Long-term trends of bottom-water concentrations of dissolved oxygen (DO), total nitrogen, and total phosphorus at 42 sites in Tokyo Bay were analyzed using monthly data from 1983 to 2018 measured by local government agencies. The results were visualized as spatial distribution maps. Bottom-water DO concentrations varied seasonally. The area of hypoxic bottom water (DO < 2 mg L⁻¹) was greatest in September of every year. Examination of interannual variations of the spatial distribution of bottom-water DO concentrations in September revealed that along the eastern side of the bay, hypoxic zones have been shrinking since the mid-2000s, but along the western side of the bay, the bottom water closest to shore is hypoxic in September even today. The areal percentage of hypoxic bottom water during September within inner Tokyo Bay has declined from 30% around 2007 to < 15% around 2015 and has not changed since then. A comparison of the spatial distributions of the frequencies of occurrence of red tides (chlorophyll-a concentrations ≥ 50 μg L⁻¹) and hypoxic water masses during 8-year intervals revealed that areas with high percentages of red tides coincided with areas with high percentages of hypoxic bottom water. The implication is that the reduction of the size of the hypoxic zones along the eastern side of the bay was due to less frequent occurrence of red tides there. Reducing the inputs of nutrients to the bay as a result of policies aimed at reducing water pollution has, therefore, contributed to the mitigation of the hypoxia problem.

使用当地政府机构测量的 1983 年至 2018 年的月度数据分析了东京湾 42 个地点的底水溶解氧 (DO)、总氮和总磷浓度的长期趋势。结果可视化为空间分布图。底水溶解氧浓度随季节变化。底水缺氧面积（DO < 2 mg L ^-1) 在每年的 9 月最大。9 月份底水溶解氧浓度空间分布的年际变化显示，海湾东侧的低氧区自 2000 年代中期以来一直在缩小，但海湾西侧的底水最近九月的岸边还是缺氧的今天。9 月份东京湾内层缺氧底水的面积百分比从 2007 年左右的 30% 下降到 2015 年左右的 < 15%，此后没有变化。赤潮发生频率的空间分布的比较（受叶绿素一个浓度≥50微克大号^-1) 和 8 年间隔内的缺氧水团显示，赤潮百分比高的区域与底水缺氧百分比高的区域重合。这意味着海湾东侧缺氧区的面积缩小是由于那里赤潮发生的频率较低。因此，由于旨在减少水污染的政策而减少对海湾的营养物质输入有助于缓解缺氧问题。