## 3 Results
### Monitoring of Marsh Shores and Coastal Wetlands  
Changes of the water level in Lake Sevan and fluctuations of its shoreline during the past 90 years led to restructuring and areal extension of coastal wetlands [38] with the formation of marsh shores [56], now occupying more than 10% of the entire shoreline, as stated in Section 3.2. Coastal wetlands of the Norashen Peninsula have an especially high conservation value as breeding grounds of the endemic and rare gill species _Larus armenicus_. The mouths of the Gavaraget, Litchk, Argitchi, and Masrik rivers together with adjacent coastal wetlands as well as the shores of the Noratus and Artanish peninsulas are considered the main breeding sites of rare and valuable fish species and essential waterbird habitats [57]. According to the existing zoning scheme of the Sevan National Park, all these coastal sites and wetlands are classified as strict nature reserves or wildlife sanctuaries [58], but they are now at risk due to changes in hydrological conditions of the shallow waters and water quality [59] as well as overexploitation of their biological resources [57]. Submergence and flooding of coastal habitats lead to active decay of biomass, which intensifies eutrophication and provokes hazardous algal blooms [39]. Only the research of marsh shores and their shoreline positions serves as a basis for assessing the influence of ongoing local hydrological and ecosystem processes upon the quality and state of the aquatic environment in Lake Sevan as a whole. Although the influence of climate change on water quality fluctuations in Lake Sevan is not entirely understood [37; 39; 40], abnormally warm conditions during 2018 and 2019 [33] were associated with large and prolonged toxic algal blooms of _Cyanophyta_[40]. For these years, we conducted time series analysis of multi-spectral images (with spatial resolution of 3 m) from PlanetScope constellation  
Figure 8: Modelling the submerged areas along the eastern coasts of Major Sevan under the scenario of increasing water level: (**a**) UAV-derived ortho-mosaic, (**b**) UAV-derived DSM, and (**c**) areas submerged under the increasing water level up to 1903.5 m.  
for two sites with marshes and coastal wetlands (Norashen peninsula on Minor Sevan and the eastern part of Major Sevan near the mouth of the Masrik River). It was found that algal blooms began five to seven days earlier in 2019 than in July 2018, which is characterized generally with lower primary production (indicated by moderate values of SABI in surface coastal waters) and shorter duration. In particular, along the coasts of Norashen Peninsula on Minor Sevan, algal blooms were recorded from 4 to 9 July of 2019 and from 9 to 25 July of 2018 (Figure 9). In both 2018 and 2019, blooms initially occurred in coastal waters of the Major Sevan where, according to data from the PlanetScope satellites constellation, the algal films were involved in large vortex structures (Figure 10). UAV-based daytime surveys (operational surveys by PlanetScope satellites are performed at 07:00 to 09:00 local time) made it possible to detect diurnal fluctuations in algal films in close proximity to lacustrine shores, which are most popular and well equipped for summer recreation. Figure 10: Development of HAB outbreaks along the south-eastern coasts of Major Sevan, near the mouth of Masrik River, according to SABI values on 5, 9, 18, and 24 July 2018 (**a–d**) and on 2, 9, 19, and 22 July 2019: (**e–h**) Index time series is constructed from multispectral surface reflectance data of PlanetScope [44]. Figure 9: Development of the HAB outbreaks along the western marsh coasts of Minor Sevan, Norashen Peninsula, according to SABI values on 4, 9, 18, and 25 July 2018 (**a–d**) and on 4, 9, 19, and 22 July 2019 (**e–h**). Index time series is constructed from multispectral surface reflectance data of PlanetScope [44].