Abstract
A scheme to semi-analytically derive waters’ Secchi depth (Zsd) from remote sensing reflectance (Rrs) considering the effects of the residual errors in satellite Rrs was developed for the China Eastern Coastal Zone (CECZ). This approach was evaluated and compared against three existing algorithms using field measurements. As it was challenging to provide the accurately inherent optical properties data for running the three existing algorithms in the extremely turbid waters, the new developed algorithm worked more effective than the latter. Moreover, with both synthetic and match-up data, the results indicated that the proposed algorithm was able to minimize some residual errors in Rrs, and thus could generate inter-mission consistent Zsd results from two ocean color missions. Finally, after application of new model to satellite images, we presented the spatial and temporal variations of Secchi depth and trophic state in the CECZ during 2002–2014. The study led to several findings: Firstly, the Zsd-based trophic state index (TSI) in the East China Sea first increased since 2002, and then gradually dropped during 2008–2014. Secondly, more and more waters within 30–35 m and 20–25 m isobaths were deteriorating from oligotrophic to mesotrophic type and from mesotrophic to eutrophic water, respectively, during 2002–2014. Lastly, the TSI increased on average 0.091 and 0.286 m per year respectively in Bohai Sea and Yellow Sea since 2002, and it might only take 14 and 67 years for Bohai Sea and Yellow Sea to deteriorate from mesotrophic to eutrophic water, following their current yearly deterioration rate and trophic trend. These results highlighted the importance to make some strict regulations for protecting the aquatic environment in the CECZ.
Highlights
The coastal zones are populated zones for human society on the earth, and the productivity, trophic state, and health of these have great impact on the lifestyle of peoples living around them
We briefly examine the respective performances of BGSD, IOPSD, and MKSD models for Zsd retrievals, and further improve these for optically complex and turbid waters using an innovative class-based scheme for Zsd (CSSD)
Using the IDAS model-derived a(488), bb(488) and Rrs data as inputs, the model shown in Figure 2d and Equation (1) was suggested as the optimal CSSD model for estimations of Zsd from the China Eastern Coastal Zone (CECZ) seas, indicating that the CSSD model was an effective algorithm for Zsd estimations with the Moderate Resolution Imaging Spectroradiometer (MODIS) spectral bands from optically complex coastal waters, whose R2 was 0.87
Summary
The coastal zones are populated zones for human society on the earth, and the productivity, trophic state, and health of these have great impact on the lifestyle of peoples living around them. The Secchi depth (Zsd) is a measurement of water transparency, which is a simple and universal method in oceanographic environmental survey. The Zsd can be cheap and easy to obtain, and there is a large archive of historical Secchi depth measurements for global oceans. They are often used as a proxy for direct measurements of trophic state index (TSI) [4,5,6,7] and primary production [4] in the environmental communication
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