<p>Installation of an Ocean Thermal Energy Conversion pilot plant (OTEC) off the Caribbean coast of Martinique is expected to use approximately 100&thinsp;000&thinsp;m³&thinsp;h^&minus;1 of deep seawater for its functioning. This study examined the potential effects of the cold nutrient-rich deep seawater discharge on the phytoplankton community before the installation of the pilot plant. Thermal effect induced by the deep seawater upwelled by the OTEC was described using the Regional Ocean Modeling System. Numerical simulations of deep seawater discharge showed that a 3.0&thinsp;°C temperature change, considered as a critical threshold for temperature impact, was never reached during an annual cycle on the top 150&thinsp;m of the water column on two considered sections centered on the OTEC. The thermal effect should be limited, less than 1&thinsp;km² on the area exhibited a temperature difference of 0.3&thinsp;°C (absolute value). The impact on phytoplankton of the resulting mixed deep and surface seawater was evaluated by <i>in situ</i> microcosm experiments. Two scenario of water mix ratio (2&thinsp;% and 10&thinsp;% of deep water) were tested at two incubation depths (deep chlorophyll-<i>a</i> maximum: DCM and bottom of the euphotic layer: BEL). The larger impact was obtained at DCM for the highest deep seawater addition (10&thinsp;%), with a development of diatoms, whereas 2&thinsp;% addition induced only a limited change of the phytoplankton community. This study suggested that the OTEC plant would significantly modify the phytoplankton assemblage only in the case of a discharge affecting the DCM and would be restricted to a local scale.</p>