Changes of seawater quality in Ofunto Bay, Iwate, after the 2011 off the Pacific coast of Tohoku Earthquake

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• 作者：Yuichiro Yamada ; Shinnosuke Kaga ; Yoshimasa Kaga ; Kimiaki Naiki…
• 关键词：The 2011 off the Pacific coast of Tohoku Earthquake ; Tsunami ; Ofunato Bay ; Breakwater ; Chlorophyll a ; Dissolved oxygen ; Heterotrophic bacteria ; Shellfish mariculture
• 刊名：Journal of Oceanography
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：73
• 期：1
• 页码：11-24
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Oceanography; Freshwater & Marine Ecology;
• 出版者：Springer Japan
• ISSN：1573-868X
• 卷排序：73

文摘

Ofunato Bay was a semi-closed area because of the breakwater effect at the entrance; however, the breakwater was destroyed by a massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake. Consequently, the physical environment of Ofunato Bay has been changed significantly, i.e., the modification of the stratified structure of seawater inside the bay and the intermittent intrusion of seawater outside the bay. These alterations of physical environment are considered to have an influence on the chemical and biological environment in Ofunato Bay. To elucidate the influence of the tsunami on the aquatic environment, we measured dissolved nutrients, chlorophyll a and dissolved oxygen concentrations, and heterotrophic bacteria abundance inside and outside of Ofunato Bay from 2012 to 2014, and compared these data with those obtained before the earthquake. As compared with before the earthquake, significant changes after the earthquake were (1) decrease of ammonium and phosphate concentrations, (2) increase of chlorophyll a concentration, (3) increase of dissolved oxygen concentration in the bottom, and (4) decrease of heterotrophic bacteria abundance. The collapse of the breakwater and consequential enhanced water exchange were considered to have brought the decrease of nutrient concentration inside the bay. Furthermore, washout of shellfish mariculture rafts by the tsunami decreased the shellfish biodeposits along with the elution of nutrients by heterotrophic bacteria. Decrease of cultivated shellfish further caused a decline in feeding pressure on phytoplankton and, subsequently, increased the phytoplankton biomass that contributed to the decrease of nutrients inside the bay.