Temporal and Spatial Distribution of COD and Its Source and Contribution to Eutrophication in the Central Bohai Sea

Abstract:

The temporal and spatial distribution of chemical oxygen demand (COD) in different water layers, and its source and contribution to eutrophication were analyzed in this study based on data collected in the Bohai Sea in all 4 seasons of 2013. The results showed that the highest average concentration of COD appeared in summer, followed by autumn; while the lowest concentration appeared in spring. The analysis about vertical distribution of COD showed that the highest and lowest concentrations of COD appeared in the surface and middle layers in spring, and they appeared in the surface and bottom layers respectively in summer; in autumn and winter they were observed in the middle and bottom layers. As for the horizontal distribution the average COD concentration was distributed quite evenly in spring; in summer COD concentration was the highest in the western sea area and displayed a gradual decrease from the west to the east; in winter the highest concentration was detected in the northern area and it dropped from the north to the south. During the survey period, the COD level in most stations met the first-class seawater quality standard. The E value ranged from 0.088 to 2.995, and the average was 0.337±0.403. There were 25 stations in which the E values were above 1, including 20 stations in autumn and 5 stations in winter. These results indicated that the surveyed regions might not undergo eutrophication. The contribution of COD to eutrophication ranged from 46.15% to 141.41%, and the average was (71.36±14.98)%, which suggested that COD played an important role in the eutrophication of the Bohai Sea.