteria) and conversely decreasing eastward for photo-heterotrophic group A (UCYN-A) (mainly UCYN-A1 and UCYN-A3), as did N2 fixation rates. UCYN-A4 and UCYN-A3 were identified for the first time in the MS. The westernmost station influenced by Atlantic waters and characterized by highest stocks of N and P displayed a patchy distribution of diazotrophic activity with an exceptionally high rate in the euphotic layer of 72.1ĝ€¯nmolNL-1d-1, which could support up to 19ĝ€¯% of primary production. At this station at 1ĝ€¯%PAR (photosynthetically available radiation) depth, UCYN-A4 represented up to 94ĝ€¯% of the diazotrophic community. These in situ observations of greater relative abundance of UCYN-A at stations with higher nutrient concentrations and dominance of NCDs at more oligotrophic stations suggest that nutrient conditions-even in the nanomolar range-may determine the composition of diazotrophic communities and in turn N2 fixation rates. The impact of Saharan dust deposition on N2 fixation and diazotrophic communities was also investigated, under present and future projected conditions of temperature and pH during short-Term (3-4ĝ€¯d) experiments at three stations. New nutrients from simulated dust deposition triggered a significant stimulation of N2 fixation (from 41ĝ€¯% to 565ĝ€¯%). The strongest increase in N2 fixation was observed at the stations dominated by NCDs and did not lead on this short timescale to changes in the diazotrophic community composition. Under projected future conditions, N2 fixation was either increased or unchanged; in that later case this was probably due to a too-low nutrient bioavailability or an increased grazing pressure. The future warming and acidification likely benefited NCDs (Pseudomonas) and UCYN-A2, while disadvantaged UCYN-A3 without knowing which effect (alone or in combination) is the driver, especially since we do not know the temperature optima of these species not yet cultivated as well as the effect of acidification. This study is a contribution to the PEACETIME project (http://peacetime-project.org, last access: 17 January 2022), a joint initiative of the MERMEX and ChArMEx components supported by the CNRS-INSU, IFREMER, CEA and Météo-France as part of the program MISTRALS coordinated by the INSU. PEACETIME was endorsed as a process study by GEOTRACES. Julie Dinasquet was funded by a Marie Curie Actions International Outgoing Fellowship (PIOF-GA-2013-629378). Søren Hallstrøm and Lasse Riemann were funded by grant 6108-00013 from the Danish Council for Independent Research. Peer reviewed

  • Open Access
    Authors: 
    P. M. Salgado-Hernanz; P. M. Salgado-Hernanz; A. Regaudie-de-Gioux; D. Antoine; D. Antoine; G. Basterretxea;
    Publisher: Copernicus GmbH
    Countries: Spain, France, France

    We estimated pelagic primary production (PP) in the coastal (300 g C m−2) associated with major river discharges to less productive provinces (<50 g C m−2) located in the southeastern Mediterranean. PP variability in the Mediterranean Sea is dominated by interannual variations, but a notable basin-scale decline (17 %) has been observed since 2012 concurring with a period of increasing sea surface temperatures in the Mediterranean Sea and positive North Atlantic Oscillation and Mediterranean Oscillation climate indices. Long-term trends in PP reveal slight declines in most coastal areas (−0.05 to −0.1 g C m−2 per decade) except in the Adriatic where PP increases at +0.1 g C m−2 per decade. Regionalization of coastal waters based on PP seasonal patterns reveals the importance of river effluents in determining PP in coastal waters that can regionally increase up to 5-fold. Our study provides insight into the contribution of coastal waters to basin-scale carbon balances in the Mediterranean Sea while highlighting the importance of the different temporal and spatial scales of variability. This article is a result of the Ministry of Economy and Competitiveness (MINECO) of Spain Project Fine-scale structure of cross-shore GRADIENTS along the Mediterranean coast (CTM2012-39476) and SifoMED (CTM2017-83774-P). P. M. Salgado-Hernanz was supported by a Ph.D. doctoral research fellowship FPI (Formación Personal Investigación) BES-2013-067305 from MINECO. Peer reviewed

  • Open Access
    Authors: 
    Loes J. A. Gerringa; Martha Gledhill; Indah Ardiningsih; Niels Muntjewerf; Luis M. Laglera;
    Publisher: Copernicus Puplications
    Country: Germany

    Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary predominantly in the choice of the added competing ligand. Although different applications show the same trends, [L] and logKcond differ between the applications. In this study, binding of two added ligands in three different common applications to three known types of natural binding ligands is compared. The applications are (1) salicylaldoxime (SA) at 25 µM (SA25) and short waiting time, (2) SA at 5 µM (SA5), and (3) 2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two with overnight equilibration. The three applications were calibrated under the same conditions, although having different pH values, resulting in the detection window centers (D) DTAC > DSA25 ≥ SA5 (as logD values with respect to Fe3+: 12.3 > 11.2 ≥ 11). For the model ligands, there is no common trend in the results of logKcond. The values have a considerable spread, which indicates that the error in logKcond is large. The ligand concentrations of the nonhumic model ligands are overestimated by SA25, which we attribute to the lack of equilibrium between Fe-SA species in the SA25 application. The application TAC more often underestimated the ligand concentrations and the application SA5 over- and underestimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the nonhumic model ligands, especially for SA5 and SA25. The estimated ligand concentrations for the humic and fulvic acids differed approximately 2-fold between TAC and SA5 and another factor of 2 between SA5 and SA25. The use of SA above 5 µM suffers from the formation of the species Fe(SA)x (x>1) that is not electro-active as already suggested by Abualhaija and van den Berg (2014). Moreover, we found that the reaction between the electro-active and non-electro-active species is probably irreversible. This undermines the assumption of the CLE principle, causes overestimation of [L] and could result in a false distinction into more than one ligand group. For future electrochemical work it is recommended to take the above limitations of the applications into account. Overall, the uncertainties arising from the CLE-AdCSV approach mean we need to search for new ways to determine the organic complexation of Fe in seawater.

  • Open Access English
    Authors: 
    Kai G. Schulz; Eric P. Achterberg; Javier Arístegui; Lennart T. Bach; Isabel Baños; Tim Boxhammer; Dirk V. Erler; Maricarmen Igarza; Verena Kalter; Andrea Ludwig; +7 more
    Countries: Germany, Denmark
    Project: EC | AQUACOSM (731065)

    Upwelling of nutrient-rich deep waters make eastern boundary upwelling systems (EBUSs), such as the Humboldt Current system, hot spots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic decomposition results in large subsurface water volumes being oxygen depleted. Under these circumstances, organic matter remineralisation can continue via denitrification, which represents a major loss pathway for bioavailable nitrogen. Additionally, anaerobic ammonium oxidation can remove significant amounts of nitrogen in these areas. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative offshore mesocosm experiment. Measured denitrification rates in incubations with water from the oxygen-depleted bottom layer of the mesocosms (via 15N label incubations) mostly ranged between 5.5 and 20 nmol N2 L−1 h−1 (interquartile range), reaching up to 80 nmol N2 L−1 h−1. However, actual in situ rates in the mesocosms, estimated via Michaelis–Menten kinetic scaling, did most likely not exceed 0.2–4.2 nmol N2 L−1 h−1 (interquartile range) due to substrate limitation. In the surrounding Pacific, measured denitrification rates were similar, although indications of substrate limitation were detected only once. In contrast, anammox (anaerobic ammonium oxidation) made only a minor contribution to the overall nitrogen loss when encountered in both the mesocosms and the Pacific Ocean. This was potentially related to organic matter C / N stoichiometry and/or process-specific oxygen and hydrogen sulfide sensitivities. Over the first 38 d of the experiment, total nitrogen loss calculated from in situ rates of denitrification and anammox was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between ∼ 1 and 5.5 µmol N L−1. This represents up to ∼  20 % of the initially bioavailable inorganic and organic nitrogen standing stocks. Interestingly, this loss is comparable to the total amount of particulate organic nitrogen that was exported into the sediment traps at the bottom of the mesocosms at about 20 m depth. Altogether, this suggests that a significant portion, if not the majority of nitrogen that could be exported to depth, is already lost, i.e. converted to N2 in a relatively shallow layer of the surface ocean, provided that there are oxygen-deficient conditions like those during coastal upwelling in our study. Published data for primary productivity and nitrogen loss in all EBUSs reinforce such conclusion.

  • Open Access English
    Authors: 
    Julie Dinasquet; Estelle Bigeard; Frédéric Gazeau; Farooq Azam; Cécile Guieu; Emilio Marañón; Céline Ridame; Ingrid Obernosterer; Anne-Claire Baudoux;
    Publisher: HAL CCSD
    Country: France

    In the oligotrophic waters of the Mediterranean Sea, during the stratification period, the microbial loop relies on pulsed inputs of nutrients through the atmospheric deposition of aerosols from both natural (e.g., Saharan dust), anthropogenic, or mixed origins. While the influence of dust deposition on microbial processes and community composition is still not fully constrained, the extent to which future environmental conditions will affect dust inputs and the microbial response is not known. The impact of atmospheric wet dust deposition was studied both under present and future environmental conditions (+3 ∘C warming and acidification of −0.3 pH units), through experiments in 300 L climate reactors. In total, three Saharan dust addition experiments were performed with surface seawater collected from the Tyrrhenian Sea, Ionian Sea, and Algerian basin in the western Mediterranean Sea during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean sea) cruise in May–June 2017. Top-down controls on bacteria, viral processes, and community, as well as microbial community structure (16S and 18S rDNA amplicon sequencing), were followed over the 3–4 d experiments. Different microbial and viral responses to dust were observed rapidly after addition and were, most of the time, more pronounced when combined with future environmental conditions. The dust input of nutrients and trace metals changed the microbial ecosystem from a bottom-up limited to a top-down controlled bacterial community, likely from grazing and induced lysogeny. The relative abundance of mixotrophic microeukaryotes and phototrophic prokaryotes also increased. Overall, these results suggest that the effect of dust deposition on the microbial loop is dependent on the initial microbial assemblage and metabolic state of the tested water and that predicted warming and acidification will intensify these responses, affecting food web processes and biogeochemical cycles.

  • Open Access
    Authors: 
    E. Marañón; F. Van Wambeke; J. Uitz; E. S. Boss; C. Dimier; J. Dinasquet; A. Engel; N. Haëntjens; M. Pérez-Lorenzo; V. Taillandier; +2 more
    Publisher: Biogeosciences
    Countries: France, France, France, France, Spain, Germany
    Project: EC | TRIATLAS (817578)

    The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant to our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both the biomass and primary production but not in the growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the ratio of carbon to chlorophyll a (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mg C m−3 at the surface to 10–15 mg C m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The ratio of fucoxanthin to total chlorophyll a increased markedly with depth, suggesting an increased contribution of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fuelled by new nutrients but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the peaks of deep primary production associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

  • Open Access English
    Authors: 
    Gonzalez-delgado, Sara; Gonzalez-santana, David; Santana-casiano, Magdalena; Gonzalez-davila, Melchor; Hernandez, Celso A.; Sangil, Carlos; Carlos Hernandez, Jose;
    Country: France

    We present a new natural carbon dioxide (CO2) system located off the southern coast of the island of La Palma (Canary Islands, Spain). Like CO2 seeps, these CO2 submarine groundwater discharges (SGDs) can be used as an analogue to study the effects of ocean acidification (OA) on the marine realm. With this aim, we present the chemical characterization of the area, describing the carbon system dynamics, by measuring pH, AT and CT and calculating Ω aragonite and calcite. Our explorations of the area have found several emission points with similar chemical features. Here, the CT varies from 2120.10 to 10 784.84 µmol kg−1, AT from 2415.20 to 10 817.12 µmol kg−1, pH from 7.12 to 8.07, Ω aragonite from 0.71 to 4.15 and Ω calcite from 1.09 to 6.49 units. Also, the CO2 emission flux varies between 2.8 and 28 kg CO2 d−1, becoming a significant source of carbon. These CO2 emissions, which are of volcanic origin, acidify the brackish groundwater that is discharged to the coast and alter the local seawater chemistry. Although this kind of acidified system is not a perfect image of future oceans, this area of La Palma is an exceptional spot to perform studies aimed at understanding the effect of different levels of OA on the functioning of marine ecosystems. These studies can then be used to comprehend how life has persisted through past eras, with higher atmospheric CO2, or to predict the consequences of present fossil fuel usage on the marine ecosystem of the future oceans.

  • Publication . Preprint . Article . Other literature type . 2021
    Open Access English
    Authors: 
    P. J. Tréguer; P. J. Tréguer; J. N. Sutton; M. Brzezinski; M. A. Charette; T. Devries; S. Dutkiewicz; C. Ehlert; J. Hawkings; J. Hawkings; +10 more
    Publisher: Copernicus GmbH
    Countries: France, Spain, France, France, Germany, Spain, France, France, France
    Project: EC | ICICLES (793962), EC | ICICLES (793962)

    The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean’s biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57% and 37% higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24% higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr-1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr-1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs D 14:8(+-2:6) Tmol Si yr-1 and outputs D 15:6(+-2:4) Tmol Si yr-1. Potential impacts of global change on the marine Si cycle are discussed This work was supported by the French National Research Agency (18-CEO1-0011-01) and by the Spanish Ministry of Science, Innovation and Universities (PID2019-108627RB-I00) 21 pages, 4 figures, 3 tables.-- This work is distributed under the Creative Commons Attribution 4.0 License Peer reviewed

  • Open Access
    Authors: 
    L. T. Bach; A. J. Paul; T. Boxhammer; E. von der Esch; M. Graco; K. G. Schulz; E. Achterberg; P. Aguayo; J. Arístegui; P. Ayón; +41 more
    Publisher: Copernicus Publications (EGU)
    Countries: Peru, Germany, Denmark

    Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can modify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes controlling organic matter cycling in the coastal Peruvian upwelling system. Eight mesocosms, each with a volume of ∼55 m3, were deployed for 50 d ∼6 km off Callao (12∘ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected subsurface waters at two different locations in the regional oxygen minimum zone (OMZ) and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the individual mesocosms. The mesocosm phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. sanguinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decoupled from surface production and sustained by the remaining plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self-shading by phytoplankton and by inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few opportunities for blooms, which represents an event where the system becomes unbalanced. Overall, our mesocosm study revealed some key links between ecological and biogeochemical processes for one of the most economically important regions in the oceans.