## 4 Discussion
### Comparison between the OLCI-Derived and MODIS-Derived \(K_{\mathrm{d}}\)(490)  
To further confirm the accuracy of the spatial distribution of the OLCI-derived \(K_{\mathrm{d}}\)(490) values, the \(K_{\mathrm{d}}\)(490) values for Lake Taihu derived from Terra-MODIS using a newly proposed \(K_{\mathrm{d}}\)(490) semi-analytical algorithm [61] were used as quasi-reference values and compared with those derived from Sentinel-3A-OLCI. A total of 18 OLCI-MODIS image matchups were used, and six matchups with large uncertainties in the MODIS atmospheric correction were excluded. The distribution of the RMSEs is shown in Figure 12a. The mean RMSE is 2.40 \(\pm\) 1.02 m\({}^{-1}\), which indicates high consistency. However, it is also worth noting that large errors occur in the land-water regions due to border effects. The MODIS-derived \(K_{\mathrm{d}}\)(490) values were resampled to a spatial resolution of 300 m, which is the same as that of the OLCI. Figure 12b shows a pixel-by-pixel scatter plot between all of the \(K_{\mathrm{d}}\)(490) products from the OLCI and MODIS. Significant correlations between the OLCI-derived and MODIS-derived \(K_{\mathrm{d}}\)(490) values were observed (\(r\) = 0.75, \(p\) < 0.001, N = 292,970):  
\[K_{\mathrm{d}}\mathrm{(490)}_{\mathrm{OLCI}}=0.62\times K_{\mathrm{d}}\mathrm{ (490)}_{\mathrm{MODIS}}-2.52 \tag{4}\]  
where \(K_{\mathrm{d}}\mathrm{(490)}_{\mathrm{OLCI}}\) and \(K_{\mathrm{d}}\mathrm{(490)}_{\mathrm{MODIS}}\) are the OLCI-derived and MODIS-derived \(K_{\mathrm{d}}\)(490) values, respectively. These results indicate good agreement between the OLCI-derived and MODIS-derived \(K_{\mathrm{d}}\)(490) values. For additional detail, the OLCI- and MODIS-derived \(K_{\rm d}\)(490) values on 14 March 2017 and 29 April 2017, which represent the typical \(K_{\rm d}\)(490) patterns in Lake Taihu under low bloom coverage conditions and significant bloom coverage conditions, respectively, were compared (Figure 13a,b). The general patterns of \(K_{\rm d}\)(490) were consistent with each other, especially under low bloom coverage conditions (Figure 13c,d). However, the OLCI-derived \(K_{\rm d}\)(490) exhibited a smoother spatial distribution and finer textual characteristics, whereas the MODIS-derived \(K_{\rm d}\)(490) had a noisy pattern. This is because the OLCI has a higher spatial and radiation resolution and a higher signal-to-noise ratio (ANR) than MODIS. Figure 13(e1,f1) show the distributions of the absolute relative differences (ARE; abs(\(K_{\rm d}\)(490)\({}_{\rm OLCI}\)-\(K_{\rm d}\)(490)\({}_{\rm MODIS}\))/\(K_{\rm d}\)(490)\({}_{\rm MODIS}\)) between the OLCI-derived and MODIS-derived \(K_{\rm d}\)(490) values. The mean absolute relative differences on 14 March 2017 and 29 April 2017 were 19.79 \(\pm\) 16.83% and 20.84 \(\pm\) 21.79%, respectively. There was good consistency between the spatial distributions of the OLCI-derived and MODIS-derived \(K_{\rm d}\)(490) values on 14 March 2017 (Figure 13(e1)) in all areas except for Meiliang Bay and Zhushan Bay, where there are large differences. In addition, there is generally good consistency between the spatial distributions of the OLCI-derived and MODIS-derived \(K_{\rm d}\)(490) values on 29 April 29 2016 (Figure 13(f1)) in all areas except for the bank areas, where there are large differences. To evaluate the consistency between the two satellite-derived \(K_{\rm d}\)(490) products, pixel-by-pixel scatter plots of the \(K_{\rm d}\)(490) values derived from the OLCI against those from MODIS are shown in Figure 13(e2,f2). The OLCI-derived and MODIS-derived \(K_{\rm d}\)(490) values are significantly correlated with each other, and the Pearson correlation coefficients (\(r\)) on 14 March 2017 and 29 April 2017 are 0.93 and 0.43, respectively, (\(p\) < 0.001). The Pearson correlation coefficient under significant bloom coverage conditions (29 April 2017) is low. Large errors occurred in both non-algal bloom-based algorithms due to adjacency effects of algal bloom areas and non-algal bloom areas.