Invited review articleTemporal records of organic carbon stocks and burial rates in Mexican blue carbon coastal ecosystems throughout the Anthropocene
Introduction
Over the last 100 years, human activities have changed the Earth's environment at unprecedented rates (Steffen et al., 2006) producing a distinct stratigraphic signature, for which the new geological time unit “Anthropocene” has been proposed (Waters et al., 2016), likely starting at the mid-20th century, synchronous with the “Great Acceleration” of population growth and industrialization (Subramanian, 2019). Between 1950 and 2010, the world population increased by ~174% (Pew Research Center, 2014), and by 2000, ~30% of the world population inhabited in low elevation coastal zones (<10 m above sea level; Neumann et al., 2015). In Mexico, the annual average growth of coastal urban population increased from 4% in 1900–1910 to 7% in 1940–1950 (Gutiérrez de MacGregor and González Sánchez, 1999), and in 2000, ~25% of total Mexican population was settled in coastal areas (Gabriel Morales and Pérez Damián, 2006).
The so-called blue carbon (BC) ecosystems, which include seagrass meadows, mangroves and salt marshes, are relevant for climate change mitigation through carbon sequestration and storage. The vegetation of these systems can effectively trap sedimentary carbon owing to their complex rooting systems (Nellemann et al., 2009). In the case of seagrasses, the canopy formed by leaves are also efficient traps for particulate material suspended in the water column, and plant refractory matter (e.g. senescent leaves and rhizomes) are also persistent in sediments (Gullström et al., 2018). Recent studies suggest that the carbonate content in seagrass meadows has implications for their carbon sink function, as carbonate structures might be developed within seagrass tissues, or by associated fauna and flora (e.g. shellfish, and calcareous benthic and epiphytic algae) (Gullström et al., 2018). Although the calcification process is a source of carbon dioxide (CO2), the role of inorganic carbon burial on CO2 emissions depends on the balance between its production and dissolution. Seagrass ecosystems, and mangroves to a lesser extent, are intense sites of inorganic carbon burial, that contribute to buffer sea-level rise supporting the role of BC ecosystems in climate change adaptation (Saderne et al., 2018, Saderne et al., 2019).
Although BC ecosystems occupy only 0.2% of the ocean surface (Duarte et al., 2013), they have the highest marine carbon burial rates (18–1713 g C m−2 yr−1) and are more efficient carbon sinks than terrestrial forests (Hori et al., 2019; Mcleod et al., 2011). On top of carbon storage and sequestration, BC ecosystems provide other important ecosystem services such as coastal protection, erosion control, maintenance of water quality, and fisheries, recreational and tourism benefits (Barbier et al., 2011; Lau, 2013). However, these habitats are threatened by land-based human activities, anthropogenic marine impacts (e.g., aquaculture) and climate change. Over the last 20–50 years, 50% of salt marshes, 35% of mangroves and 29% of seagrasses have been lost worldwide (Himes-Cornell et al., 2018 and references therein).
The anthropogenic conversion rate of BC ecosystems worldwide is 0.4–3% per year, accounting for the release of 30–813 Gg of CO2 per year (Howard et al., 2017 and references therein). Mexico is one of the five countries with largest mangrove extent worldwide (775,555 ha in 2015) and also faces a high loss rate (9% between 1970 and 2015; Valderrama-Landeros et al., 2017). Mexican salt marshes are ~5% of the global salt marsh area (2725 ha; Mcowen et al., 2017), although the extent of salt marshes and seagrass meadows has not yet been thoroughly evaluated. For the Mexican Caribbean, estimates suggest that up to 50% of the original seagrass cover has been lost (INECC, 2018).
Conservation of BC ecosystems significantly contribute to climate change mitigation by avoiding carbon emission from plant tissues and sediments; this occurs when marine vegetated communities are disturbed and Corg stored in sediments and tissues is oxidized and released as CO2 to the atmosphere (Pendleton et al., 2012; Kuwae et al., 2016). Both the amount of Corg accumulated in the sediments per unit area within a defined depth (Corg stock) and burial rates (the rate at which Corg is accumulated in sediments) can be affected by human activities.
Land use change and habitat conversion directly impact BC ecosystems, by reducing their ability to sequester Corg (Coverdale et al., 2014). In seagrass meadows, mechanical destruction by dredging, mooring, and nutrients excess from agriculture lead to a reduction of seagrass cover and Corg stocks (Marbà et al., 2015; Serrano et al., 2016). Mangroves degraded by deforestation, agriculture and industry, or affected by aquaculture, also display significantly lower Corg contents than undisturbed areas (Suárez-Abelenda et al., 2014; Weiss et al., 2016). In salt marshes, the excess of nutrients may increase above-ground leaf biomass and decrease the dense below-ground biomass roots, thus promoting bank-destabilization and erosion (Deegan et al., 2012), and also may change the dynamics of microbial activities, stimulating denitrifying bacteria and fungi, which enhance organic matter decomposition and alter the form of available carbon (Kearns et al., 2019).
In this work we evaluated Corg stocks at 1 m depth and the temporal changes of Corg stocks and burial rates in mangroves, salt marshes and seagrass meadows in different sites representative of key Mexican marine coastal systems by using 210Pb-dated sediment cores, under the hypothesis that Corg stocks and burial rates have declined since the beginning of the Anthropocene (1950s) owing to the increase of multiple anthropogenic stressors.
Section snippets
Study sites
Most of the sedimentary records referred here have already been used to estimate Corg stocks and burial rates in different ecosystem types (Cuellar-Martinez et al., 2019; Vázquez-Molina, 2019; Ruiz-Fernández et al., 2018a; López-Mendoza et al., 2020; Ruiz-Fernández et al., 2020; Aldana-Gutiérrez et al., 2020).
San Quintín Bay (SQB, Fig. 1) is in the northern Pacific coast of the Baja California Peninsula, and is a Ramsar site since 2008 (Martínez-Ríos, 2007). It has 42 km2 surface, a mean depth
210Pb dating and MAR
According to 210Pb chronologies, sediment records spanned from 35 to >100 years. In most cores, 137Cs activities were below the detection limit (<2 Bq kg−1), which precluded their use to validate 210Pb dating. However, in the salt marsh cores from SQB (M1 and M2; Cuellar-Martinez et al., 2019) and the mangrove core UH1 from CRM (Vázquez-Molina, 2019) 137Cs maxima were consistent with the period of maximum 137Cs fallout (1962–1964). Also, in cores from SQB (P1, P2), CRM (PCm1, JB1) and TL
210Pb age models validation
The lack of detectable 137Cs has been reported for other Mexican coastal areas (Ruiz-Fernández and Hillaire-Marcel, 2009 and references therein; Carnero-Bravo et al., 2016; Cuellar-Martinez et al., 2017; Ruiz-Fernández et al., 2019). This is attributed to a low radioactive atmospheric fallout in the zone, the time elapsed since the period of maximum fallout from nuclear testing (which implies that ~70% of the initially deposited 137Cs has already decayed), high solubility in seawater, potential
Conclusions
Temporal evolution of Corg stocks and burial rates through the last century were evaluated in 210Pb-dated sediment cores from mangroves, seagrass meadows and salt marshes in Mexico. High MAR variability was observed within and among habitats. Organic carbon stocks(1 m) were comparable between mangroves and salt marshes, and both higher than those in seagrass meadows. Organic carbon stocks(1 m) and burial rates were within the reported global ranges. Organic carbon stock(100 years) were highest
Declaration of Competing Interest
None.
Acknowledgements
This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT, Mexico) grants PDCPN 2015/1-473, CB2010/153492 and SEMARNAT-2016-01-278634. The authors thank M.G. Barba-Santos, J.A. Reda Deara, H. Álvarez Guillén, M. A. Gómez-Ponce (sampling), H. Bojórquez-Leyva and S. Rendón-Rodríguez, L. Felipe Álvarez (UNINMAR) for technical assistance, and E. Cruz-Acevedo, G. Ramírez-Reséndiz and C. Suárez, for data management.
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