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Capítulo 12 ALMACENES Y FLUJOS EN LAGUNAS COSTERAS: LAGUNA CELESTÚN
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INTRODUCCIÓN A lo largo de casi un tercio de los 10 000 km de longitud que conforman la banda litoral de México se contabilizan aproximadamente 164 cuerpos de agua costeros (De la Lanza-Espino et al., 2013). Abonando a la generalidad, más que a las particularidades morfológicas y geológicas que estos cuerpos de agua costeros pudieran albergar, se denomina lagunas costeras a las bahías someras, sondas, bocas, esteros, estuarios y caletas (Figura 1). En el interior de las lagunas costeras coexisten productores primarios como fitoplancton, microfitobentos y pastos marinos, favoreciendo una alta productividad durante todo el ciclo anual, aún en el marco de un ambiente fluctuante como el que caracteriza a estos sistemas, el cual selecciona bajo una alternancia programática, al mayor contribuyente de entre esta variedad de productores primarios (Medina-Gómez y Herrera-Silveira, 2006). Particularmente, una alta biodiversidad del componente de vegetación acuática sumergida (VAS) brinda no sólo áreas críticas para numerosas especies, sino también insumos de materia y energía que abastecen procesos de transformación biogeoquímica que ocurren tanto en la columna de agua, como en la biota y los sedimentos. A pesar de la gama de ecosistemas que se agrupan en la categoría de lagunas costeras, un rasgo esencial en todos ellos es la presencia de diversos sistemas circundantes, tanto hacia el continente (manglares y marismas) como oceánicos (mar de plataforma), así como el transporte activo de materiales a través de las fronteras que estos sistemas comparten y de las múltiples interfaces internas (aire-columna de agua, sedimento-columna de agua, agua dulce-salobre-marina) que resultan de la contigüidad con dicho mosaico de ecosistemas. Esta conexión está determinada por la geomorfología de cada ecosistema, la cual concierne a la intermitencia o permanencia de apertura de la boca lagunar y otras fronteras que guarda con ecosistemas vecinos, desempeñando un papel importante al materializar el acoplamiento con aquellos. También, los patrones de circulación del agua dentro de dichos ecosistemas controlan la tasa y magnitud del intercambio de materiales con los distintos ambientes costeros, definiendo al contexto climatológico y la hidrodinámica como reguladores preponderantes del grado de interacción a lo largo de este continuo ecológico. La interacción entre los diferentes ecosistemas, llamada conectividad, es evidente en las lagunas costeras, las cuales constituyen fundamentalmente un escenario ambiental que recibe el suministro de distintos aportes, entre ellos carbono inorgánico disuelto y carbono orgánico particulado (CID y COP, respectivamente). Se calcula en 1.0 Pg C año-1 el incremento de los flujos de carbono hacia aguas epicontinentales debido a alteraciones de origen humano desde la era industrial, principalmente asociadas a actividades de explotación del suelo (Reigner et al., 2013). Se considera que las lagunas costeras desempeñan un papel crucial en el denominado Carbono Azul, a consecuencia de su alta capacidad para secuestrar este insumo de carbono (C), ya sea en material vivo o como
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Purpose of Review
We use the ‘seascape’ concept to explore how interactions between mangrove forests, tidal marshes and seagrass influence the storage of carbon in these ecosystems. Mangrove forests, with the other two ‘blue carbon’ habitats, are exceptionally powerful carbon sinks. Maintaining and enhancing these sinks is an emerging priority in climate change mitigation. However, managing any one ecosystem on its own risks is ignoring important contextual drivers of carbon storage emerging from its place in the seascape. We consider how interactions between these coastal habitats directly or indirectly affect the amounts of carbon they can store.
Recent Findings
The export of carbon from seagrasses may occur over hundreds or thousands of kilometres, much further than reported for mangroves or tidal marshes. Seagrasses may buffer mangroves from wave impacts, assisting forest regeneration. Trophic cascades supported by contiguous blue carbon habitat may limit excessive herbivory and bioturbation in them but evidence is limited.
Summary
Direct transfers of carbon between blue carbon habitats are common and are likely to enhance total carbon storage, but our understanding of their contribution to carbon stocks at the seascape level is elementary. There is evidence for indirect enhancement of carbon storage at the seascape by close association of habitats, mostly through the creation and maintenance of propitious conditions by one ecosystem for another. Protection from waves of mangroves by seagrass and protection from excess nutrients and sediment of seagrass by mangroves and tidal marsh are key mechanisms. There is little evidence or theory suggesting negative effects on carbon storage of one blue carbon habitat on another.
The role of mangroves in the blue carbon stock is critical and requires special focus. Mangroves are carbon-rich forests that are not in steady-state equilibrium at the decadal time scale. Over the last decades, the structure and zonation of mangroves have been largely disturbed by coastal changes and land use conversions. The amount of time since the last disturbance is a key parameter determining forest structure, but it has so far been overlooked in mangrove carbon stock projections. In particular, the carbon sequestration rates among mangrove successional ages after (re)establishment are poorly quantified and not used in large scale estimations of the blue carbon stock. Here, it is hypothesized that ecosystem age structure significantly modulates mangrove carbon stocks. We analysed a 66-year chronosequence of the aboveground and belowground biomass and soil carbon stock of mangroves in French Guiana, and we found that in the year after forest establishment on newly formed mud banks, the aboveground, belowground and soil carbon stocks averaged 23.56±7.71, 13.04±3.37 and 84.26±64.14 (to a depth of 1 m) Mg C ha⁻¹, respectively. The mean annual increment (MAI) in the aboveground and belowground reservoirs was 23.56×Age−0.52 and 13.20×Age−0.64 Mg C ha⁻¹ yr⁻¹, respectively, and the MAI in the soil carbon reservoir was 3.00±1.80 Mg C ha⁻¹ yr⁻¹. Our results show that the plant carbon sink capacity declines with ecosystem age, while the soil carbon sequestration rate remains constant over many years. We suggest that global projections of the above- and belowground reservoirs of the carbon stock need to account for mangrove age structures, which result from historical changes in coastal morphology. Our work anticipates joint international efforts to globally quantify the multidecadal mangrove carbon balance based on the combined use of age-based parametric equations and time series of mangrove age maps at regional scales.
Wetlands are highly productive ecosystems that can sequester large quantities of carbon. However, variation in physiological potential can alter their capacity to sequester carbon. To examine variation in ecosystem physiological capacity, we established three coastal marsh sites in Mississippi and Alabama spanning a productivity gradient. Over 1 year, we measured ecophysiological activity and spectral indices in two vegetation zones within each marsh to develop a better understanding of variation in ecosystem responses and health. Gross ecosystem exchange of carbon and ecosystem respiration rates (Reco) differed significantly among sites, with the highest activity at Grand Bay, Mississippi and Point Aux Pines, Alabama and lower ecophysiological activity at Dauphin Island, Alabama. Net ecosystem exchange was similar for all three study areas because greater carbon assimilation was negated by higher levels of respiration. Spectral indices and leaf area were significantly different by marsh vegetation zone, suggesting that alterations in species composition and plant productivity can have important implications for carbon sequestration. While limited to 1 year, this study establishes a foundation by which to evaluate future research conducted over greater temporal and spatial scales, thereby enhancing our understanding of marsh physiological activity.
Subterranean estuaries extend inland into density-stratified coastal carbonate aquifers containing a surprising diversity of endemic animals (mostly crustaceans) within a highly oli-gotrophic habitat. How complex ecosystems (termed anchialine) thrive in this globally distributed, cryptic environment is poorly understood. Here, we demonstrate that a microbial loop shuttles methane and dissolved organic carbon (DOC) to higher trophic levels of the anchialine food web in the Yucatan Peninsula (Mexico). Methane and DOC production and consumption within the coastal groundwater correspond with a microbial community capable of methanotrophy, heterotrophy, and chemoautotrophy, based on characterization by 16S rRNA gene amplicon sequencing and respiratory quinone composition. Fatty acid and bulk stable carbon isotope values of cave-adapted shrimp suggest that carbon from methano-trophic bacteria comprises 21% of their diet, on average. These findings reveal a heretofore unrecognized subterranean methane sink and contribute to our understanding of the carbon cycle and ecosystem function of karst subterranean estuaries.
Wetlands are often considered as nature-based solutions that can provide a multitude of services of great social, economic and environmental value to humankind. Changes in land-use, water-use and climate can all impact wetland functions and services. These changes occur at scales extending well beyond the local scale of an individual wetland. However, in practical applications, engineering and management decisions usually focus on individual wetland projects and local site conditions. Here, we systematically investigate if and to what extent research has addressed the large-scale dynamics of landscape systems with multiple wetlands, hereafter referred to as wetlandscapes, which are likely to be relevant for understanding impacts of regional to global change. Although knowledge in many cases is still limited, evidence suggests that the aggregated effects of multiple wetlands in the landscape can differ considerably from the functions observed at individual wetland scales. This applies to provisioning of ecosystem services such as coastal protection, biodiversity support, groundwater level and soil moisture regulation, flood regulation and contaminant retention. We show that parallel and circular flow-paths, through which wetlands are interconnected in the landscape, may largely control such scale-function differences. We suggest ways forward for addressing the mismatch between the scales at which changes take place and the scale at which observations and implementation are currently made. These suggestions can help bridge gaps between researchers and engineers, which is critical for improving wetland function-effect predictability and management.
Globally, seagrass ecosystems are considered major blue carbon sinks and thus indirect contributors to climate change mitigation. Quantitative estimates and multi-scale appraisals of sources that underlie long-term storage of sedimentary carbon are vital for understanding coastal carbon dynamics. Across a tropical–subtropical coastal continuum in the Western Indian Ocean, we estimated organic (Corg) and inorganic (Ccarb) carbon stocks in seagrass sediment. Quantified levels and variability of the two carbon stocks were evaluated with regard to the relative importance of environmental attributes in terms of plant–sediment properties and landscape configuration. The explored seagrass habitats encompassed low to moderate levels of sedimentary Corg (ranging from 0.20 to 1.44% on average depending on species- and site-specific variability) but higher than unvegetated areas (ranging from 0.09 to 0.33% depending on site-specific variability), suggesting that some of the seagrass areas (at tropical Zanzibar in particular) are potentially important as carbon sinks. The amount of sedimentary inorganic carbon as carbonate (Ccarb) clearly corresponded to Corg levels, and as carbonates may represent a carbon source, this could diminish the strength of seagrass sediments as carbon sinks in the region. Partial least squares modelling indicated that variations in sedimentary Corg and Ccarb stocks in seagrass habitats were primarily predicted by sediment density (indicating a negative relationship with the content of carbon stocks) and landscape configuration (indicating a positive effect of seagrass meadow area, relative to the area of other major coastal habitats, on carbon stocks), while seagrass structural complexity also contributed, though to a lesser extent, to model performance. The findings suggest that accurate carbon sink assessments require an understanding of plant–sediment processes as well as better knowledge of how sedimentary carbon dynamics are driven by cross-habitat links and sink–source relationships in a scale-dependent landscape context, which should be a priority for carbon sink conservation.
The largest estuarine flow restoration project is the Comprehensive Everglades Restoration Plan (CERP). In 2012, one of its first phases, the C-111 Project, was initiated and designed to increase freshwater flow into northern Florida Bay. Effects of changes in flow and associated nutrients on phytoplankton abundance and composition were studied in two sets of interconnected, quasi-enclosed saline lakes at the southern end of the Everglades that discharge into northern Florida Bay. After C-111 implementation, total dissolved nitrogen (TDN) decreased, driven by especially large declines in NO 2 x. In spite of N reduction, those lakes that has been oligotrophic prior to the project experienced an approximate doubling of phytoplankton biomass as nutrient ratios converged on Redfield proportions, and a shift to smaller sized cells and a loss of diatoms as NO 2 x availability declined. The other set of these lakes, previously highly eutrophic, sustained a significant, nearly 50%, decline in overall phyto-plankton biomass, particularly larger sized cells, which, in turn, increased downstream. A chemostat model was invoked to explain these changes based on changes in flow and nutrients. In both systems the proportions of picocyanobacteria increased in the upper lakes as the relative availability of chemically reduced forms of N increased. Both lake systems may export algal biomass, changing the phytoplankton assemblage in northern Florida Bay. These lakes may also serve as buffers, damping the effects in Florida Bay itself, and they illustrate broad implications for estuaries that are impacted by natural or anthropogenic changes in flow.
Methane concentrations in the water column and emissions to the atmosphere were determined for three tropical coastal lagoons surrounded by mangrove forests on the Yucatán Peninsula, Mexico. Surface water dissolved methane was sampled at different seasons over a period of two years in areas representing a wide range of salinities and anthropogenic impacts. The highest surface water methane concentrations (up to 8,378 nM) were measured in a polluted canal associated with Terminos Lagoon. In Chelem Lagoon, methane concentrations were typically lower, except in the polluted harbor area (1,796 nM). In the relatively pristine Celestún lagoon, surface water methane concentrations ranged from 41 to 2,551 nM. Methane concentrations were negatively correlated with salinity in Celestún, while in Chelem and Terminos high methane concentrations were associated with areas of known pollution inputs, irrespective of salinity. The diffusive methane flux from surface lagoon water to the atmosphere ranged from 0.0023 to 15 mmol CH4 m–2 d–1. Flux chamber measurements revealed that direct methane release as ebullition was up to three orders of magnitude greater. Coastal mangrove lagoons may therefore be an important natural source of methane to the atmosphere despite their relatively high salinity. Pollution inputs are likely to substantially enhance this flux. Additional statistically rigorous data collected globally are needed to better consider methane fluxes from mangrove-surrounded coastal areas in response to sea level changes and anthropogenic pollution in order to refine projections of future atmospheric methane budgets.