# Exploring the Spectral Variability of Estonian Lakes Using Spaceborne Imaging Spectroscopy
## 4 Conclusions  
Imaging spectroscopy is emerging as a functional technique for assessing the status of inland waters. Although this technique can provide useful information for studying the optical characteristics of aquatic ecosystems, it requires considerable research and development in terms of the used methodology and its application, with several challenges such as atmospheric correction still to be addressed (see, e.g., [44]). In this study, a preliminary analysis of PRISMA and EnMAP hyperspectral data was conducted on two Estonian lakes for which the water quality is greatly influenced by several inflows and meteorological variables, such as precipitation, wind speed, and lake water temperature. These lakes also exhibit complex optical properties and, to date, have been studied using mainly multispectral data [58; 70], which might be suboptimal in terms of distinguishing spectral features at specific narrow bands over a variety of surface properties and OWTs. A satellite dataset consisting of 12 images (nine gathered from PRISMA and three from EnMAP) was collected, enabling a total of nine different applications to be developed. A subset of the images was used to evaluate the quality of Rrs data, given the availability of coincident in situ and multispectral measurements as a reference. For the PRISMA images of Lake Peipsi, atmospheric correction was performed with POLYMER while, for Lake Vortsjarv and in the case of the EnMAP images, standard Rrs products were used. The spectral comparison demonstrated the ability of hyperspectral sensors to capture the spectral signatures of the reference measurements in terms of their magnitude and shape (SA below 16\({}^{\circ}\)), with larger differences at the shortest wavelengths as observed in previous studies [44; 46]. In particular, the spectral signatures provided a qualitative estimate of the main optical characteristics of the lakes, as influenced by river inputs or environmental factors, such as increasing temperature, in agreement with the existing literature [92]. The high spectral and spatial resolutions enabled the characterization of OWTs, the detection of PC and macrophytes, and the riverine influences on water Rrs. In Applications n.5 and n.7, the PC concentrations retrieved from PRISMA were consistent with our knowledge. The spectral and spatial resolutions of the PRISMA and EnMAP imagery were also exploited for the detection of aquatic vegetation association groups. This study demonstrated that PRISMA and EnMAP hyperspectral data, complemented by both in situ measurements and Sentinel imagery, can support the comprehensive evaluation of optically complex water ecosystems such as the two largest Estonian lakes. The limited availability of in situ measurements and the frequent presence of cloud cover prevented the development of more quantitative applications and the investigation of critical events (e.g., surface runoff). These limitations underscore the importance of a high-frequency revisit time for the development of aquatic ecosystem-related applications, as well as higher number of in situ measurements for validation. The findings of this study offer valuable information to guide the development of future hyperspectral missions, providing useful insights for the selection of observation windows to capture key seasonal dynamics in lake ecosystems. This information, together with the expansion of automated hyperspectral sensor networks (e.g., HYPERNETS) in inland water bodies, has the potential to significantly improve the ability of future missions--such as ESA's Copernicus Hyperspectral Imaging Mission for the Environment (CHIME [102])--to monitor inland water quality on a large scale. Conceptualization, A.F., M.B., A.P., C.G. and K.A. ; methodology, A.F., M.B., A.P. and C.G. ; software, A.F., A.P., A.J.G. and F.S. ; validation, A.F., A.P. and L.P.; formal analysis, A.F., M.B., A.P., L.P. and C.G. ; investigation, A.F., K.K. and K.A. ; data curation, A.F., L.P., A.J.G., F.S. and J.K.; writing--original draft preparation, A.F. and A.P.