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Simulation of an artificial upwelling using immersed in situ phytoplankton microcosms

Giraud, Mélanie, Boye, Marie, Garçon, Véronique, Donval, Anne, de la Broise, Denis
Journal of experimental marine biology and ecology 2016 v.475 pp. 80-88
Bacillariophyceae, Haptophyta, Prochlorococcus, biogeochemistry, bottles, chlorophyll, coasts, energy conversion, phytoplankton, seawater, thermal energy, Caribbean, Martinique
The inflow of deep seawater in the surface layer by an Ocean Thermal Energy Conversion (OTEC) plant will generate artificial upwelling. In order to study the potential impact on biogeochemical processes that could result, in situ microcosms were designed to simulate seawater plant discharge and these were deployed off the Caribbean coast of Martinique. Seawater was collected in ultra-clean conditions at maximum chlorophyll a concentrations (45m depth). The water was then mixed with either 2% or 10% deep seawater (1100m depth) and put in 2.3L polycarbonate bottles. These microcosms were immersed for 6days at 45m depth on a 220m mooring. Samples from the surrounding environment and from the microcosms were analyzed by pigment quantification, counting of picophytoplankton groups and macronutrient analyses. Similar trends in the evolutions of the phytoplankton populations were observed over time between the control microcosms (without addition of deep seawater) and the surrounding environment, suggesting that these microcosms can be used as a realistic representation of the natural surrounding waters over a 6-day incubation period. Microcosm enrichment with 10% deep seawater induced a shift in the phytoplankton assemblage towards the development of diatoms, haptophytes, and Prochlorococcus, whereas 2% enrichment only led to an increase in the Prochlorococcus population.