Devresse, Quentin (2023) Impact of cyclonic eddies on organic matter fate in the eastern tropical North Atlantic Ocean. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 195 pp.

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Abstract

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.

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