Eddy enhanced primary production accelerates bacterial growth in the Eastern Tropical North Atlantic

Mesoscale eddies play essential roles in modulating the ocean's physical, chemical, and biological properties. In cyclonic eddies (CE) nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophic bacterial activities (biomass production and respiration) and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To address this gap, we investigated the horizontal and vertical variability of phytoplankton and heterotrophic bacterial activity along ~900 km zonal corridor between the coast of Mauretania and the Cape Verde Islands in the eastern tropical North Atlantic (ETNA). We additionally collected samples from a CE along this transect at high spatial resolution. Our results show cascading effects of physical disturbances induced by a CE on phyto- and bacterioplankton biomass and metabolic activities. Specifically, the injection of nutrients into the sunlit surface resulted in enhanced autotrophic plankton abundance and activity as indicated by Chlorophyll a (Chl-a) concentration, DOM exudation, and primary productivity (PP). However, the detailed eddy survey revealed an uneven distribution of these parameters with, for example, the highest Chl-a concentrations and PP rates near and just beyond the CE’s periphery. The heterotrophic bacterial activity was similarly variable. Optode-based bacterial respiration (BR) and 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 studied here that was closely related to variability in autotrophic production. This was supported by a significant positive correlation between concentrations of semi-labile organic carbon (SL-DOC; the sum of dissolved hydrolyzable amino acids and combined carbohydrates) and BR measurements. Bacterial growth efficiency (BP/(BR+BP)) was variable (1.4–10.5 %) within the CE and carbon exudation was not always sufficient to compensate the bacterial carbon demand (BR+BP; 28.3–114.5 %). We have additionally estimated the metabolic state in our samples, which showed that the CE carried a strong autotrophic signal (PP/(BR+BP) > 1). Overall, our results show that submesoscale (0–10 km) processes lead to highly variable metabolic activities of both phototrophic and heterotrophic microbes, which has implications for biogeochemical models estimating oceanic carbon fluxes. Additionally, we revealed that the CE not only traps and transports coastal nutrients and carbon to the open ocean but also stimulates phytoplankton growth generating freshly produced organic matter during westward propagation. This organic matter may fuel heterotrophic processes in the open ocean and may help to explain the often-observed net heterotrophic metabolic state of these environments

Eddy Study to Understand Physical-Chemical-Biological Coupling and the Biological Carbon Pump as a Function of Eddy Type off West Africa, Cruise No. M160, 23.11.2019 - 20.12.2019, Mindelo (Cabo Verde) - Mindelo (Cabo Verde)

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

Impact of cyclonic eddies on organic matter fate in the eastern tropical North Atlantic Ocean

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

Impact of cyclonic eddies on organic matter fate in the eastern tropical North Atlantic Ocean

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

Microbial activity and nutrients measured from water bottle samples in a cyclonic eddy during METEOR cruise M156, off Mauritania

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

Bacterial production, community respiration and primary production rates measured from water bottle samples during METEOR cruise M160

This dataset includes measured and calculated data over the epi-mesopelagic layer (0-450 m depth) of 29 stations with 4 stations within a cyclonic eddy nearby Brava island, 5 stations within a cyclonic eddy nearby Sal island, and one station within an anticyclonic eddy around the Cabo Verde islands in the eastern Tropical North Atlantic during the M160 cruise on the RV Meteor from November 22nd to December 20th, 2019. 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). 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 measured from optodes based method (Wikner et al., 2013; https://doi.org/10.4319/lom.2013.11.1)

Chromophoric and fluorescent dissolved organic matter measured from water bottle samples during METEOR cruise M156

This dataset includes measured data over the epi-mesopelagic layer (0-800 m depth) of 27 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. Chromophoric and fluorescent dissolved organic matter were measured with a Shimadzu® 1800 UV−VIS double beam spectrophotometer and a Cary Eclipse Fluorescence Spectrophotometer respectively. The data included the absorption coefficient at 325 nm (a325) and the four fluorescent components obtained from PARAFAC analysis

Nutrients measured from water bottle samples during METEOR cruise M156

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. 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)

Temperature, salinity and oxygen measured with CTD in selected depths during METEOR cruise M156

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. Temperature, salinity, and oxygen were obtained from a Seabird 911 plus CTD system equipped with two independently working sets of temperature–conductivity–oxygen