渤海中部海域低氧区的发生记录

2014年8月对渤海中部海域的水文(温度、盐度)、化学(溶解氧DO、营养盐和化学耗氧量COD)和生物要素(叶绿素a Chl-a)的空间分布进行了调查。研究了该海域底层水体低氧(DO<3.0 mg/L)的分布特征，深入分析了低氧区发生的关键因素。结果显示，底层水体DO浓度最小值为2.30 mg/L，低氧面积达1200 km2，呈西北–东南走向。调查海域的西部和西南海域呈现出明显的温度层化，尤其在低氧区附近形成了一个表层与底层水体温度差(δT)>5℃的区域，δT 最高值达到7.3℃。水体密度层化与温度层化特征相似，在低氧区附近形成了一个底表层密度差(δρ)>2 g/L的等值线闭合圈。温度层化是低氧产生的主要物理因素。表层水体 COD 高值区主要分布在调查海域的西部，覆盖大部分的低氧海域。表层水体中的Chl-a(>4μg/L)和PO43--P (>6μg/L)浓度高值区主要分布在调查海域的西南部，部分与低氧区重合。本研究可为探索渤海海域富营养化演变过程提供借鉴。

Report on the Occurrence of Hypoxia in the Central Bohai Sea

Hypoxia in the central areas of the Bohai Sea was reported for the first time in this study. The survey in August 2014 suggested that the hypoxic zone was 1200 km2 in area and the minimum DO concentration was 2.30 mg/L. The hypoxic zone was on the southwest side of the investigated area (119.1°–119.6°E, 38.3°–38.8°N). The DO (4 mg/L) isoline indicated that the hypoxic layer reached 10 m under the water. There were differences in the temperature and density between the surface and the bottom layer (δT andδρ), implying the formation of strong pycnocline in the hypoxic zone. TheδT isoline of 5℃mainly occurred in the vicinity of hypoxic zone with the highestδT value of 7.3℃. The density difference between the bottom and the surface layer has a similar spatial pattern withδT. By contrast, the salinity difference between the bottom and the surface layer was small (< 0.8) in the investigated areas. These results suggested that thermal stratification could be more influential than saline stratification in controlling the occurrence of hypoxia. The COD concentration was high (< 0.9 mg/L) on the southwest side of the investigated area that accounted for the majority of the hypoxic zone. In addition, high level of Chl-a (> 4μg/L) and PO43--P (> 6μg/L) was also observed in this area. We proposed that the formation of hypoxia could be a result of combined factors including the inflow from the Yellow Sea, the topography, and especially, the thermal stratification andin situ production.