## 4 Discussion  
### UAV Surveys as a Part of Multi-Platform Environmental Monitoring of the Lake Sevan Coastal Zone  
Water withdrawal for agriculture, runoff from livestock farms, pastures, croplands, fish farms, sewage, wastes from residential areas and industrial units, and heavy recreational loads in a coastal zone are the main drivers of water supply and water quality issues of Lake Sevan. All these human activities pertain to the most important sectors of Armenia's economy. Thus, no measures to reduce their environmental impacts are expected in the near future. Furthermore, within the agarian Gegharkunik Province, only 56.8% of the total population was economically active in 2018, and improved employment prospects are connected only with agriculture, fisheries (Lake Sevan is the main supplier of fish for the whole country), and recreation [60]. Our large-scale mapping of point and non-point sources of anthropogenic impacts demonstrated that, in a coastal zone and drainage basin of Minor Sevan, the built-up areas and recreational facilities prevailed. Major Sevan is more affected by agricultural runoff from the drainage basin, but its coasts are not so widely-used for organized recreation (Figure 4). Recreational services are regarded by Armenia's government as one of its main income sources and will, therefore, be promoted in the future. It is possible to reduce the negative impact of tourism only by offering tourism services of higher quality. There are three challenges. 1. Illegal land development;
2. Discharge of untreated sewage water;
3. Environmentally unsound behavior of local people and tourists. For the area in the study, these challenges are exacerbated by insufficient or absent topical fine-scaled spatial data [42]. As an example, in the course of an inspection organized by the Ministry of Environment in 2020, about 130 cases of illegal economic activity, unauthorized construction, and 150 cases of contractual renting without official registration were identified within the coastal zone of Lake Sevan [58]. Of 300 sites along Lake Sevan shores equipped for recreation and entertainment, only 17 have local wastewater treatment facilities. Using highly detailed UAV-derived DSMs for two extensive parts of coastal zone, we primarily identified residential buildings, recreational facilities, and fragments of roads within the area of expected flooding under the planned water level rise up to 1903.5 m. Bearing in mind that most of them are not properly documented [42], such acquired results have a high significance for coastal management and cadastral accounting. The identification and large-scale delineation of natural coastal habitats under the risk of waterlogging and submerging is necessary for spatial planning of clearing such sites to prevent the decay of biomass as an additional driver of lake eutrophication. Agricultural lands, sites for endemic aquaculture, and fish net poaching have undeniable impacts on water quality. However, to date, contribution of such runoff to lake eutrophication and HAB issues have not been well-studied either qualitatively or quantitatively [37; 41]. Eutrophication and increasing HAB's outbreaks have clear consequences for bathing, irrigation, water sources for animals, and nutrition for fish, but these issues have not been well-studied [40]. For the time being, toxicity parameters are not included in development plans for the territory around Lake Sevan [42]. Complex use of UAV-derived data and multispectral satellite imagery allowed us to distinguish the spatio-temporal pattern of HABs at a large scale in 2018-2019 specific to both Minor Sevan and Major Sevan. Under conditions of divergent human-induced changes of the water level, severe and disparate impacts associated with human activities, low efficiency of environmental policies, and a lack for adequate and detailed spatial data, there is evidently a need to devise and implement a remote sensing system for regular monitoring of the environmental situation in the Lake Sevan coastal zone.