Study area

The study was conducted in the northern part of BT (Table 1; Figure 1) and in the El Sargento estuary (Figure 2) in Sonora, Mexico. For both sites, ^{García and CONABIO (1998}) report a BW(h')hw climate, very dry and warm with mean annual temperature >22 °C, warm summer and intermediate rainfall regime between summer and winter. White (Laguncularia racemosa [L.] Gaertn), red (Rhizophora mangle L.) and black (Avicennia germinans [L.] L.) mangrove species, listed as "threatened" in NOM-059-SEMARNAT-2010 (^{Secretaría del Medio Ambiente y Recursos Naturales [SEMARNAT], 2019}), are distributed in the area.

BT is characterized as a resting, refuge and breeding site for marine species, migratory and resident waterbirds (^{WHSRN, 2019}). However, BT is deteriorating because its freshwater supply was channeled to irrigation district 041, while it receives untreated wastewater from agricultural and urban activities (containing phosphates, nitrates and nitrites) (^{Domínguez, 2010}). In addition, El Sargento is a site for the development, reproduction and shelter of several biological species (^{López-Medellín et al., 2009}), which shows deterioration due to contamination and siltation (^{Domínguez, 2010}).

Figure 1 Location of Bahía del Tóbari (Sonora, Mexico) and changes in coverage 2015-2020. Source: compiled by the authors with information from ^{CONABIO (2021)}. 

Figure 2. Location of El Sargento (Sonora, Mexico) and changes in coverage 2015-2020. Source: Compiled by the authors with information from ^{CONABIO (2021)}. 

Biomass and carbon stock estimation defining the 2015 baseline

The study by ^{Bautista-Olivas et al. (2018}) describes in detail the field phase and estimates of aboveground biomass of BT and El Sargento, according to the technique of ^{Kauffman, Donato, and Adame (2013}) and the allometric equations of ^{Day, Conner, Ley-Lou, Day, and Navarro (1987}). At each site, the aboveground biomass of mangrove species was determined according to their density per unit area (ha), while the sum of their average values indicated the biomass per ha. This value was multiplied by the conversion factor 0.5 and thereby the C stock was obtained (^{Intergovernmental Panel on Climate Change [IPCC], 2013}).

Estimates of biomass and C stock per ha for each site were extrapolated to their respective total areas covered by mangrove, determined by ^{CONABIO (2016)} and cross-checked in this study with Landsat 8 satellite images (^{United States Geological Survey [USGS], 2022}) (Table 2). In this way, reference characteristics, which form part of the "baseline (2015)" for both sites, were established.

Estimation of aboveground biomass and carbon stock for 2020

The information reported by ^{CONABIO (2021)}, which indicated gain, loss or unchanged permanence of wetland, was used to know the behavior of the area covered by mangrove in both sites in 2020. These areas were compared with Landsat 8 satellite images (^{USGS, 2022}) (Table 2). Based on this information and the baseline of 2015, the biomass and C stocks of 2020 were recalculated.

Avoided CO₂e emissions (2020) and economic valuation

As C sequestration has a market price in terms of tCO₂e, the determined C content was transformed into those units with the factor 3.7 (^{Howard et al., 2014}), while the EV (USD) of the reserve was calculated with a direct method using the following formula (^{Valera & Carvajal, 2014}):

where,

CO₂e = avoided emissions (t·ha^-1)

TA = total area covered by mangrove (ha)

Vc = marginal cost of reducing one tCO₂e (^{Hernández-Blanco et al., 2018}).

World Bank (WB, 1.9 USD) (^{World Bank Group [WGB], 2019}) and global VM (2.6 USD) (^{Ecosystem Marketplace, 2018}) average prices were considered.

Statistical analysis

Statistically significant differences between the means of the C contents of both study sites, regarding each mangrove species and total stores, were estimated with the t-student test for independent samples. The homoscedasticity of the data was evaluated with the F-test (P ≤ 0.05), to estimate heterogeneity in variances, using the Statistical Package for the Social Sciences software ver. 23.0 (^{IBM Corp. Released, 2015}).

Biomass and carbon stock for the baseline for 2015

The areas covered by mangrove at BT and El Sargento are smaller than their geographic boundaries (Table 1). In BT, densities were 3 162 trees∙ha^-1 and 1 147 144.8 trees∙TA^-1 (362.79 ha) with biomass of 104.1 ± 19.0 Mg∙ha^-1 and 37 756.7 ± 6 901.6 Mg∙TA^-1 and dominance of A. germinans (83.4 %) over R. mangle (16.6 %) (Table 3).

The densities at El Sargento were 4 406 trees∙ha^-1 and 2 063 432 trees∙TA^-1 (468.32 ha); i.e., 1.4 times more than BT (same ratio compared to mangrove area), which was attributed to its pristine character. The species L. racemosa dominated (60.1 %) over A. germinans (34.0 %) and R. mangle (5.8 %). Biomass was estimated at 143.3 ± 31.9 Mg∙ha^-1 and 67 119.7 ± 14 948.4 Mg∙TA^-1.

When comparing the mangroves of Balandra, Zacatecas and Enfermería (Baja California Sur, Mexico), it was found that these had lower tree densities (12 924, 11 094 and 5 775 trees∙ha^-1, respectively) and biomasses (68.7, 51.7 and 29.6 Mg∙ha^-1 and 1 662.5, 931.32 and 56.24 Mg∙TA^-1, respectively) than BT and El Sargento (^{Ochoa-Gómez et al., 2019}), which was related to their respective areas and coverages (24.2 ha and 66 %; 18 ha and 77 %; 1.9 ha and 14 %).

C stores in BT were estimated at 52.1 ± 9.5 MgC∙ha-¹ and 71.7 ± 16 MgC∙ha^-1 in El Sargento, while TA recorded 18 901 ± 3 446.5 MgC and 33 579 ± 7 493.2 MgC, respectively (Table 3). Like biomass, C stock in A. germinans was 2.2 times higher than in R. mangle in BT, and L. racemosa was 2.7 times higher than A. germinans at El Sargento

When contrasting the mean values of C content between BT and El Sargento, no statistically significant differences were found for total stores (t = 1.41; P = 0.17 and F = 0.048) and A. germinans (t = -1.58; P = 0.12 and F = 0.82). The opposite was observed for R. mangle (t = -2.57; P = 0.02 and F = 9.4 x 10^-39), which was associated with the disproportionate abundance in BT (16.6 %) and El Sargento (5.82 %); L. racemosa was excluded from the analysis because it was only found in El Sargento.

Baseline (2015) - prediction for 2020

Mangrove cover changed from 2015 to 2020 in both study sites; in the case of BT there was an increase of 68.77 ha. Because atmospheric CO₂ fixation in mangrove depends on density, productivity, basal area, height and age of trees (^{Sitoe, Comissario, & Guedes, 2014}), and in the absence of such information to define concrete values of biomass and C stocks in the area gained in BT, this was excluded from the prediction for 2020; however, for the area that remained constant, a biomass of 37 756.7 ± 6 901.6 Mg∙TA ^-1 was reported and a C reserve of 18 901.4 ± 3 447 Mg∙TA ^-1 (Table 4). According to ^{Kauffman et al. (2016}), mangrove loss emits GHG amounts greater than 66.3 %.

Avoided CO₂e emissions and economic valuation (2015-2020)

In 2015, estimated avoided CO₂e emissions in BT were higher in A. germinans than in R. mangle, and together totaled 191.2 ± 41.1 tCO₂e∙ha^-1 and 69 368.2 ± 14 912.2 tCO₂e∙AT^-1 (Table 5). At El Sargento, emissions were higher for L. racemosa and lower for R. mangle; together they totaled 263.2 ± 77.8 tCO2e∙ha^-1 and 123 253 ± 36 452.9 tCO2e∙AT^-1. The highest amount of avoided CO₂e emissions placed Sargento above BT.

In 2020, the EV of avoided emissions of tCO2e∙ha^-1 in BT were 131 799.5 ± 28 333.1 USD (WB) and 180 357.21 ± 38 771.6 USD (VM) (Table 5); because of its abundance, A. germinans represented the largest economic contribution in both markets (Figures 3 and 4). For El Sargento (2020), EV were 221 722.2 ± 65 575.8 USD (WB) and 303 409.4 ± 89 735.3 USD (VM) (Table 5); by dominance, L. racemosa had the highest EV in both markets (Figures 3 and 4). As in 2015, BT achieved a lower price than El Sargento in both markets (Table 5), despite changes in TA.

Figure 3 Economic value of avoided CO₂e emissions in mangroves Bahía del Tóbari and El Sargento, Sonora (2020) based on World Bank price (1.9 USD∙tCO₂e^-1). Bars indicate standard error of the mean; 95 % confidence interval. 

Figure 4 Economic value of CO₂e emissions avoided in the mangroves Bahía del Tóbari and El Sargento, Sonora (2020) based on the voluntary carbon market price (2.6 USD∙tCO₂e^-1). Bars indicate standard error of the mean, 95 % confidence interval. 

Reduction of mangrove cover in El Sargento implied a C loss of 1 786.4 ± 398.6 MgC∙TA^-1, equivalent to emissions of 6 556.1 tCO₂e∙TA^-1 to the atmosphere, with an economic loss in C stock of 12 458.5 ± 3684.7 USD according to the WB or 17 048.4 ± 5 042.2 USD according to the VM (Table 6).

^{Herrera et al. (2016}) found that the C stored in the biomass of BT and El Sargento was 1.24 times higher (in both cases) compared to the overall assessment of the North Pacific mangroves. For comparison purposes, this study took the C reference values reported by Herrera et al. (2016) and estimated avoided CO₂e emissions. Thus, the North Pacific recorded 154.47 t∙ha^-1, while the extreme values corresponded to the Yucatan Peninsula (171.68 tCO₂e∙ha^-1) and the South Pacific (512.52 tCO₂e∙ha^-1) (Table 7). While the calculated EVs for the North Pacific were 401.6 USD (WB) and 293.5 tCO₂e USD (VM) (Table 7), the estimates for BT and El Sargento were 1.24 and 1.7 times higher. After the North Pacific region, the Yucatan Peninsula recorded the lowest EV (446.4 USD∙ tCO₂e^-1 [VM] and 326.2 USD∙ tCO₂e^-1 [WB]) and the South Pacific the highest.

In other North Pacific mangroves, such as the Marismas Nacionales Biosphere Reserve (Nayarit), aboveground biomass was estimated at 361 ± 296 and 43.83 ± 21.39 Mg∙ha^-1 in the northern and southern zones, respectively, with reserves of 159. 9 ± 132.6 and 19.73 ± 9.5 MgC∙ha^-1 in the north and 586.30 ± 486.20 and 72.34 ± 34.83 tCO₂∙ha^-1 in the south (^{Quintero-Morales, Plata-Rocha, Olimón-Andalon, Monjardín-Armenta, & Nemiga-Antonio, 2021}). These biomasses are higher compared to BT (104.1 ± 19.0 Mg∙ha^-1) and El Sargento (143.3 ± 31.9 Mg∙ha^-1), even though the reserve experiences anthropogenic pressure and frequent hydrometeorological events. Biomass, C stocks and avoided CO₂ emissions were higher in the northern zone of that reserve, where A. germinans predominated; these results are contrary to those of the present study, since this species dominated in BT where the lowest values were recorded.

According to ^{Agraz-Hernández et al. (2020}), the EV of avoided CO₂ emissions in the reserve amounts to 4 431,826.7 USD, based on a market price per tCO₂e of 5.5 USD and a TA of 3 280 ha. This area is higher than that of BT (362.8 ha) and El Sargento (443.31 ha), as well as the tCO₂e price considered in this study (WB and VM, 1.9 and 2.6 USD, respectively).

The C stock of the mangrove biomass of the Nuevas Grandes-La Isleta Bay (in Cuba) was calculated at 13.27 t∙ha^-1, and the VE of the TA (2 631.3 ha) at 1 353 372.55 Cuban pesos (^{Díaz, González, Pérez, & Reyes, 2018}), without specifying the tCO₂e price. That C stock is lower than that recorded for BT and El Sargento (52.1 ± 9.5 and 71.7 ± 16 MgC∙ha^-1 respectively).

Access to financial mechanisms

In order to access financing mechanisms, it is necessary to demonstrate that the C stock can be increased, and CO₂ emissions can be reduced with additional economic incentives in BT and El Sargento. Therefore, the C baseline is defined (^{Herold et al., 2011}) and the amount of C stored in the area in a certain time, with and without project (conservation, maintenance, rehabilitation or recovery). The CEC (2017) highlights mangrove financing if linked to climate change mitigation and adaptation strategies (contemplated in the National Climate Change Strategy) and biodiversity (National Biodiversity Strategy), because of the importance and vulnerability of priority ecosystems and their role as biodiversity "aggregators". Although the quantification of edaphic C stores is recommended to complement the present study, neither the type of reservoir nor the extension of the mangrove are limiting factors for requesting financing, since it can be managed under maintenance and conservation strategies as occurs with mangroves in other countries (Table 8).

Limitations of this study include the lack of annual field measurements to generate a continuous record of biomass and soil C increments and determine the annual sequestration rate, as well as the lack of standardized methods for carbon quantifications and EV. This topic is particularly important because it allows the establishment of a baseline and the determination of C sequestration over time that is comparable between regions.

This suggests the development of the following lines of research: (i) edaphic C baseline and continuous monitoring of C in biomass and soil, since considering all C stocks would provide an integral evaluation and, with it, a complete scenario of C stock and its trend over time; (ii) integral EV of C stocks (biomass and soil) that would increase their cost and facilitate their insertion in the market; iii) standardized methodologies for the EV of this environmental service, which would avoid under- or overvaluation of the C stock and, consequently, favor an impartial market; and iv) development and implementation of strategies for accessing blue C financing mechanisms, which would benefit the conservation and maintenance of mangrove ecosystem services, including climate change.