**1. Introduction**

Coastal lagoons, as transitional environments between land and sea, occupy 14% of the world's coastlines [1]. Due to their shallow waters, morphology, trophic status, and physicochemical processes in a semienclosed system, they are considered one of the most productive habitats on Earth [2,3]. These areas play a significant conservational, ecological, and protective role and are home to an important part of global biodiversity [4]. They underpin human livelihoods, well-being, and welfare and provide several ecosystem services, including tourism, fisheries, aquaculture, and industrial, recreational or navigational activities. Coastal lagoons are subject to diverse transformations and uncoordinated management plans by different agencies and stakeholders from the local to national scale, which might, in some cases, degrade their ecological values. A wide range of anthropogenic activities such as urbanization, agriculture, aquaculture or industry use a variety of organic substances and pollutants, which can reach semienclosed bays, inland waters, and lagoons [5]. Considering

**Citation:** Caballero, I.; Roca, M.; Santos-Echeandía, J.; Bernárdez, P.; Navarro, G. Use of the Sentinel-2 and Landsat-8 Satellites for Water Quality Monitoring: An Early Warning Tool in the Mar Menor Coastal Lagoon. *Remote Sens.* **2022**, *14*, 2744. https:// doi.org/10.3390/rs14122744

Academic Editors: Alban Kuriqi and Luis Garrote

Received: 3 May 2022 Accepted: 4 June 2022 Published: 7 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that these environments are extremely sensitive and vulnerable, they usually show signs of deterioration, pollution, biodiversity loss, alteration of their ecological functioning, and limited ecosystem services [6]. Habitat destruction, water withdrawal, overexploitation, and chemical and biological pollution, such as invasive species, are the main causes of their deterioration, making them one of the most threatened ecosystems in the world [7]. The conservation of coastal lagoons is crucial for their ecological value and the significant number of services they provide [8]. Furthermore, in the context of climate change, coastal lagoons are sentinel systems with an essential role in controlling the fluxes of water, organisms, and nutrients between land, rivers, and oceans, as well as eutrophication and pollution processes [8,9]. Therefore, there is an urgent need to advance monitoring, mapping, and management tools in order to improve the knowledge of these strategic systems, prevent their environmental degradation, and increase their future protection [10].

This is the case of Mar Menor, the largest hypersaline lagoon in the Western Mediterranean Sea (Figure 1) and one of the most iconic and emblematic natural areas in Spain due to its significance in terms of habitats and species, its ecological value, and the uniqueness of its ecosystem. Mar Menor, its surrounding wetlands, and natural areas, are of vital natural importance and a protected landscape as a Wildlife Protection Area, Natura 2000 network, Wetland of International Importance (RAMSAR Convention), and a Regional Park. Moreover, the area is a key component of the regional economy, development, and policy plans due to the variety of uses and human activities developed there. The lagoon, with a maximum depth of 6.5 m and a surface area of 135 km2, presents a long sand bar called "La Manga" acting as a barrier between the Mediterranean Sea and the lagoon, only connected through five shallow inlets called "golas" [11]. The lagoon is close to the Campo de Cartagena region, one of the most intensive agricultural areas in Europe. Several ephemeral wadis drain into the western part of the lagoon, transporting nutrient-enriched waters from agricultural runoff after rainy periods, the Albujon watercourse (Figure 1) being the main collector of the Campo de Cartagena drainage basin and the only permanent wadi flowing into the lagoon [11–16]. Therefore, most of the discharges are located in the southern half of the lagoon where the Albujon watercourse maintains a regular flux of water, albeit depending on the torrential and sporadic rainfall regime, as occurred in September 2019 during one of the most extreme storms, known as the "Cold Drop" [13].

**Figure 1.** Location of the Mar Menor coastal lagoon on the southeastern coast of Spain and Sentinel-2 image captured on 21 March 2021 indicating the final transect of the Albujon watercourse flowing into the lagoon.

Nowadays, this ecosystem is a cause for international concern due to the drastic modifications of its natural and physical status caused by anthropogenic activities. The main impacts causing acute degradation of Mar Menor are those from mining, agriculture, tourism, and urban development [11,14]. The land-use modifications that occurred in the watershed during the 1980s and 1990s with relation to agriculture, from dry land to intensively irrigated vegetable crops (Figure 2), have produced a severe excess of nutrients and fertilizers draining into the lagoon from the freshwater discharge [11], which clearly affect the environmental health of the lagoon (Corine Land Cover datasets, https://centrodedescargas.cnig.es/CentroDescargas/; accessed on 1 January 2022). The current problems affecting the lagoon are increased turbidity and chlorophyll-a, resulting in an acute eutrophication process, silting, a general loss of sediment and seawater quality, and the deterioration of submerged seagrass and animal communities. The degradation of the coastal lagoon has also influenced conventional fishing that has been carried out in the lagoon since ancient times [17].

**Figure 2.** Land cover classification for the Mar Menor lagoon and its surroundings according to Corine Land Cover datasets (https://centrodedescargas.cnig.es/CentroDescargas/; accessed on 1 January 2022) for 1990 and 2018.

The effects of these massive contributions of nutrients in the lagoon ecosystem have been cushioned by its elements that, for decades, have acted as mechanisms of homeostasis and resilience, preventing an excess of nutrients from being available to opportunistic phytoplankton organisms. However, despite the capacity of Mar Menor to resist the effects of elevated nutrient concentration, a succession of catastrophic events have occurred (Figure 3a–c) since 2016 [18–20]. In August 2021, the latest environmental crisis caused alarm and considerable concern and was considered worse than previous eutrophication events. The excess of nutrients and organic matter caused anoxia in the deep layer and massive mortality of benthic flora and fauna during several weeks (Figure 3d), causing an impact on public opinion at the local, national, and international level. Images of dying wildlife traumatized citizens and occupied the public agenda, raising questions about the cause of this ecological disaster that keeps getting worse year after year. The Spanish Institute of Oceanography (IEO-CSIC) highlighted the main cause as pollution and the entry of fertilizers and nutrients into the lagoon from intensive agriculture and other human activities, causing the aquatic ecosystem to collapse [21]. This has led to a clear transformation in the regime of the lagoon, from an apparently stable state (with frequent symptoms of eutrophication in recent decades) to an altered and highly unstable state, much more vulnerable to changes in the environment, especially extreme weather events, which are clearly more intense and frequent as a result of global climate change [22]. In fact, in recent years, it has been observed that when the chlorophyll-a values in the lagoon exceed 3 mg/m3, a process of eutrophication occurs immediately [21]. Several administrations and public authorities responsible for managing the lagoon have been developing initiatives in an attempt to solve the problem. The regional government maintains an open-access network with a few constant sampling sites and field-based campaigns for monitoring variables related to water quality, such as water clarity, turbidity or chlorophyll-a, an indicator of phytoplankton biomass in seawater, temperature, salinity, and dissolved oxygen [23]. However, sound management and deterioration control need improved characterization of the spatial variability and distribution of the water quality, in particular in the western area of Mar Menor, where the Albujon watercourse flows into the lagoon. This information remains key not only for long-term monitoring but also for quick emergency response as an early warning system.

**Figure 3.** (**a**) Sentinel-2 scene in Mar Menor after the extreme weather event known as the "Cold Drop" and the catastrophic flooding on 13 September 2019; (**b**) surface of the water in the lagoon in July 2016 during the environmental crisis; (**c**) typical resuspension of sediments and increased turbidity during strong winds; (**d**) latest environmental catastrophe with massive dead fish and crustacean in August 2021 (authorship: Greenpeace); and (**e**) field campaign carried out in the lagoon in March 2021.

This study examined the evolution of the main biogeochemical parameters of the seawater quality in the coastal lagoon using the Landsat-8 and Sentinel-2 satellite missions in tandem, both with high spatial resolution. Remote sensing technologies can advance current management and monitoring strategies providing synoptic information of the lagoon, as well as provide insights into past, present, and future eutrophication events, in particular during the severe environmental crisis that occurred in summer 2021. Although ocean color sensors provide a distinct picture of the seawater bio-optical status across several scales not achievable with traditional in situ surveying techniques, application in coastal lagoon biogeochemical monitoring is challenging. In this sense, there are studies that focus on mapping water quality, phytoplankton blooms, and eutrophication events in Mar Menor with traditional ocean color sensors at the moderate spatial resolution of 300–1000 m [24–26]. RGB composite images on 3 August 2021 of the Sentinel-3 satellite (300 m spatial resolution), Landsat-8 satellite (30 m spatial resolution), and Sentinel-2 satellite (10 m spatial resolution) are shown in Figure 4. Sentinel-3 is the ocean color mission developed by the European Union's Copernicus programme to support ocean forecasting systems, environmental and climate monitoring with high accuracy and reliability. However, this example highlights that the moderate spatial resolution of Sentinel-3 might not be adequate in complex coastal areas, such as Mar Menor. In recent years, some studies suggested that in order to appropriately determine the ecological conditions of complex inland or coastal water areas by means of remote sensing tools, improved temporal and spatial capabilities are required [27–32], in particular in Mar Menor [13,33–35]. Conceived in the first instance to monitor land cover, the notably enhanced spectral and spatial resolution and minor footprint of both Landsat-8 and Sentinel-2 platforms provide the opportunity to evaluate terrestrial–aquatic interfaces and their dynamic spatial heterogeneity at local, regional, or global scales [36,37].

**Figure 4.** RGB (Red–Green–Blue) composite image on 3 August 2021 of (**a**) Sentinel-3 satellite (300 m spatial resolution), (**b**) Landsat-8 satellite (30 m spatial resolution), and (**c**) Sentinel-2 satellite (10 m spatial resolution).

The main aims of this study are: (1) to use a consistent atmospheric and sunglint correction strategy with Landsat-8 and Sentinel-2 imagery in Mar Menor; (2) to validate the satellite-derived chlorophyll-a and turbidity retrievals with in situ data; (3) to detect the spatiotemporal fluctuations of the biogeochemical parameters during the study period with the multisensor approach; and (4) to evaluate and identify the critical zones in the context of the most recent ecological catastrophe in 2021. This combined information can allow enhanced temporal mapping and predictability of the water mass degradation as an early warning system. Remote sensing technology has large-area and real-time advantages in promoting the monitoring and forecasting of coastal disasters, providing information about when and where the chlorophyll-a values in the lagoon exceed 3 mg/m<sup>3</sup> [21]. These tools can be applied in parallel to regular on-site sampling campaigns in order to reduce the detrimental effects of high levels of phytoplankton, algae, and turbidity on the vulnerable lagoon system and to continuously calibrate/validate the different water quality algorithms for more reliable results.
