Variability of Chlorophyll-a and Secchi Disk Depth (1997–2019) in the Bohai Sea Based on Monthly Cloud-Free Satellite Data Reconstructions

Junting Guo,Xianqing Lv,Yuming Zhang,Daosheng Wang,Jingfang Lu,Chen Zhou,Shufang Zhang

doi:10.3390/rs14030639

Abstract

Ocean colour data are crucial for monitoring and assessing marine ecosystems. In this study, the Data Interpolating Empirical Orthogonal Functions (DINEOF) approach was applied to the Ocean Colour Climate Change Initiative (OC-CCI), chlorophyll-a (Chl-a) and Secchi disk depth (Zsd) to completely reconstruct the missing pixels in the Bohai Sea during 1997–2019. The results of cross-validation demonstrate that the DINEOF reconstructed data have a good agreement with the satellite-measured data. Based on monthly cloud-free satellite data reconstructions, the Zsd series showed high negative correlation with log10 (Chl-a). The Zsd as a function of log10 (Chl-a) can be well fitted by the cubic polynomial in the offshore waters. The Chl-a in the entire Bohai Sea showed a significant decreasing trend (−0.013 mg/m3/year), while the Zsd exhibited a significant increasing trend (0.0065 m/year), and both had regional-seasonal variations. In addition, the ensemble empirical mode decomposition (EEMD) results reveal highly nonlinear trends of Chl-a and Zsd. The linear and nonlinear trends of Chl-a and Zsd suggest the deterioration of water quality in the Bohai Sea was not continued over the past two decades. This study presents the first simultaneous investigation of Chl-a and Zsd using the 23 years of cloud-free reconstructions in the Bohai Sea.

Highlights

Introduction iationsChlorophyll-a (Chl-a) and Secchi disk depth (Zsd) are two important indicators to quantify water quality
The monthly Chl-a and Zsd. For the Chl-a (Zsd) data of the Bohai Sea were produced with the European Space Agency (ESA)-Ocean Colour Climate Change Initiative (OC-CCI) data from 1997–2019, defined by the location 37◦ –41◦ N and 117◦ –122◦ E
The Data Interpolating Empirical Orthogonal Functions (DINEOF) method was used to reconstruct the missing data caused by the cloud cover, high sun-glint contamination and other reasons

Summary

Introduction

Chlorophyll-a (Chl-a) and Secchi disk depth (Zsd) are two important indicators to quantify water quality. Chl-a is considered a proxy for phytoplankton biomass in primary productivity studies [1,2,3]. The Zsd is the oldest oceanographic variable, recorded since the nineteenth century to provides a reading of water transparency [4,5,6]. Traditional measurements of Chl-a and Zsd are sparse and laborious, and are not helpful for performing spatio-temporal analyses of large-scale water bodies. With the development of ocean color satellite sensors, remote sensing techniques have been widely used to investigate the Chl-a and Zsd, providing global and repetitive measurements [7,8,9]. Lee et al [10]

Methods

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Discussion

Conclusion