Inland surface waters form valuable ecosystems of great importance for global biodiversity^[1]. Karst lakes are lenitic ecosystems, with waters rich in bicarbonates, which develop on sedimentary rocks. As they are linked with areas where karstification phenomena are very active, these types of lakes frequently have rounded shapes, steep sides, lack of freshwater inbound or outbound and a relatively high depth. Therefore, the waters are often thermally stratified, giving rise to water compartments with different physicochemical and biological conditions^[2]. Due to the oligotrophic characteristics of the waters, they allow the development of communities dominated by charophytes. Underwater meadows of Chara spp. are a good bio-indicator of water quality as their populations decline when water eutrophy increases^[3]. 

The Cruz Lake, presents a biogenic meromixis, due to the enrichment of the monimolimnion water (the deep layer not mixed which remains without any kind of exchange with the upper waters, except for slow diffusion processes^[4] with calcium, magnesium, and iron bicarbonates^[5]. These physicochemical changes are evident every summer, at the end of July, when the blue-green waters of the lake acquire a milky appearance (figure 1a) due to calcium carbonate precipitation favoured by high temperatures, high pH and/or an increase in primary production^[6]. In Cañada del Hoyo there are other lakes that also show impressive colours changes like Lagunillo de las Tortugas, which acquires a reddish colour (figure 1b) and Cardenillas in greenish color.

The aim of the work is to carry out a monitoring of the whiting, reddish and greenish phenomenon of the karstic lakes of Cañada del Hoyo through a thematic analysis of the optical properties of the water bodies using Sentinel-2 satellite images. In this way, it is possible to collect updated data on the colouring phenomenon of the small lakes, dates when it starts and check its duration in time.

Figure 1. Comparison of the two colours of the lakes. (A) picture of the La Cruz during the whiting phenomenon in summer of 2020. (B) Tortugas Lake during the reddish colour in autumn 2020. (Source: M.R.).