Corg Stocks Research Articles

Seagrass losses since mid-20th century fuelled CO2 emissions from soil carbon stocks.

Seagrass meadows store globally significant organic carbon (Corg) stocks which, if disturbed, can lead to CO2 emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO2 emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil Corg erosion following eutrophication‐driven seagrass loss in Cockburn Sound (23 km2 between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher Corg stocks in 50 cm thick soils (4.5 ± 0.7 kg Corg/m2) than previously vegetated counterparts (0.5 ± 0.1 kg Corg/m2). In deeper areas (>5 m), however, soil Corg stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg Corg/m2, respectively). The soil Corg sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g Corg m−2 year−1, respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil Corg but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil Corg erosion. We estimate, from field studies and satellite imagery, that soil Corg erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06–0.14 Tg CO2‐eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11–21 Tg CO2 ‐eq since the 1950s, increasing cumulative CO2 emissions from land‐use change in Australia by 1.1%–2.3% per annum. The patterns described serve as a baseline to estimate potential CO2 emissions following disturbance of seagrass meadows.

Temporal trends of organic carbon accumulation in seagrass meadows from the northern Mexican Caribbean

Carbon sequestration in seagrass meadows mitigates the currently rising CO2 atmospheric levels, as these ecosystems are highly productive and preserve sediment organic carbon over long periods. Five 210Pb dated sediment cores, collected from seagrass meadows dominated by Thalassia testudinum in the northern Mexican Caribbean touristic corridor Cancun-Riviera Maya, were used to establish temporal trends of organic carbon (Corg) content, burial rates and stock variation over the past 100 years. Corg contents (0.17–1.94%) and burial rates (2.0–252 g m−2 yr−1) were generally within the lower end range reported for seagrass meadow sediments worldwide. Corg stock gradually increased over the past decades, associated with increasing sediment accumulation, possibly caused by inland erosion promoted by agricultural and forestry activities and, in the recent decades, by a rapid increase of urban development due to a fast growth of the tourist industry and population, likely causing fertilization of coastal waters. Seagrass meadow sediments in the northern Mexican Caribbean coastline showed long-term Corg storage and preservation, since ~ 40 to ~ 100 years depending on the core, which emphasizes the need to protect these ecosystems against impacts of global change, and their incorporation into climate change mitigation strategies should be considered.

Soil Carbon Stocks Vary Across Geomorphic Settings in Australian Temperate Tidal Marsh Ecosystems

Tidal marshes rank among the ecosystems with the highest capacity to sequester and store organic carbon (Corg) on earth. To inform conservation of coastal vegetated ecosystems for climate change mitigation, this study investigated the factors driving variability in carbon storage. We estimated soil Corg stocks in tidal marshes across temperate Western Australia and assessed differences among geomorphic settings (marine and fluvial deltas, and mid-estuary) and vegetation type (Sarcocornia quinqueflora and Juncus kraussii) linked to soil biogeochemistry. Soil Corg stocks within fluvial and mid-estuary settings were significantly higher (209 ± 14 and 211 ± 20 Mg Corg ha−1, respectively; 1-m-thick soils) than in marine counterparts (156 ± 12 Mg Corg ha−1), which can be partially explained by higher preservation of soil Corg in fluvial and mid-estuary settings rich in fine-grained (< 0.063 mm) sediments (49 ± 3% and 47 ± 4%, respectively) compared to marine settings (23 ± 4%). Soil Corg stocks were not significantly different between S. quinqueflora and J. kraussii marshes (185 ± 13 and 202 ± 13 Mg Corg ha−1, respectively). The higher contribution of tidal marsh plus supratidal vegetation in fluvial (80%) and intermediate (76%) compared to marine (57%) settings further explains differences in soil Corg stocks. The estimated soil Corg stocks in temperate Western Australia’s tidal marshes (57 Tg Corg within ~ 3000 km2 extent) correspond to about 2% of worldwide tidal marsh soil Corg stocks. The results obtained identify global drivers of soil Corg storage in tidal marshes and can be used to target hot spots for climate change mitigation based on tidal marsh conservation.

Factors Influencing Carbon Stocks and Accumulation Rates in Eelgrass Meadows Across New England, USA

Increasing the protection of coastal vegetated ecosystems has been suggested as one strategy to compensate for increasing carbon dioxide (CO2) in the atmosphere as the capacity of these habitats to sequester and store carbon exceeds that of terrestrial habitats. Seagrasses are a group of foundation species that grow in shallow coastal and estuarine systems and have an exceptional ability to sequester and store large quantities of carbon in biomass and, particularly, in sediments. However, carbon stocks (Corg stocks) and carbon accumulation rates (Corg accumulation) in seagrass meadows are highly variable both spatially and temporally, making it difficult to extrapolate this strategy to areas where information is lacking. In this study, Corg stocks and Corg accumulation were determined at 11 eelgrass meadows across New England, representing a range of eutrophication and exposure conditions. In addition, the environmental factors and structural characteristics of meadows related to variation in Corg stocks were identified. The objectives were accomplished by assessing stable isotopes of δ13C and δ15N as well as %C and %N in plant tissues and sediments, measuring grain size and 210Pb of sediment cores, and through assessing site exposure. Variability in Corg stocks in seagrass meadows is well predicted using commonly measured environmental variables such as grain size distribution. This study allows incorporation of data and insights for the northwest Atlantic, where few studies on carbon sequestration by seagrasses have been conducted.

Temporal records of organic carbon stocks and burial rates in Mexican blue carbon coastal ecosystems throughout the Anthropocene

Blue carbon (BC) ecosystems provide important and multiple ecosystem services, including climate change mitigation through carbon storage. However, these ecosystems are under the pressure of numerous anthropogenic stressors, such as population growth and industrialization, which jeopardize the benefits they provide. Although the common methodology is based on the use of a linear depth scale, this work highlights the need of sediment dating to evaluate temporal variation of organic carbon (Corg) stocks and burial rates, since sedimentation patterns can be variable even between sites within a single ecosystem. Temporal records of Corg stocks and burial rates during the past 100 years were determined in mangrove, seagrass and salt marsh ecosystems, from five representative areas of the Mexican marine coastal systems, by using 210Pb-dated sediment cores. Carbon stocks during the past 100 years (Corg stock(100 years)) ranged from 67 to 269 Mg Corg ha−1 in mangroves, 8–53 Mg Corg ha−1 in seagrasses and 32–56 Mg Corg ha−1 in salt marshes, and burial rates were 8–426 g m−2 yr−1, 2–252 g m−2 yr−1 and 1–60 g m−2 yr−1, respectively. Most of the stored Corg (60–86%) accumulated during the Anthropocene (since 1950), and in mangrove cores recent Corg stocks were up to six times higher than before the Anthropocene onset (1900–1950), most likely as a result of land use changes. Increased Corg burial rates in seagrass ecosystems were mostly driven by mass accumulation rate changes (promoted by catchment erosion and sea level rise), and in mangroves and salt marshes by changes in Corg content, likely promoted by an increase in productivity associated with nutrient enrichment by fertilizer runoff and sewage disposal. Despite the multiple anthropogenic stressors that positively affected Corg stocks and burial rates in BC ecosystems, worldwide destruction of these valuable habitats will cause losses of long-term buried Corg through significant CO2 releases, thus conservation and/or restoration are effective actions for climate change mitigation.

Increasing salinization and organic carbon burial rates in seagrass meadows from an anthropogenically-modified coastal lagoon in southern Gulf of Mexico

Intertidal ecosystems, such as marshes, mangroves, and seagrasses, contribute to the global carbon storage, helping to regulate atmospheric CO2 concentrations. Organic carbon (Corg) data from seagrass meadows in the Mexican coastline are still scarce, preventing appropriate evaluations of its contribution to mitigate global climate change. Recent temporal changes (≤100 years) of Corg concentrations and burial rates were assessed in 210Pb dated cores from seagrass meadows at Terminos Lagoon, the largest coastal lagoon in the southern Gulf of Mexico. Irrespective of the predominant seagrass species, the cores showed comparable Corg concentrations (0.04–3.9%) and burial rates (2.5 – 66.7 g m−2 yr−1). Increasing salinization, likely caused by sea level rise, favoured Corg concentrations and burial rates in sediments with Halodule wrightii, while in sediments with Thalassia testudinum only Corg burial rates increased, owing to increasing mass accumulation rates, likely promoted by erosion induced by land use changes, and sea level rise. The mean Corg stock (1 m depth; 130 ± 29 Mg ha−1) was comparable with the global average Corg stock for seagrasses in other tropical areas around the world. Our results suggested that, under the current scenario of land use change and marine transgression, the seagrass meadows in Terminos Lagoon preserve the capacity for Corg accumulation and long-term preservation. However, both factors are also known to reduce seagrass resilience caused by water quality deterioration, thus these ecosystems must be monitored and protected from global change impacts, as part of climate change mitigation efforts.

The greenhouse gas offset potential from seagrass restoration

Awarding CO2 offset credits may incentivize seagrass restoration projects and help reverse greenhouse gas (GHG) emissions from global seagrass loss. However, no study has quantified net GHG removal from the atmosphere from a seagrass restoration project, which would require coupled Corg stock and GHG flux enhancement measurements, or determined whether the creditable offset benefit can finance the restoration. We measured all of the necessary GHG accounting parameters in the 7-km2Zostera marina (eelgrass) meadow in Virginia, U.S.A., part of the largest, most cost-effective meadow restoration to date, to provide the first seagrass offset finance test-of-concept. Restoring seagrass removed 9,600 tCO2 from the atmosphere over 15 years but also enhanced both CH4 and N2O production, releasing 950 tCO2e. Despite tripling the N2O flux to 0.06 g m−2 yr−1 and increasing CH4 8-fold to 0.8 g m−2 yr−1, the meadow now offsets 0.42 tCO2e ha−1 yr−1, which is roughly equivalent to the seagrass sequestration rate for GHG inventory accounting but lower than the rates for temperate and tropical forests. The financial benefit for this highly successful project, $87 K at $10 MtCO2e−1, defrays ~10% of the restoration cost. Managers should also consider seagrass co-benefits, which provide additional incentives for seagrass restoration.

Blue carbon of Mexico, carbon stocks and fluxes: a systematic review.

Mexico has more than 750,000 ha of mangroves and more than 400,000 ha of seagrasses. However, approximately 200,000 ha of mangroves and an unknown area of seagrass have been lost due to coastal development associated with urban, industrial and tourist purposes. In 2018, the approved reforms to the General Law on Climate Change (LGCC) aligned the Mexican law with the international objectives established in the 2nd Article of the Paris Agreement. This action proves Mexico’s commitment to contributing to the global target of stabilizing the greenhouse gas emissions concentration in the planet. Thus, restoring and conserving mangrove and seagrass habitats could contribute to fulfilling this commitment. Therefore, as a first step in establishing a mitigation and adaptation plan against climate change with respect to conservation and restoration actions of these ecosystems, we evaluated Mexican blue carbon ecosystems through a systematic review of the carbon stock using the standardized method of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). We used the data from 126 eligible studies for both ecosystems (n = 1220). The results indicated that information is missing at the regional level. However, the average above and below ground organic carbon stocks from mangroves in Mexico is 113.6 ± 5.5 (95% CI [99.3–118.4]) Mg Corg ha−1 and 385.1 ± 22 (95% CI [344.5–431.9]) Mg Corg ha−1, respectively. The variability in the Corg stocks for both blue carbon ecosystems in Mexico is related to variations in climate, hydrology and geomorphology observed along the country’s coasts in addition to the size and number of plots evaluated with respect to the spatial cover. The highest values for mangroves were related to humid climate conditions, although in the case of seagrasses, they were related to low levels of hydrodynamic stress. Based on the official extent of mangrove and seagrass area in Mexico, we estimate a total carbon stock of 237.7 Tg Corg from mangroves and 48.1 Tg Corg from seagrasses. However, mangroves and seagrasses are still being lost due to land use change despite Mexican laws meant to incorporate environmental compensation. Such losses are largely due to loopholes in the legal framework that dilute the laws’ effectiveness and thus ability to protect the ecosystem. The estimated emissions from land use change under a conservative approach in mangroves of Mexico were approximately 24 Tg CO2e in the last 20 years. Therefore, the incorporation of blue carbon into the carbon market as a viable source of supplemental finance for mangrove and seagrass protection is an attractive win-win opportunity.

Improving soil carbon estimates of mudflats in Araçá Bay using spatial models that consider riverine input, wave exposure and biogeochemistry

A persistent challenge to managing coastal wetlands in a way that will mitigate climate impacts is understanding how soil organic carbon (Corg) stocks change over spatial and temporal scales. This study tested how seasonal change in riverine flows and soil biogeochemical attributes (grain-size, degree of wave disturbance and microphytobenthic productivity) can influence the Corg stock estimates of an unvegetated tidal flat within a Brazilian subtropical bay. We used a Generalized Additive Modelling (GAM) approach to accurately predict and map subtle changes in the Corg pools of these expansive, but often overlooked coastal ecosystems. The most important predictors across seasons were sediment grain-size and wave exposure. Seasonally important covariates were riverine flow and microphytobenthic productivity (during the warm and wet summer season). We then compared the high-resolution map outputs derived from models with more traditional approaches (interpolation and extrapolation) to see how these different methods might influence estimates of the carbon stocks for the entire bay (i.e., an area covering 115 ha). Modelled estimates for the bay (1945 ± 45 Mg Corg) where more conservative and had lower associated error than interpolated (2196 ± 185 Mg) and extrapolated estimates (2535 ± 117 Mg). Given the current urgency to understand the carbon sequestration potential of coastal ecosystems, the present findings are important and draw attention to the often overlooked, but important carbon pools of intertidal mudflats.

Estuarine submerged aquatic vegetation habitat provides organic carbon storage across a shifting landscape

Submerged aquatic vegetation (SAV) thrives across the estuarine salinity gradient providing valuable ecosystem services. Within the saline portion of estuaries, seagrass areas are frequently cited as hotspots for their role in capturing and retaining organic carbon (Corg). Non-seagrass SAV, located in the fresh to brackish estuarine areas, may also retain significant soil Corg, yet their role remains unquantified. Given rapidly occurring landscape and salinity changes due to human and natural disturbances, landscape level carbon pool estimates from estuarine SAV habitat blue carbon estimates are needed. We assessed Corg stocks in SAV habitat soils from estuarine freshwater to saline habitats (interior deltaic) to saline barrier islands (Chandeleur Island) within the Mississippi River Delta Plain (MRDP), Louisiana, USA. SAV habitats contain Corg stocks equivalent to those reported for other estuarine vegetation types (seagrass, salt marsh, mangrove). Interior deltaic SAV Corg stocks (231.6 ± 19.5 Mg Corg ha−1) were similar across the salinity gradient, and significantly higher than at barrier island sites (56.6 ± 10.4 Mg Corg ha−1). Within the MRDP, shallow water SAV habitat covers up to an estimated 28,000 ha, indicating that soil Corg storage is potentially 6.4 ± 0.1 Tg representing an unaccounted Corg pool. Extrapolated across Louisiana, and the Gulf of Mexico, this represents a major unaccounted pool of soil Corg. As marshes continue to erode, the ability of coastal SAV habitat to offset some of the lost carbon sequestration may be valuable. Our estimates of Corg sequestration rates indicated that conversion of eroding marsh to potential SAV habitat may help to offset the reduction of Corg sequestration rates. Across Louisiana, we estimated SAV to offset this loss by as much as 79,000 Mg C yr−1 between the 1960s and 2000s.

Contribution of Seagrass Blue Carbon Toward Carbon Neutral Policies in a Touristic and Environmentally-Friendly Island

Estimates of organic carbon (Corg) storage by seagrass meadows which consider inter-habitat variability are essential to understand their potential to sequester carbon dioxide (CO2) and derive robust global and regional estimates of blue carbon storage. In this study, we provide baseline estimates of seagrass extent, and soil Corg stocks and accumulation rates from different seagrass habitats at Rottnest Island (in Amphibolis spp., Posidonia spp., Halophila ovalis and mixed Posidonia/Amphibolis spp. meadows). The Corg stocks in 0.5 m thick seagrass soil deposits, derived from 24 cores, were 5.1+-0.7 kg Corg m-2 (mean+-S.E, ranging from 0.05 to 12.9 kg Corg m-2), accumulating at 23.2+-3.2 g Corg m-2 yr-1 (ranging from 0.22 to 58.9 g Corg m-2 yr-1) over the last decades. There were significant differences in Corg content (%) and stocks (mg Corg cm-3), stable carbon isotope composition of the soil organic matter (d13C) and soil grain size among the seagrass meadows studied, highlighting that biotic and abiotic factors influence seagrass soil Corg storage. Mixed meadows of Posidonia/Amphibolis spp. and monospecific meadows of Posidonia spp. and Amphibolis spp. had the highest Corg stocks (ranging from 6.2 to 6.4 kg Corg m-2), while Halophila spp. meadows had the lowest Corg stocks (1.2+-0.6 kg Corg m-2). We estimated a total soil Corg stock of 48.1±8.5 Gg Corg beneath the 755 ha of Rottnest Island’s seagrasses, and a Corg sequestration capacity of 0.81+-0.06 Gg Corg yr-1, which is equivalent to the sequestration of ~22% of the island’s current annual CO2 emissions. Our results contribute to the existing global dataset on seagrass soil Corg storage and show a significant potential of seagrass to sequester CO2, which are particularly relevant in the context of achieving carbon neutrality through conservation actions in environmentally-marketed, tourist destinations such as Rottnest Island.

Changes in the stocks of soil organic carbon, total nitrogen and phosphorus following afforestation of post-arable soils: A chronosequence study

The objective of this study was to investigate the effect of afforestation of post-arable sandy soils (Dystric Arenosols) with Scots pine (Pinus sylvestris L.) on the amount and distribution of Corg, Ntot and Ptot stocks between genetic soil horizons. The study was performed at three locations with five classes of afforestation each: 10-, 20-, 30-, 40-, 50-year-old stands, which additionally included the arable and the continuous forest soils as reference. The soil was sampled by genetic horizon, including the organic one, down to 100 cm, from its whole thickness, and from A horizon of the afforested soils from: 0–5, 5–10 and 10–20 cm. In the organic horizon of the afforested soils, the stock of Corg, Ntot and Ptot increased significantly with stand age, and an average rate of accumulation was 33.6, 1.30 and 0.04 g m−2 year−1 respectively. Corg stocks in the former plough layer were found to decrease within the first decade of afforestation, and subsequently, the values gradually increased. With organic horizon included the respective stocks reached the level comparable with the related horizon of the arable soil after approx. 20–30 years. Subsoil B, BC and C horizons together accounted for approx. 35% of the Corg stocks in the entire profile. Ntot stocks in mineral soil horizons initially dropped in the first decade after afforestation, which was subsequently followed by an increase; however, after 50 years, still the observed values were lower in comparison with both the respective arable and continuous forest soils. Ptot stocks in the mineral soil horizon declined over the chronosequence, and in the profile at 50-year-old stands, they were lower in comparison with both the arable and the continuous forest soils. Stand age and sampling by genetic horizons, including the organic horizon, from the entire soil profile should be considered for estimation of changes in Corg, Ntot and Ptot stocks following afforestation of agricultural soils.

The Sources of Organic Matter in Seagrass Sediments and Their Contribution to Carbon Stocks in the Spermonde Islands, Indonesia

Seagrass ecosystems have a potential role in climate change mitigation due to their ability to store high amount of carbon, particularly in the sediment. Studying the factors and mechanisms responsible for this storing capacity is essential to understand seagrass carbon sink function. Therefore, in this study, we identified the sources of organic carbon (Corg) in seagrass sediments and the implication to Corg stocks from four islands in the Spermonde Islands that located at different zones. We used the Bayesian stable isotope mixing model to estimate the proportional contribution of different sources to sediment carbon. Seagrass meadows that located in adjacent to high anthropogenic activities (deforestation and aquacultures) with direct exposure to wave actions, such as on the Bauluang Island, accumulated organic carbon that derived from multiple sources, where phytoplankton contributed the highest, while on the other three islands that are relatively protected from wave actions, the highest contribution (~ 75%) was from autochthonous production (seagrass-derived). Sediment Corg stocks vary spatially, ranging from 11.9 to 32.1 Mg C ha−1 (based on the obtained depth of 20–55 cm), or 40.5 to 83.5 Mg C ha−1 if extrapolated to 1 m depth. The variability of sediment properties and Corg stocks in this study is not solely determined by the geographical differences (inshore, nearshore and offshore islands), but also influenced by other local factors such as hydrodynamics that control the distribution of carbon sources, anthropogenic pressures and species composition. These factors should be taken into account when developing coastal management strategies, as efforts are being undertaken to include coastal ecosystems (including seagrass ecosystems) on the National Green House Gasses Reduction Strategy.

Exotic Halophila stipulacea is an introduced carbon sink for the Eastern Mediterranean Sea

Carbon and nitrogen storage in exotic Halophila stipulacea were compared to that in native Posidonia oceanica and Cymodocea nodosa meadows and adjacent unvegetated sediments of the Eastern Mediterranean Sea and to that in native H. stipulacea of the Red Sea at sites with different biogeochemical conditions and level of human pressure. Exotic H. stipulacea possessed considerable storing capacity, with 2-fold higher Corg stock (0.71 ± 0.05 kg m−2 in the top 20 cm of sediment) and burial (14.78 gCorg m−2 y−1) than unvegetated areas and C. nodosa meadows and, surprisingly, comparable to P. oceanica. N (0.07 ± 0.01 kg m−2) and Cinorg (14.06 ± 8.02 kg m−2) stocks were similar between H. stipulacea and C. nodosa or unvegetated sediments, but different to P. oceanica. Corg and N stocks were higher in exotic than native H. stipulacea populations. Based on isotopic mixing model, organic material trapped in H. stipulacea sediments was mostly allochthonous (seagrass detritus 17% vs seston 67%). Corg stock was similar between monospecific and invaded C. nodosa meadows by H. stipulacea. Higher stocks were measured in the higher human pressure site. H. stipulacea introduction may contribute in the increase of carbon sequestration in the Eastern Mediterranean.

Carbon and Nitrogen Concentrations, Stocks, and Isotopic Compositions in Red Sea Seagrass and Mangrove Sediments

Coastal vegetated ecosystems are intense global carbon (C) sinks; however, seagrasses and mangroves in the Central Red Sea are depleted in organic C (Corg). Here, we tested whether Corg depletion prevails across the whole Red Sea, or if sediment Corg and nitrogen (N) stocks reflect the latitudinal productivity gradient of the Red Sea. We assessed Corg and N concentrations, stocks, isotopic compositions (δ13C and δ15N), and the potential contribution of primary producers to the organic matter accumulation in seagrass and mangrove sediments along the Eastern coast of the Red Sea. Sediment Corg concentration was higher in mangroves than seagrasses, while N concentrations were similar, resulting in higher C/N ratios in mangrove than seagrass sediments. Mangrove Corg stocks (integrated over the top 10 cm) were two-fold higher than those of seagrasses, respectively. N concentrations and stocks decreased from south to north in seagrass sediments matching the productivity gradient while Corg concentrations and stocks were uniform. The δ15N decreased from south to north in seagrass and mangrove sediments, reflecting a shift from nitrate and nitrite as N sources in the south, to N2 fixation towards the north. Stable isotope mixing models showed that seagrass leaves and macroalgae blades were the major contributors to the organic matter accumulation in seagrass sediments; while mangrove leaves were the major contributors in mangrove sediments. Overall, vegetated sediments in the Red Sea tend to be carbonate-rich and depleted in Corg and N, compared to coastal habitats elsewhere. Specifically, mean Corg stocks in Red Sea seagrass and mangrove sediments (7.2 ± 0.4 and 14.5 ± 1.4 Mg C ha-1, respectively) are lower than previously reported mean global values. This new information of Blue Carbon resources in the Red Sea provides a background for Blue Carbon programs in the region while also helping to balance global estimates.

Relevance of carbon burial and storage in two contrasting blue carbon ecosystems of a north-east Pacific coastal lagoon

Coastal vegetated ecosystems constitute very productive habitats, characterized by efficient Corg sequestration and long-term preservation in sediments, so they play an important role in climate change mitigation. The temporal evolution of Corg content, stocks and burial rates were evaluated in seagrass and salt marsh habitats in San Quintin Bay (northeast Pacific, Mexico) by using 210Pb-dated sediment cores. Salt marsh cores were characterized by fine-grained sediments, higher salinities, lower terrigenous input and lower mass accumulation rates (MAR: 0.01–0.03 g cm−2 yr−1) than seagrass cores (MAR: 0.02–3.21 g cm−2 yr−1). Accumulation rates in both habitats steadily increased throughout the past century most likely because of soil erosion promoted by land use changes in the surroundings. The Corg stocks were highest in salt marsh cores (12.2–53.6 Mg ha−1 at 10 cm depth; 259–320 Mg ha−1 at 1 m depth) than in seagrass cores (5.7–14.4 Mg ha−1, and 80–98, Mg ha−1, respectively), whereas Corg burial rates were considerably lower in salt marsh (13–60 g m−2 yr−1) than in seagrass (9–144 g m−2 yr−1) habitats, and the temporal variations observed in Corg burial rates were mostly driven by changes in the accumulation rates. The overall Corg stock (485 ± 51 Gg C) for both habitats together was comparable to the carbon emissions of a major city nearby. Our results highlight the need to protect these environments as relevant carbon reservoirs.

Variability in the organic carbon stocks, sources, and accumulation rates of Indonesian mangrove ecosystems

Mangrove ecosystems are an important natural carbon sink that accumulate and store large amounts of organic carbon (Corg), in particular in the sediment. However, the magnitude of carbon stocks and the rate of carbon accumulation (CAR) vary geographically due to a large variation of local factors. In order to better understand the blue carbon sink of mangrove ecosystems, we measured organic carbon stocks, sources and accumulation rates in three Indonesian mangrove ecosystems with different environmental settings and conditions; (i) a degraded estuarine mangrove forest in the Segara Anakan Lagoon (SAL), Central Java, (ii) an undegraded estuarine mangrove forest in Berau region, East Kalimantan, and (iii) a pristine marine mangrove forest on Kongsi Island, Thousand Islands, Jakarta. In general, Corg stocks were higher in estuarine than in marine mangroves, although a large variation was observed among the estuarine mangroves. The mean total Corg stock in Berau (615 ± 181 Mg C ha−1) is twice as high as that in SAL (298 ± 181 Mg C ha−1). However, the Segara Anakan Lagoon displayed large within-system variation with a much higher Corg stock in the eastern (483 ± 124 Mg C ha−1) than in the central lagoon (167 ± 36 Mg C ha−1). The predominant accumulation of autochthonous mangrove organic matter likely contributed to the higher Corg stocks in Berau and the eastern SAL. Interestingly, the CAR distribution pattern in SAL is opposite to that of its Corg stocks. The central SAL that receives high sediment inputs from the hinterland has a much higher CAR than the eastern SAL (658 ± 311 g C m−2 yr−1 and 194 ± 46 g C m−2 yr−1, respectively), while Berau has one of the highest CAR (1722 ± 183 g C m−2 yr−1) ever measured. It appears that these large differences are driven by the environmental setting and conditions, mainly sediment dynamics and hydrodynamics, landform, and vegetation conditions. It is inferred that quantifying carbon accumulation in sediments is a useful tool in estimating the present-day carbon storage of mangrove ecosystems. This is a precondition for taking measures under REDD+ (Reducing Emissions from Deforestation and Forest Degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries) schemes.

Reviews and syntheses: &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb-derived sediment and carbon accumulation rates in vegetated coastal ecosystems – setting the record straight

Abstract. Vegetated coastal ecosystems, including tidal marshes, mangroves and seagrass meadows, are being increasingly assessed in terms of their potential for carbon dioxide sequestration worldwide. However, there is a paucity of studies that have effectively estimated the accumulation rates of sediment organic carbon (Corg), also termed blue carbon, beyond the mere quantification of Corg stocks. Here, we discuss the use of the 210Pb dating technique to determine the rate of Corg accumulation in these habitats. We review the most widely used 210Pb dating models to assess their limitations in these ecosystems, often composed of heterogeneous sediments with varying inputs of organic material, that are disturbed by natural and anthropogenic processes resulting in sediment mixing and changes in sedimentation rates or erosion. Through a range of simulations, we consider the most relevant processes that impact the 210Pb records in vegetated coastal ecosystems and evaluate how anomalies in 210Pb specific activity profiles affect sediment and Corg accumulation rates. Our results show that the discrepancy in sediment and derived Corg accumulation rates between anomalous and ideal 210Pb profiles is within 20 % if the process causing such anomalies is well understood. While these discrepancies might be acceptable for the determination of mean sediment and Corg accumulation rates over the last century, they may not always provide a reliable geochronology or historical reconstruction. Reliable estimates of Corg accumulation rates might be difficult at sites with slow sedimentation, intense mixing and/or that are affected by multiple sedimentary processes. Additional tracers or geochemical, ecological or historical data need to be used to validate the 210Pb-derived results. The framework provided in this study can be instrumental in reducing the uncertainties associated with estimates of Corg accumulation rates in vegetated coastal sediments.

Response of Seagrass ‘Blue Carbon’ Stocks to Increased Water Temperatures

Seagrass meadows are globally important sinks of ‘Blue Carbon’, but warming water temperatures due to climate change may undermine their capacity to sequester and retain organic carbon (Corg). We tested the effects of warming on seagrass Corg stocks in situ by transplanting seagrass soil cores along a thermal plume generated by a coal-fired power plant in a seagrass-dominated estuary (Lake Macquarie, Australia). Transplanted cores were subjected to temperatures 2 and 4 °C above ambient temperatures and Corg content was measured after 7, 30, 90 and 180 days. We were unable to detect any significant effect of warming on Corg concentration, stocks, chemical composition (as measured by labile, recalcitrant, refractory ratios), or microbial abundance at any time point. In fact, Corg levels were temporally variable. These findings contrast those of previous studies (mostly laboratory-based) that have reported increases in microbial remineralisation (breakdown) of Corg in response to warming. To explain the lack of any detectable warming effect, we suggest that higher temperatures, longer durations of warming exposure, or additional stressors (e.g., oxygen exposure) may be needed to overcome microbial activation barriers and stimulate Corg remineralisation.

Carbon stocks and accumulation rates in Red Sea seagrass meadows

Seagrasses play an important role in climate change mitigation and adaptation, acting as natural CO2 sinks and buffering the impacts of rising sea level. However, global estimates of organic carbon (Corg) stocks, accumulation rates and seafloor elevation rates in seagrasses are limited to a few regions, thus potentially biasing global estimates. Here we assessed the extent of soil Corg stocks and accumulation rates in seagrass meadows (Thalassia hemprichii, Enhalus acoroides, Halophila stipulacea, Thalassodendrum ciliatum and Halodule uninervis) from Saudi Arabia. We estimated that seagrasses store 3.4 ± 0.3 kg Corg m−2 in 1 m-thick soil deposits, accumulated at 6.8 ± 1.7 g Corg m−2 yr−1 over the last 500 to 2,000 years. The extreme conditions in the Red Sea, such as nutrient limitation reducing seagrass growth rates and high temperature increasing soil respiration rates, may explain their relative low Corg storage compared to temperate meadows. Differences in soil Corg storage among habitats (i.e. location and species composition) are mainly related to the contribution of seagrass detritus to the soil Corg pool, fluxes of Corg from adjacent mangrove and tidal marsh ecosystems into seagrass meadows, and the amount of fine sediment particles. Seagrasses sequester annually around 0.8% of CO2 emissions from fossil-fuels by Saudi Arabia, while buffering the impacts of sea level rise. This study contributes data from understudied regions to a growing dataset on seagrass carbon stocks and sequestration rates and further evidences that even small seagrass species store Corg in coastal areas.