Blue carbon of Mexico, carbon stocks and fluxes: a systematic review.

Mexico has more than 750,000 ha of mangroves and more than 400,000 ha of seagrasses. However, approximately 200,000 ha of mangroves and an unknown area of seagrass have been lost due to coastal development associated with urban, industrial and tourist purposes. In 2018, the approved reforms to the General Law on Climate Change (LGCC) aligned the Mexican law with the international objectives established in the 2nd Article of the Paris Agreement. This action proves Mexico's commitment to contributing to the global target of stabilizing the greenhouse gas emissions concentration in the planet. Thus, restoring and conserving mangrove and seagrass habitats could contribute to fulfilling this commitment. Therefore, as a first step in establishing a mitigation and adaptation plan against climate change with respect to conservation and restoration actions of these ecosystems, we evaluated Mexican blue carbon ecosystems through a systematic review of the carbon stock using the standardized method of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). We used the data from 126 eligible studies for both ecosystems (n = 1220). The results indicated that information is missing at the regional level. However, the average above and below ground organic carbon stocks from mangroves in Mexico is 113.6 ± 5.5 (95% CI [99.3-118.4]) Mg Corg ha-1 and 385.1 ± 22 (95% CI [344.5-431.9]) Mg Corg ha-1, respectively. The variability in the Corg stocks for both blue carbon ecosystems in Mexico is related to variations in climate, hydrology and geomorphology observed along the country's coasts in addition to the size and number of plots evaluated with respect to the spatial cover. The highest values for mangroves were related to humid climate conditions, although in the case of seagrasses, they were related to low levels of hydrodynamic stress. Based on the official extent of mangrove and seagrass area in Mexico, we estimate a total carbon stock of 237.7 Tg Corg from mangroves and 48.1 Tg Corg from seagrasses. However, mangroves and seagrasses are still being lost due to land use change despite Mexican laws meant to incorporate environmental compensation. Such losses are largely due to loopholes in the legal framework that dilute the laws' effectiveness and thus ability to protect the ecosystem. The estimated emissions from land use change under a conservative approach in mangroves of Mexico were approximately 24 Tg CO2e in the last 20 years. Therefore, the incorporation of blue carbon into the carbon market as a viable source of supplemental finance for mangrove and seagrass protection is an attractive win-win opportunity.

Terrestrial and oceanic influence on spatial hydrochemistry and trophic status in subtropical marine near-shore waters.

Terrestrial and oceanic influences like groundwater discharges and/or oceanic upwelling define the hydrochemical and biological characteristics of near-shore regions. In karst environments, such as the Yucatan Peninsula (SE Mexico), the balance between these two influences on spatial and temporal scales is poorly understood. This study focused on near-shore waters within 200 m offshore along the Yucatan coast. The trophic status and hydrochemical zones of the study area were determined as a function of physical and nutrient data collected from 2005 to 2006. The main terrestrial influence was groundwater discharge, while the most important marine influence was related to seasonal changes in water turbulence. Spatial differences (p < 0.05) were observed among salinity, light extinction coefficient (k), NO(3)(-), NH(4)(+), and Chl-a. Seasonal differences were observed for all variables except for k. During the dry season, terrestrial influences are the dominant factor on near-shore hydrochemistry. The region around Dzilam exhibited the maximum influence of groundwater discharge estimated by salinity dissolution (δ). During the rainy and "nortes" seasons, there is a balance between oceanic and terrestrial influences. The trophic status measured using the TRIX index, indicated that near-shore waters were mainly oligo-mesotrophic; with a meso-eutrophic status in areas with documented anthropogenic impacts. Four hydrological zones were identified by a Canonical Variate Analysis (CVA) using salinity, NO(2)(-), k and NH(4)(+) as the main discriminating variables. Zones I and II showed almost pristine conditions, with well-balanced terrestrial-oceanic influences. In Zone III, terrestrial influences such as groundwater discharges and inland pollution suggesting human impacts were dominant respect to the effects of oceanic influences like upwelling and sediment resuspension caused by winds and oceanic currents. Zone IV received enhanced groundwater and associated nutrients. Anthropogenic activities have led to ecosystem degradation but the speed at which this occurs depends on local and regional characteristics. Therefore, this study has defined those characteristics so as to enact better management policies.

Evaluation of the health status of a coastal ecosystem in southeast Mexico: Assessment of water quality, phytoplankton and submerged aquatic vegetation.

The coastal environment of the Yucatan Peninsula (SE, Mexico) includes a wide variety of ecosystems ranging from mangroves to coral reefs, resulting in a heterogeneous landscape. Specifically, the marine system is characterized by environmental differences which respond to regional and local forcing functions such as marine currents and groundwater discharges (GD). Such functional characteristics were used here to define four subregions across the Yucatan coast and diagnose the health status of this coastal marine ecosystem. To achieve this goal, we conducted an analysis and integration of water quality variables, an eutrophic assessment, evaluated changes in submerged aquatic vegetation (SAV), and analyzed the community structure and distribution of harmful phytoplankton. The first step was to determine the reference values for each subregion based on data previously collected from 2002 to 2006 along the coast of Yucatan, 200m offshore. The trophic index (TRIX) and Canadian index for aquatic life (CCMEWQI) were used to diagnose each subregion and then the ASSETS approach was conducted for Dzilam and Progreso, sampling localities on each end of the health status continuum (those with the best and worst conditions). Overall, results indicated that the marine coastal ecosystem of Yucatan is in good condition; however, differences were observed between subregions that can be attributed to local forcing functions and human impacts. Specifically, the central region (zone HZII, Progreso-Telchac) showed symptoms of initial eutrophication due to nutrient inputs from human activities. The eastern region (zone HZ III, Dzilam-Las Bocas) showed a meso-eutrophic condition linked to natural groundwater discharges, while the other two subregions western (zone HZI Celestun-Palmar) and caribbean (zone HZ IV Ria Lagartos-El Cuyo) exhibited symptoms of oligo-mesotrophic condition. These findings may be considered baseline information for coastal ecosystem monitoring programs in Yucatan, and the approach used could be replicated for other coastal areas.