Oxygen-deficient water volume and marine lower trophic processes in Dokai Bay after large decreases in nutrient concentrations

• Published in: Bulletin on coastal oceanography Vol. 48; no. 1; pp. 29 - 36
• Main Authors: Hamada, Kenichiro, Ueda, Naoko, Yamada, Machiko, Tada, Kuninao, Montani, Shigeru
• Format: Journal Article
• Language: Japanese
• Published: 01.01.2010
• Subjects: Marine; INW, Japan, Kyushu; INW, Japan, Kyushu, Fukuoka Prefecture, Dokai Bay; INW, Japan, Kyushu, Fukuoka Prefect., Dokai Bay
• ISSN: 1342-2758

Dokai Bay is a small enclosed bay, located in the northern part of Kyushu Island, Japan. This bay is hypertrophic, as evidenced by extraordinarily high total N and P concentrations (TN & TP). TN and TP environmental quality standards (EQSs, TN: less than 1.0 mg L super(-1), TP: less than 0.09 mg L super(-1)) were applied to Dokai Bay in 1997. Through reductions in nitrogen loading, TN and TP concentrations had declined to below the EQSs by 2001. In this study, we report the decrease of nutrients and change in phytoplankton biomass in an oxygen-deficient water mass in Dokai Bay. After the reduction of nitrogen loading, the annual range of NH sub(4)-N concentrations remarkably decreased from 4.97-1,590 mu M to 0.120-63.0 mu M. However, Chl a concentrations remained relatively unchanged, from 1.5-146 mu g L super(-1) before the implementation of EQSs to 1.4-218 mu g L super(-1) afterward. Furthermore, the quality and quantity of particulate organic matter did not change. This result shows that even such a remarkable decrease in nutrient concentrations did not inhibit phytoplankton growth in a hypertrophic bay. The oxygen-deficient water mass volume was approximately the same: 521 x 10 super(4) m super(3) before the implementation of EQSs and 508 x 10 super(4) m super(3) after. However, we presume that the duration of the oxygen-deficient water mass was shorter after EQS-implementation. Moreover, we think that the oxygen-deficient water mass in Dokai Bay was affected by the bay sediment. These findings show that the oxygen-deficient water mass volume and marine lower trophic process did not change after the implementation of EQSs.