Carbon accumulation and storage capacity in mangrove sediments three decades after deforestation within a eutrophic bay

Phosphate buffering in mangrove sediment pore water under eutrophication and deforestation influences

Phosphorus (P) behavior was evaluated in mangrove wetlands impacted by urban sewage, including a deforested site. Sediment cores were analyzed for grain size, organic carbon, total nitrogen, stable isotopes (δ13C and δ15N), P contents, and pore water PO43- concentrations and net consumption/production rates. Under stronger eutrophication influence, significantly higher P (1390 vs. <1000 μg/g), δ15N (8.9 vs. <6.7 ‰) and algal material contents (with lower C/N ratio and heavier δ13C) occurred. Depth-integrated PO43- consumption rates in eutrophicated sites were up to two orders of magnitude higher (at the deforested site) than in a moderately preserved mangrove. The whole core of the moderately preserved site presented no saturation of PO43- buffering capacity, while more eutrophicated sites developed buffering zones saturated at ∼18-26 cm depth. Contrasting to nearby subtidal environments, eutrophication did not cause larger pore water PO43- concentration, evidencing the role of PO43- buffering on P filtering by mangrove wetlands.

Blue carbon stock heterogeneity in Brazilian mangrove forests: A systematic review

This is the first systematic review and meta-analysis of blue carbon stocks in Brazilian mangroves. We evaluated the effect of characteristics and site status (impacted versus non-impacted) on carbon stocks found in the various compartments on total ecosystem carbon stock (TECS). TECS followed an inverse trend with the latitudinal position: the highest values were found on the North coast (mean 511 Mg C ha-1), followed by the Northeast and Southeast. A similar latitudinal trend was observed for sediment and above-ground biomass carbon stocks. Site status also significantly affected TECS and sediment carbon stocks. The heterogeneity observed in carbon stocks suggests that tidal regime, latitudinal position, climate, and human impacts jointly drive the processes related to sequestration and storage. Brazilian mangroves store ~0.44 PgC, representing 10-12 % of the world TECS. This highlights Brazilian mangroves as a global blue carbon hotspot, and as an efficient nature-based solution for carbon dioxide removal.

Increasing carbon, nutrient and trace metal accumulation driven by development in a mangrove estuary in south Asia

Mangrove forests sequester organic carbon, nutrients and toxic metals sorbed to fine sediment, and thus restrict the mobility of pollutants through estuarine environments. However, mangrove removal and environmental degradation caused by industrial activity and urban growth can impact the ability of mangrove communities to provide these critical ecosystem services. Here, we use sediment profiles from an impacted tropical estuary in southwest India to provide a c. 70-year record of carbon, nutrient and trace metal burial in the context of rapid urban development and the systemic removal of mangrove communities. Our results show that carbon and nutrient accumulation rates increase sharply during the 1990's in accordance with the high rates of deforestation. Nitrogen and phosphorus accumulation rates increased fourfold and twofold, respectively, during the same period. Organic carbon accumulation was fivefold higher than the global average during this period, reflecting intense deforestation during the last three decades. The enrichment of Hg, Zn, Pb, Mo, Ni, Cu and Mn demonstrate clear anthropogenic impact starting in the 1950's and peaking in 1990. Mercury, the trace metal with the highest enrichment factor, increased sevenfold in the most recent sediments due to increased fossil fuel emissions, untreated water and incineration of medical waste and/or fertilizers used in aquaculture. Organic carbon isotope (δ¹³C) and C:N molar ratios indicate shifts to more terrestrial-derived source of organic matter in the most recent sediments reflecting growing deforestation of which may be prevalent in southeast Asia due to increasing development. This study emphasizes the critical role played by mangrove ecosystems in attenuating anthropogenically-derived pollutants, including carbon sequestration, and reveals the long-term consequences of mangrove deforestation in the context of rapidly developing economies.

Assessment of the temporal retention of mercury and nutrient records within the mangrove sediments of a highly impacted estuary

Mangrove ecosystems are dynamic and biodiverse environments with the capacity to sequester more organic carbon per unit area, per time, than terrestrial forests, yet are among one of the most heavily degraded ecosystems on Earth. Here, we quantify trace metal, nutrient and carbon accumulation rates in a tropical mangrove environment in northeast Brazil, a region that has been rapidly developed over the past seven decades. Carbon accumulation rate results show modest or no increase since the 1950's, when major development occurred in the region. Organic carbon isotope (δ¹³C) and C:N molar ratios indicate that the OM is primarily derived from autochthonous C₃ plant sources. However, the most recent sediments revealed changes from terrestrial to alga-derived source of OM, which is consistent with the increase of total nitrogen, δ¹⁵N and total phosphorous content in the last seven decades, suggesting anthropogenic impact. Furthermore, the Hg enrichment factor (EF) in mangrove sediments is shown to have increased 13-fold since the 1960's, highlighting the ability of tropical mangrove systems in trap filtering pollutants from proximal urban development.

Anthropogenic and environmental influences on nutrient accumulation in mangrove sediments

Here we provide a global review on nutrient accumulation rates in mangroves which were derived from sixty-nine dated sediment cores, addressing environmental and anthropogenic influences. Conserved mangroves presented nitrogen and phosphorous accumulation rates near to 5.8 ± 2.1 and 0.8 ± 0.5 g m^-2 yr^-1, respectively. These values were significantly lower than those observed for mangroves impacted by coastal eutrophication, which were found to bury 21.5 ± 8.6 and 17.9 ± 2.4 g m-2 yr^-1, of nitrogen and phosphorous respectively. Moreover, higher nutrient accumulation rates were found in mixed mangroves as compared to monospecific forests, and higher values were noted within vegetated areas as compared to mudflats. For South America and Asia, mangroves impacted by anthropogenic activities may result in up to seventeen-fold higher nitrogen and phosphorous accumulation rates in comparison with values under conserved conditions. For Oceania, these differences may be up to fivefold higher in impacted as compared to the conserved ecosystems in this region.

Trajectory of coastal wetland vegetation in Xiangshan Bay, China, from image time series

Coastal wetland vegetation is crucial for providing multiple ecosystem services. However, accurate assessment of wetland vegetation is problematic due to the challenging coastal environment. Using Xiangshan Bay (XB) in China as a typical case study, we developed a time series biological phenological approach to classifying coastal wetland vegetation using Landsat time-series images from 1984 to 2018. The results demonstrate that the total vegetation area of coastal wetlands in XB in 2018 was ~85.3 km². The interannual dynamics of coastal wetland vegetation area in XB in the last 35 years can be divided into three periods: increasing volatility (1984-1998), decreasing (1999-2004), and increasing volatility (2005-2018). Our results emphasize the potential of the use of the time-series biological phenological approach for monitoring coastal wetland vegetation, which can contribute to the sustainable management of coastal ecosystems.

Nitrogen mineralization and eutrophication risks in mangroves receiving shrimp farming effluents

Nitrogen (N) inputs originated from shrimp farming effluents were evaluated for potential changes in the net N mineralization for mangrove soils from Northeastern Brazil. Our study provides notable information and assessment for the potential enhancement of N mineralization in preserved and shrimp-impacted semi-arid mangrove soils of the Jaguaribe River estuary, which is one of the largest shrimp producers of Brazil, using an analytical and daily tidal variation experimental approach. Nitrogen-rich effluents promoted a significant (p value < 0.001) increase of the total soil N content (1998 ± 201 mg kg^-1 on average) compared with the preserved sites (average: 1446 ± 295 mg kg^-1). The effluents also increased the N mineralization in the shrimp-impacted sites (N-min: 86.6 ± 37.5 mg kg^-1), when compared with preserved mangroves (N-min: 56.5 ± 23.8 mg kg^-1). Over a daily tidal variation experiment, we found that just 30% (36.2 ± 20.6 mg kg^-1) of mineralized N remains stored in the soil, whereas 70% (102.9 ± 38.8 mg kg^-1) was solubilized in tidal waters. Therefore, the N mineralization process may trigger eutrophication by increasing N inorganic bioavailability in mangrove soils receiving N-rich effluents from shrimp ponds, which in turn might increase primary producers' activity. This approach has not been studied so far in semi-arid mangroves, where the shrimp farming activity is one of the most important economic activities.

Global Significance of Mangrove Blue Carbon in Climate Change Mitigation

Mangrove forests store and sequester large area-specific quantities of blue carbon (Corg). Except for tundra and peatlands, mangroves store more Corg per unit area than any other ecosystem. Mean mangrove Corg stock is 738.9 Mg Corg ha−1 and mean global stock is 6.17 Pg Corg, which equates to only 0.4–7% of terrestrial ecosystem Corg stocks but 17% of total tropical marine Corg stocks. Per unit area, mangroves sequester 179.6 g Corg m−2a−1 and globally about 15 Tg Corg a−1. Mangroves sequester only 4% (range 1.3–8%) of Corg sequestered by terrestrial ecosystems, indicating that mangroves are a minor contributor to global C storage and sequestration. CO2 emissions from mangrove losses equate to 0.036 Pg CO2-equivalents a−1 based on rates of C sequestration but 0.088 Pg CO2-equivalents a−1 based on complete destruction for conversion to aquaculture and agriculture. Mangrove CO2 emissions account for only 0.2% of total global CO2 emissions but 18% of CO2 emissions from the tropical coastal ocean. Despite significant data limitations, the role of mangrove ecosystems in climate change mitigation is small at the global scale but more significant in the tropical coastal ocean and effective at the national and regional scale, especially in areas with high rates of deforestation and destruction.

Global Significance of Mangrove Blue Carbon in Climate Change Mitigation (Version 1)

Mangrove forests store and sequester large area-specific quantities of blue carbon (Corg). Except for tundra and peatlands, mangroves store more Corg per unit area than any other ecosystem. Mean mangrove Corg stock is 738.9 Mg Corg ha−1 and mean global stock is 6.17 Pg Corg, which equates to only 0.4–7% of terrestrial ecosystem Corg stocks but 17% of total tropical marine Corg stocks. Seagrasses sequester more Corg per unit area than mangroves (179.6 g Corg m−2·a−1) but twice the Corg sequestered by mangroves globally (15 Tg Corg a−1). Mangroves sequester only 4% (range 1.3–8%) of Corg sequestered by terrestrial ecosystems, indicating that mangroves are a minor contributor to global C storage and sequestration. CO2 emissions from mangrove losses equate to 0.036 Pg CO2-equivalents a−1 based on rates of C sequestration but 0.088 Pg CO2-equivalents a−1 based on complete destruction for conversion to aquaculture and agriculture. Mangrove CO2 emissions account for only 0.2% of total global CO2 emissions but 18% of CO2 emissions from the tropical coastal ocean. Despite significant data limitations, the role of mangrove ecosystems in climate change mitigation is globally insignificant but may be more significant and effective at the national and regional scale.

Factors influencing organic carbon accumulation in mangrove ecosystems

There is growing interest in the capacity of mangrove ecosystems to sequester and store 'blue carbon'. Here, we provide a synthesis of 66 dated sediment cores with previously calculated carbon accumulation rates in mangrove ecosystems to assess the effects of environmental and anthropogenic pressures. Conserved sedimentary environments were found to be within the range of the current global average for sediment accretion (approx. 2.5 mm yr-1) and carbon accumulation (approx. 160 g m-2 yr-1). Moreover, similar sediment accretion and carbon accumulation rates were found between mixed and monotypic mangrove forests, however higher mean and median values were noted from within the forest as compared to adjacent areas such as mudflats. The carbon accumulation within conserved environments was up to fourfold higher than in degraded or deforested environments but threefold lower than those impacted by domestic or aquaculture effluents (more than 900 g m-2 yr-1) and twofold lower than those impacted by storms and flooding (more than 500 g m-2 yr-1). These results suggest that depending on the type of impact, the blue carbon accumulation capacity of mangrove ecosystems may become substantially modified.