Total carbonic acid, total alkalinity, phosphoric acid, silicic acid and total inorganic nitrogen of DOW were measured once a month at all the three stations mentioned above. CO₂ emission caused by DOW being pumped up to the surface was estimated based on these measurements. Total carbonic acid concentrations of DOW pumped up from a depth of ca.320 m in Kochi and Toyama were approximately 2,200 - 2,300 μmol・kg^-1, while they were around 1,900 - 2,000 μmol・kg^-1 for surface water. This indicates a possibility that 200 - 300 μmol・kg^-1 of CO₂ will be released into the atmosphere by warming the temperature of DOW to surface temperature after pumping up. Pumping up of one million t・d^-1 of DOW for cooling a 600 MW LNG power plant might release 2,800 - 3,200 tCO₂・year^-1 .

　

On the other hand, using cold DOW as a coolant for an electric power generator is highly likely to increase the power generation efficiency, which will reduce fuel consumption and CO₂ emissions from the power plant. According to Kadoyu (2000, 2001), it is estimated that the power generation efficiency of a power plant would increase by 3%. This improvement in power generation would reduce CO₂ release about 12,000 tCO₂・year^-1 for a LNG power plant with a 50% duty cycle. This means that even if DOW pumped up releases CO₂ into the atmosphere, the total amount of CO₂ emissions would actually be reduced by saving fuel consumption through improved power generation efficiency (Table 2). Furthermore, if DOW is discharged into sea areas after use, growth of phytoplankton and seaweed will absorb CO₂ through photosynthesis by use of the excess nutrients in DOW, which can further contribute to reducing CO₂ in the atmosphere (Table 2).

　

　

　

Since DOW has different characteristics than surface water, DOW after uses needs to be carefully discharged into the natural environment not to cause serious impacts. Our research was at first focused on the behavior of DOW water mass, in which DOW warmed in a power plant, was discharged into a coastal environment.

　

Hydraulic tests showed clear differences in the dispersal pattern of DOW depending upon differences in internal Froude number, where larger Froude numbers increased dilution of DOW (Fig.2). This suggests how to discharge DOW after use for possible impacts of DOW such as low water temperature and some others on organisms living in the coastal regions. For enhancing phytoplankton productivity by excess nutrients of DOW after use, DOW needs to be discharged in order to stay in the euphotic zone.

　

　

　

Since the actual evaluation of possible effects due to discharged DOW on phytoplankton under natural environment is not easy with the limited amount of DOW discharge currently occurring at each pumping station (Ishizaka et al., 1983; Furuya et al., 1993), we approached estimating the probable effects by culture experiments in the laboratory. We focused particularly attention on the specific growth rate of phytoplankton, using natural assemblages collected from surface at sites in Kochi Prefecture where DOW is actually discharged.

　

Culture experiments were conducted in a laboratory incubator under light with DOW mixed with the surface water containing natural phytoplankton assemblages in several different mixing proportions of 90, 75, 50, 25 and 0 percent. The specific growth rate of the total phytoplankton assemblage evaluated from chlorophyll a changes increased with the amounts of DOW up to 25% dilution. Specific growth rates of 1.16 to 1.80 d^-1 in May and August, and 0.38 to 0.57 d^-1 in November and February were obtained at 25% dilution of DOW (Fig.3).

　

　

To evaluate possible effects of nutrients and temperature of DOW on phytoplankton, further culture experiments were conducted at two different temperatures, in which different amounts of NaN0₃ were added to the surface water. The following relationships were experimentally obtained between the specific growth rate of total phytoplankton (μ, d^-1) and the two environmental parameters, dissolved inorganic nitrogen (DIN) as nitrate and water temperature (WT) .

　

For DIN (μs),

　

　

where S represents DIN concentration as nitrate in μM.

　

　

For WT(μ_T),

　

where T represents water temperature in ℃.

　

Relationships between DOW discharged and the specific growth rate of total phytoplankton were estimated by use of equations (1) and (2), and the obtained results under an assumption that either nitrate concentration and/or temperature control the growth of the phytoplankton assemblage are graphically shown in Figure 4. Growth of the phytoplankton assemblage was enhanced both in the summer and in winter by the addition of DOW. It was also expected that DOW without warming could depress the growth of phytoplankton in the water even in winter because of lower temperature of DOW compared to the surface water.

　

DOW contains large amounts of nutrients and is characterized with high concentration of Si such as N:P:Si = 12:1:24. It has been reported that diatom species grow faster if the Si/P ratio is 10 or more in a mixed culture of a red tide alga of Alexandrium tamarense and a diatom species (Yamamoto et al., 1999). Diatoms generally tend to dominate in our culture experiments using DOW even the surface phytoplankton assemblages were dominated by other phytoplankton species than diatoms at starting. Thus, it can be concluded that DOW could specifically accelerate the growth of diatoms in natural phytoplankton assemblages.