# Google Earth Engine for Monitoring Marine Mucilage: Izmit Bay in Spring Bay in Spring 2021
## 4 Results
In addition to the clouds, their shadows also created problems when extracting information underneath. All index formulations were applied to the image and the results were thus produced (Figure 5). All indices except for AFAI detected clouds as dense nucilage coverage and the whole area was defined as nucilage at different degrees of densities. Considering that the index values over 0.5 represent moderate and high densities of nucilage aggregates, almost the whole Bay was under the effect of nucilage bloom. In the thematic maps produced by the NDWI and \(\mathrm{NDWI_{Gao}}\) indices, cloud-covered areas were determined as solid polygons with the highest density of nucilage. On the other hand, compared with the original image by visual interpretation, the AFAI index correctly detected the nucilage formations with high density, and other parts of the Bay were described as very low risk of nucilage coverage. In terms of its performance in cloud-covered areas, while disregarding the shadow effect, it identified dense cloudy zones as high-density nucilage. Overall, the best performance was again performed by the AFAI index compared to the performances of the other indices. Temporal analysis of nucilage coverage on Izmit Bay was conducted using the best performing AFAI index by merging the detected nucilage areas on the seven dates considered in this study (Figure 6). Since the results were overlaid to each other, areas detected as nucilage by different indices were shown with the color of the latest date, which results in suppression of previous dates. For instance, eastern coastal areas were shown with blue color showing the nucilage-covered areas on 28 July, but these zones were also detected by the index from the 13 and 18 July images. As shown in the figure, a nucilage bloom was mostly observed on the north-west coast of the Bay, and then a thin creamy surface layer with a few kilometers long appeared throughout the Bay on 19 May. On 24 May, nucilage reached its maximum level covering a large proportion of the water surface. Figure 5: Thematic maps produced for (**a**) cloudy 13 July Sentinel-2 imagery using (**b**) AFAI; (**c**) MI; (**d**) NDTI; (**e**) NDWI; (**f**) \(\mathrm{NDWI_{Gao}}\). Mucilage entrance to the Bay from the Sea of Marmara due to the dominant streams and winds should be particularly mentioned for this time period. According to the map of currents in the Sea of Marmara (Figure 7), main current enters the Izmit Bay and secondary currents leaves the Bay, which can explain the movement of mucilage towards the Bay. Wind speed gradually increased mainly in the eastern direction between 19 and 21 May, then started to increase again after 21 st of May. Irregularly changing its direction, winds were mostly in the eastern and southern directions between 22 and 24 May, which resulted in the movement of mucilage towards the Bay (Figure 8). Although the mucilage in the shape of waves continued to enter the Bay from the Sea of Marmara, the AFAI index failed to properly discriminate the clouds from mucilage; thus, cloud covered areas were detected as mucilage on the coasts of Karamursel district of Kocaeli on the image acquired on 13 June. After this date, mucilage lost its effect, particularly on the open waters, but gathered on the shores of the Izmit Bay. Figure 6: Temporal analysis of mucilage coverage on Izmit Bay using AFAI index. Figure 7: Surface currents in Sea of Marmara ([50]). After the production of index maps, areal estimations for coverage of mucilage aggregates were conducted by considering index values over 0.5 as mucilage and values lower than 0.5 as water. In addition to averages, ratios of mucilage areas in relation to the whole water surface of Izmit Bay were estimated (Table 3). According to the results, whilst the highest mucilage concentration was found on the 24 May image, which corresponds to almost 13% of the whole study area, the least mucilage coverage was detected on 18 July, corresponding to approximately 1% of the study area.