## 1 Introduction
However, an integration with remote sensing technology can allow these gaps to be filled and, due to its repeatability and synoptic view, it can be a reliable and cost-effective solution for monitoring and research activities, also in compliance with certain European Directive strategic objectives (i.e., the Habitat Directive (92/43/EEC) [22] and the Water Framework Directive (2000/60/EC) [24]). The sampling protocols to detect macrophyte areas proposed by the European WFD (e.g., [25; 26; 27]) require large and costly efforts, which can be excessive in large, deep lakes [6]. Satellite sensors with adequate revisit time can be used for multitemporal studies to provide reliable information for understanding macrophyte dynamics and spatial landscape trends or changes [28]. Aquatic vegetation and its characteristics (e.g., biomass, coverage, structural stand complexity, and composition) can be retrieved from specific spectral properties of reflectance spectra [29; 30]. The major technical issues are presented by submerged or flooded vegetation, as water can alter the spectral characteristics of the images [31]. Lake Garda is the largest Italian Lake, one of the most important lacustrine ecosystems in Europe [23]. It is located in one of the most densely populated and industrialized regions of Italy representing an essential resource due to the multiple uses of its waters, as well as being a strategic area due to its ecological, conservational, and socioeconomic value [23; 32]. In particular, this study is focused on the Sirimione Peninsula located in the southern portion of the lake. This area includes key habitats, such as common need stands and extensive submerged macrophyte meadows, in addition to hosting the most diverse populations of overwintering water birds in Italy. The colonization patterns of the aquatic vegetation of this portion of the Lake Garda were previously studied by [PERSON] et al. (2007) [33] and [PERSON] et al. (2012) [6], and described the macrophyte coverage evolution around the Sirimione Peninsula. [PERSON] et al. (2007) [33] reported, for the 8-year interval analyzed by airborne imaging spectrometry, a decline in the submerged macrophyte meadows from 1997 to 2005 (from 72% to 52%) with a concomitant increase in sandy substrates. [PERSON] et al. (2012) [6] used in situ and airborne hyperspectral data to analyze the variation of colonized substrates in relation to hydrological and physicochemical variables. They found a substantial modification of the structural complexity of macrophyte communities and of the colonized areas: 98% of macrophyte meadows were lost, mainly the dense beds (density over 70%), which were replaced by moderate to rare communities with lower density (range 10-40%). Transparency, water level fluctuation, trophic status, grazing by herbivous aquatic birds, and number of boats were identified as the main factors to explain the spatiotemporal variability of macrophyte communities in the period investigated (13 years, from 1997 to 2010). The present case study fits into a broader discussion about the global trends of macrophytes in lakes [34] which are currently undergoing an accelerating rate of decrease, with losses especially acute for submerged species in lakes larger than 50 km\({}^{2}\). Previous evidence indicated that in some eutrophic lakes the current phytoplankton dominance can be a result of submerged macrophyte decline rather than the cause [35; 36], further suggesting that these phenomena are a conclusion of a process of years (over perhaps 10-100s), driven by many pressures rather than a rapid change. In fact, several studies have reported the occurrence of long-term trends of macrophyte diversity and abundance loss in many shallow lakes affected by eutrophication [37; 38] and spatial and seasonal variation of macrophytes over the long term [7; 39; 40]. At the same time, the reduction of human-mediated pressures exerted on lake systems can stimulate the recovery of macrophytes, as in the case of Lake Constance [41].