The real sea experiment of seaweed growing with DOW

摘要

Deep Ocean Water (DOW: the sea water below 200m depth) which has three major characteristics, Low Temperature, Rich Nutrient and Higher Cleanliness is expected as a future renewable resources of the ocean. Toward the era of environment and low carbon society, utilizing the ocean renewable resource is absolutely important because the land base resources are now peaking out. In order to making use of DOW effectively and economically, multi-purpose utilizing of DOW is recommended. However, utilizing the nutrient of DOW to increase the primary production in the sea is not so popular, comparing with the low temperature and higher cleanliness. Generally, the photosynthesis in the sea is limited by the luck of inorganic nutrient salts such as Nitrogen, Phosphorus, etc. so that the supply of these nutrient salts is very important to increase the primary production in the sea, especially in the stratified water of equatorial zone where there are very few vertical mixture between poor nutrient surface water and rich nutrient DOW. The experimental facility for increasing primary production such as producing seaweed forest in the sea using DOW was planned and constructed at Maja Fishery Port (Area: about 10,000m2) in Kumejima Island Okinawa-prefecture Japan where there is the OTEC (Ocean Thermal Energy Conversion) plant who is discharging waste DOW which is still having a rich nutrient from its condenser cooling system. The experimental container (Inner capacity: about 6m3) which is filled with DOW constantly by the transfer pump from the OTEC plant was set up on the sea floor bottom of the Maja Fisheries Port. The ambient water temperature is about 22~30°C and the DOW temperature is 12 ~16°C, so that higher density remains in the experimental container stably. The outline of the facility is shown in Figure below. The seaweed growing tests of Japanese Wakame (Undaria pinnatifida) which lives in northern region of Japan were carried out using the facility. As a results of the tests it is confirmed that even the seaweed living in cold region can grow in a semi-equatorial region such as Okinawa using waste DOW from the OTEC plant.