Mangrove sinkholes (cenotes) of the Yucatan Peninsula, a global hotspot of carbon sequestration

Precision of mangrove sediment blue carbon estimates and the role of coring and data analysis methods

Carbon accumulation in coastal wetlands is normally assessed by extracting a sediment core and estimating its carbon content and bulk density. Because carbon content and bulk density are functionally related, the latter can be estimated gravimetrically from a section of the core or, alternatively, from the carbon content in the sample using the mixing model equation from soil science. Using sediment samples from La Paz Bay, Mexico, we analyzed the effect that the choice of corer and the method used to estimate bulk density could have on the final estimates of carbon storage in the sediments. We validated the results using a larger dataset of tropical mangroves, and then by Monte Carlo simulation. The choice of corer did not have sizable influence on the final estimates of carbon density. The main factor in selecting a corer is the operational difficulties that each corer may have in different types of sediments. Because of the multiplication of errors in a product of two variables subject to random sampling error, when using gravimetric estimates of bulk density, the dispersion of the data points in the estimation of total carbon density rises rapidly as the amount of carbon in the sediment increases. In contrast, the estimation of total carbon density using only the carbon fraction as a predictor is very precise, especially in sediments rich in organic matter. This method, however, depends critically on the accurate estimation of the two parameters of the mixing model: the bulk density of pure peat and the bulk density of pure mineral sediment. The estimation of carbon densities in peaty sediments can be very imprecise when using gravimetric bulk densities. Estimating carbon density in peaty sediments using only the estimate of organic fraction can be much more precise, provided the model parameters are estimated with accuracy. These results open the door for simplified and precise estimates of carbon dynamics in mangroves and coastal wetlands.

Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests

Mangroves store blue carbon (693 Mg CORG ha−1) disproportionate to their small area, mainly (74%) in deep soil horizons. Global stock estimates for mangroves (5.23–8.63 Pg CORG) are equivalent to 15–24% of those in the tropical coastal ocean. Carbon burial in mangrove soils averages 184 g CORG m−2 a−1 with global estimates (9.6–15.8 Tg CORG a−1) reflecting their importance in carbon sequestration. Extreme weather events result in carbon stock losses and declines in carbon cycling and export. Increased frequency and ferocity of storms result in increasingly negative responses with increasing strength. Increasing temperatures result in increases in carbon stocks and cycling up to a critical threshold, while positive/negative responses will likely result from increases/decreases in rainfall. Forest responses to sea-level rise (SLR) and rising CO2 are species- and site-specific and complex due to interactive effects with other drivers (e.g., temperature, salinity). The SLR critical threshold is ≈ 6 mm a−1 indicating survival only under very low-low CO2 emissions scenarios. Under low coastal squeeze, landward migration could result in sequestration and CO2 losses of 1.5 and −1.1 Pg C with net stock gains and losses (−0.3 to +0.5 Pg C) and CO2 losses (−3.4 Pg) under high coastal squeeze.

Role of Mangrove Rehabilitation and Protection Plans on Carbon Storage in Yanbu Industrial City, Saudi Arabia: A Case Study

Mangroves are one of the main considerations that might be used to mitigate the effects of climate change in coastal areas. Mangrove populations can be affected by industrial and civil activities on coasts. According to the Kyoto Protocol, protection and rehabilitation programs may play a pivotal role in conserving mangroves in industrial areas. Therefore, this study was designed to examine and evaluate the possible impact of conservation plans, implemented by the Royal Commission of Yanbu, in preserving mangrove trees’ ability to store carbon in the soil. Soil and plant samples were collected from three distinct locations, including a mangrove conservation site in Yanbu and natural unprotected sites in Umluj and Ar-Rayis. Organic-carbon (OC) stock, in both soil and plants, was calculated. Our results showed that at different depths, soil bulk density (SBD) in Yanbu ranged between 0.32–0.94 g cm−3. In Ar-Rayis and Umluj, SBD ranged between 1.43 to 1.99 and 0.90 to 1.57g cm−3, respectively. The average SBD values in Yanbu, Umluj, and Ar-Rayis were 0.68, 1.71, and 1.20 g cm−3, respectively. Similarly, the average soil OC density in Yanbu, Umluj, and Ar-Rayis was 165.19, 30.82, and 18.90 g C cm−3, respectively. Generally, the conserved mangrove tress grown in Yanbu industrial city showed higher (P ≤ 0.001) soil OC stock (0.39 t C ha−1) compared to the unprotected trees grown in Umluj (0.12 t C ha−1) and Ar-Rayis (0.11 t C ha−1) cities. Similarly, the highest (P ≤ 0.001) plant OC stocks (13.93 t C ha−1) were observed in protected mangroves of Yanbu, compared to the plant OC stocks observed in Umluj (8.06 t C ha−1) and Ar-Rayis (8.80 t C ha−1) cities. The results of the current study showed that the protected mangrove trees grown in Yanbu industrial city store more carbon in their sediments than those grown in the Umluj and Ar-Rayis sites without conservation or rehabilitation. These findings may provide evidence for the beneficial role of protecting mangrove forests in mitigating the effects of climate change.

Extant Earthly Microbial Mats and Microbialites as Models for Exploration of Life in Extraterrestrial Mat Worlds

As we expand the search for life beyond Earth, a water-dominated planet, we turn our eyes to other aquatic worlds. Microbial life found in Earth's many extreme habitats are considered useful analogs to life forms we are likely to find in extraterrestrial bodies of water. Modern-day benthic microbial mats inhabiting the low-oxygen, high-sulfur submerged sinkholes of temperate Lake Huron (Michigan, USA) and microbialites inhabiting the shallow, high-carbonate waters of subtropical Laguna Bacalar (Yucatan Peninsula, Mexico) serve as potential working models for exploration of extraterrestrial life. In Lake Huron, delicate mats comprising motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis and pigment-free chemosynthetic sulfur-oxidizing bacteria lie atop soft, organic-rich sediments. In Laguna Bacalar, lithification by cyanobacteria forms massive carbonate reef structures along the shoreline. Herein, we document studies of these two distinct earthly microbial mat ecosystems and ponder how similar or modified methods of study (e.g., robotics) would be applicable to prospective mat worlds in other planets and their moons (e.g., subsurface Mars and under-ice oceans of Europa). Further studies of modern-day microbial mat and microbialite ecosystems can add to the knowledge of Earth's biodiversity and guide the search for life in extraterrestrial hydrospheres.