# Google Earth Engine for Monitoring Marine Mucilage: Izmit Bay in Spring Bay in Spring 2021
###### Abstract  
Global warming together with environmental pollution threatens marine habitats and causes an increasing number of environmental disasters. Periodic monitoring of coastal water quality is of critical importance for the effective management of water resources and the sustainability of marine ecosystems. The use of remote sensing technologies provides significant benefits for detecting monitoring, and analyzing rapidly occurring and displaced natural phenomena, including mucilage events. In this study, five water indices estimated from cloud-free and partly cloudy Sentinel-2 images acquired from May to July 2021 were employed to effectively map mucilage aggregates on the sea surface in the Izmit Bay using the cloud-based Google Earth Engine (GEE) platform. Results showed that mucilage aggregates started with the coverage of about 6 km\({}^{2}\) sea surface on 14 May, reached the highest level on 24 May and diminished at the end of July. Among the applied indices, the Adjusted Floating Algae Index (AFAI) was superior for producing the mucilage maps even for the partly cloudy image, followed by Normalized Difference Turbidity Index (NDTI) and Mucilage Index (MI). To be more specific, indices using green channel were found to be inferior for extracting mucilage information from the satellite images. marie mucilage; remote sensing; Google Earth Engine; Sentinel-2; mucilage index; AFAI Article  
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Although marine mucilage events have been observed in the oceans since 1729, uncertainty remains about the reasons for their emergence under different water and environmental conditions. They generally appear after sudden changes in the biogeochemical properties of the water and occur with long-lasting calm weather conditions, weak water circulation in spring and fall terms, and reduced levels of nutrient (nitrogen and phosphorus) supply coming from rivers. Therefore, stability along the water column and sudden increase in water temperature can be given as the leading causes of mucilage events when intensive production processes and stable meteorological conditions exist [4; 5; 6; 7]. For the continuity of mucilage events, temperature changes, wind speed, and amount of rainfall are the most effective factors [8]. With frequent appearances in the Adriatic and Tyrrhenian Seas, mucilage events have been observed in different parts of the world [7; 9; 10; 11; 12]. In the Turkish seas, after the first reported mucilage occurrence observed in the water column around Erdek Bay in 1992, a massive mucilage event occurred in the Sea of Marmara between October 2007 and February 2008, severely affecting the Bosporus and Dardanelles Straits [13; 14; 15]. In the Canakkale Strait, mucilage events were also observed between September 2007 and July 2008, and then more severely in March 2021. Another mucilage outbreak was reported in the water column of the Dardanelles in December 2020 [16]. Following the extreme fishery activities in 2018, increased populations of Salpa colonies, _Rhizostoma pulmo, Aurelia aurita_ species, and decreased phytoplankton and zooplankton populations were observed in the Sea of Marmara [2]. In Spring 2021, a major mucilage event was recorded in the Sea of Marmara that turned into a natural catastrophe. The organic structure of the mucilage cover was much more intense, and the effect was more severe compared to 2007-2008 [17]. Floating sticky and gelatinous mucilage aggregates controlled by the wind and currents accumulated on the coasts and diminished recreational suitability. Intensive fieldwork was conducted by both by Turkish Ministry of Environment, Urbanization and Climate Change (TMEUCC) and the municipalities to collect the aggregated from the sea surface (>11,000 m\({}^{3}\)). Underwater observations conducted for several months revealed that the mucilage was spread throughout the water column, decreasing the visibility, and then collapsed to the seabed.