Optimization of Shanghai marine environment monitoring sites by integrating spatial correlation and stratified heterogeneity

Abstract

The water quality grades of phosphate (PO4-P) and dissolved inorganic nitrogen (DIN) are integrated by spatial partitioning to fit the global and local semi-variograms of these nutrients. Leave-one-out cross validation is used to determine the statistical inference method. To minimize absolute average errors and error mean squares, stratified Kriging (SK) interpolation is applied to DIN and ordinary Kriging (OK) interpolation is applied to PO4-P. Ten percent of the sites is adjusted by considering their impact on the change in deviations in DIN and PO4-P interpolation and the resultant effect on areas with different water quality grades. Thus, seven redundant historical sites are removed. Seven historical sites are distributed in areas with water quality poorer than Grade IV at the north and south branches of the Changjiang (Yangtze River) Estuary and at the coastal region north of the Hangzhou Bay. Numerous sites are installed in these regions. The contents of various elements in the waters are not remarkably changed, and the waters are mixed well. Seven sites that have been optimized and removed are set to water with quality Grades III and IV. Optimization and adjustment of unrestricted areas show that the optimized and adjusted sites are mainly distributed in regions where the water quality grade undergoes transition. Therefore, key sites for adjustment and optimization are located at the boundaries of areas with different water quality grades and seawater.