A blooming disputing: green energy in the oligotrophic ocean

Ocean Thermal Energy Conversion (OTEC) produces electricity using the temperature difference between deep ocean waters and tropical surface waters and is considered a clean energy source. However, effects on microorganisms of OTEC discharging cold nutrient rich water within warm oligotrophic surface water is relatively unknown.

Giraud et al (2019) investigated this, using in situ microcosm incubations of various seawater depth mixes, simulating deep-sea water discharge. Carbon and Nitrate uptake was measured using 13C/15N isotopic label technique. Microbial effects were limited with 2% deep water addition and negligible with discharge at the bottom of euphotic zone. However, a shift in assemblage to larger phytoplankton with 10% addition at the DCM was shown due to inputs of PO₄³⁻ and NO₃­­­^-.   

This change to a more nutrient rich environment with a higher light exposure has shown to cause shifts in assemblage potentially elevating risks of eutrophication, harming surrounding communities. This puts into question the effectiveness of this green energy source and its effects on microbial communities. A weakness of this paper is its lack of scope, only looking at the changes over 6-days periods, if the study had been carried out for a longer period the results would be more convincing.

Giraud, M., Garçon, V., de la Broise, D., L’Helguen, S., Sudre, J., & Boye, M. (2019). Potential effects of deep seawater discharge by an Ocean Thermal Energy Conversion plant on the marine microorganisms in oligotrophic waters. Science of The Total Environment, 693, 133491.