## 1 Introduction  
The overgrowth of algae has been frequently observed in rivers and lakes of Korea owing to abnormal weather conditions and climate change. The growth and death of these algae is greatly influenced by solar radiation, water temperature, nutrients, and residence time. However, domestic rivers and lakes are rich in nutrients, and hence, the occurrence of algae is mainly affected by the seasons. From spring to summer, green algae and blue-green algae grow rapidly and show a maximum growth rate in midsummer [1]. The growth rate of green algae and blue-green algae significantly increases between summer and autumn. During midsummer, green algae and blue-green algae dominate rivers and reservoirs, causing algae bloom to occur, and the dominant species change according to the proliferation of diatom increases in winter [2]. One of the problems caused by the large-scale occurrence of blue-green algae in the summer across the globe, including Korea, is the generation of odors and toxic substances [3]. In addition to the odor, blue-green algae have air bubbles in their cells; consequently, when their specific gravity is less than that of water, they float to the surface and form scun on the surface of the water, which further results in aesthetic displeasure [4]. Due to the construction of eight large-scale reservoirs for flood control in Korea, the river environment, including water quality characteristics, has been greatly affected by changes in hydraulic characteristics, such as water depth, flow velocity, and residence time, as well as changes in river topography [5]. As a result, water quality has been significantly impacted, leading to the occurrence of harmful algal blooms and the manifestation of characteristics of stagnant water bodies [6]. Algae in rivers are mainly classified into green algae, blue-green algae, and diatoms, and exist mixed with the river, which makes it difficult to distinguish them. Algae have very small cells (!1-10 um), and blue-green algae form filaments or colonies, so taxonomic knowledge, time, and effort are required to identify species under a microscope [7]. In Korea, the current status of algae growth is identified through the constant monitoring of blue-green algae in accordance with the Water Quality and Ecosystem Conservation Act. Algal warning systems and water quality forecast systems have also been implemented in the country. The issuance criteria for the algae warning system are based on the number of harmful blue-green algae cells and the system for the water quality forecast is based on the concentration of harmful blue-green algae cells and Chlorophyll a [8]. Currently, the algae survey method used in Korea is performed once or twice a week, and the analysis includes sample collection from the water body, laboratory analysis, algae identification, and cell count microscopy. These steps can take days to several weeks [9]. The limitations of this survey method make it difficult to immediately classify and quantify algae species in the field, which also makes it difficult to respond immediately when algal bloom occurs [10]. Recently, remote sensing has been actively used to conduct research on algae. Previous studies have identified the parameters that cause changes in the absorption characteristics of algae; these parameters include Chlorophyll a, a pigment of green algae, and Phycocyanin, a pigment of blue-green algae [11]. Thus, algal concentration can be determined by measuring Chlorophyll a, Phycocyanin, and Tripton for the spectral absorption and scattering coefficients of algae [12]. Based on this, a model for predicting Chlorophyll a and Phycocyanin contained in algae is being developed using satellite images. Furthermore, the use of UAVs (unmanned aerial vehicle) is being investigated to overcome the differences in field-measured data that are attributed to temporal and spatial limitations [13].