Long-term observations of iron-oxyhydroxide-rich reddish-brown water in Nagahama Bay, Satsuma Iwo-Jima Island, Kagoshima, Japan

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Nagahama Bay, in the southern part of Satsuma Iwo-Jima Island (38 km south of Kyushu Island, Japan), contains an active hydrothermal system along the beach and at the fishing port. The construction ofa breakwater around the bay has resulted in a semi-closed body of water with an entrance from the south. From this bay, reddish-brown seawater enriched in iron-oxyhydroxides is discharged to the Pacific Ocean constantly. The greenish-blue seawater of the Pacific Ocean occasionally enters the bay and mixes with the reddish-brown water. The daily flow pattern of the reddish-brown seawater in the bay has been monitored previously during storms and periods of heavy rain. However, greenish-blue seawater is sometimesobserved in the bay at low tide. Therefore, we monitored the color of the sea surface and water at the seafloor for periods of 26 and 12 days during spring and autumn, respectively, using automatic still cameras to compare the water color with meteorological data recorded at Iwo-Jima Island. The results show that a southerly wind is associated with the trapping of reddish-brown surface water in the bay. In contrast, a northerly wind brings greenish-blue water from the open ocean into the bay, even at low tide. The factors that control the color of seawater in Nagahama Bay are not only the tidal cycle andstorm waves, but also wind speed and direction.
2012-03-01