This paper describes the modeling work by Makai Ocean Engineering, Inc. to simulate the biochemical effects of of the nutrient-enhanced seawater plumes that are discharged by one or several 100 megawatt OTEC plants. The modeling is needed to properly design OTEC plants that can operate sustainably with acceptably low biological impact.
Summary of Findings: This study has showed that the biochemical response of OTEC discharges can be modeled, quantified, and dynamically visualized for OTEC plants having different discharge configurations. We now have an extremely useful tool for use by OTEC regulators and designers. In all cases modeled (discharge at 70 meters depth or more), no perturbation occurs in the upper 40 meters of the ocean's surface. The picoplankton response in the 110 - 70 meter depth layer is approximately a 10-25% increase, which is well within naturally occurring variability. The nanoplankton response is negligible. The enhanced productivity of diatoms (microplankton) is small, but this additional "standing stock" may potentially enhance growth if the plume water subsequently advects into nearshore water. Another significant finding is that detecting the plume of an OTEC pilot plant, as envisioned for the "NAVFAC OTEC Pilot Plant", will require many more samples in time and space than was originally envisioned, because ocean variability is greater than anticipated. Finally, the model does not attempt to calculate the higher order trophic levels where fauna consume the phytoplankton, but these results could be readily extended to this purpose. The subtle phytoplankton increase of our baseline design suggests that higher-order effects will be very small.