Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Mesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post‐upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with parallel factor analysis, two humic‐like, and two protein‐like components. Humic‐like FDOM components correlated to optode‐based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g., temperature) and to dissolved inorganic nitrogen. The tryptophan‐like FDOM components correlated with the carbon and nitrogen content of semi‐labile DOM, phytoplankton biomass, community respiration, and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies.〈/p〉

Description: Plain Language Summary: Mesoscale eddies are ubiquitous circulation features in the ocean with horizontal scales on the order of 100 km and lifetimes of days to months. Their swirling motion can cause nutrients from deeper waters to be transported to the surface, stimulating phytoplankton biomass and resulting in the production of dissolved organic matter. However, these effects are difficult to quantify and proxies (biomarkers) are needed to monitor the impact of eddies at high resolution. In this work, we used the optical properties of the dissolved organic matter, especially the fraction capable of fluorescence (FDOM) as biomarker in two cyclonic eddies, one formed in an eastern boundary upwelling system and one formed offshore by winds/Island interaction. We identified four FDOM components, among which an indicator of cyclonic eddy productivity and two indicators of dissolved organic matter recycling, which also tracked nutrient transport in the offshore cyclonic eddy. Our study highlights that continuous FDOM data obtained with sensors could help to follow eddy development and influence on seawater biogeochemistry.〈/p〉

Description: Key Points: 〈list list-type="bullet"〉 〈list-item〉 〈p xml:lang="en"〉Four fluorescent dissolved organic matter (FDOM) components were studied in two cyclonic eddies (CEs) in the Eastern Tropical North Atlantic〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Tryptophan‐like FDOM was an indicator of the CEs' productivity as it correlated with semi‐labile dissolved organic matter and microbial metabolic activities〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Humic‐like FDOM was a by‐product of microbial respiration; its distribution within an offshore CE covaried with nutrient upwelling〈/p〉〈/list-item〉 〈/list〉 〈/p〉

Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Description: Cruise M160 is part of concerted MOSES/REEBUS Eddy Study featuring three major research expeditions (M156, M160, MSM104). It aims to develop both a qualitative and quantitative understanding of the role of physical-chemical-biological coupling in eddies for the biological pump. The study is part of the MOSES “Ocean Eddies” event chain, which follows three major hypotheses to be addressed by the MOSES/REEBUS field campaigns: (1) Mesoscale and sub-mesoscale eddies play an important role in transferring energy along the energy cascade from the large-scale circulation to dissipation at the molecular level. (2) Mesoscale and sub-mesoscale eddies are important drivers in determining onset, magnitude and characteristics of biological productivity in the ocean and contribute significantly to global primary production and particle export and transfer to the deep ocean. (3) Mesoscale and sub-mesoscale eddies are important for shaping extreme biogeochemical environments (e.g., pH, oxygen) in the oceans, thus acting as a source/sink function for greenhouse gases. In contrast to the other two legs, MOSES Eddy Study II during M160 did not include any benthic work but focused entirely on the pelagic dynamics within eddies. It accomplished a multi-disciplinary, multi-parameter and multi-platform study of two discrete cyclonic eddies in an unprecedented complexity. The pre-cruise search for discrete eddies suitable for detailed study during M160 had already started a few months prior to the cruise. Remote sensing data products (sea surface height, sea surface temperature, ocean color/chlorophyll a) were used in combination with eddy detection algorithms and numerical modelling to identify and track eddies in the entire eddy field off West Africa. In addition, 2 gliders and 1 waveglider had been set out from Mindelo/Cabo Verde for pre-cruise mapping of the potential working area north of the Cabo Verdean archipelago. At the start of M160, a few suitable eddies – mostly of cyclonic type – had been identified, some of which were outside the safe operation range of the motorglider plane. As technical problems delayed the flight operations, the first eddy (center at 14.5°N/25°W) for detailed study was chosen to the southwest of the island of Fogo. It was decided to carry out a first hydrographic survey there followed by the deployment of a suite of instruments (gliders, waveglider, floats, drifter short-term mooring). Such instrumented, we left this first eddy and transited – via a strong anticyclonic feature southwest of the island of Santiago – to the region northeast of the island of Sal, i.e. in the working range of the glider plane. During the transit, a full suite of underway measurements as well as CTD/RO section along 22°W (16°-18.5°N) were carried in search for sub-surface expressions of anticyclonic eddy features. In the northeast, we had identified the second strong cyclonic eddy (center at 18°N/22.5°W) which was chosen for detailed study starting with a complete hydrographic survey (ADCP, CTD/RO, other routine station work). After completion of the mesoscale work program, we identified a strong frontal region at the southwestern rim of the cyclonic eddy, which was chosen for the first sub-mesoscale study with aerial observation component. There, the first dye release experiment was carried out which consisted of the dye release itself followed by an intense multi-platforms study of the vertical and horizontal spreading of the initial dye streak. This work was METEOR-Berichte, Cruise M160, Mindelo – Mindelo, 23.11.2019 4 – 20.12.2019 supported and partly guided by aerial observation of the research motorglider Stemme, which was still somewhat compromised by technical issues and meteorological conditions (high cloud cover, Saharan dust event). Nevertheless, this first dye release experiment was successful and showed rapid movement of the dynamic meandering front. After completion of work on this second eddy and execution of a focused sampling program at the Cape Verde Ocean Observation, RV METEOR returned to the first eddy for continuation of the work started there in the beginning of the cruise. This was accompanied by a relocation of the airbase of Stemme from the international airport of Sal to the domestic airport of Fogo. The further execution of the eddy study at this first eddy, which again included a complete hydrographic survey followed by a mesoscale eddy study with dye release, was therefore possible with aerial observations providing important guidance for work on RV METEOR. Overall, M160 accomplished an extremely intense and complex work program with 212 instrument deployments during station work, 137 h of observation with towed instruments and a wide range of underway measurements throughout the cruise. Up to about 30 individually tracked platforms (Seadrones, glider, wavegliders, drifters, floats) were in the water at the same time providing unprecedented and orchestrated observation capabilities in an eddy. All planned work components were achieved and all working groups acquired the expected numbers of instrument deployments and sampling opportunities.

Description: Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B-DOM), the quality of which (as enrichment in humic or protein-like substances) differed between strains. B-DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B-DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B-DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.

Description: Mesoscale eddies modulate the ocean's physical, chemical, and biological properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophy and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To fill this gap, we investigated the horizontal and vertical variability in auto- and heterotrophic microbial activity (biomass production and respiration) within a CE that formed off Mauritania and along the ∼ 900 km zonal corridor between Mauritania and the Cape Verde islands in the Eastern Tropical North Atlantic (ETNA). Our results show how the physical disturbances caused by the CE affected the biomass distribution of phyto- and bacterioplankton and their metabolic activities. The injection of nutrients into the sunlit surface resulted in enhanced autotrophic pico- and nanoplankton abundance and generally increased autotrophic activity as indicated by chlorophyll a (Chl a) concentration, primary production (PP), and extracellular release rates. However, the detailed eddy survey also revealed an uneven distribution of these variables with, for example, the highest Chl a concentrations and PP rates occurring near and just beyond the CE's periphery. The heterotrophic bacterial activity was similarly variable. Optode-based community respiration (CR), bacterial respiration (BR) estimates, and bacterial biomass production (BP) largely followed the trends of PP and Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial abundance and activities occurred within the CE that was closely related to variability in autotrophic production. Consistent with this, we found a significant positive correlation between concentrations of semi-labile dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable amino acids and dissolved combined carbohydrates) and BR estimates. Extracellular release of carbon as indicated by primary production of dissolved organic carbon (PPDOC) was variable with depth and laterally and not always sufficient to compensate the bacterial carbon demand (BCD: BR + BP), with PPDOC accounting for between 28 % and 110 % of the BCD. Bacterial growth efficiency (BGE: BP / BCD) ranged between 1.7 % and 18.2 %. We estimated the metabolic state to establish whether the CE was a source or a sink of organic carbon. We showed that the CE carried a strong autotrophic signal in the core (PP  CR 〉 1). Our results suggest that submesoscale (0–10 km) processes lead to highly variable metabolic activities in both photoautotrophic and heterotrophic microorganisms. Overall, we revealed that the CEs not only trap and transport coastal nutrients and organic carbon to the open ocean but also stimulate phytoplankton growth, generating freshly produced organic matter during westward propagation. This drives heterotrophic processes and may contribute to the previously observed net heterotrophy in open Atlantic surface waters.

Description: Mesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post-upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with parallel factor analysis, two humic-like, and two protein-like components. Humic-like FDOM components correlated to optode-based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g., temperature) and to dissolved inorganic nitrogen. The tryptophan-like FDOM components correlated with the carbon and nitrogen content of semi-labile DOM, phytoplankton biomass, community respiration, and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies. Key Points: - Four fluorescent dissolved organic matter (FDOM) components were studied in two cyclonic eddies (CEs) in the Eastern Tropical North Atlantic - Tryptophan-like FDOM was an indicator of the CEs' productivity as it correlated with semi-labile dissolved organic matter and microbial metabolic activities - Humic-like FDOM was a by-product of microbial respiration; its distribution within an offshore CE covaried with nutrient upwelling

Description: The Eastern Tropical North Atlantic Ocean (ETNA) includes the oligotrophic waters of the Atlantic Gyre and the productive waters of the Canary Current system off Northwest Africa, where upwelling of nutrient−rich waters stimulates primary productivity. The offshore waters of the ETNA are predominantly heterotrophic, as the consumption of organic matter in the euphotic zone is greater than its production. In the ETNA, phytoplankton and bacteria compete for access to inorganic nutrients to produce and degrade organic matter. Mesoscale eddies (10 − 100 km) are ubiquitous features connecting highly productive eastern boundary upwelling system to their adjacent oligotrophic Gyres. Eddies are vectors of organic matter supply that maintain heterotrophy, but they are also responsible for local variability in the balance of trophic metabolism (autotrophy/heterotrophy) as their swirling motion impact water productivity. Eddies influence the stratification of the water column by lifting deep water rich in inorganic nutrients to the surface (upwelling) or by bringing nutrient−poor surface water to the depths (downwelling). In the Canary Current system, coastal upwelling promotes eddy formation. How phytoplankton and heterotrophic bacteria respond to eddy−induced sub−mesoscale (〈10 km) disturbances is not yet well defined. In this regard, this thesis is part of the REEBUS (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems) project, which aims to study how oceanic eddies affect the physical, biogeochemical and biological properties of ETNA waters. The data included in this thesis contribute to a better understanding of the sub−mesoscale impacts of cyclonic eddies on the biogeochemistry and fate of organic matter in the ocean. Three cyclonic eddies (CEs) were investigated in this thesis, one, was sampled off the Mauritanian coast during summer (Mau), one was sampled offshore during winter (Sal) and one was sampled in the vicinity of Brava island during winter (Brava). The three CEs sampled showed varying degrees of upwelling signal impacting phytoplankton distribution and abundances. The rates of primary production and consumption of organic matter were variable within the Mau eddy (Chapter III). Overall, the Mau eddy carried an autotrophic signal with more organic carbon produced than consumed yet with spatial variability in the ratio. The carbon exuded by phytoplankton in the eddy covered 28−110% of the carbon requirements of the metabolic activities (biomass production and respiration) of heterotrophic bacteria. Dissolved organic matter (DOM) optical properties were used to track DOM production and remineralization and nutrient transport within the Mau and Brava eddies (Chapter IV). The analysis of the fluorescent properties of DOM (FDOM) allowed us to find four ubiquitous fluorophores. Two fluorophores were humic−like components and the other two were amino acid−like (tryptophan−like, tyrosine−like) components. Tryptophane−like FDOM component was a good indicator of the productivity associated with the CEs and of semi−labile dissolved organic nitrogen concentration. The humic−like FDOM components were good indicators of DOM remineralization and of nutrient transport within eddies. Polysaccharidic transparent exopolymer particle (TEP) and proteinaceous coomassie stainable particles (CSP) roles in biogeochemical processes such as the carbon cycle and sea−air gas and particle exchanges. Their abundance were investigated within the three CEs (Chapter V). In general, TEP and CSP were higher in CEs compared to their respective surrounding waters, yet they were also affected by seasonal dynamics. Samples collected during summer had on average 4−fold more TEP than those collected during winter. TEP and CSP in epipelagic waters were on average 4 and 2−fold higher within Mau, 2.5 and 5−fold higher within Sal, and 2.4 and 2.4−fold higher higher within Brava respectively than in their respective surrounding waters.

Description: The iron(II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (9 around Cape Verde, 11 around the Canary Islands, and 5 around Madeira). In a physicochemical context, experiments were carried out to study the pseudo-first-order oxidation rate constant (k′, min-1) over a range of pH (7.8, 7.9, 8.0, and 8.1) and temperature (10, 15, 20, and 25 °C). Deviations from the calculated kcal′ at the same T, pH, and S were observed for most of the stations. The measured t1/2 (ln 2/k′, min) values at the 25 stations ranged from 1.82 to 3.47 min (mean 1.93 ± 0.76 min) and for all but two stations were lower than the calculated t1/2 of 3.21 ± 0.2 min. In a biogeochemical context, nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM) were analyzed to explain the observed deviations. The application of a multilinear regression model indicated that k′ can be described (R = 0.921 and SEE = 0.064 for pH = 8 and T = 25 °C) from a linear combination of three organic variables, k′OM = kcal′-0.11∗ TDN + 29.9*bDOM + 33.4*C1humic, where TDN is the total dissolved nitrogen, bDOM is the spectral peak obtained from colored dissolved organic matter (DOM) analysis when protein-like or tyrosine-like components are present, and C1humic is the component associated with humic-like compounds obtained from the parallel factor analysis of the fluorescent DOM. Results show that compounds with N in their structures mainly explain the observed k′ increase for most of the samples, although other components could also play a relevant role. Experimentally, k′ provides the net result between the compounds that accelerate the process and those that slow it down.

Description: Transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP), two prominent classes of gel−like particles in the ocean primarily produced by phytoplankton, play crucial roles in ecological and biogeochemical processes, influencing microbial nutrition, growth, and particle aggregation. The distribution of these particles is intricately linked to the spatiotemporal dynamics of phytoplankton. Mesoscale cyclonic eddies (CEs) are known to stimulate phytoplankton growth and influence particle transport, but their effects on TEP and CSP remain to be determined. In the Eastern Tropical North Atlantic (ETNA), we examined three CEs: one off the Mauritanian coast during summer (Mau), one offshore during winter (Sal), and another near Brava island during winter. Mau and Brava CEs were in their intensification/maturity phase, while the Sal CE was in its decay phase. Both TEP and CSP concentrations correlated with primary productivity, but TEP increased with chlorophyll−a concentration, whereas elevated CSP coincided also with the highest abundance of pico−nanophytoplankton (〈20 µm), mainly Synechococcus. Both gels exhibited a positive correlation with bacterial biomass production, indicating their consumption by heterotrophic bacteria. TEP total area in the epipelagic waters of all CEs (Mau, Brava, and Sal) was elevated compared to surrounding waters, with on average 4, 2.5, and 1.6−fold higher values, respectively. However, no significant difference in TEP size distribution was observed within any CEs and their surroundings. Similarly, CSP total area increased in the epipelagic waters of Mau and Brava CEs, with on average 5 and 2.4−fold higher values, respectively, compared to surrounding waters. CSP particles were notably larger in these two eddies, while the Sal CE showed no significant difference from surrounding waters in CSP abundance and size. Overall, TEP and CSP exhibited distinct responses to CEs, with increased concentrations during their intensification/maturation stage and remineralization dominating during their decaying stage.

Description: Auto- and heterotrophic microbial activity (biomass production and respiration) were investigated in a cyclonic eddy that formed off Mauritania along the ∼ 900 km zonal corridor between Mauritania and the Cabo Verde islands in the eastern Tropical North Atlantic during the M156 cruise on the RV Meteor from July 3rd to August 1st 2019. The dataset includes measured and calculated data over the epipelagic layer (0-200 m depth) of 25 stations with 14 of them inside or in the vicinity of a cyclonic eddy. Temperature, salinity, and oxygen were obtained from a Seabird 911 plus CTD system equipped with two independently working sets of temperature–conductivity–oxygen. Seawater samples were collected using 10 L Niskin bottles attached to the CTD Rosette. Ammonium was analysed based on Solórzano (1969, https://doi.org/10.4319/lo.1969.14.5.0799) and nitrate, nitrite, phosphate and silicate were measured photometrically with continuous-flow analysis on an auto-analyser based on Hansen and Koroleff, (1999, https://doi.org/10.1002/9783527613984.ch10). To estimate the fraction of semi-labile dissolved organic carbon, we determined high-molecular-weight (〉1 kDa) dissolved combined carbohydrates (dCCHO) based on Engel and Händel (2011, https://doi.org/10.1093/plankt/fbq122) and dissolved hydrolysable amino acids (dHAA) based on Lindroth and Mopper (1979, https://https://doi.org/10.1021/ac50047a019) and Dittmar et al, (2009). The analysis of DCCHO detected 11 monomers: and the dHAA analysis classified 13 monomers. The calculations for the carbon content of dCCHO and dHAA were based on carbon atoms contained in the identified monomers. The sum of dCCHO and dHAA carbon content is referred to as SL-DOC. Chlorophyll a was measured from photometric analysis based on Evans et al, (1987). Heterotrophic bacteria, photosynthetic bacteria (Prochlorococcus and Synechococcus), and autotrophic pico and nanoplankton (〈20 μm) abundances were measured by flow cytometry. We converted the cell abundance of the different autotrophic pico- and nanoplankton populations into biomass based on Hernández-Hernández et al. (2020, https://doi.org/10.3389/fmars.2020.00667). Extracellular release rates, dissolved-, particulate- and total- primary production rates were determined from 14C incorporation according to Nielsen (1952, https://doi.org/10.1093/icesjms/18.2.117) and Gargas (1975). Community respiration was obtained from optode-based method from incubations by measuring changes in dissolved oxygen over 24–36 h. Bacterial biomass production rates were measured through the incorporation of labelled leucine (3H) using the microcentrifuge method (Kirchman et al., 1985, https://doi.org/10.1128/aem.49.3.599-607.1985; Smith and Azam, 1992, ). Community respiration and bacterial biomass production were converted to rates at 22°C using equations from Regaudie-De-Gioux and Duarte (2012, https://doi.org/10.1029/2010GB003907) and from López-Urrutia and Morán (2007, https://doi.org/10.1890/06-1641) respectively. Community respiration rates were converted to bacteria respiration rates based on Aranguren-Gassis et al, (2012, https://doi.org/10.3354/meps09707). Bacteria carbon demand and growth efficiency were calculated from bacterial production and respiration rates. Dittmar, T., Cherrier, J., and Ludwichowski, K. U.: The analysis of amino acids in seawater, in: Practical guidelines for the analysis of seawater, edited by: Wurl, O., 67–78, CRC Press, Boca Raton, ISBN: 978-1-4200-7306-5, 2009. Evans, C. A., O'Reily, J. E., and Thomas, J. P.: A handbook for measurement of Chl a and primary production, College Station, TX, Texas A and M University, ISBN:9780948277078, 0948277076, 1987. Gargas, E.: A Manual for Phytoplankton Primary Production Studies in the Baltic, The Baltic Marine Biologists, 2, 88 pp. Ed. Gargas E. (Hørsholm, Denmark: Water Quality Institute), 1975. Smith, D. and Azam, F.: A simple, economical method for measuring bacterial protein synthesis rates in seawater using, Mar. Microb. Food Webs, 6, 107–114, 1992.

Description: This dataset includes measured and calculated data over the epipelagic layer (0-200 m depth) of 25 stations with 14 of them inside or in the vicinity of a cyclonic eddy that formed off Mauritania along the ∼ 900 km zonal corridor between Mauritania and the Cabo Verde islands in the eastern Tropical North Atlantic during the M156 cruise on the RV Meteor from July 3rd to August 1st 2019. Chlorophyll a was measured from photometric analysis based on Evans et al, (1987).We converted the cell abundance of the different autotrophic pico- and nanoplankton populations into biomass based on Hernández-Hernández et al. (2020, https://doi.org/10.3389/fmars.2020.00667). Extracellular release rates, dissolved-, particulate- and total- primary production rates were determined from 14C incorporation according to Nielsen (1952, https://doi.org/10.1093/icesjms/18.2.117) and Gargas (1975).