Evaluation de l'impact potentiel d'un upwelling artificiel lié au fonctionnement d'une centrale à énergie thermique des mers sur le phytoplancton

Abstract : As part of the implementation of an Ocean thermal energy conversion (OTEC) pilot plant planned off the Caribbean coast of Martinique by 2020, this thesis aims to assess the potential impacts of deep seawater discharge at the surface on the phytoplankton. The offshore pilot plant NEMO, designed by DCNS and implemented by Akuo Energy anticipates production of approximately 10 MW. The cold and nutrient-rich waters that are pumped in the bottom and discharged at the surface by the heat engine with a flow of roughly 100 000 m3 h-1 should enrich surface waters of the study site, which are particularly poor in nitrate and phosphate. Two campaigns of field measurements in two contrasting seasons (the dry season in November 2013 and the wet season in June 2014) have allowed the description of different physical and biogeochemical parameters that may induce changes in the phytoplankton community. Marked seasonal variability in stratification and biogeochemical parameters occurred, with strong oceanic influences (advection of waters from the Amazon and Orinoco) and atmospheric influences (African dust) potentially enriching the surface layer in nutrients and trace metals during the wet season. In situ microcosms were designed to simulate the discharge of bottom waters into the surface layer under different scenarios. Surface water collected at the chlorophyll maximum(45 m, where the phytoplankton is the most abundant), and at the base of the euphotic layer (80 m, where the phytoplankton is present, but in very low abundance) was enriched with either a weak (2%) or strong (10%) input of bottom waters (1100 m), and incubated for 6 days. Primary production was estimated in both the natural environment, and in the microcosms. These experiments have shown that high input (10%) stimulates the development of micro-phytoplankton, especially diatoms, to the detriment of Prochlorococcus. The response of diatoms could be linked to the input of nitrate and phosphate by the deep seawater.Primary production would be dependent on the composition of the phytoplankton assemblage rather than on the intensity of deep water discharge. Finally, thermal disturbances linked to the discharge of cold water at the surface were assessed using a numerical model (ROMS), which considered the thermal impact threshold of -3°C as recommended by the World Bank Group, and -0.3 °C, corresponding to a 2% dilution with deep water. Even at the lowest threshold (-0.3 °C), the area impacted in the first 150 m of the water column was too low to be detected by the simulation, regardless of the discharge depth. The thermal impact of cold water should therefore be negligible and limited to an area of less than 3 km2. This work provides the first critical step in understanding how bottom water discharge may impact the ecosystem in the longer-term.
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Mélanie Giraud. Evaluation de l'impact potentiel d'un upwelling artificiel lié au fonctionnement d'une centrale à énergie thermique des mers sur le phytoplancton. Océan, Atmosphère. Université de Bretagne occidentale - Brest, 2016. Français. ⟨NNT : 2016BRES0007⟩. ⟨tel-02183899⟩

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