Water clarity in the Bohai Sea during 2003-2014

Water clarity, which is commonly and widely represented by the Secchi depth (Z_sd), is an important quality parameter for all aquatic environments. To evaluate the variation of water clarity of the Bohai Sea (BHS) in the past decade (2003-2014), Z_sd is derived from MODIS-Aqua ocean color data with a mechanistic model developed based on an innovative underwater visibility theory. The resulted Z_sd product from MODIS is found within 23% difference (root mean square difference is 0.126 in log scale) when compared with match-up in situ measurements that span a range of 2 -12 m. Based on this validated MODIS Z_sd product of the BHS, some valuable findings regarding water quality of the BHS over the period of 2003-2014 were obtained for the first time. Most importantly, monthly climatology of Z_sd in the central BHS during summer of 2003-2014 (~4 m) is found strikingly lower than that observed in situ in 1982-1983 (~9 m), and the summer climatology of 1959-1987 (~10 m). This implicates substantial deterioration of water clarity in the central BHS during the past decades, likely an outcome of the rapid economic development since the late 1980s. However, no significant changes of Z_sd in the central BHS is detected for the period of 2003-2014, suggesting no further deterioration of water clarity occurred in the past decade. Environmental protection schemes such as control of pollutant discharge and river damming may have helped.