# Exploring the Spectral Variability of Estonian Lakes Using Spaceborne Imaging Spectroscopy
## 3 Results and Discussion
Indeed, historical data demonstrate a consistent link between rising water temperatures and the increased frequency of cyanobacterial blooms, particularly during the summer months. Moreover, models predict that global warming will lead to earlier and longer blooms, along with shifts in the seasonal dynamics of rivers and lakes [92; 93; 94; 95]. Laboratory analysis confirmed that, at that time, _Aphanizomenon flos-aquae_ Rafs was dominating in the lake. Figure 6: (**a**) EnMAP image acquired on 16 May 2024 at the confluence where the Emajögi River flows into Lake Peipsi, (**b**) spectral signatures extracted along a transect crossing the study area. Each curve (corresponding to the points marked in the image using the same color scheme) represents the Rrs values recorded at successive points along the transect. This result refers to Application n.4. Figure 7: (**a**) PRISMA image acquired on 16 August 2022 over Lake Peipsi, (**b**) PC concentration map, (**c**) spectral comparison between PRISMA L1 corrected with POLYMER and S2 (17 August 2022), displayed in green and blue, respectively. These results refer to Application n.5. Figure 8 shows Application n.7, in which the concentration of PC was estimated (using the MDN model) for a PRISMA image acquired on 4 April 2020 over Lake Vortsjarv. In the westernmost region of the lake, elevated concentrations of PC were observed. These concentrations are also discernible in the spectral signatures obtained from PRISMA imagery; however, due to the lower spatial resolution, this feature was not detectable in S3 data. Laboratory analysis confirmed that, at that time, _Limnothrix planctonica_ Meffert and _L. redkei_ Meffert were dominant in the lake. It is worth noting that the comparable spectral configurations of the PRISMA and EnMAP missions can facilitate the adaptation of the MDN model--originally calibrated and validated for PRISMA--to EnMAP data, thereby laying the foundation for its application to other current and future hyperspectral satellite platforms [47]. In Figure 9 regarding Application n.8, the capability of the EnMAP image to distinguish between emergent plants, common needs, floating plants, submerged plants, and water pixels is demonstrated. This analysis was conducted near the station \"Ohne\" (58.202 N, 26.018 E) identified in [96]. Based on [97], it is possible to assume that the aquatic vegetation species identified in the image could be _Piragmites australis_ (emergent), _Nuphar lutea_ (floating), and _Myriophyllum spicatum_ (the dominating species among the submerged plants, for at least 20 years [98]). Figure 8: (**a**) PRISMA image acquired on 4 April 2020 over Lake Vortsjarv, (**b**) PC concentration map, (**c**) spectral comparison between PRISMA L2C and S3 (4 April 2020), displayed in green and blue, respectively. These results refer to Application n.7. As mentioned in Section 2.1, Lake Vortsjarv is covered by ice for a period of around 131 days from November to April [64]. Furthermore, from January to March, snow coverage reduces incident light by more than 40%, and the attenuation of light by ice varies from 4% in December to around 26% in March [99]; in addition, air temperatures range around 0.6 \({}^{\circ}\)C [63]. Application n.9 (Figure 10) shows the map of the integral of reflectance from 450 to 1000 nm (R\({}_{450-1000}\)) generated from a PRISMA image acquired on 18 March 2022. The spectral signatures extracted from the image likely correspond to lake cover in terms of snow and ice, which potentially represent other physical properties such as snow impurities and thickness [100; 101]. Figure 10: (**a**) PRISMA image acquired on 18 March 2022 over Lake Vortsjarv, (**b**) map of the integral of reflectance, (**c**) spectral signatures extracted from the three ROIs displayed in (**a**), following the same colors. These results refer to Application n.9. Figure 9: (**a**) EnMAP image acquired on 21 July 2024 over Lake Vortsjarv, (**b**) spectral signatures extracted from the five ROIs displayed in (**a**), following the same color scheme.