open access

https://hdl.handle.net/2115/56488

Oxygen-deficient Water Volume and Marine Lower Trophic Processes in Dokai Bay after Large Decreases in Nutrient Concentrations(Symposium: Hypoxic Water in Coastal Seas)

日本語

環境基準

2010-08-30

日本海洋学会沿岸海洋研究会

沿岸海洋研究

48(1)

29-36

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^<-1>, TP : less than 0.09 mg L^<-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_4-N concentrations remarkably decreased from 4.97-1,590μM to 0.120-63.0μM. However, Chl a concentrations remained relatively unchanged, from 1.5-146μg L^<-1> before the implementation of EQSs to 1.4-218μg L^<-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 : 521×10^4 m^3 before the implementation of EQSs and 508×10^4 m^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 oxygendeficient water mass volume and marine lower trophic process did not change after the implementation of EQSs.

学術雑誌論文

Version of Record

1342-2758

AN10492259