*2.1. Study Area*

The area considered in this work is La Carbonera lagoon, in the northeast of the Yucatan Peninsula, Mexico (21◦13 41.80"–21◦14 4.79 N, 89◦53 21.66–89◦54 0.45 W) (Figure 1) [24]. This coastal lagoon lies in a region of karst characteristics and is approximately 16.5 km<sup>2</sup> in area. More detailed information concerning the geology of the region can be found in Appendix A. The channel connecting it to the sea, the Gulf of Mexico, is quite recent, a result of the passage of Hurricane Gilbert, in 1988. The fresh water of the system comes from submarine groundwater discharges, or springs, carrying continental water, which is the result of regional precipitation. The main inlet of fresh water, locally called a 'peten', is located southwest of the system. 'Peten' is a colloquial Mayan name that refers to islands of vegetation which are associated with freshwater springs that allow the

development of perennial rainforests, frequently exposed to flooding [25]. It is a shallow lagoon, 0.30–2.0 m, and as such is very much influenced by the local atmospheric climate. The bathymetry of the lagoon and the coastal zone are presented in Appendix A. The mean annual rainfall is 1025 mm and there are three seasons: dry (March to June), rainy (July to October), and 'Nortes', characterised by a decrease in temperature, storm clouds, and heavy rains (November to February) [26–28]. The region is influenced by the transit of tropical storms during the summer months, which commonly intensify into hurricanes. Appendix A contains more detailed information on the climate of the study area.

**Figure 1.** Location of La Carbonera lagoon, Yucatan, Mexico, showing areas with significant saline properties, the location of the CTD sensors (red dots), and sites proposed for the RED power plant (A and B). The polygons SZR1, SZAE1, and SZUR6 are areas with protection categories within the Cienegas and Manglares State Reserve of the North Coast of Yucatán (RECMY).

La Carbonera is part of a system of wetlands along the coast of Yucatan. It has a sand bar of 1.5 km in length, a sandy beach, and coastal dunes of medium height. Mangroves lie behind the dunes, around the lagoon, and sometimes further inland. There are springs and an area of swampland around the lagoon [29]. Geological data on the lagoon are given in Appendix A.4.

On the coast, the waves have low energy, except in the Nortes season or tropical storm or hurricane conditions, when the waves, currents, wind, and precipitation are extreme. The tidal regime is mixed, predominantly diurnal, with tides ranging from 0.40 m spring tides to 0.08 m neap tides. Appendix A has more information on the hydrodynamics, currents, and tides. The mean air temperature is between 24 ◦C and 26 ◦C, with variations

of up to 10 ◦C throughout the day, minimums in December and January and maximums in July and August [30].

#### *2.2. Legal Framework*

This lagoon system is part of a Natural Protected Area (NPA) in the region known as the Cienegas and Manglares State Reserve of the North Coast of Yucatan (RECMY), which has a total surface area of 55,000 ha and covers several municipalities of the State of Yucatan. The aim of officially recognising the ecological importance of the region was to protect the coastal ecosystems there, which are mainly well-conserved mangroves. La Carbonera is in the west of the reserve, and within it, several polygons have been assigned categories according to their uses. The lagoon is within polygon SZR1, which has the category of Buffer Zone/Public Use. In this zone, current or future actions are given permits if they lead to sustainable development and, at the same time, create conditions to conserve the reserve s ecosystems in the long term. Recreation, leisure activities, group or individual tours are allowed in designated sites, approved for this purpose. Overnight stays and camping are permitted, as well as the development of low-impact infrastructure, in accordance with the Ecological Use Plan for the Coastal Territory of Yucatan. Such infrastructure must be subject to the corresponding authorisations and permits in terms of land and environmental use; including lodging infrastructure, walkways, trails, conditioning of water crossings, signage, and surveillance, which are aimed at sustainable use and the inspection and surveillance of such sites. The construction of new infrastructure, as well as actions that have an effect on hydrological flows, must comply with the relevant environmental impact requirements (Figure 1, polygon SZR1).

Within polygon SRZ1, there is a smaller polygon of category Core/Subzone Restricted Use Zone, SZUR6. This is a peten zone within the lagoon. It is better conserved, or little altered; an area that contains ecosystems, natural phenomena, and geohydrological processes of special interest, as well as species of flora and fauna that have special protection. In this polygon, only exceptional activities that do not modify the ecosystems and that are subject to strict control and supervision measures are allowed, subject to having a permit from the Reserve's authorities [31].

There is also another polygon with the category of Buffer Zone/Subzone of Special Use, SZAE1. This is where the main groundwater discharge to the lagoon is located. It is an area composed of vegetation mosaics with a certain degree of conservation. It contains natural resources which are essential for the social development of the inhabitants of the area. The exploitation of these resources must be carried out without damaging the ecosystem and without substantially modifying the landscape or causing irreversible environmental impacts, in appropriate ways, subject to limited, supervised load capacities. Ecotourism is therefore allowed if it is sustainable and compatible with the environment. For this, operators must have the corresponding permits and management plans, from the Reserve's authorities. The promotion of environmental management units for the intensive and extensive use of wild flora and fauna is also allowed, with the corresponding registers and authorised management plans. Similarly, artisanal and subsistence fishing activities are permitted, subject to surveillance and supervision, with fishing gear that has been authorised, for each case, in the specific sites of these subzones [31].
