Water clarity variations in Jiaozhou Bay over 39 years based on satellite observations

Water clarity is an important indicator for the water quality of aquatic ecosystems and measured as Secchi disk depth (Z_SD). In this study, images from Landsat Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), and Operational Landsat Imager (OLI) over the period from 1984 to 2022 are used to study the spatiotemporal variation in Jiaozhou Bay, a typical semi-enclosed bay in China, and to gain the insights into the drivers for the variation during the study period. The satellite images are first processed to obtain Z_SD, and corrected against the field measurements to ensure the Z_SD to be high-quality, and then the spatial and temporal distributions of Z_SD are analysed. The results show that the regional monthly average Z_SD over the study period are the highest in May and lowest in January. From long-term seasonal observation, it declines most significantly in Summer. In terms of inter-annual variation, the results clearly indicate that 2015 was a turning point, as the regional average Z_SD decreased significantly from 1984 to 2014 at a rate of approximately 0.022 m/year, but significantly increased from 2015 to 2022 with a rate of about 0.049 m/year. The turning trend of the water clarity in Jiaozhou Bay can be mainly attributed to the increasing sunshine duration during 2015–2022, as well as rapidly reduction emissions of industrial waste from Qingdao city during 2015–2016.