Satellite-observed significant improvement in nearshore transparency of the Bohai Sea during pollution control
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Abstract: The Bohai Sea (BS) is the unique semi-closed inland sea of China, characterized by degraded water quality due to significant terrestrial pollution input. In order to improve its water quality, a dedicated action named “Uphill Battles for Integrated Bohai Sea Management” (UBIBSM, 2018–2020) was implemented by the Chinese government. To evaluate the action effectiveness toward water quality improvement, variability of the satellite-observed water transparency (Secchi disk depth, ZSD) was explored, with special emphasis on the nearshore waters (within 20 km from the coastline) prone to terrestrial influence. (1) Compared to the status before the action began (2011–2017), majority (87.3%) of the nearshore waters turned clear during the action implementation period (2018–2020), characterized by the elevated ZSD by 11.6% ± 12.1%. (2) Nevertheless, the improvement was not spatially uniform, with higher ZSD improvement in provinces of Hebei, Liaoning, and Shandong (13.2% ± 16.5%, 13.2% ± 11.6%, 10.8% ± 10.2%, respectively) followed by Tianjin (6.2% ± 4.7%). (3) Bayesian trend analysis found the abrupt ZSD improvement in April 2018, which coincided with the initiation of UBIBSM, implying the water quality response to pollution control. More importantly, the independent statistics of land-based pollutant discharge also indicated that the significant reduction of terrestrial pollutant input during the UBIBSM action was the main driver of observed ZSD improvement. (4) Compared with previous pollution control actions in the BS, UBIBSM was found to be the most successful one during the past 20 years, in terms of transparency improvement over nearshore waters. The presented results proved the UBIBSM-achieved remarkable water quality improvement, taking the advantage of long-term consistent and objective data record from satellite ocean color observation.
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Key words:
- Secchi disk depth /
- transparency /
- water quality /
- nearshore /
- Bohai Sea /
- satellite ocean color remote sensing /
- pollution control
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Figure 1. Location of Bohai Sea and the surrounding TPOM. The study area is nearshore waters (the dark blue region), which is within 20 km from the coastline, with average depth of about 11 m. Purple circles and crosses represent the sites with Rrs(λ) (N = 7) and ZSD (N = 45) observations, respectively. It is noted that these 2 measurements are available at 5 stations.
Figure 2. Validation of MODIS Rrs(λ) in the nearshore waters of the BS. a. Scatter plot at major ocean color bands; b. comparison of spectral shapes. Error bars denote the standard deviations of multiple observations.
Figure 3. Diagram of ZSD retrieval model. In the blue box,
$\mathrm{Min}\left[{K}_{{\rm{d}}}\left({{\text{λ}} }_{1},{{\text{λ}} }_{2},{{\text{λ}} }_{3},\cdots \right)\right]$ represents the minimum at these bands and${R}_{{\rm{rs}}}^{{\rm{pc}}}$ is the remote sensing reflectance of the corresponding wavelength.
Figure 4. Comparison between satellite-derived ZSD and in situ measurements. Solid line is the 1:1 line. Dashed lines are the 1:2 and 2:1 lines.
Figure 5. Scatter plot of satellite-observed and DINEOF-reconstructed ZSD based on cross-validation dataset compiled from the nearshore waters of the BS.
Figure 6. Spatial distribution of ZSD variability over BS nearshore waters during 2018 and 2020, as compared to that during 2011 and 2017, ZSD improvement of Liaoning, Hebei, Shandong and Tianjin were 13.2% ± 11.6%, 13.2% ± 16.5%, 10.8% ± 10.2%, and 6.2% ± 4.7%, respectively (a); spatial distribution of nearshore waters during 2018 and 2020 with ZSD increase of higher than 20% (in blue) and lower than 20% (in green), respectively (b); and average ZSD during 2011 and 2017 (c). The “improved significantly” refers to ZSD improvement satisfying 95% significance level.
Figure 7. BEAST-derived change trend (black line) of the ZSD over the BS nearshore waters from 2011 to 2020 and the detected change point with the highest probability of abrupt change. The shaded part highlights the UBIBSM duration, and the red line indicates the detected change point.
Figure 8. Annual averaged ZSD of the BS nearshore waters from 2003 to 2020. Error bars denote the standard deviations of monthly transparency.
Figure 9. Coverage percentage of nearshore waters with improved ZSD (2018–2020) for the entire BS and TPOM.
Figure 11. Spatial distribution of ZSD variability over the BS nearshore waters of Liaoning Province during 2018 and 2020, as compared to that during 2011 and 2017 (a) and coverage percentages of BS nearshore waters with improved ZSD (2018–2020) for cities of the Liaoning Province (b).
Figure 12. Spatial distribution of ZSD variability over the BS nearshore waters of Hebei Province during 2018 and 2020, as compared to that during 2011 and 2017 (a) and coverage percentages of BS nearshore waters with improved ZSD (2018–2020) for cities of the Hebei Province (b).
Figure 13. Spatial distribution of ZSD variability over the BS nearshore waters of Shandong Province during 2018 and 2020, as compared to that during 2011 and 2017 (a) and coverage percentages of BS nearshore waters with improved ZSD (2018–2020) for cities of the Shandong Province (b).
Figure 14. Comparison of wind speed averaged over 2018–2020 and 2011–2017 (a) and scatter plot of in situ measured monthly wind speed and satellite-derived ZSD over the BS nearshore waters (b).
Figure 15. SPM concentrations (the ratio of total sediment discharge to runoff) of major rivers flowing into the BS, including the Huanghe River, Liaohe River and Haihe River. The Huanghe River contributes the majority of the sediment discharged into the BS.
Figure 16. Number of dust storm events in spring (March to May) over China mainland from 2011–2020.
Figure 17. Percentages of water quality classes at state-monitored cross-sections of rivers entering the BS from Liaoning Province (a), Hebei Province (b), Shandong Province (c), and Tianjin Municipality (d) during 2016−2020. Note that in the study area there is no waters belonging to Class Ⅰ.
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