# Exploring the Spectral Variability of Estonian Lakes Using Spaceborne Imaging Spectroscopy
## 3 Results and Discussion  
The PRISMA and EnMAP reflectance data, which have already been validated in previous studies (e.g., [44; 48]), also showed consistent results with both in situ and S2 and S3 data in this study. Examples of radiometric validation of satellite data are shown below (Figures 2 and 3). In Figure 2, Rrs data gathered from fieldwork in Lake Peipis were compared with the PRISMA L1 product corrected using the POLYMER model, showing a level of agreement with SA equal to 15.6\({}^{\circ}\). PRISMA data, similar to the in situ measurements, captured greater variability around the 700 nm peak--related to the presence of Chl-a (average in situ value: 36.6 mgm\({}^{-3}\))--while presenting a slight overestimation in the blue region of the spectrum. In the case of Lake Vortsjarv, there was a higher level of agreement between the in situ data provided by the HYPSTAR instrument and EnMAP L2A reflectance data, with SA value equal to 6.4\({}^{\circ}\). It is interesting to note that the sensor adequately captured the absorption feature in the blue region of the spectrum, due to the presence of CDOM (average in situ value: 2.6 m\({}^{-1}\)) in the lake. In the second example (Figure 3), the comparisons between PRISMA L1 products corrected using POLYMER and multispectral S3 and between EnMAP L2A and multispectral S2 data are reported: the results showed a level of agreement with MAPD of less than 25% and an average SA of 9.8\({}^{\circ}\). In general, as can be seen from the reference measurements and confirmed by the satellite products, these lakes are characterized by high optical variability, mainly due to factors such as seasonality, precipitation, and wind speed. Figure 2: From left to right: spectral comparisons between PRISMA data corrected with POLYMER and in situ measurements in Lake Peipis (six ROI selected during the fieldwork on 18 July 2021); spectral comparison between EnMAP L2A data and in situ measurements (HYPSTAR data acquired on 20 June 2024 and 21 July 2024) in Lake Vortsjarv. Blue and green curves represent in situ data and hyperspectral spaceborne data (mean + st.dev. ), respectively. These results refer to Applications n.1 and n.6. In the following paragraph, different applications are detailed for each image in the dataset, including spectral signature characterization and the generation of advanced products. Starting from the true color composition, each satellite image was stretched to enhance the features relevant to the specific application. Figure 4 presents Application n. 2 (see Figure 1 and Table 1), applied to the PRISMA image acquired on 19 September 2020 over Lake Lammijarv. A map was generated based on OWT, and the spectral signatures corresponding to four main distinct classes are shown: OWT 4a, 4b, 5a (the latter being associated with the majority of water pixels in the scene), and 6 [81]. This outcome is consistent with the results reported in [57], which stated that 65% of Lake Peipsi belongs to moderately turbid water and only 5% to brown water, which occurs in the narrowest part of the Lake Lammijarv. The first three classes presented characteristics typical of green waters and differed from each other in terms of phytoplankton load and short-wave absorption due to CDOM. OWT 4b may be indicative of Coccolithophore blooms, which can significantly influence vertical mixing processes and have a substantial impact on the vertical transport of nutrients [86]. OWT 5a exhibited a spectral peak around 700 nm, suggesting a high concentration of Chl-a--this is typically associated with eutrophication, a phenomenon that requires close monitoring due to its detrimental effects on the lake ecosystem. OWT 6, on the other hand, presented the typical spectral signature of brown waters with a high debris load. Within the area identified by OWT 6, optically shallow waters are present [63], with resuspension caused by wind playing a key role in the determination of optical characteristics.