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The diffuse attenuation coefficient for the downwelling irradiance K d z , λ is a critical parameter in terms of the optical properties of the ocean. In the northwestern South China Sea, there are complex physical processes, and the accurate estimation of K d z , λ in the northwestern South China Sea is critical for the study and application of the underwater light field and water constituents. In this study, using Hydrolight 6.0 (HL60) software, K d z , λ was simulated based on the inherent optical properties (IOPs) and chlorophyll a concentration dataset in the northwestern South China Sea. The simulations were in good agreement with the results calculated by the model of Lee (2005), and the spectral characteristics of K d z , λ were consistent with several oceanic types according to Jerlov's classification. The horizontal and vertical distribution characteristics of K d (z , 490) were studied in the two typical upwelling areas of eastern Hainan Island and eastern Vietnam. K d (z , 490) in eastern Hainan Island exhibited an overall decreasing trend from west to east at the same depth, while the vertical depth of the maximum value of K d (z , 490) in eastern Hainan Island was found to increase from west to east, which was significantly associated with the distribution trend of the temperature and salinity. K d (z , 490) in eastern Vietnam exhibited unique horizontal and vertical distribution characteristics due to upwelling, with a low temperature and high salinity. A satisfactory linear relationship between K d (z , 490) and K d z , λ was found from 420   t o   580   n m with R 2 > 0.76, root mean square (RMSE) ≤ 0.010 m − 1 , and mean absolute percentage error (MAPE) < 9%, and this result indicated that K d z , λ from 420 ~ 580   n m could be estimated with K d (z , 490) . The regression accuracy sharply decreased after 580 nm, indicating that K d z , λ estimation based on K d (z , 490) can be more suitably achieved from 420~580 nm and becomes inaccurate after 580 nm. Based on the simulations, an empirical relationship for Z S D D estimation involving K d , 10 % 490 was developed, and Z S D D in the northwestern South China Sea was calculated, with a range of 5–23 m and a suitable agreement with Z S D D obtained via the method of Lee (2018). [ABSTRACT FROM AUTHOR]

Marine zoning has been widely used to effectively manage the relationship between economic development and marine environmental protection. However, it remains challenging for marine environmental management to integrate land–sea coordination and effectively implement spatially differentiated control under the existing technical framework. This study aims to explore the development and practice of marine zoning in China. This paper discusses the technical framework and key technical methods of marine zoning under the "Three Lines One Permit" policy. The main effectiveness and shortcomings of this strategy are described in the Hainan Free Trade Port case. The Hainan Free Trade Port has 276 marine control units, of which 36% are priority protection units and 6% are priority control units. These marine control units help improve the orderly development and utilization of land and sea resources and promote construction of an ecological security pattern of land–sea coordination. The marine zoning under the "Three Lines One Permit" policy complements and enhances the existing marine zoning system and promotes the potential for integrated coastal zone management. Suggestions are proposed to promote a more efficient marine zoning strategy towards land–sea integration and science-management integration. • Discussed the technical framework of marine zoning based on the land–sea coordination under the "Three Lines One Permit" policy. • First proposed a technical framework of marine zoning using the four-step method of "challenge-goal-zoning-regulation". • Established a quantitative response relationship between pollution discharge and marine environmental quality. [ABSTRACT FROM AUTHOR]

The light scattering and backscattering coefficients of particles (b p (λ) and b bp (λ)) and their association with particle concentration, size distribution, and composition off the eastern Hainan coast in the South China Sea (SCS) were analyzed between August and September 2013. Variations in b p (532) and b bp (532) were correlated to changes in the LISST-based total cross-sectional area and volume concentration of particles (AC and VC). Two significantly different covariations between b bp (λ) and b p (λ) and chlorophyll a concentration (Chla) were observed, and all samples were clearly separated into two groups (Group A and Group B) using the threshold relationship between b bp and Chla. Samples in Group B, most of which were collected in the nepheloid layer, showed higher b p /AC and b bp /AC than those in Group A. This indicates the presence of inorganic particles with higher n p in Group B than in Group A. The relationship between b bp and Chla for Group A could be well fitted by a published model involving a constant backscattering background independent of Chla. A significant relationship (R2 = 0.85) was observed between b bp and Chla in the Chla maximum layer, which was different from that in other layers. This finding might imply that both an increase in the micro-phytoplankton community and a decrease in the contribution of background backscattering to b bp at the Chla maximum layer. Understanding this variability of beam-scattering properties off the eastern Hainan coast can improve our understanding of marine biogeochemical processes at a regional scale, and can also promote the application of these regional bio-optical relationships for ocean color. • Both b bp (532) and b p (532) could serve as an indicator for LISST-based VC and AC. • Different covariations between b bp (λ) or b p (λ) and Chla were observed in surface, Chla maximum layer and bottom. • Relationships of b bp to Chla or LISST-based particle size parameters are affected by the contribution of small particles. [ABSTRACT FROM AUTHOR]

Mangrove forests play an important role in absorbing carbon, filtering pollution, and protecting coastal residents from storms. However, mangrove forests are most susceptible to human interference and suffer more than one-third of losses globally. Therefore, quantifying their spatial-temporal changes and identifying their potential distributions are necessary for protection, restoration, and management. We first mapped and analysed the spatial-temporal changes in mangrove forests from 1993 to 2021 in the Xinying-Bay region (XYB), Hainan Province, China. Then, we predicted suitable habitats for mangrove forests by using the maximum entropy (MaxEnt) model based on mangrove distribution data for 2021 and related environmental data for the Circum-Xinying-Bay region (CXYB). The results show remarkable changes in mangrove forests in the XYB from 1993 to 2021, with the loss rate reaching 56 ha/y from 1998 to 2002. The MaxEnt model performs well, with the curve (AUC) extending beyond 0.9. Water quality and topography determined the distribution of mangrove forests, with the total contribution exceeding 65%. The six most important influential factors were salinity, DEM, precipitation in the driest quarter, pH, isothermality, and winter sea surface temperature. The area of mangroves highly suitable in the CXYB is approximately 3880 hm2 and is mainly located in the town of Xinzhou, the northeast area of the town of Mutang and the southwest area of the town of E'man. This suggests that areas prioritized for protection should include the northern and western areas of Xinzhou, northeastern areas of Mutang, and northern and western areas of E'man. These findings provide further understanding of mangrove distribution and can guide mangrove restoration and management. • Remarkable changes in mangrove forests in the XYB from 1993 to 2021. • Water quality and topography determined the distribution of mangrove forests. • The area of high mangrove forests restoration potential is approximately 2106 hm2. • A reasonable restoration strategy is the key to mangrove forests restoration. • More social groups should be encouraged to participate in the construction and management of mangrove forests. [ABSTRACT FROM AUTHOR]