Ocean Thermal Energy Conversion : the potential impact on microplankton of bottom water discharge at subsurface

Marie Boye1, Mélanie Giraud1,2,3, Véronique Garçon2, Morgane Lejart3, Cédric Auvray4, Marc Bœuf 3, Denis De La Broise1 1LEMAR ‐ UMR 6539 (Plouzané, France), 2LEGOS ‐ UMR 5566 (Toulouse, France), 3France Energies Marines (Brest, France), 4DCNS(Brest, France) Contacts: marie.boye@univ‐brest.fr, melanie.giraud@france‐energies‐marines.org  

Simulation of an upwelling with in situ microcosms 

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[NO3‐] (µM) 

Depth (m

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Temperature (°C)  Warm sub‐surface seawater (SSW) Nutrient‐depleted 

Cold Deep seawater (DSW) Nutrient‐rich 

In situ microcosm experiments: simulation of the seawater plant discharge off Martinique.

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Chl a (µg/L) 

Depth (m

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PAR 

Fluorescence peak 

Control 100% SSW 

Control 100% SSW 

Control 100% SSW 

Control 100% SSW 

CONTROL 

98% SSW 2% DSW 

98% SSW 2% DSW 

98% SSW 2% DSW 

98% SSW 2% DSW 

DSW 

2% 

90% SSW 10% DSW 

90% SSW 10% DSW 

90% SSW 10% DSW 

90% SSW 10% DSW 

DSW 

10% 

2,3 L transparent polycarbonate bottle 

Bottom of euphotic layer 

Results Potential impact of bottom water discharge at sub-surface Comparison between the natural environment and the microcosms

Significant shifts over time in the natural phytoplankton assemblage for diatoms (fucoxanthin), cyanobacteria (zeaxanthin) and prochlorophytes (divinyl-Chl a).

Similar range of variability in microcosms, with a stronger decrease in the abundance of the fragile prochlorophytes in the microcosm.

Significant increases with 10% enrichment of sub-surface waters with bottom waters, for diatoms (fucoxanthin), pelagophytes (19’BF), diadinoxanthin, Chl c2 and c3, and b-carotene.

Lower effect with 2% enrichment and only for the b-carotene.

- Evolution over time of the phytoplankton communities grown in microcosms control was close to one outside the microcosms, suggesting that microcosms can be used to assess the impact of bottom water discharge at sub-surface. - Enrichment of the sub-surface waters by deep seawater (10%) induces a significant shift in the phytoplankton assemblage towards the development of diatoms. This could have biogeochemical and ecological consequences since diatoms are major drivers of the biological carbon pump in the ocean. - There is no significant impact if the discharge takes place below the subsurface. However, the impact of the functioning of a continuous OTEC remains to study.

Conclusion 

Sampling of subsurface seawater at 2 depths (maximum of chlorophyll and bottom of euphotic layer).

Deep SeaWater (DSW) addition in poor nutrient Subsurface SeaWater (SSW): sampling of DSW and mixing in 3 proportions (0%, 2% and 10% of DSW) with SSW.

Impact assessment of the enrichments with bottom water on the phytoplankton community: pigment analyses using HPLC, flow cytometry, nutrients analyses in the microcosms, and natural variability of the phytoplankton assemblage (days 0 and 6).

Immersion of bottles for 6 days on a 250 meters mooring at these two depths.

Distribution in polycarbonate bottles (4 replicates per condition).

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Pigm

ent con

centratio

n (µg/L) 

microcoms control Day 6  microcosms 2% of Deep SeaWater Day 6 microcosms 10% of Deep SeaWater Day 6 

Cooled warm water

Warm cold water

Cold water intake -1100 m

Deep seawater discharge 100/150 m

Warm seawater intake 5 m

Background 

Scientific objectives 

Industrial NEMO project (DCNS , France) : Pilot plant for Ocean Thermal Energy Conversion (OTEC) offshore the Martinique Island. IMPALA project : Potential impact on the microplankton of bottom water discharge at surface generated by OTEC.

OTEC 

100 000 m3/hour of deep cold seawater will be rejected in the surface oligotrophic waters generating an artificial upwelling :

What are the impacts on microplankton?

How to experimentally reproduce the operating conditions of an artificial upwelling? What are the anticipated effects on the phytoplankton communities and their distribution?

Operating principle of an OTEC

Bathymetry of the study area