A fundamental study on behavior of deep ocean water discharged for seaweed bed generation

Abstract

In order to investigate the effectiveness of deep ocean water (DOW) on development of seaweed bed, a field experiment has been conducted in a semi-enclosed port near the pumping facility of DOW in Kume Island in Okinawa, Japan. A cuboid pool in which DOW is discharged was installed on the bottom of the port to retain low temperature and high nutrient concentration of DOW effectively. In the field experiment, water temperature and nutrients concentrations in and out of the pools, and current velocity on the top of the pool have been measured to detect the behavior of discharged DOW. Through the experiment, it is clarified that the cuboid pool installed on the bottom of the port is quite effective to retain the discharged DOW. The monitoring results also indicate that the strong horizontal current could mix DOW in the experimental pool with the seawater above the pool. Additionally, it is found that DOW hardly affect surrounding environment since it will rapidly mix with ambient seawater when it goes out of the pool. Hydrodynamic simulations are also carried out to investigate the flow and temperature fields around the pool. The simulations showed that the water in the pool keep low temperature under small horizontal current velocities. When the velocity of horizontal current exceeds a threshold, the temperature in the pool gets higher because the discharged DOW mixes with the ambient seawater due to the vorticities induced at the upper surface of the pool.