Optimization of marine environmental monitoring sites in the Yangtze River estuary and its adjacent sea, China

► Seawater Environmental Quality Index (SEQI) is firstly proposed. ► SEQI is calculated to describe the seawater environmental quality of the study area. ► Kriging variance analysis method is used to optimize monitoring sites. ► The number of monitoring sites is increased from 42 to 59 by using kriging variance analysis method. ► This study gives an optimal design of seawater quality monitoring network in the Yangtze River estuary and its adjacent sea, China.