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Recent Advances in Marine Science and Technology, 2002

 事業名 海洋科学技術に関する太平洋会議の開催
 団体名 国際海洋科学技術協会 注目度注目度5


ASSESSMENT ON ENVIRONMENTAL EFFECTS OF DEEP OCEAN WATER DISCHARGED INTO COASTAL SEA
 
Masatoshi Hayashi1, Tomoji Ikeda1, Koji Otsuka2 and Masayuki Mac Takahashi3
 
1Kansai Environmental Engineering Center, Co., Ltd.
Azuchi-machi, JAPAN
hayashi_masatoshi@kanso.co.jp
 
2Osaka Prefecture University
Osaka, JAPAN
 
3Graduate School of Arts and Sciences, University of Tokyo
Tokyo, JAPAN
 
ABSTRACT
 
Deep ocean water (DOW) has some superior characteristics in comparison with surface seawater such as coldness, rich nutrients and cleanliness. Some of those characteristics of DOW have recently been used for many practical purposes. However environmental impacts due to DOW discharged into coastal sea after uses have not yet been evaluated. This will be one of the important subjects to be clarified for a possible great increase in DOW usage in the near future. Following points such as biological entrainment during pumping up of DOW, CO2 emission into the atmosphere from DOW by pressure reduction and warming, fertilization of phytoplankton and seaweed and the other effects of DOW discharged into coastal sea have been given particular attention as probable impacts in this study. It is also intended to assess probable environmental impacts of pumping up and discharging used DOW into the natural environment.
 
INTRODUCTION
 
Deep ocean water (DOW) has recently received particular attention in Japan as a new natural resource having clean, cold and nutrient-rich characteristics (Takahashi, 2000), and many practical applications of DOW characteristics have now been made in various commercial uses such as foods, drinks, cosmetics and so on (Takahashi, 2002). There are nine pumping stations of DOW being operated in Japan, with pumping capacities ranging from 60 to 13,000 t.d-1. Two pumping stations, Mitsu and Takaoka in Kochi Prefecture, discharge DOW directly into the surrounding coastal sea without any special care, although actual amounts of discharge are art of DOW pumped up. At Namerikawa in Toyama Prefecture, most DOW of the ca 3000 t.d-1 pumped up is discharged into Namerikawa Harbor after being mixed with river water. While on Kume Island in Okinawa Prefecture, where the large quantity of 13,000 t.d-1 of DOW is pumped up. DOW is discharged using two pipes reaching beyond the coral reefs to avoid direct exposure of discharged DOW to corals.
 
Although existing pumping capacity at a given station is 13,000 t.d-1 or less in Japan, there is a possibility that the pumping capacity will be greatly increased in the near future, if we start to use the low temperature and nutrients contained in DOW resources commercially. For such a large use of DOW, we need to establish how to discharge DOW after use (Komatsu, 2000), and how to make the necessary assessment of probable impacts of DOW discharge into the natural coastal environment (Otsuka, 2001).
 
One of the most probable impacts of DOW discharges will be local fertilization by the excess nutrients contained in DOW as well as lowering water temperature, which could give us some benefits if they could enhance local production of commercially valuable fish and shellfish (Ryther, 1969; Furuya, 1993; Ouchi, 2001 ; Matsuda, 2002). However, it could change the environment, which will be considered detrimental to the conservation of the existing natural environment. Furthermore, we have to evaluate not to fertilize the area will developing harmful red tide organisms and creating anoxic water.
 
There are also other possible impacts, such as entrainment of organisms living in deep depths by pumping up of DOW to the surface (Daniel, 1992) and probable CO2 emission into the atmosphere (Harada, 2000). DOW contains higher concentrations of carbonic acid than that of surface seawater, and the excess CO2 will be released into the atmosphere since DOW is reduced pressure and warmed at the surface. Since at least these two impacts cannot be avoided, we have to minimize the impacts. For the biological entrainment, we need to know what kinds of organisms might be entrained with DOW and their amounts. For the CO2 release, we can minimize the impact or may change to a positive impact by absorbing excess CO2 by accelerating the productivity of phytoplankton and seaweed using nutrients contained in DOW.
 
A national research project, in which a large quantity of DOW such as 1 million t・d-1 is planned for cooling a 600 MW commercial electric power generator, has been carried out since 1999 for the five-year program (Takahashi, 2002). The present study has been made a part of the national project mentioned above for making the necessary assessment of environmental impacts of DOW discharged into coastal area after uses. For establishing a suitable assessment, methodology on environmental impacts of large quantities of DOW is another target in this study, too.
 
This paper particularly focuses on probable impacts of the following factors: biological entrainment during pumping up of DOW, CO2 emission into the atmosphere from DOW by warming, fertilization of phytoplankton and seaweed and several other effects of discharging DOW into coastal sea.
 
METHODS
 
Samples of biological entrainment were collected once a month (October 2000-September 2001) at the two prefectural deep seawater laboratories of Kochi and Toyama, and the Kinki University Fisheries Research Laboratory. DOW and surface seawater samples pumped up in the three DOW pumping stations were filtered through 92μm nylon gauze mesh for 24 hours. Three quarters of each sample was immediately fixed with 5% neutral formalin and then species were identified and the number of individuals was counted. The rest of the sample was filtered onto 50μm nylon gauze mesh filter and organic carbon of retained materials on the filter was then measured.
 
Samples to estimate CO2 balance were collected once a month (October 2000-September 2001) at the two prefectural deep seawater laboratories mentioned above. The method for estimating CO2 balance followed with Lewis and Wallace (1 998). The hydraulic test was carried out in a horizontal flume being 7 m in length, 3 m in width and with 1/25 slope in bottom. Salinity of seawater in the tank was measured and for estimating the level of dilution.
 
Natural surface phytoplankton assemblages collected near the Kochi Prefectural Deep Seawater Laboratory were cultured in DOW mixed with surface seawater at levels of 90, 75, 50, 25 and 0% four times a year (May, August, November and February). Culture experiments for investigating possible effects of nutrients were also conducted by adding different amounts of nitrate, phosphate and silica to the surface water containing 1 % of DOW. Possible effect of temperature was also examined. Specific growth rate was estimated based upon chlorophyll-a of phytoplankton assemblage determined every day. Culture of seaweed was examined in DOW mixed with surface seawater at levels of 100, 80, 60, 40, 20 and 0%. Relative growth rate of seaweed was estimated based upon wet weight measured every day.
 
RESULTS
 
Biological entrainment during pumping up of DOW
 
Biological entrainment due to pump up of DOW has been investigated by 24 hours accumulated sample monthly at three DOW pumping stations during one year. Kochi Prefectural Deep Seawater Research Laboratory pumped from ca.320 m by two pipes of inside diameter of 0.125m at 0.43 m・s-1 of pumping velocity, Toyama Prefectural Fisheries Research Laboratory from ca.320 m by a pipe of 0.25 m inside diameter at a pumping speed of 0.7 m・s-1 and Fisheries Research Laboratory in Toyama of the Kinki University from ca, 100 m by a pipe of 0.45 m inside diameter at a pumping speed of 0.125 m・s-1. Entrainment surveys have also been performed for the surface seawater pumped.
 
Table 1. Particulate organic carbon (POC) retained on 50 μm netting in DOW and surface seawater samples during 24 hours continuous sampling once month throughout one year
(Enlarge: 27KB)
 
Monthly total amounts of biological entrainment during 24 hours continuous net sampling in DOW for each station were generally smaller than those for surface seawater. The main group of organisms collected from DOW was copepods of Arthropoda (Fig.1). No actual entrainment of commercially valuable organisms was seen in the DOW samples collected during 24 hours in each month over one year at all three sampling stations. Organic carbon (POC) retained on 50μm mesh netting was lower in the DOW samples than that of the surface seawater samples at the three DOW pumping stations (Table 1). POC in DOW in Kochi was at least 1/10 of that in Toyama. These results show that the total amounts of entrainment in DOW pumped up were consistently low compared with the surface seawater. Although there is a possibility to entrain large size Organisms such as fish, it has not been evaluated in the present study because of less frequency of sampling compared to probable entrainment probability of those large organisms.
 
(Enlarge: 51KB)
Figure 1. Biological entrainment expressed with individual numbers of different taxonomical groups due to pump up of DOW and of surface seawater; S: surface seawater, D: DOW







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