Changes in water clarity of the Bohai Sea: Observations from MODIS

Water clarity, which is commonly and widely represented by the Secchi depth (Zsd), 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), Zsd is derived from MODIS-Aqua ocean color data with an innovative mechanistic model. The resulted Zsd product from MODIS is found within 23% differences of match-up in situ measurements that span a range of 2-12 m. Based on this validated MODIS Zsd product of the BHS, valuable findings regarding water clarity of the BHS over the period of 2003-2014 are obtained for the first time. For the central BHS, the climatological mean Zsd in August of 2003-2014 (3.9 m) is found strikingly lower than that observed in situ in August of 1982-1983 (8.7 m), 1959-1979 (8-12 m) and 1972-1987 (8-14 m). This implies substantial deterioration of water clarity in the central BHS after the late 1980s, an unpleasant byproduct of rapid economic development in the BHS surrounding regions initiated in the mid-1980s. On the other hand, no significant variations in Zsd in the central BHS is detected for the period of 2003-2014, suggesting deterioration of water clarity of this water body is not continued. Results from this effort highlight the value of both the analytically-derived Zsd product and satellite ocean color remote sensing in monitoring water quality of coastal seas.