P.I.: c/o Luis A. Vega, Ph.D.
Latest Information from OES AnnexIV of the International Energy Agency (IEA) on the effects of marine renewable energy devices on the marine environment was published February 2016 ( Executive-Summary_Effects of marine renewable energy devices in the marine environment ).
Previously, HINMREC tried working in coordination with federal regulatory agencies (FERC, BOEM, and NOAA) to define differences between ocean energy systems and already established regulated industrial activities. HINMREC concluded that: (i) Discharge of deep seawater below the photic zone is the OTEC differentiator; and, (ii) The effect of arrays/farms over large coastal regions (spacing and quantity) the WEC differentiator.
The OTEC environmental baseline database, for a potential site off Kahe Point in Oahu, has been documented and is available upon request ( Environmental Assessment of OTEC in Hawaii.) Key oceanographic parameters to be considered in assessing the OTEC environmental impact have been identified and can be grouped as follows:
Nutrients and Biological
– Chlorophyll a
– Dissolved Oxygen
– Dissolved inorganic carbon
The primary indicators of impact during plant operations are: Chlorophyll a; CDT Data; and, pH. These should be monitored at the discharged plume Neutral-Buoyancy-Depth as well as the Far-Field and compared to baseline conditions.
Generalities: Wave Energy Devices Among the various possible impacts to be considered for each specific device are: electromagnetic effects on sharks, acoustic effects on whales, bird-strikes, entanglement, benthic effects of anchors, and aggregation of animals. Objects placed in the ocean often become powerful fish aggregators. The depths proposed for wave energy conversion (WEC) devices are less than 80m. In Hawai’i, these water depths are not major habitats for the commercial pelagic fishes such as tunas. There is a network of Fish Aggregation Devices (FADs) in Hawaiian waters than are installed for the purpose of aggregating tunas and other commercial species, and these FADS are in waters much deeper than 100m (http://www.hawaii.edu/HIMB/FADS/). The WECs under consideration may aggregate small schooling fishes such as opelu. The opelu is one of the main bait fishes used in fisheries for larger species such as tunas. Therefore, fishermen may be attracted to these shallow water facilities to catch bait.
Generalities: OTEC Operations Deep seawater used in OTEC operation contains elevated levels of dissolved inorganic nutrients, primarily phosphate, nitrate and silicate, which could be expected to promote blooms of photosynthetic organisms if and only if the seawater is discharged and contained within the upper ocean or in coastal waters. However, the density of the deep seawater is higher than that of surface waters, and thus deep seawater discharged above the thermocline would sink, mitigating this effect. Deep seawater also contains elevated levels of dissolved carbon dioxide, which would lead to the release of carbon dioxide to the atmosphere if and only if discharged water was allowed to come in contact with the atmosphere.
Two additional points are worth noting: (i) discharges of deep seawater within the photic zone of the ocean, but below the surface mixed layer, should result in photosynthetic production that would remove both the dissolved nutrients and the dissolved carbon dioxide at approximately the same stoichiometric ratio as they are elevated in deep seawater; thus, the only large-scale environmental impact would involve the fate of the resulting photosynthetically produced organic matter; and, (ii) the reduction in pressure of deep seawater as it is brought to the surface will lead to an increase in its pH, offering some relief to the acidification of seawater due to global increases in atmospheric carbon dioxide.
Modeling work by Makai Ocean Engineering: A report describing the modeling work by Makai Ocean Engineering, Inc. to simulate the biochemical effects of the nutrient-enhanced seawater plumes that are discharged by one or several 100 MW OTEC plants is available at: http://www.osti.gov/scitech/servlets/purl/1055480
The modeling is needed to properly design OTEC plants that can operate sustainably with acceptably low biological impact. The report shows that the biochemical response of OTEC discharges can be modeled, quantified, and dynamically visualized for OTEC plants having different discharge configurations. 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.
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 in their baseline design suggests that higher-order effects will be very small.