Regular ArticleMineralization of Organic Matter in Intertidal Sediments of a Tropical Semi-enclosed Delta
Abstract
Rates and pathways of organic matter decomposition in sediments were examined during winter and summer in two mangrove forests and two accreting mud flats in Hinchinbrook Channel, north-eastern Australia. Rates of O2consumption (range: 2·8–61·0 mmol m−2day−1) and CO2release (range: 1·8–21·9 mmol m−2day−1) were faster in winter than in summer. Low respiratory quotients (CO2/O2range: 0·24–1·08) and a comparison of other metabolic pathways with total carbon oxidation (Tcox) in summer suggests that most O2was used in oxidizing reduced solutes. Rates of total carbon oxidation were greater in the mangrove sediments than in the mud flats. In winter, sulphate reduction in the mangroves (range: 4·9–8·3 mmol S m−2day−1) accounted for a much greater proportion (45–78%) of Tcoxthan in the mud flats where sulphate reduction (range: 0·8–1·0 mmol S m−2day−1) was a minor (12–14% of Tcox) metabolic pathway. In summer, sulphate reduction accounted for a greater (62–>100%) proportion of Tcoxin both mangrove (range: 7·9–10·1 mmol S m−2day−1) and mud flat (range: 0·8–2·3 mmol S m−2day−1) sediments. Rates of denitrification in summer were rapid but highly variable at all four sites (range: 2·9–6·9 mmol N2m−2 day−1). Rates of Fe and Mn reduction were slow at all four sites (range: 0·0–0·27 mmol C m−2day−1) suggesting that metal reduction was a minor decomposition pathway. No methane was detected in the porewater or released from sediments at any of the sites. There was no net microalgal primary production at any of the sites. Mineralization efficiency of organic carbon was low (9–14%) at the most sheltered mud flat, but efficiencies were greater (44–60%) and nearly equivalent at less sheltered mud flat and mangrove habitats, respectively. Imbalances of mineralized carbon between the sum of the various pathways and Tcox, particularly at the mangrove sites, suggest unaccounted losses of dissolved carbon. Large seasonal changes in porewater Cl−concentrations and lack of solute concentration differences with overlying tidal water, imply dilution and tidal advection and/or drainage of interstitial water by intense summer rains.
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Quantifying blue carbon stocks and the role of protected areas to conserve coastal wetlands
2023, Science of the Total EnvironmentVegetated coastal ecosystems, in particular mangroves, tidal marshes and seagrasses are highly efficient at sequestering and storing carbon, making them valuable assets for climate change mitigation and adaptation. The state of Queensland, in northeastern Australia, contains almost half of the total area of these blue carbon ecosystems in the country, yet there are few detailed regional or state-wide assessments of their total sedimentary organic carbon (SOC) stocks. We compiled existing SOC data and used boosted regression tree models to evaluate the influence of environmental variables in explaining the variability in SOC stocks, and to produce spatially explicit blue carbon estimates. The final models explained 75 % (for mangroves and tidal marshes) and 65 % (for seagrasses) of the variability in SOC stocks. Total SOC stocks in the state of Queensland were estimated at 569 ± 98 Tg C (173 ± 32 Tg C, 232 ± 50 Tg C, and 164 ± 16 Tg C from mangroves, tidal marshes and seagrasses, respectively). Regional predictions for each of Queensland's eleven Natural Resource Management regions revealed that 60 % of the state's SOC stocks occurred within three regions (Cape York, Torres Strait and Southern Gulf Natural Resource Management regions) due to a combination of high values of SOC stocks and large areas of coastal wetlands. Protected areas in Queensland play an important role in conserving SOC assets in Queensland's coastal wetlands. For example, ~19 Tg C within terrestrial protected areas, ~27 Tg C within marine protected areas and ~ 40 Tg C within areas of matters of State Environmental Significance. Using multi-decadal (1987–2020) mapped distributions of mangroves in Queensland; we found that mangrove area increased by approximately 30,000 ha from 1987 to 2020, which led to temporal fluctuations in mangrove plant and SOC stocks. We estimated that plant stocks decreased from ~45 Tg C in 1987 to ~34.2 Tg C in 2020, while SOC stocks remained relatively constant from ~107.9 Tg C in 1987 to 108.0 Tg C in 2020. Considering the level of current protection, emissions from mangrove deforestation are potentially very low; therefore, representing minor opportunities for mangrove blue carbon projects in the region. Our study provides much needed information on current trends in carbon stocks and their conservation in Queensland's coastal wetlands, while also contributing to guide future management actions, including blue carbon restoration projects.
Linking eutrophication to carbon dioxide and methane emissions from exposed mangrove soils along an urban gradient
2022, Science of the Total EnvironmentCitation Excerpt :Concerning n-alkanes, the CPI index higher than 1.0 in all studied mangrove soils, confirmed a large contribution of biogenic sources rather than petrogenic sources (Kumar et al., 2019). In particular, LMW:HMW ratio lower than 1.0 and TAR values above 1 at all sampling sites confirm a dominance of high molecular OM weight from mangrove trees (Gearing et al., 1976; Wang et al., 2006), as previously reported in mangroves globally (Alongi et al., 1999; Kristensen et al., 2008a). Another proxy of OM sources here was the overall predominance of long and odd chain of n-alkanes (Cn25–31), still indicating a large contribution from mangrove tree leaves (Eglinton et al., 1962; Nishigima et al., 2001).
Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH4) and carbon dioxide (CO2), are still poorly understood. Here, we aim to evaluate the relationships between CO2 and CH4 efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO2 and CH4 fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δ13C and δ15N), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δ15N (proxy of anthropogenic N enrichment), less negative δ13C, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO2 efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m−2 h−1 from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO2 fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH4 emissions were relatively low (three orders of magnitude lower than CO2 fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.
Degraded mangroves as sources of trace elements to aquatic environments
2022, Marine Pollution BulletinMangrove forests have been reported as sinks for metals because of the immobilization of these elements in their soils. However, climate change may alter the functioning of these ecosystems. We aimed to assess the geochemical dynamics of Mn, Cu, and Zn in the soils of a mangrove forest dead by an extreme weather event in southeastern Brazil. Soil samples were collected from dead and live mangroves adjacent to each other. The physicochemical parameters (total organic carbon, redox potential, and pH), total metal content, particle size, and metal partitioning were determined. Distinct changes in the soil geochemical environment (establishment of suboxic conditions) and a considerable loss of fine particles was caused by the death of the mangroves. Our results also showed a loss of up to 93 % of metals from soil. This study highlights the paradoxical role of mangroves as potential metal sources in the face of climate change.
Litho-climatic characteristics and its control over mangrove soil geochemistry: A macro-scale approach
2022, Science of the Total EnvironmentBrazil hosts an extensive coastal area, marked by a great diversity of geoenvironments. The present study evaluated the role of geoclimatic factors in the geochemistry of mangrove soils by using wet extractions and several physical and chemical parameters. Soil samples were collected in 11 mangrove forests from NE (n = 94) and SE Brazil (n = 230). Our results show an important effect of the surrounding geology and climate on the geochemistry of the mangrove soils. NE mangroves are dominated by suboxic soils (mean: Eh of +150 ± 174 mV and pH 7.1 ± 0.5, respectively) while anoxic conditions prevail in the SE mangrove soils (mean: Eh −46 ± 251 mV and pH 6.5 ± 0.5). In the NE region, a period of several months without rainfall and high temperatures leads to soil suboxic conditions. Conversely, at the SE coast, the surrounding mountain range contributes to well-distributed rain favoring anoxic conditions. The contrasting geochemical environment caused differences in the geochemistry of elements such as C, Fe, and S. Significantly higher Fe (193 ± 24 μmol g−1) and organic carbon contents (6.9 ± 7.1%) were recorded in the SE coast. The higher organic carbon contents are possibly related to Fe organo-mineral associations. These differences are ultimately associated with the contrasting geological surroundings (crystalline massifs at the SE and the iron poor sedimentary formations at the NE). The higher contents of reactive Fe and organic carbon also triggered more intense pyritization in the SE mangroves (pyritic Fe: 93 ± 63 μmol g−1). Our results demonstrate that climate and geological surroundings create identifiable patterns at a regional level and, thus, studies should take these factors into account on future global modelling approaches.
Seasonal changes in metal and nutrient fluxes across the sediment-water interface in tropical mangrove creeks in the Amazon region
2022, Applied GeochemistryMangrove creeks are considered important routes between terrestrial and adjacent coastal waters regarding the transport of dissolved material to oceans. The present study assessed if Amazonian seasonal rainfall patterns affect the pore water biogeochemistry and the intensity and direction of nutrient (NH4+ and PO43−) and metal (Fe2+ and Mn2+) exchanges from intertidal creek mudflats fringed by pristine mangroves. The results indicate that mangrove-fringed mudflats are effective in retaining iron and nutrients in solid sediment phases compared to export to coastal waters, also potentially comprising a significant manganese contributor to coastal waters. However, nutrient and metal retention are lower during the wet season, as intense rainfall periods reduce pore water salinity and promote increased reducing sediment conditions. Such conditions enhance organic matter degradation and pore water NH4+, PO43−, Fe2+ and Mn2+ concentrations just below the sediment-water interface, generating higher effluxes during this period. Our findings demonstrate that seasonal variabilities drive substantial physicochemical property and pore water biogeochemistry changes, affecting the efficiency of mudflat sediments retaining and exporting nutrients and metals.
Effects of diverse mangrove management practices on forest structure, carbon dynamics and sedimentation in North Sumatra, Indonesia
2021, Estuarine, Coastal and Shelf ScienceFor decades, mangrove forests have been under tremendous pressure due to deforestation and conversion. To sustainably manage the mangroves that remain, an ecosystem approach to management is essential. Two different management regimes – conservation and restoration – were assessed, looking at their respective effects on forest structure and carbon cycling capacity, when compared with degraded mangrove. We found that mangrove restoration enhanced tree density, while mangrove conservation was able to maintain species diversity. In terms of carbon budgets, aboveground carbon was lower in restored mangrove (79.40 ± 37.41 Mg C ha−1) when compared with conserved mangrove (92.26 ± 22.65 Mg C ha−1), but was almost double that found in degraded mangrove (39.89 ± 27.49 Mg C ha−1). Although conserved mangrove had higher aboveground carbon, lower amounts of soil carbon were found in conserved mangrove (127.49 ± 33.21 Mg C ha−1) than in restored and degraded mangrove (236.26 ± 20.33 Mg C ha−1 and 139.17 ± 25.44 Mg C ha−1, respectively). The elevation change was highest in degraded mangrove (41.7 ± 24.0 mm yr−1), followed by restored (20.7 ± 14.6 mm yr−1) and conserved mangrove (12.2 ± 3.9 mm yr−1). Carbon burial in conserved mangrove (1.20 ± 1.90 Mg C ha−2 yr−1) was double that of degraded mangrove (0.63 ± 0.60 Mg C ha−2 yr−1). Ultimately, we conclude that although a conserved mangrove is not always the end result of mangrove restoration and sustainable management, finding balance between structural development and ecosystem function is essential to serve different objectives, including biodiversity maintenance.