Ocean Thermal Energy Conversion: The Potential Impact of Bottom Water Discharge at Subsurface on Microphytoplankton in the Caribbean Sea

Mélanie Giraud^1,2,3, Véronique Garçon³, Marie Boye¹

¹LEMAR - UMR 6539 Plouzané France, ²France Energies Marines Brest France, ³LEGOS/CNRS Toulouse -France

Part of the solar energy can be harvested and used in different processes. Taking advantage of the natural temperature gradient between the surface and deep ocean, the Ocean Thermal Energy Conversion (OTEC) process fulfills this goal. The IMPALA project (Impacts of artificial upwelling on microplankton) aims to study the potential environmental impacts of releasing, below the surface, deep seawater flowing out of a scheduled OTEC pilot plant offshore the Martinique Island in the Caribbean Sea.

The OTEC installed off the Caribbean coast of Martinique is expected to pump approximately 100 000 m³/h of deep seawater for its functioning. The resulting impact of this cold nutrient-rich deep seawater discharge on the phytoplankton community of the nutrient depleted surface water is not fully understood. Therefore, a study was initiated before the installation of the pilot plant.

The vertical and horizontal seawater plume dispersion of the deep seawater upwelling was described using the Regional Ocean Modeling System (ROMS). Field data allowed describing the expected chemical characteristics of the resulting mixed seawaters. The potential impact on the phytoplankton community was evaluated by in situ microcosm experiments. Two deep seawater ratio (2% and 10%) and two incubation depths (deep chlorophyll maximum and euphotic layer deep limit) were tested, showing a larger impact at the deep chlorophyll maximum and for the higher deep seawater concentration (10%). The microcosm enrichment by 10% of deep seawater induced a significant shift in the phytoplankton assemblage, supporting diatoms development, whereas the enrichment by 2% of deep seawater only showed a small shift in phytoplankton population.

Experimental and modelling results were combined in order to identify the deep seawater discharge depth exhibiting the lower potential effects on the phytoplankton community.