Introduction

Wetlands are ecosystems that, from their origin, shape, biodiversity, water and biogeochemical system, are classified as unique; they are also considered transition ecosystems between aquatic and terrestrial systems (^{Cowardin et al., 1979}), and intervene directly or indirectly in the global water cycle (^{Convención de Ramsar sobre los Humedales, 2018}). The ecosystem services that they provide place them in a central part for the sustainability of some societies, since fish products, algae, rice, fiber and other direct and indirect inputs are obtained from them. Only in 2020, 214 million tons of fishery products were obtained from wetlands, which yielded $ 151 billion US dollars for the same year (^{FAO, 2022}).

Other intrinsic goods to wetlands are linked to social and cultural well-being, as confirmed by studies on water recycling, recreational and aesthetic satisfaction, climate regulation, transmission of knowledge values, among others (^{Rochgiyanti and Susanto, 2017}; ^{Clarke et al., 2021}; ^{Agaton y Guila, 2023}).

The contribution of wetland services, however, is at risk on a global scale, due to the continuous conversion of this ecosystem towards "other productive uses", for which, frequently, they are drained or modified for agricultural uses or human settlements (^{Amenu and Mamo, 2018}). Stopping or reversing negative impacts on wetlands requires global action, for this reason, in 1971 the Convention on Wetlands (Ramsar Convention) was created, which became effective in 1975. The central objective of this agreement between countries is the conservation and rational use of wetlands, based on the idea of rational use as the maintenance of their ecological characteristics. The Ramsar Convention identifies wetlands of international relevance, especially those that provide habitat for waterfowl (^{Ramsar, 2023a}).

According to the Ramsar website, Mexico currently has 144 sites designated as wetlands of international importance (Ramsar sites), covering an area of 8 721 911 ha (^{Ramsar, 2023b}), which is equivalent to 1.69 % of its total territorial surface. They represent wetlands of different origin and water regulation, seven of them are located in Northeastwen Mexico (^{Ramsar, 2023c}); site No. 1981 (Figure 1) stands out for being a source of sulfurous waters that are home to endemic species of fish and habitat for migratory birds, such as the sandhill crane (Antigone canadensis, Linnaeus 1758) (^{eBird, 2023}). In addition, it is an emblem in region, its hydrogeochemical system is unique in Nuevo León and allows the formation of microbial mats (Chacón-Baca et al., 2015), as well as the presence of endemic ichthyo-species (^{Lozano-Vilano et al., 2021}), however, agricultural and livestock pressure has modified its landscape in recent decades (^{Benítez, 1997}).

Ramsar Site No. 1981.

Figure 1 Geographical location of the study site in the Linares municipality, Nuevo León. 

The scarce information that is generated in Mexico about wetlands, specifically inland (not coastal), increases the uncertainty of its future. The present study focuses on analyzing the change in land use at the Ramsar No. 1981 site (Figure 1) in a period of 44 years (1975 to 2019), in order to determine the deterioration and significant changes that modify its surface and identify disturbances in its area of influence.

Materials and methods

Delimitation of the study area

A Raster Tile layer was imported from the ^{Open Street Map (2021)} website, which contains the polygon of Ramsar Site No. 1981. In QGIS 3.32.2 (^{QGIS Development Team, 2021}) a vector layer was created vector in ESRI^® Shapefile format (.shp) with polygon geometry and WGS 84/UTM Zone 14N projection. In it, the study area was drawn with the "add polygon" tool that was supported by the green lines marked by the OpenStreetMap map (Figure 2).

The green outline shows the polygon of the Ramsar Site. Blue barred area represents the body of water.

Figure 2 OpenStreetMap image used in the delimitation of the Ramsar Site No. 1981. 

To verify that the surface was correct, the field calculator tool was used. Because this program works with meters as the default unit, the formula $area/10 000 was used in the field calculator to express the value obtained in hectares. The resulting value was 4 225.40 hectares, which agrees with the surface established as the study area and validates the polygon created.

Results

In the year 1975 there are no records of the presence of the class without apparent vegetation (WAV) and infrastructure, due to the lack of detail in the map of origin, however, it is feasible to appreciate the substantial reduction and increase in the natural coverage of the site and the expansion of anthropic activity (Table 3; Figure 3).

Figure 3 Change in land use per year, taking 1975 as the baseline. 

The most outstanding changes are shown in Figure 3. In it, it can be seen that the most notable occur with the loss of scrub cover and floodable area for an increase of the same magnitude in agricultural and grassland areas, respectively; they occur mainly in the period from 1975 to 2003 (Figure 3a-3h). The minimal or little apparent changes are presented in bodies of water, infrastructure and WAP for the periods analyzed (Figure 3i-3n).

The analysis of the change in land use between the years 1975 and 2003 (Figure 4) shows relevant modifications of scrub cover (-22 %), floodable area (-7.6 %), grassland (+7 %) and agricultural area (+twenty-one %). Minor change in water and infrastructure and moderate in WAV.

WAV = Without Apparent Vegetation. Negative values indicate a loss in coverage type.

Figure 4 Coverage changes at Ramsar Site No. 1981 during the 1975-2003 period, expressed in hectares. 

The agricultural expansion continued during the period between 2003 and 2019, with an increase of 7 % and the same proportion of loss for scrubland. The grassland and water remained without significant changes in their coverage, as well as infrastructure and the flood zone (0.22 %), WAV was modified in small amounts (2 %) (Figure 5).

WAV = Without Apparent Vegetation.

Figure 5 Coverage changes at Ramsar Site No. 1981 during the 2003-2019 period, expressed in hectares. 

The total changes in coverage that occurred in a period of 44 years (1975-2019) are considerable, as shown in Figure 6. The largest increases occur in agriculture (from 253 to 1 384 ha) and grassland (158 to 448 ha), which contrasts with the notorious loss of scrub (from 3 414 to 2 185 ha) and floodable area (from 400 to 70 ha). Small gains are shown for infrastructure (11 to 14 ha) and water (0.23 to 1 ha).

WAV = Without Apparent Vegetation.

Figure 6 Total coverage changes at Ramsar Site No. 1981 over a 44-year period (1975-2019). 

The changes observed in the mentioned period are significant in most land use classes (p=0.000001; Table 4). Only the bodies of water (water) remained without apparent modification, probably because they were supplied mostly underground.

Discussion

The results confirm a significant loss of coverage at the Ramsar Site No. 1981. An almost extinction of the floodable area was verified by losing 84 % of its surface and being replaced by grassland. The expansion of the agricultural frontier was the cause of the loss of scrub (36 %), initially occupying 6 % of the total surface of the area up to 33 % in 2019. In addition to the loss of the original cover and in accordance with the generated images, a modification of the landscape is noticed, mainly in the vicinity of what was previously the floodable area, which probably interrupted the natural flow of water towards the floodable area and the pools, which also modified the levels of evaporation and erosion protection.

This degradation and modifications in the Ramsar Site No. 1981 agree with other investigations carried out in Mexican wetlands, in which desiccation and loss of native cover due to the effects of urbanization, agriculture, aquaculture and industry are mentioned (^{Merlín-Uribe et al., 2012}; ^{Castellanos-Navarrete, 2021}; ^{Ballut-Dajud, 2022}), a process that occurs in most of these ecosystems in the world (^{Convención de Ramsar sobre los Humedales, 2018}). The foregoing coincides with was mentioned by ^{Benítez (1997)}, who states that the greatest change in land use in the study area was due to an increase in agricultural areas, and confirms that the reduction in scrub cover was caused due to clearing, the selective extraction of woody elements and extensive overgrazing practices.

To date, the most successful cases of recovery of the ecological integrity of these sites involve the inclusion of local residents who use the resources generated by these ecosystems. An example of this are the Rarámuri communities and the landowners in Chihuahua State, where together with NGO´s, they raise awareness among the local population to optimize the use of water from the groundwater table and store rainwater to avoid water stress, carry out concrete actions to mitigate the erosion (living fences) (^{Briggs et al., 2020}) and/or vary agricultural practices (^{Sarkar et al., 2020}). Empowering direct stakeholders based on the resource and the quality of life that wetlands provide seems to be the best way to conserve and perpetuate the environmental services linked to this ecosystem (^{Rochgiyanti and Susanto, 2017}; ^{Pedersen et al., 2019}).

The governance of the Ramsar Site No. 1981 is essential to recover its biological integrity, which must be inclusive, transparent and equitable with the populations in the area of influence (^{Mauerhofer, 2015}); their activities have a direct impact on hydrology by verifying that the use of peripheral land notoriously fragmented the landscape.

The use of water for agricultural purposes also has an important impact on its hydrology, due to changes in the flow caused by agricultural areas, as has been reported in the past for other regions (^{McCauley and Jenkins, 2005}). More detailed studies are required to know the underground water supply of the Ramsar Site No. 1981. The loss of native vegetation and the gain of agricultural surface reflect the lack of concrete actions that are observed in the National Wetlands Policy, whose objective is to conserve the hydrological basins and their biological component (^{Semarnat, 2018}). This is a reflection of legislation rarely observed, in addition to not being specific to these ecosystems and which unfortunately does not present comprehensive and cross-cutting actions, which makes coordination for their conservation difficult (^{CEDRSSA, 2021}).

A quick review of the historical satellite images available in Google Earth^® and the consultation of the historical precipitation data from the Cerro Prieto weather station (1950-2010) (^{Servicio Meteorológico Nacional, 2011}), confirm a prolonged drought during the last decades, which potentially influenced the water system, drying out the soil and evaporating surface water. The hydric changes experienced in recent decades due to inadequate management expose the risks of climate change and deforestation, including the loss of endemic species (^{Lozano-Vilano et al., 2021}), the reduction of semi-aquatic plant species present in the flooded areas such as Carex L. spp. and Cyperus L. spp., confirm the loss of biological diversity that gradually occurs in the site.

^{Pérez (2017)} proposed that the underground flow that controls the Ramsar Site No. 1981 site has its origin in the Sierra Madre Oriental (SMO), which is consistent with the conclusions of ^{Cantú and González (2002)} that mention the importance of oak, pine and pine-oak forests for runoff and water use in the SMO. This binding information makes it clear that the conservation of vegetation covers in the basin where the Ramsar Site No. 1981 is located is a priority if the objective is to preserve its functionality.

According to ^{Landgrave and Moreno-Casasola (2012)}, Mexico has lost 62 % of its wetlands, and Nuevo León is one of the states with the greatest decline (>50 %). At present there is no study that updates the conditions of these sites in the country; the National Inventory of Wetlands compiles and integrates the most complete database, however, there is no periodic monitoring of them at the national level. Therefore, the small or large contributions provided by studies such as this one will help to identify their current state, mainly those in the interior (non-coastal), whose information is very scarce.

It is essential to recognize that a Ramsar system (or Protected Natural Area) is not enough to stop or reverse the processes of environmental deterioration. Specific laws and actions are required to guarantee the perpetuity of freshwater reservoirs. Protected natural areas are subject to various threats (^{Maass et al., 2010}). In addition, they are vulnerable to the management schemes of their adjacent surfaces, and even within the same protected area due to the fact that on many occasions, there is inappropriate infrastructure within them, which occurs in the Ramsar Site No. 1981 when increasing in 3 ha the existing infrastructure. Part of the study site has gone from being an ejidal property to a private one, specifically the flood prone area and part of the scrub that surrounds it, which isolates the main pool and the smaller pools from the wetland.

The first action was to surround the property and restrict the passage of the public and ejidatarios in general, which sends the wrong signal to the inhabitants of the area. Socially, disconnecting the local population from the actions that are carried out does little to favor their conservation, since the basin that supplies water to the wetland is essential. An approach with the main actors of the rural settlements will allow finding holistic solutions that benefit all those involved and pay off in the recovery of the site.

Establishing clear management and use actions that are compatible and pertinent with the Ramsar declaration would avoid the continuous growth of the agricultural frontier in the protected area, in addition, the conservation of the vegetation cover of the basin would reduce the water stress detected and supply those wetlands present.

Conclusions

The results prove a significant change in the landscape structure of the Ramsar Site No. 1981 during the period from 1975 to 2019. The drastic reduction in the floodable area reflects the lack of surface water flow, as well as the low soil saturation, which subsequently produces a slight reduction of water in the main pool of the wetland.

Although the exposed results show clear variations between the periods analyzed with the near extinction of the floodable area (16 % remaining). The absence of quality information currently makes it difficult to carry out work with a high degree of precision in the study area. Added to this, is the owner's unwillingness to grant permission to obtain field data, which makes it difficult to corroborate in situ the aforementioned losses. However, what is exposed in this study is worrying, due to the low groundwater levels, as the water level of the main pool decreases and small springs within the site become extinct. The loss of scrub alters runoff and water infiltration, agriculture increases the albedo of the area and with it evaporation, coupled with a prolonged drought that accentuates the gradual loss of surface area of the wetland.

The basin is still under anthropic pressure, whether due to agriculture, livestock or use of groundwater tables, ordering activities within the basin is a priority. At the moment there is no management program that focuses on the current situation of climate change and land use, its protection is a priority to conserve not only the hydrology of the basin, but also to protect the biota present and that depends on sulfur ecosystems. and hot springs.

Promoting specific laws for both coastal and inland wetlands will be a task for the current and future legislatures, as well as supporting basic research on monitoring and inventory of wild populations dependent on these ecosystems. The current scenario makes it clear that the established policies hardly benefited our study area, however, each water reservoir is a priority for the health and quality of life of the inhabitants of Mexico.