## 4 Discussion  
### Feasibility and Necessity of the Gaofen-4 (GF-4) PMS-1 Model in the Bohai Sea  
The reflectivity of water body is far lower than that of other surface features, especially for a clear water body. Except for strong reflection in the blue and green wave bands, the absorption is obvious in the other optical wave range, and the absorption is more intense in the near-infrared wave band [15].  
However, when the water is mixed with other substances, the reflection spectral curve of the water will change due to the spectral interference of other substances. Chla has unique reflection spectral characteristics, and the absorption peak appears at the wavelength of 440 nm and 678 nm, and when the content of Chla is high, the absorption valley appears near the two wavelengths [11]. Due to the weak absorption of Chla and carotene and the scattering effect of cells, reflection peaks appeared near 550 nm. For water containing algae, its spectral curve has remarkable characteristics, presenting obvious fluorescence peak within the range of 685-715 nm. The concentration of Chla will affect  
Figure 7: **(a1)–(c1)**, **(a2)–(c2)**: Chla concentration distribution at different times of the day. **(a1)–(a2)**: the data at 11:12:34 and 11:18:08 on 3 March 2017, respectively; **(b1)–(b2)**: the data at 12:46:36 and 12:47:29 on 9 June 2017, respectively; **(c1)–(c2)**: the data at 07:32:34 and 07:33:41 on 6 September 2018, respectively; **(a3)–(c3)**: changes in Chla concentration at two different times of the day. Colored squares: Chla change areas.  
the location and value of the peak, since the change of Chla concentration in seawater can induce the change of water spectral characteristics [54]. Based on the change of spectral reflectance value obtained by the PMS sensor, the sensitivity of band 2-5 of PMS image to the concentration of Chla in Bohai Sea waters was analyzed.  
Prior studies found that, when constructing the model for inversion of Chla concentration with characteristic bands, the accuracy of the model obtained from the combination of different bands is higher than that of the model constituted by a single band [55]. Therefore, we applied band combination to build the Chla inversion model.  
GF-4 can g perform continuous observation during marine disasters, and obtain remote-sensing information at high frequency and efficiency. Because of its high spatial resolution, GF-4 has great potential for remote-sensing applications in the marine environment [56]. However, the existing conventional Chla algorithms, such as OC3, OC4, and GSM, are not suitable for GF-4 band settings [28]. Therefore, a set of inversion algorithms suitable for GF-4 satellite remote-sensing data is urgently needed. According to the reflectance value of the satellite data and the in situ Chla concentration values, a regression model was established for GF-4 PMS.  
The article combines in situ data and reflectance to analyze the correlation between single-band and multi-band combinations respectively, and find the regression equation with the greatest correlation and fit. The perform error analysis on the results confirmed the feasibility of empirical models. Prior studies showed that the difference of Chla concentration was obvious at different locations in the study area, with the higher being around 14 \(\upmu\)g/L, appears in coastal areas, and the lower value being around 3 \(\upmu\)g/L, appears in the distant sea and the low-value region is widely distributed [16; 27]. Our modeled results show a good consistency with prior studies. This is the first domestic use of GF-4 remote-sensing data in the Bohai Sea to establish a suitable model for Chla concentration, which can provide effective technical support for marine civilization construction and marine disaster investigation.