Distribution and influencing factors of COD, along with the relationship with eutrophication in Tianjin coastal area

The spatiotemporal variation of chemical oxygen demand (COD) and the eutrophication status were determined in spring(May), summer(August)and autumn(October) 2014, in Tianjin coastal area, Bohai Bay. Based on the seawater quality monitoring database at 18 stations in this area, the relationship between COD and nutrient quality index (NQI) were investigated, as well as the variations of COD during the occurrence of a red tide. Significant seasonal variation of COD was found, with that higher in summer and lower in spring and autumn. COD decreased from nearshore to offshore and from south to north. NQI index ranged from 1.68 to 12.50, with the average of 4.46, reflecting serious eutrophication. To some extent, the significant positive correlation between COD and NQI index could reflect the eutrophication status in seawater. Significantly and positively, COD was correlated with red tide biological density and Chl a, which plays an important role in reflecting the red tide generating and vanishing process.