# Google Earth Engine for Monitoring Marine Mucilage: Izmit Bay in Spring Bay in Spring 2021
## 4 Results  
To meet the objectives of this study, five spectral indices were firstly derived from six cloud-free Sentinel-2 images obtained in May and July 2021. The indices were calculated using equations given in Table 2 for all the images on the Google Earth Engine platform. Specific GEE code was developed to produce the index map and then reclassify the pixel values into 0-1 range, which ensured standardized representation of all figures with the same scale. Whilst pixel values of 0 represent no nucilage, values of 1 show high nucilage density shown with red color. Through the developed code, all index maps were created separately and then combined to make common analyses and interpretations. Whilst the thematic maps of AFAI and MI are given in Figure 3, those of NDTI, NDWI, and NDWI\({}_{\text{Gao}}\) are shown in Figure 4. It should be noted that some thematic maps have a diagonal line, resulting from the combination of different tracks by the Sentinel data center. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Index** & **Equation** & **Reference** \\ \hline AFAI & \(B8-B4-0.5\times(B11-B4)\) & [46] \\ MI & \((((B8+B3)/2)-B2)/B11\) & [44] \\ NDTI & \((B4-B3)/(B4+B3)\) & [49] \\ NDWI & \((B3-B8)/(B3+B8)\) & [48] \\ NDWI\({}_{\text{Gao}}\) & \((B8-B11)/(B8+B11)\) & [47] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Water-related indices formulated with Sentinel-2 spectral bands and used in this study. Figure 3: Estimated thematic maps of AFAI and MI from Sentinel-2 images acquired on (**a**) 14 May; (**b**) 19 May; (**c**) 24 May; (**d**) 13 July; (**e**) 18 July; (**f**) 28 July 2021. Thematic maps produced for the 14 and 19 May images showed a starting stage of mucilage bloom densified in several locations in the Bay. For these particular days, the clearest detection of mucilage aggregates was performed by the AFAI. Although the MI detected the same locations much more clearly, it identified more mucilage accumulated on the east coast (Figure 4). According to the results, the highest mucilage coverage was on May 24, when the whole Bay was under the severe effect of a mucilage bloom. Once again, the thematic map produced by FAI indicated the dense population of mucilage more correctly. It was also observed that the NDWI index detected denser mucilage formations accumulated in the southern part of the Bay. When the thematic maps produced for July 13 and 18 were considered, it was noticed that the mucilage bloom diminished although the images show high turbidity, which can result from winds. Due to the existence of turbidity in the water, NDTI produced the most acceptable results for these days. Compared with these images, the image taken on July 28 was clearer and had very limited turbidity. However, all indices detected distinct features on the water surface, which could be high algal concentrations just after the mucilage appearance. It can be concluded from the produced results that the index of AFAI produced the clearest maps of mucilage for both high-and low-density mucilage coverages, especially in the blooming period. Although the index of MI was proposed especially for the detection of mucilage-covered areas, it was found to be inferior to AFAI. Another important result is that the indices using the green channel in their formulations (i.e., MI, NDTI, NDWI) had confusion in the separation of mucilage and algal bloom pixels since they have similar reflectances in the green channel compared to the blue and red channels. For the second part of the study that aimed toe test the performance of indices for an image including clouds and their shadows, an image taken on 13 June 2021 was employed. There were two groups of cumulus clouds on the southern part of the Izmit Bay, which are  
Figure 4: Estimated thematic maps of NDTI, NDWI and NDWICao from Sentinel-2 images acquired on (**a**) 14 May; (**b**) 19 May; (**c**) 24 May; (**d**) 13 July; (**e**) 18 July; (**f**) 28 July 2021.  
indicated by red circles.