In the coastal sea in front of the Kochi Prefectural Deep Seawater Laboratory, several hundred t・d^-1 of original and used DOW have been discharged over ten years. Active developments of Sargassum spp. and Ecklonia cave have been noticed in the area around the outlets where DOW has been discharged particularly large coverage of Sargassum spp (Fig.5). This suggests a possibility for creating a seaweed bed by discharged DOW. However, it is still not clear whether the growth of algae is due to nutrients or low temperature of DOW, yet.

　

　

"No DOW" represents the specific growth rates determined in surface seawater containing 2μM of DIN at 28℃ for summer, and 2μM of DIN at 12℃ for winter. "DOW" represents different additions of DOW containing 25μM at 10℃. "25℃ DOW" represents DOW heated up to 25℃ with different additions.

　

Then we conducted a culture experiment to determine possible relationships between the relative growth rate of algae and nutrient concentrations at a constant water temperature in an incubator under light in the laboratory. Nutrient concentrations were controlled by mixing DOW containing high concentrations of nutrients with surface seawater. Gelidium sp. showed almost similar growth rate above 40% additions of DOW although it gradually decreased with less amounts of DOW below 40% of DOW (Fig.6). On the other hand, Ecklonia sp. showed a consistent increase of the relative growth rate with increasing proportion of DOW up to 100% (Fig.6). These results indicate that nutrients in DOW could enhance the growth of seaweed. According to another culture, experiments conducted using Gelidium sp. and Ecklonia sp. at different temperatures, Gelidium sp. Showed the fastest growth rate at 23℃ but Ecklonia sp. was at 15℃. Based on the results of the culture experiments, the following relations were obtained for the relative growth rates (%) of Gelidium sp. and Ecklonia sp. with dissolved inorganic nitrogen and water temperature.

　

　

For Gelidium sp.,

　

　

For Ecklonia sp.,

　

　

where S represents DIN concentration in μM as nitrate and T represents water temperature in ℃.

　

By using the relationships obtained, the growth of Gelidium sp. and Ecklonia sp. can be estimated under different DIN and temperature influenced by DOW (Fig.7). Although the relative growth rates of Gelidium sp. and Ecklonia sp. were low under no addition of DOW but they generally increased their growth rate with the increase of DOW proportion. But the growth of Gelidium sp. was inhibited by the addition of too much DOW because of its low temperature, while DOW heated to 25℃ enhanced the algal growth to the maximum at the highest proportion of DOW.

　

　

"No DOW" represents relative growth rates determined in surface seawater containing 2μM of DIN at 28℃ for summer, and 2μM of DIN at 12℃ for winter. "DOW" represents different additions of DOW containing 25μM at 10℃. "25℃ DOW" represents DOW heated up to 25℃.

　

　

Compared to using surface seawater varying temperature depending on season for a coolant of electric power plants, deep ocean water (DOW) having low temperature with no or small seasonal changes provides some improvements of power generating efficiency and results in reducing CO₂ emission as well as requiring small amount of cooling water with small size of heat exchangers (Kadoyu, 2000, 2001). It is also possible to eliminate low-temperature effects of DOW by warming up in power plants, and could be discharged at similar temperature around the surrounding environment.

　

Very little biological entrainment of DOW mainly small size zooplankton found in this study, as well as the characteristics of almost no biofouling of DOW which has been found in Japan and Hawaii (Daniel, 1992), are very helpful for its use as a coolant for power plants and other facilities. Although less frequent entrainment of large size organisms such as fish is still remained for future evaluation.

　

Another influences of DOW on the environment are enhancements of biological productivity of phytoplankton and seaweed. The specific growth rate of phytoplankton increased with the increasing percentage of DOW. Thus there are possibilities to enhance biological production by DOW discharged which could lead into some increases of commercially valuable fish and shellfish production (Ouchi et al., 2001; Ogiwara et al., 2001). DOW discharge may also be effective for restoration and development of seaweed beds (Fujita, 2001; Watanabe et al., 2000). Increased biological production will also be beneficial for CO₂ absorption from the atmosphere.

　

Concerning a possible large increase of DOW utilization in the near future, it is important to study the environmental impacts of the pumping-up and discharge of DOW. We still need further considerations on most of the processes in this study and the other possible processes, which were not taken into account in the present study, finally, to put all the processes affecting the environment together by using a suitable approach such as an ecosystem model with sufficient hydrodynamic considerations for the thorough evaluation of effects by DOW.

　

　

This study has been carried out by the project team of the Japan Ocean Industries Association (JOIA), supervised by the New Energy and Industrial Technology Development Organization (NEDO) and supported financially by the Ministry of Economy, Trade and Industry of Japan.

　

　

　

Fujita, D. 2001. Algal recovery on coralline-covered cobbles collected from an urchin-dominated barren ground in flowing deep-sea water. Deep Ocean Water Research. 2:57-64 (In Japanese with English summary).

Furuya, K., H. Tsuzuki, K. Iseki and A. Kawamura. 1993. Growth response of natural phytoplankton assemblages in artificially induced upwelling in Toyama Bay. Japan. Bulletin of Plankton Society of Japan. 40:109-125.

　

　

　

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Kadoyu, M. 2001. Research on deep ocean water utilization for cooling water of power plant. Report of Central Research Institute of Electric Power Industry (In Japanese).

　

　

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Ogiwara, S., Y. Awashima, H. Miyabe and K. Ouchi. 2001. Conceptual design of a deep ocean water upwelling structure for development of fisheries. Proc. 4th ISOPE Ocean Mining Symp. ISOPE-OMS-2001, pp.150-157.

　

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Watanabe, M., M. Taniguchi, T. Ikeda, M. Komatsu, K. Takatsuki and S. Kanamaki. 2000. Fertilization at coastal zone using deep ocean water. Gekhan kaiyo, Special volume 22:160-169 (In Japanese).