Report on the Occurrence of Hypoxia in the Central Bohai Sea

Abstract:

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 and δρ), implying the formation of strong pycnocline in the hypoxic zone. The δT isoline of 5℃ mainly occurred in the vicinity of hypoxic zone with the highest δT value of 7.3℃. The density difference between the bottom and the surface layer has a similar spatial pattern with δT. 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 (> 4 μg/L) and PO43--P (> 6 μg/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 and in situ production.