Although the potential environmental effects of OTEC plant construction and operation were evaluated in the 1980s as part of earlier OTEC development, recent OTEC efforts have led to the re-examination of the issues involved. During the intervening years we have significantly increased our understanding of the oceans, and our ability to observe and model the marine environment has improved markedly. For example, OTEC environmental assessments have traditionally included the effects of discharging deep seawater, with its elevated levels of dissolved inorganic nutrients and dissolved inorganic carbon, and depleted levels of dissolved oxygen, into the upper water column. However, the role of trace elements in controlling marine primary production rates is now widely accepted, and their natural vertical distribution in the ocean needs to be considered. Our expanded understanding of ocean biogeochemistry also makes environmental assessment more complicated. For example, 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 dissolved nutrients and dissolved carbon dioxide at approximately the same stoichiometric ratio as they are elevated in deep seawater; thus, the only large-scale related environmental impact would involve the fate of the resulting photosynthetically produced organic matter. Similarly, our improved knowledge of marine physical chemistry allows a better understanding of OTEC’s potential impact on the ocean’s inorganic carbon chemistry. For example, the reduction in pressure of deep seawater as it is brought to the surface, and the increase in temperature due to OTEC heat exchange, will both lead to an increase in the deep water’s pH; opposite effects will occur in the shallow seawater used by OTEC. Determination of the net effect will require modeling using predicted pumping rates for warm and cold seawater, the planned intake and discharge depths and temperatures, the inorganic carbon chemistry at the specific site, and recently refined inorganic carbon equilibria data. Ecological data (e.g., primary productivity, the biomass of various trophic levels, biota attraction to floating objects, etc.) should also be updated with the results from more contemporary studies. Additional factors that should be examined include electromagnetic effects of cabling, alterations in the bio-physical coupling of water column as a result of the discharge plume, potential harmful algal bloom development, and low-frequency noise production. Moreover, new ocean observation techniques such as gliders and AUVs allow large areas of the ocean to be monitored in 3-D for extended periods of time. Similarly, new marine modeling techniques, such as regional ocean modeling systems (ROMS), allow OTEC plumes to be studied in the context of a 3-D dynamic ocean, including such features as internal tides and mesoscales eddies, and allow assimilation of 3-D data to improve model performance. As an early step in these efforts, we have used HOT time-series data to determine patterns of seasonal variability in the upper ocean (warm water intake and discharge zone) and in the deep ocean (cold water uptake) near the site for the proposed Kahe Point, Oahu OTEC demonstration plant.