Potential effects of deep seawater discharge by an OTEC plant on the marine microorganisms in oligotrophic waters

In relation to the for 2020 scheduled 10 MW OTEC plant offshore the Martinique Island, this project called ‘IMPALA’ (Impacts of artificial upwelling on microplankton) aims to study the potential environmental impacts of releasing, below the surface, deep seawater flowing out of OTEC plant. This OTEC plant will pump approximately 100,000 m3.h−1 of deep seawater at 1100 m depth. In order to optimize the energy efficiency, the deep seawater should be rejected close to the surface. However, this large discharge could induce important disturbances on the upper ocean ecosystem, and this impact should be estimated.

Environmental assessment of OTEC functioning was studied since the 1980’s (NOAA, 1981, 2010). The deep seawater discharge was described as one of the major drivers impacting the marine environment in OTEC plant. However, 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.

Graphical image of the cold deep seawater discharge generated by an OTEC plant and its thermic effect on the phytoplankton assemblage. Source: 2019 Elsevier BV

Graphical image of the cold deep seawater discharge generated by an OTEC plant and its thermic effect on the phytoplankton assemblage. Source: 2019 Elsevier BV

Numerical simulations of deep seawater upwelled by the OTEC, showed that a 3.0 °C temperature change, considered as a critical threshold for temperature impact, was never reached during an annual cycle on the top 150 m of the water column on two considered sections centered on the OTEC. The thermal effect should be limited, <1 km2 on the area exhibited a temperature difference of 0.3 °C (absolute value), producing a negligible thermic impact on the phytoplankton assemblage. The impact on phytoplankton of the resulting mixed deep and surface seawater was evaluated by in situ microcosm experiments. Two scenarios of water mix ratio (2% and 10% of deep water) were tested at two incubation depths (deep chlorophyll-maximum:  and bottom of the euphotic layer: BEL).  Temperature differences between numerical simulations without and with the deep seawater discharge were compared on the upper 150 m of a vertical section.

The main conclusions:

  • Cold deep seawater discharge generated by an OTEC plant has negligible thermic impact on the phytoplankton assemblage
  • Discharge of 10% deep seawater at chlorophyll-a maximum depth induces large shift in the phytoplankton community structure.
  • Microbial effect was limited with 2% deep water addition and negligible with discharge at the bottom of euphotic layer depth
  • First environmental standards for discharge depth to exploit OTEC plant without harmful effects on the microbial community

Since the lower impact on the phytoplankton assemblage was obtained at the bottom of the euphotic layer (BEL) this depth can be recommended for the discharge of the deep seawater to exploit the OTEC plant.

Source:

ScienceDirect – 2019 Elsevier B.V.

Authors: Mélanie Giraud, Véronique Garçon, Denis De La Broise, Stéphane L’Helguen,Joël Sudre, and Marie Boye

https://www.sciencedirect.com/science/article/pii/S0048969719334114

 

 

 

 

 

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