Spatiotemporal variations of total suspended matter in complex archipelagic regions using a sigmoid model and Landsat-8 imagery

Total suspended matter (TSM) is an important parameter of ocean water quality and for aquatic environmental assessment. The Zhoushan Islands are near the estuarine and coastal regions of the Yangtze River and Qiantang River and TSM occurs at very high concentrations and in long-term suspended states. The distribution and variation of TSM in the Zhoushan Islands has great impacts on its coastal water quality and activities such as ferry services, fisheries, and tourism. Based on Landsat-8’s blue and near-infrared bands, this study developed a sigmoid TSM inversion model that was more accurate and robust than the other common linear and exponential models. We also checked three types of atmospheric correction models, FLAASH, 6S, and ACOLITE and determined that ACOLITE is better than the other two methods when using Landsat-8 in ocean color in estuarine and coastal regions. The sigmoid model was then applied to a series of Landsat-8 images covering the Zhoushan Islands from 2013 to 2018, and the image-derived results show that TSM absorption coefficients in the Zhoushan Islands are extremely high and range from 0.26 to 444.29 m−1 with a mean of 113.14 m−1. Furthermore, they have a clear downward gradient from the Hangzhou Bay to the open sea. In addition, the TSM concentrations in winter are usually much higher than those in summer. Our results indicate that TSM spatiotemporal variations in the Zhoushan Islands are not only controlled by many natural environmental factors, such as riverine discharge, but are also significantly influenced by many human activities such as transportation, tourism, and fisheries.