Using Landsat-8 to Remotely Estimate and Observe Spatio-Temporal Variations of Total Suspended Matter in Zhoushan Coastal Regions
PANG Shu-na1, ZHU Wei-ning1*, CHEN Jiang2, SUN Nan3, HUANG Li-tong1, ZHANG Yu-sen1, ZHANG Ze-liang1
1. Ocean College, Zhejiang University, Zhoushan 316021, China
2. School of Remote Sensing and Information Engineering, Wuhan University, Wuhan 430079, China
3. Second Institute of Oceangraphy, Ministry of Natural Resources, Hangzhou 310012, China
Abstract：Total suspended matter (TSM) is one of the important parameters for ocean water quality and aquatic environment assessment. Zhoushan Islands is located on the edge of Hangzhou Bay, where sediments present very high concentration and TSM shows suspended status for long-term. The distribution and variation of TSM in Zhoushan Islands have great impacts on its coastal water quality, ferry, fishery and tourism. Because of its high spatial resolution and open access, Landsat-8 imagery has been widely used for ocean color studies and hence providing good spectral information for observing variations of TSM in Zhoushan coastal regions. This study used in-situ measured TSM absorption coefficient (ap(440), m-1) and water surface spectra to develop the TSM remote sensing model based on Landsat-8 imagery, and results show that the S-function using blue and near-infrared bands of Landsat-8 performed better than the other functions. The S-function is with the form ap=3.72/(0.009+e-5.249B5/B2). This function overcomes the shortcomings of previous inversion functions (e. g., linear, logarithmic, and exponential functions) which performed well for modeling datasets, but often failed when they were applied to real satellite images since spectral values in images are often much larger than those in the modeling dataset. Another problem of coastal and inland water color remote sensing is the atmospheric correction. The previous studies usually used a specific atmospheric correction method, but it might not return the best results. In this study, we tested and compared three types of atmospheric corrections models, FLAASH, 6S, and ACOLITE, and found that ACOLITE was better than the other two methods for ocean color remote sensing using Landsat-8, especially in blue band, where ACOLITE performed better than the FLAASH and 6S methods. The S-model was used to a series of Landsat-8 images covering the Zhoushan Islands from 2013 to 2018. The field measured and image-derived results show that TSM absorption coefficients in Zhoushan Islands were extremely high, ranging from 1.64 to 417.04 m-1, with mean 118.47 m-1, which account for more than 90% of total water absorption coefficients. The measured above-surface spectra demonstrated typical spectral signatures of complex turbid coastal water, with two peaks within green and red bands and relatively high reflectance within red and near-infrared bands. Due to the high concentrations of the riverine discharged TSM, remote sensing reflectance in estuarine and coastal regions were much higher than those in open sea. TSM concentrations illustrated a clear downward gradient from Hangzhou Bay to open sea, and TSM in coastal areas such as Qushan, Yangshan and Ningbo were much higher than those in open sea area such as Dongji and Shengsi. TSM distribution pattern in coastal regions were usually more complex than those in offshore regions. TSM concentrations in winter were usually much higher than those in summer, where the highest was found in December with value of 413.32 m-1, and the lowest was found in August with value of 3.69 m-1. There are also local peaks of TSM during May and October. Our results indicate that the TSM spatio-temporal variations in Zhoushan Islands are not only controlled by many natural environmental factors, such as currents, tides, typhoons, monsoons, but also aresignificantly influenced by many human activities, such as ferry, ocean shipping, harbor projects and island tourism.
Key words：Ocean color remote sensing; Total suspended matter; Landsat-8; Coastal regions; spatio-temporal variations
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