## 2 Data Materials and Preprocessing
### Sentinel-3 Image Dataset  
Fifteen level 2 water full resolution (WFR) images of Laguna Lake, acquired by the ocean and land color instrument (OLCI) sensor of Sentinel-3, were utilized. A Sentinel-3 \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Campaign \#**} & \multirow{2}{*}{**Date (in 2019)**} & \multirow{2}{*}{**\# of Samples**} & \multicolumn{4}{c}{**Chla Concentration Statistics (\(\mu\)g/L)**} \\ \cline{3-6}  & & & **Min.** & **Max.** & **Mean** & **Std.** \\ \hline
1 & 11 Jan & 35 & 9.072 & 13.235 & 11.391 & 0.655 \\ \hline
2 & 29 Mar & 74 & 7.378 & 8.076 & 7.906 & 0.163 \\ \hline
3 & 6 Apr & 98 & 6.980 & 10.970 & 8.459 & 1.218 \\ \hline
4 & 26 Apr & 22 & 6.731 & 7.692 & 7.254 & 0.315 \\ \hline
5 & 30 Apr & 48 & 7.856 & 11.295 & 9.613 & 0.801 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistical summary of the in situ samples from Laguna Lake. “Min”, “Max”, Mean” and “Std.” represent the minimum, maximum, mean, and standard deviation of the Chla concentrations, respectively. Figure 2: Lake Victoria and in situ samples. The locations of samples are marked by red dots, and the sample information is provided. WFR image contains 16 atmosprically-corrected bands, excluding bands 13-15 (\(\lambda_{761}\), \(\lambda_{764}\), \(\lambda_{767}\)) and bands 18-19 (\(\lambda_{885}\), \(\lambda_{900}\)), which are mainly designed for atmospheric correction [39]. The water-leaving reflectance in Sentinel-3 WFR images is further divided by \(\pi\) to derive the remote-sensing reflectance \(R_{rs}\). In addition, the Sentinel-3 WFR product also contains several water quality parameters, including the Chla concentrations estimated by using an inverse radiative transfer model-neural network (IRTM-NN) [40]. The Chla concentrations from the IRTM-NN were regarded as a simulated dataset in this study and were used for model pretraining. In the image data preprocessing, cloud-free water pixels in the \(R_{rs}\) images and their neighboring local patches of the spatial size \(7\times 7\) were extracted. Image patches containing non-water pixels, such as cloud, and pixels with negative \(R_{rs}\) values due to imprecise atmospheric correction or cloud shadow, were excluded from the dataset, forming full-water \(R_{rs}\) image patches. The summary of the \(R_{rs}\) image patches is presented in Table 3. Similarly, the cloud-free Sentinel-3 image patches corresponding to the locations with the simulated Chla concentrations generated by IRTM-NN were extracted. These image patches with simulated Chla concentrations were used in the model pretraining. The \(R_{rs}\) water patches and their corresponding simulated Chla data were used as a training set. The training set is denoted as \(\{(\textbf{P}_{i},s\_chla_{i})\}_{i=1}^{n}\), where \(n\) denotes the number of simulated labelled data, and \(\textbf{P}_{i}\) and \(s\_chla_{i}\) represent the \(i\)-th \(R_{rs}\) water patch and its corresponding simulated Chla concentration, respectively. There were a total of 47,231 simulated labelled data. In addition, 275 in situ Chla data over Laguna Lake and their corresponding \(R_{rs}\) water patches were used as the retraining dataset. The retraining dataset is denoted as \(\{(\textbf{K}_{i},t\_chla_{i})\}_{i=1}^{m}\), where \(n\) represents the number of in situ Chla samples, and \(\textbf{k}_{i}\) and \(t\_chla_{i}\) represent the \(i\)-th \(R_{rs}\) water patch and its corresponding in situ Chla concentration, respectively. In addition, one Sentinel-3 WFR image \(R_{rs}\) located in Lake Victoria was also obtained, and the acquisition date of the image was 15 June 2019. A \(R_{rs}\) water patch of the size \(7\times 7\) located at the field measurement point LV\({}_{1}\) was extracted. As for the field measurement LV\({}_{2}\), which was taken on 16 June 2019, the water patch was extracted from the Sentinel-3 image acquired on 15 June 2019. This means that the estimation was conducted using the image acquired one day before the field measurement in LV\({}_{2}\).