The landscape of the BSMR is dominated by agriculture practices but maintains a notable proportion of coastal wetlands. The current spatial heterogeneity patterns of the landscape are due to natural and anthropogenic drivers operating at different spatiotemporal scales (Bertolo et al., 2012; Roy & Tomar, 2001). In the case of the BSMR, the coast is of primary origin and rapid sediment accumulation during its formation favored the creation of a broad coastal plain in which Vertisols predominated. Since the 1930s, the agrarian reforms of the Mexican government stimulated agriculture development with the creation of hydraulic infrastructure (Berlanga-Robles & Ruiz-Luna, 2011). As a result, agriculture in the BSMR is currently the prevailing land use, and this trend will probably continue for the next 20 years, even though the extent of agricultural land has remained relatively constant since the mid-1990s.
As detected in this and other studies in the region (Gurrola, 2000; García, 2005), the expansion of agriculture has come at the expense of natural terrestrial vegetation (e.g., dry forests and xerophytic shrubs) and saltmarshes. In addition, notable changes in natural covers are due to shrimp farming, with facilities being built primarily on saltmarshes with low mangrove deforestation rates.
Saltmarshes offer advantages when constructing shrimp ponds, such as flatness (slope < 5°), soils with fine textures and low infiltration rates (e.g., Solonchak and Vertisols), a minimal presence of trees, and the proximity to lagoons and estuaries that simplifies water supply and drainage operations. Moreover, the legislation to protect and regulate the use of saltmarshes in Mexico is ambiguous and only specifies some restrictions when saltmarshes are near mangroves, which further facilitates the creation of shrimp farms in these habitats (Berlanga-Robles et al., 2011a; 201b).
The same geological process that allowed the coastal plain to advance in central and northernSinaloa (including BSMR) also allowed for large areas of intertidal wetlands to form as saltmarshes. In Sinaloa, there are between 115,300–159,400 ha of saltmarshes (Berlanga-Robles & Ruiz-Luna, 2011; Hernández-Guzmán et al., 2021), representing 42–63% of the total saltmarsh cover in Mexico (272,527 ha; Mcowen et al. 2017). As such, the saltmarsh cover of the BSMR coastal lagoon system represents around 10% of that reported for all Mexico in 2017. However, by 2037, the BSMR saltmarsh cover could be reduced to 6%.
To date, most of this reduction has been due to shrimp farming growth, which was responsible for the loss of around 19,000 ha from 1985 to 2017. Shrimp farming is also expected to result in additional losses in cover of ~ 8500 ha by 2037. Moreover, this activity has substantially impacted the connectivity of the entire BSMR wetland complex, which is probably a consequence of the linear infrastructure (e.g., roads and channels) required for shrimp farming rather than of the ponds themselves (Berlanga-Robles et al. 2011b).
These changes also alter the capacity of wetlands to deliver ecosystem services and highlight the interdependence of humans and nature and the benefits that ecosystems provide to societies
(Lebreton et al., 2019; Zorrilla-Miras et al., 2014). The monetary value of ecosystems has been increasingly used to support sustainable socio-ecosystem management policies, but for saltmarshes, these assessments may underestimate their value due to a lack of understanding of their ecological functions and the ecosystem services that they offer.
In addition, identifying the interdependence between humans and nature is complex in ecosystems that are highly connected to other nearby ecosystems (e.g., saltmarshes). Likewise, valuations of ecosystem services require the segmentation of ecosystem components, and thus any services associated with the emergent properties of a coastal ecosystem complex cannot be assessed (Lebreton et al., 2019). Despite these limitations, a negative balance can be identified when saltmarshes (natural wetlands) are converted to shrimp farms (artificial wetlands).
In 2007, shrimp farming in Mexico produced 111,787 t from ~ 72,500 ha of shrimp ponds (Berlanga-Robles et al., 2011a), which was valued at 421,206,200 (2007 USD; FAO, 2022). This amounts to 5,810 (2007 USD)/ha/year, which is 39% less than the value estimated for saltmarsh ecosystem services by Camacho-Valdez et al. (2014) of 9,544 (2007 USD) per ha/year. Therefore, from the start of shrimp farming in the BSMR region until 2017, a loss of 70,946,000 (2007 USD) due to the conversion of saltmarshes to shrimp farms has accrued. From 2017 to 2037, this loss is expected to grow to 31,739,000 (2007 USD). These losses are based on the direct elimination of saltmarshes and do not consider either other indirect effects due to their disappearance, such as fragmentation, or emergent effects that depend on the entire coastal system.
Activities that increase the provisioning of services of high economic value at the expense of regulating services, such as saltmarsh-to-shrimp farm conversions, are major drivers of LULCC worldwide (Zorrila-Miras et al., 2014). Moreover, while many saltmarsh ecosystem services come in the form of collective goods, the economic benefits of shrimp farming are mainly limited to farm owners, with narrow benefits spreading to local communities in the form of taxes or temporary jobs.
According to the official statistics for the municipality of Angostura, which includes most of the BSMR wetland system, the social impacts of shrimp farming are difficult to assess, especially considering recent methodological changes to measure poverty and well-being in Mexico. Even so, the number of people in poverty in Angostura decreased from 48% in 2010 to 26% in 2020, although this reduction was accompanied by a decrease in population size of 14.5% from 52,438 to 44,814 inhabitants (CONEVAL, 2022). Moreover, a notable rise in the number of people employed in the fishing and aquaculture industry can be observed from 1998 (2,124) to 2003 (3,596), although this number fell once again in 2008 (2,156; INEGI, 2015). At the national level, around 19,000 workers were employed by 955 shrimp farming operations in 2018 (INEGI, 2021).
The frequency and duration of tidal flooding, which primarily define the structure and ecological functions of saltmarshes, are also affected by changes in saltmarsh cover (Meijer et al., 2021; Roman & Burdick, 2012; Schutte et al., 2019). When saltmarshes are restricted, their biophysical processes are affected, resulting in disturbances to vegetation patterns, fish and avian communities, biogeochemical cycling, the links among physical and biological factors, and the marine-coastal-terrestrial environmental gradient (Meijer et al., 2021; Roman & Burdick, 2012; Schutte et al., 2019). The interruption of tidal flows in saltmarshes threatens various biological and biogeochemical processes, such as the vertical and horizontal migrations of benthic microalgae, and benthic-pelagic coupling that supports high primary productivity and sustains food webs (Kuwae et al., 2021; Lebreton et al., 2019). Coastal wetlands, such as saltmarshes and mangroves, may also be responsible for ~ 50% of the carbon that is sequestered each year in the oceans (Adam, 2019). However, changes in the natural hydrology of wetlands due to fragmentation alter the balance between methane emissions and CO2 uptake, reducing the efficiency of carbon sequestration (Mitsch, 2016). In addition, the direct impacts of fragmentation in saltmarshes may lead to the release of carbon into the atmosphere that had been previously sequestered for hundreds of years (Adam, 2019).
The saltmarshes and other coastal wetlands in BSMR host critical wintering habitats for migratory waterbirds and shorebirds, supporting nearly one-third of all wintering shorebirds in the North American region of the Pacific Flyway (Engilis et al., 1998; Kramer & Migoya, 1989; Morrison et al., 2009). Consequently, these coastal wetlands host notable portions of the global populations of several shorebird species.
Given their importance, these coastal wetlands have been designated as priority sites for conservation in several national and international bird conservation initiatives (Enríquez-Andrade et al., 2005). Some of these distinctions offer various levels of legal protection in national and international frameworks (Boere & Piersma, 2012). However, the degradation of coastal wetlands affects habitat availability for migratory shorebird species. As a result, shorebird populations are declining due to the limited amount of available space and the loss of habitat quality in coastal non-breeding areas (Piersma et al., 2017; Rosenberg et al., 2019), which also impacts the food chain and associated biogeochemical cycles.
Besides shrimp farming, which is clearly one of the main drivers of landscape fragmentation and the loss of saltmarshes in the BSMR, other threats to wetlands in the study area are present and include sewage discharge, which increases nutrient concentrations and the eutrophication of the lagoon system (Gurrola et al., 2016). This threat is particularly severe in the southern region of the BSMR coastal lagoon system due to the discharge of polluted water from industrial, agricultural, livestock, and urban sources in the neighboring municipalities (Leyva et al., 2020). In semi-intensive shrimp farming systems, such as those of the BSMR coastal lagoon system, the discharge of nitrogen and phosphorus per harvest cycle has been estimated to be 45 and 12.9 kg/ha, respectively (Páez-Osuna, 2001). Thus, according to this and the extent of the shrimp farming estimated in this study, nutrient discharge from this activity increased from 0 t in 1985 to 1834 t (N) and 526 t (P) in 2017. Moreover, these values may increase to 2621 t (N) and 751 t (P) by 2037, considering an average of two shrimp production cycles each year. Moreover, shrimp farming contributes 10.2% and 3.3% of the nitrogen and phosphorus that are annually discharged into the Gulf of California, while agriculture contributes 67.2% and 62.5%, respectively (Miranda et al., 2009).
This study highlights the importance of coastal wetlands, particularly that of saltmarshes, in the BSMR coastal lagoon system. At a regional level, this study also highlights how these systems have been impacted by LULCC, especially those due to agriculture and shrimp farming. Thus, instruments are needed to regulate the territorial expansion of these activities while current management efforts should be strengthened (Mcowen et al., 2017). Economic activities that impact the natural systems and biodiversity of the BSMR coastal lagoon system have benefited local communities; however, shrimp production has been mainly designed to satisfy external markets, leading to the overexploitation of natural resources. In particular, shrimp farming has affected the biodiversity in the region, and the resulting land use changes have caused notable wetland losses, particularly those of saltmarshes.
As a result of the associated problems in the BSMR coastal lagoon system, stakeholders, agencies, and non-governmental civil organizations have worked to mitigate the impacts of shrimp farming through restoration, conservation, and environmental education efforts. Currently, governance has strengthened through the formation of committees to design and implement strategies that mitigate the environmental and social impacts caused by the different economic activities conducted in wetlands. In addition, participatory action schemes in communities are available, although it is limited to the islands of the system.
At the international level, programs have been established to promote the active involvement of local people in environmental protection efforts (Artigas et al., 2014), but it is still necessary to foster interactions among stakeholders to encourage engagement in participatory processes whose outcomes must be monitored and supported with legal backing. Current and future management efforts in the BSMR coastal lagoon system and similar regions should also consider adopting integrated coastal management programs that rely on dynamic decision-making to ensure the sustainable use, development, and protection of coastal and marine areas and their resources (Cisin-Sain & Kenecht, 1998).
Given the changes in the coastal landscape of the BSMR region, restoration strategies that ensure the coexistence of natural and artificial wetlands are needed. While a recovery of the saltmarshes is unlikely, coastal wetland conservation and land-use planning in the BSMR region may prevent further LULCC in this important wetland system. Likewise, interventions are required that restore hydrological flows and that manage tidal activity and freshwater runoff to safeguard the ecological functioning of the remaining saltmarshes and extended wetland network and their ecosystem services. According to Berlanga et al. (2011b), these interventions should be made over the linear infrastructure (e.g., roads and canals) rather than the shrimp farms themselves. In addition, the regulation of economic activities at the regional level is necessary, as the BSMR coastal wetlands are located in the lower portion of an anthropized basin with more than 60% of its surface affected by LULCC (García, 2005). Management proposals must also consider pollution and wastewater from productive activities and human settlements. In doing so, nutrient loads and eutrophication may be reduced.