Article

Impacts of wetland dieback on carbon dynamics: A comparison between intact and degraded mangroves

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Mangroves are effective blue carbon sinks and are the most carbon rich ecosystems on earth. However, their areal extent has declined by over one-third in recent decades. Degraded mangrove forests result in reduced carbon captured and lead to release of stored carbon into the atmosphere by CO2 emission. The aim of this study was to assess changes in carbon dynamics in a gradually degrading mangrove forest on Bonaire, Dutch Caribbean. Remote sensing techniques were applied to estimate the distribution of intact and degraded mangroves. Forest structure, sediment carbon storage, sediment CO2 effluxes and dissolved organic and inorganic carbon in pore and surface waters across intact and degraded parts were assessed. On average intact mangroves showed 31% sediment organic carbon in the upper 30 cm compared to 20% in degraded mangrove areas. A loss of 1.51 MgCO2 ha⁻¹ yr⁻¹ for degraded sites was calculated. Water samples showed a hypersaline environment in the degraded mangrove area averaging 93 which may have caused mangrove dieback. Sediment CO2 efflux within degraded sites was lower than values from other studies where degradation was caused by clearing or cutting, giving new insights into carbon dynamics in slowly degrading mangrove systems. Results of water samples agreed with previous studies where inorganic carbon outwelled from mangroves might enhance ecosystem connectivity by potentially buffering ocean acidification locally. Wetlands will be impacted by a variety of stressors resulting from a changing climate. Results from this study could inform scientists and stakeholders on how combined stresses, such as climate change with salinity intrusion may impact mangrove's blue carbon sink potential and highlight the need of future comparative studies of intact versus degraded mangrove stands.
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... For example, one of the largest mangrove diebacks globally was caused by hypersalinity, which resulted from impaired hydrological connection after construction of causeways in combination with warmer temperatures (Jaramillo et al., 2018). As impacts of combined stressors are likely to intensify with climate change, more research is needed on comparing intact and degraded mangrove (Senger et al., 2021). ...
... Hypersalinity, defined as salinity > 40 (Whitfield et al., 2012;Tweedley et al., 2019), has caused mangrove dieback around the world (Barreto, 2004;Lovelock et al., 2017a;Jaramillo et al., 2018;Senger et al., 2021). Mangroves are adapted to live in saline habitats with regular tidal inundation (Krauss et al., 2008;Lovelock et al., 2016). ...
... While A. marina is adapted to high salinities, widespread canopy loss (leaves and branches) occurred with porewater salinities of 68.5 (Lovelock et al., 2017a). Mortality of other mangrove was observed at salinities of >74 (Cardona and Botero, 1998), >80 (Barreto, 2004), or >93 (Senger et al., 2021). ...
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Mangrove forests provide essential ecosystem services, but are threatened by habitat loss, effects of climatic change and chemical pollutants. Hypersalinity can also lead to mangrove mortality, although mangroves are adapted to saline habitats. A recent dieback event of >9 ha of temperate mangrove (Avicennia marina) in South Australia allowed to evaluate the generality of anthropogenic impacts on mangrove ecosystems. We carried out multidisciplinary investigations, combining airborne remote sensing with on-ground measurements to detect the extent of the impact. The mangrove forest was differentiated into “healthy,” “stressed,” and “dead” zones using airborne LIDAR, RGB and hyperspectral imagery. Differences in characteristics of trees and soils were tested between these zones. Porewater salinities of >100 were measured in areas where mangrove dieback occurred, and hypersalinity persisted in soils a year after the event, making it one of the most extreme hypersalinity cases known in mangrove. Sediments in the dieback zone were anaerobic and contained higher concentrations of sulfate and chloride. CO2 efflux from sediment as well as carbon stocks in mangrove biomass and soil did not differ between the zones a year after the event. Mangrove photosynthetic traits and physiological characteristics indicated that mangrove health was impacted beyond the immediate dieback zone. Normalized Difference Vegetation Index (NDVI), photosynthetic rate, stomatal conductance and transpiration rate as well as chlorophyll fluorescence were lower in the “stressed” than “healthy” mangrove zone. Leaves from mangrove in the “stressed” zone contained less nitrogen and phosphorous than leaves from the “healthy” zone, but had higher arsenic, sulfur and zinc concentrations. The response to extreme hypersalinity in the temperate semi-arid mangrove was similar to response from the sub-/tropical semi-arid mangrove. Mangrove in semi-arid climates are already at their physiological tolerance limit, which places them more at risk from extreme hypersalinity regardless of latitude. The findings have relevance for understanding the generality of disturbance effects on mangrove, with added significance as semi-arid climate regions could expand with global warming.
... In this respect, the carbon stock of mangroves ecosystems is site-specific. It depends on the method used, latitudes, and site characteristics that influence mangroves' community composition and productivity (Pérez et al., 2018;Atwood et al., 2017;Senger et al., 2021). For instance, the results obtained from Al-Qurm Nature Reserve in Oman (Al-Nadabi and Sulaiman, 2018), coastal Red sea of Saudi Arabia , and Khor Kalba in UAE (Crooks et al., 2019) of carbon sequestration by mangroves explain the enormous differences obtained and the shortcoming of absolute use of these values. ...
... Thus, a comparison of the findings is not possible. However, the total carbon stock of mangrove in Tubli Bay (i.e., 106.5 Mg C ha −1 ) is within the range reported in other regions (e.g., 45 Mg C ha −1 in north-west Australia (Hickey et al., 2018), 60-140 Mg C ha −1 in Bonaire, Dutch Caribbean (Senger et al., 2021). ...
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The grey mangrove Avicenna marina, constructs one of the most critical coastal ecosystems in the Kingdom of Bahrain which has been severely deteriorated by increasing anthropogenic pressures. This study aimed to assess the spatiotemporal changes in the mangrove habitat around Tubli Bay, Kingdom of Bahrain, over the last 50 years through achieving the following: (1) detect the progressive reduction in the mangrove cover using Geographic Information Science and Systems (GISs) techniques and remote sensing data, (2) estimate the changes of above-below ground (AGB-BGB) carbon sequestered in the mangroves using a GIS-based spatial analysis approach, and (3) estimate the potential carbon emission change from the loss of original mangrove habitats. Various GIS and remotely sensed data were employed in the study, including high-resolution satellite images from Worldview-3. Worldview-2, IKONOS, and QuickBird, coupled with true-color orthorectified aerial photographs. Additional data was acquired from fieldwork and the ancillary GIS maps. Image processing of the satellite data was conducted using ENVI 5.5 software. ArcGIS 10.8 was used for digitizing the mangrove areas in all satellite imagery. The final maps were used to assess and calculate mangrove area changes. The spatiotemporal process was used to calculate carbon values and estimate the amount of carbon loss due to land reclamation activities. Our results indicated that Bahrain lost more than 95% of the natural mangrove cover during the period from 1967 to 2020. The net loss in the mangrove extent area reached 280 ha during 1967–2020, as it declined from 328 ha in 1967 to 48 ha in 2020. The primary cause of the decline was land reclamation associated with urban development. The rates and causes of the loss varied both spatially as well as temporally. Due to land clearing, the total carbon stored in the mangrove habitat declined from 34932.2 Mg C ha ⁻¹ in 1967 to 5112 Mg C ha ⁻¹ in 2020. Consequently, the potential carbon Sequestration decreased from 128,200.44 Mg CO2 eq. ha-1 in 1967 to 18,761.04 Mg CO2 eq. ha ⁻¹ in 2020. Our study urges for more efficient conservation of the remaining mangroves in Bahrain to sustain their valuable ecosystem services particularly the Sequestration of carbon.
... Investigations of GHG fluxes across different restoration stages of mangrove forests have revealed variable responses of CO 2 fluxes to deforestation. Higher, lower and no difference from deforested/ degraded or bare mangrove sediment compared to forested mangrove sediment have been observed (Alongi et al., 1998;Bulmer et al., 2017;Castillo et al., 2017;Hien et al., 2018a;Senger et al., 2021). Lower CH 4 fluxes and emissions have been observed from deforested mangrove sediment and mudflats compared to forested mangrove sediments (Castillo et al., 2017;Soper et al., 2019). ...
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... Mangrove forests (MFs) are of great ecological and economic importance to humans, especially from coastal communities [1]; they are highly productive [2][3][4], generate large amounts of nutrients and organic matter that subsequently contributes to soil formation and coastal erosion control [5,6], they are ecosystems that provide food, shelter and breeding to a wide variety of flora and fauna including commercial species, and they are key to reductions in the greenhouse gases (GHG) effect. The conditions of the floodable soil of MFs allow GHG to be purified and hijacked in the soil by biogeochemical processes, degradating organic matter which is then incorporated into the soil [7,8]. ...
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Lac Bay of Bonaire is a shallow non-estuarine lagoon of about 700 hectares, separated from the open sea by a shallow coral barrier-reef. It possesses the only major concentration of seagrass beds and mangroves of the island. It is a designated Ramsar wetland of international significance, an Birdlife International IBA (Important Bird Area) and also fulfills a critical fish nursery function for the reefs of the island. The bay has consequently been designated as a protected area and is managed by Stinapa-Bonaire. The bay has been losing effective seagrass nursery habitat surface and quality as a consequence of mangrove-driven land acclamation. This in-turn is potentially being exacerbated by human-mediated eutrophication and erosion caused by agricultural and animal husbandry in the wider watershed, as well as other factors. The number of visitors to Bonaire and to Lac has been increasing dramatically over the last decades particularly from cruise ships. Yet little has been done to document and map the various types of human use that occur on and in the vicinity of the bay which might affect the ecological carrying capacity of the bay and the critical roles it plays. In this survey we do preliminary mapping and analysis of the level and distribution of human activity in and around Lac and discuss what possible threats these may entail for the environment of the bay.
Technical Report
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This report focuses on the management of natural coastal carbon sinks. The produc�on of the report has been s�mulated by an apparent lack of recogni�on • and focus on coastal marine ecosystems to comple- ment ac�vi�es already well advanced on land to ad- dress the best prac�ce management of carbon sinks. The produc�on of this report is �mely as a number of Governments are now introducing legisla�on to tackle climate change. In the UK, for example, the Climate Change Act sets out a statutory responsibility to quan- �fy natural carbon sink as part of the overall carbon • accoun�ng process. It is important that such quan��- ca�ons and processes work with the latest science and evidence. To construct this report we asked leading scien�sts for their views on the carbon management poten�al of a number of coastal ecosystems: �dal saltmarshes, mangroves, seagrass meadows, kelp forests and coral • reefs. The resultant chapters wri�en by these scien�sts form the core of this report and are their views on how well such habitats perform a carbon management role. These ecosystems were selected because the belief from the outset was that they are good at sequestering carbon, and are located in situa�ons where manage- ment ac�ons could secure the carbon sinks. There are of course other features of our ocean that are already • established as good carbon sinks – the key focus for this ini�al work has, however, been on those ecosystems where management interven�on can reasonably read- ily play a role in securing and improving the future state of the given carbon sinks. If proven this work could ex- pand the range of global op�ons for carbon manage- ment into coastal marine environments, unlocking many possibili�es for ac�on and possible �nancing of new management measures to protect the important • carbon sinks. The key �ndings of this report are: These key marine ecosystems are of high im- portance because of the signi�cant goods and services they already provide as well as the carbon management poten�al recog- nised in this report, thus providing new con- vergent opportuni�es to achieve many po- li�cal goals from few management ac�ons. The carbon management poten�al of these se- lected marine ecosystems compares favourably with and, in some respects, may exceed the po- ten�al of carbon sinks on land. Coral reefs, rather than ac�ng as ‘carbon sinks’ are found to be slight ‘carbon sources’ due to their e�ect on local ocean chemistry The table below highlights some of the key car- bon sink data documented in this report for these coastal habitats. It provides summary data on the comparison of carbon stocks and long-term accu- mula�on of carbon in the coastal marine ecosys- tems. Comparisons with informa�on on terrestrial carbon sinks are provided in the body of this report. The chemistry of some speci�c marine sediments (for example salt marshes) suggests that whilst such habitats may be of limited geographical ex- tent, the absolute compara�ve value of the car- bon sequestered per unit area may well outweigh the importance of similar processes on land due to lower poten�al for the emission of other powerful greenhouse gases such as methane. Alongside the carbon management poten�al of these ecosystems, another key �nding of this report is the lack of cri�cal data for some habitat types. Having comprehensive habitat inventories is cri�cally important and this report highlights the urgent need, alongside recognising the carbon role of such ecosystems, to ensure that such inventories are completed for saltmarsh and kelp forests and then all such inventories are e�ec�vely maintained over �me. • These coastal marine ecosystems are also vital for the food security of coastal communi�es in developing countries, providing nurseries and �shing grounds for ar�sanal �sheries. Furthermore, they provide natural coastal defences that mi�gate erosion and storm ac�on. Therefore, be�er protec�on of these ecosystems will not only make carbon sense, but the co-bene�ts from ecosystem goods and services are clear. • Signi�cant losses are occurring in the global extent of these cri�cal marine ecosystems due to poor management, climate change (especially rising sea levels), coupled to a lack of policy priority to address current and future threats. • Certain human impacts – notably nutrient and sediment run-o� from land, displacement of mangrove forests by urban development and aquaculture, and over-�shing - are degrading these ecosystems, threatening their sustainability and compromising their capacity to naturally sequester carbon. The good news is that such impacts can be mi�gated by e�ec�ve management regimes. • Management approaches already exist that could secure the carbon storage poten�al of these ecosystems, and most governments have commitments to put such measures in place for other reasons. These include biodiversity protec�on or achieving sustainable development. Agreed management approaches that would be e�ec�ve include Marine Protected Areas, Marine Spa�al Planning, area-based �sheries management approaches, bu�er zones to allow inland migra�on of coastal carbon sinks, regulated coastal development, and ecosystem rehabilita�on. • Greenhouse gas emissions that occur as a result of the management of coastal and marine habitats are not being accounted for in interna�onal climate change mechanisms (ie UNFCCC, Kyoto, CDM, etc) or in Na�onal Inventory Submissions. Not only does this mean that countries are under- es�ma�ng their anthropogenic emissions, but also that the carbon savings from measures to protect and restore coastal and marine habitats will not count towards mee�ng interna�onal and na�onal climate change commitments. This report provides the essen�al evidence needed to mo�vate discussions and ini�a�ves on how such coastal ecosystems should be incorporated into interna�onal and na�onal emission reduc�on strategies, na�onal greenhouse gas inventories and, poten�ally, carbon revenues schemes. The la�er could take the marine equivalent of the Reducing Emissions from Deforesta�on and Forest Degrada�on (REDD) scheme on land to safeguard these cri�cal coastal carbon sinks. Don’t just think REDD, think coastal too! The evidence presented here makes clear why moving forward with Marine Protected Areas, Marine Spa�al Planning and area-based �sheries management techniques is not only a poli�cal impera�ve for biodiversity conserva�on, food security, and shoreline protec�on, but also now for helping mi�gate climate change.
Article
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Carbon fluxes from a mangrove creek with adjacent seagrass meadows and coral reefs (at 4 km from the creek) were investigated in Gazi Bay (Kenya). Analysis of the stable isotope signature of sediment carbon in the seagrass zone and data on the sediment carbon content indicate that outwelling of particulate organic matter (POM) from the mangrove forest occurs, but that deposition of this POM rapidly decreases away from the forest. No evidence for any input of mangrove POM in the seagrass zone was found at a distance of 3 km from the mangrove creek, near the reefs. The gradient in sediment deltaC-13 values in the seagrass zone was paralleled by a similar gradient of deltaC-13 values in Thalassodendron ciliatum, the dominant subtidal seagrass. This gradient probably reflects the availability of respiratory CO2 derived from mangrove POM as a carbon source for the seagrass. Analysis of C:N ratios of particulate material (< 1 mm) collected with sediment traps in the seagrass zone yielded values ranging from 8.5 to 11.2. This range is remarkably low compared to C:N ratios of plant material produced in the mangrove forest, and suggests that some of the mangrove-derived organic particles deposited in the seagrass zone have gone through a phase of intensive processing. During flood tides conspicuous decreases were found in deltaC-13 values of seston flowing over the seagrass zone, coinciding with significant increases in the carbon content of the seston. These findings point to a reversed flux of organic particles from the seagrass zone to the mangrove forest. Our data indicate that, as far as POM fluxes are concerned, the mangrove forest and adjacent seagrass meadows are tightly coupled, but that the nearby coral reefs may exist in relative isolation.
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Carbon gas balance was evaluated in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. Exchange of carbon dioxide (CO2) was measured for inundated and air-exposed sediments during day and night using in situ and laboratory incubations. In situ methane (CH4) emissions were measured in the dark during air exposure only. Emission of CO2 and CH4 from open waters (e.g. creeks) was estimated from diurnal measurements of CO2, partial pressure (pCO2) and CH4 concentrations. CO2 emission from darkened sediments devoid of biogenic structures was comparable during inundation and air exposure (28 to 115 mmol m–2 d–1) with no differences between mangrove forests. Benthic primary production was low with only occasional net uptake of CO2 by the sediments. Emissions of CH4 from air-exposed sediment were generally 3 orders of magnitude lower than for CO2. Presence of pneumatophores and crab burrows increased low tide emissions several fold. Emissions from open waters were dependent on tidal level and wind speed. Lowest emission occurred during high tide (1 to 6 mmol CO2 m–2 d–1; 10 to 80 µmol CH4 m–2 d–1) and highest during low tide (30 to 80 mmol CO2 m–2 d–1; 100 to 350 µmol CH4 m–2 d–1) when supersaturated runoff from the forest floor and porewater seepage reached the creek water. Based on global average primary production and measured gas emissions, the carbon gas balance of the 2 mangrove forests was estimated. The densely vegetated Ras Dege forest appears to be an efficient sink of greenhouse carbon gases, while extensive clear-cutting at the Mtoni forest apparently has reduced its capacity to absorb CO2, although it is seemingly still a net sink for atmospheric CO2.
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Coastal wetlands can have exceptionally large carbon (C) stocks and their protection and restoration would constitute an effective mitigation strategy to climate change. Inclusion of coastal ecosystems in mitigation strategies requires quantification of carbon stocks in order to calculate emissions or sequestration through time. In this study, we quantified the ecosystem C stocks of coastal wetlands of the Sian Ka'an Biosphere Reserve (SKBR) in the Yucatan Peninsula, Mexico. We stratified the SKBR into different vegetation types (tall, medium and dwarf mangroves, and marshes), and examined relationships of environmental variables with C stocks. At nine sites within SKBR, we quantified ecosystem C stocks through measurement of above and belowground biomass, downed wood, and soil C. Additionally, we measured nitrogen (N) and phosphorus (P) from the soil and interstitial salinity. Tall mangroves had the highest C stocks (987±338 Mg ha) followed by medium mangroves (623±41 Mg ha), dwarf mangroves (381±52 Mg ha) and marshes (177±73 Mg ha). At all sites, soil C comprised the majority of the ecosystem C stocks (78-99%). Highest C stocks were measured in soils that were relatively low in salinity, high in P and low in N∶P, suggesting that P limits C sequestration and accumulation potential. In this karstic area, coastal wetlands, especially mangroves, are important C stocks. At the landscape scale, the coastal wetlands of Sian Ka'an covering ≈172,176 ha may store 43.2 to 58.0 million Mg of C.
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A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this ‘‘missing carbon’’ is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore-water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and d13C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon (222Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and d13C values displayed consistent tidal variations, and mirrored the trend in 222Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar d13C value in summer and winter (, 230%). The exported d13C-DIC showed no difference between summer and winter and had a d13C value slightly more enriched (, 222.5%) than the exported DOC. The imported POC had differing values in summer (, 216%) and winter (, 222%), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222Rn and carbon model showed that 93–99% of the DIC and 89–92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m22 d21 and was an order of magnitude higher than DOC export, and similar to global estimates of the mangrove missing carbon (i.e., , 1.9–2.7 g C m22 d21).
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The assessment of net ecosystem exchange (NEE) and respiration of ecosystem (Reco) of terrestrial ecosystems is necessary to improve our knowledge about the carbon cycle. The aims of this paper were to present reliable measurements of CO2 fluxes of a temperate bog ecosystem located in Poland using a closed dynamic chamber system and to obtain a daily dynamic course of CO2 fluxes over the 2007 vegetation season. Measurements of CO2 fluxes were carried out at Rzecin peatland ecosystem located in northwestern Poland using the set of two chambers (dark and transparent). Reco during the experiment period ranged 2.65 to 14.76 μmolCO2·m-2·s-1. The daily run of NEE was inversed to PPFD and the values of NEE varied from 0.06 to -11.82 μmolCO2·m-2·s-1. We found differences between NEE and Reco in the wetland ecosystem with respect to term of measurements. The PPFD, air and soil temperatures explain most temporal variability of CO2 fluxes at Rzecin. But vegetation structure, its phenology and water-level depth seem also to play important roles. The chamber technique is a useful tool for determining carbon dioxide exchange between wetland surface and the atmosphere.
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Mangrove forests occur along ocean coastlines throughout the tropics, and support numerous ecosystem services, including fisheries production and nutrient cycling. However, the areal extent of mangrove forests has declined by 30-50% over the past half century as a result of coastal development, aquaculture expansion and over-harvesting. Carbon emissions resulting from mangrove loss are uncertain, owing in part to a lack of broad-scale data on the amount of carbon stored in these ecosystems, particularly below ground. Here, we quantified whole-ecosystem carbon storage by measuring tree and dead wood biomass, soil carbon content, and soil depth in 25 mangrove forests across a broad area of the Indo-Pacific region--spanning 30° of latitude and 73° of longitude--where mangrove area and diversity are greatest. These data indicate that mangroves are among the most carbon-rich forests in the tropics, containing on average 1,023Mg carbon per hectare. Organic-rich soils ranged from 0.5m to more than 3m in depth and accounted for 49-98% of carbon storage in these systems. Combining our data with other published information, we estimate that mangrove deforestation generates emissions of 0.02-0.12Pg carbon per year--as much as around 10% of emissions from deforestation globally, despite accounting for just 0.7% of tropical forest area.
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Mangroves, the only woody halophytes living at the confluence of land and sea, have been heavily used traditionally for food, timber, fuel and medicine, and presently occupy about 181 000 km2 of tropical and subtropical coastline. Over the past 50 years, approximately one-third of the world's mangrove forests have been lost, but most data show very variable loss rates and there is considerable margin of error in most estimates. Mangroves are a valuable ecological and economic resource, being important nursery grounds and breeding sites for birds, fish, crustaceans, shellfish, reptiles and mammals; a renewable source of wood; accumulation sites for sediment, contaminants, carbon and nutrients; and offer protection against coastal erosion. The destruction of mangroves is usually positively related to human population density. Major reasons for destruction are urban development, aquaculture, mining and overexploitation for timber, fish, crustaceans and shellfish. Over the next 25 years, unrestricted clear felling, aquaculture, and overexploitation of fisheries will be the greatest threats, with lesser problems being alteration of hydrology, pollution and global warming. Loss of biodiversity is, and will continue to be, a severe problem as even pristine mangroves are species-poor compared with other tropical ecosystems. The future is not entirely bleak. The number of rehabilitation and restoration projects is increasing worldwide with some countries showing increases in mangrove area. The intensity of coastal aquaculture appears to have levelled off in some parts of the world. Some commercial projects and economic models indicate that mangroves can be used as a sustainable resource, especially for wood. The brightest note is that the rate of population growth is projected to slow during the next 50 years, with a gradual decline thereafter to the end of the century. Mangrove forests will continue to be exploited at current rates to 2025, unless they are seen as a valuable resource to be managed on a sustainable basis. After 2025, the future of mangroves will depend on technological and ecological advances in multi-species silviculture, genetics, and forestry modelling, but the greatest hope for their future is for a reduction in human population growth.
Article
Water-to-air carbon dioxide fluxes from tropical coastal waters are an important but understudied component of the marine carbon budget. Here, we investigate drivers of carbon dioxide partial pressure (pCO2) in a relatively pristine mangrove-seagrass embayment on a tropical island (Bali, Indonesia). Observations were performed over eight underway seasonal surveys and a fixed location time series for 55 hours. There was a large spatial variability of pCO2 across the continuum of mangrove forests, seagrass meadows and the coastal ocean. Overall, the embayment waters surrounded by mangroves released CO2 to the atmosphere with a net flux rate of 18.1 ± 5.8 mmol m-2 d -1. Seagrass beds produced an overall CO2 net flux rate of 2.5 ± 3.4 mmol m-2 d -1, although 2 out of 8 surveys revealed a sink of CO2 in the seagrass area. The mouth of the bay where coral calcification occurs was a minor source of CO2 (0.3 ± 0.4 mmol m-2 d -1 ). The overall average CO2 flux to the atmosphere along the transect was 9.8 ± 6.0 mmol m-2 d -1, or 3.6 x 103 mol d-1 CO2 when upscaled to the entire embayment area. There were no clear seasonal patterns in contrast to better studied temperate systems. pCO2 significantly correlated with antecedent rainfall and the natural groundwater tracer radon (222Rn) during each survey. We suggest that the CO2 source in the mangrove dominated upper bay was associated with delayed groundwater inputs, and a shifting CO2 source-sink in the lower bay was driven by the uptake of CO2 by seagrass and mixing with oceanic waters. This differs from modified landscapes where potential uptake of CO2 is weakened due to the degradation of seagrass beds, or emissions are increased due to drainage of coastal wetlands.
Article
Seasonal variations of CO2 and CH4 fluxes were investigated in a Rhizophora mangrove forest that develops under a semi-arid climate, in New Caledonia. Fluxes were measured using closed incubation chambers connected to a CRDS analyzer. They were performed during low tide at light, in the dark, and in the dark after having removed the top 1–2 mm of soil, which may contain biofilm. CO2 and CH4 fluxes ranged from 31.34 to 187.48 mmol m−2 day−1 and from 39.36 to 428.09 μmol m−2 day−1, respectively. Both CO2 and CH4 emissions showed a strong seasonal variability with higher fluxes measured during the warm season, due to an enhanced production of these two gases within the soil. Furthermore, CO2 fluxes were higher in the dark than at light, evidencing photosynthetic processes at the soil surface and thus the role of biofilm in the regulation of greenhouse gas emissions from mangrove soils. The mean δ13C-CO2 value of the CO2 fluxes measured was −19.76 ± 1.19‰, which was depleted compared to the one emitted by root respiration (−22.32 ± 1.06‰), leaf litter decomposition (−21.43 ± 1.89‰) and organic matter degradation (−22.33 ± 1.82‰). This result confirmed the use of the CO2 produced within the soil by the biofilm developing at its surface. After removing the top 1–2 mm of soil, both CO2 and CH4 fluxes increased. Enhancement of CH4 fluxes suggests that biofilm may act as a physical barrier to the transfer of GHG from the soil to the atmosphere. However, the δ13C-CO2 became more enriched, evidencing that the biofilm was not integrally removed, and that its partial removal resulted in physical disturbance that stimulated CO2 production. Therefore, this study provides useful information to understand the global implication of mangroves in climate change mitigation.
Article
The blue carbon paradigm has evolved in recognition of the high carbon storage and sequestration potential of mangrove, saltmarsh and seagrass ecosystems. However, fluxes of the potent greenhouse gases CH4 and N2O, and lateral export of carbon are often overlooked within the blue carbon framework. Here, we show that the export of dissolved inorganic carbon (DIC) and alkalinity is approximately 1.7 times higher than burial as a long-term carbon sink in a subtropical mangrove system. Fluxes of methane offset burial by approximately 6%, while the nitrous oxide sink was approximately 0.5% of burial. Export of dissolved organic carbon and particulate organic carbon to the coastal zone is also significant and combined may account for an atmospheric carbon sink similar to burial. Our results indicate that the export of DIC and alkalinity results in a long-term atmospheric carbon sink and should be incorporated into the blue carbon paradigm when assessing the role of these habitats in sequestering carbon and mitigating climate change.
Article
Although mangrove forests are efficient natural carbon sinks, most of the atmospheric carbon dioxide (CO2) fixed by its vegetation is believed to be exported via tidal exchange, rather than stored in the vegetative biomass and sediment. However, the magnitude of tidal export is largely unknown because direct measurements are scarce. We deployed a novel experimental design that combined automated high-resolution measurements of hydrodynamic, hydrogeochemical and biogeochemical parameters during the dry season in a mangrove tidal creek in the Can Gio Mangrove Forest in Vietnam. The objective was to quantify the tide-controlled water, porewater, DIC and DOC exchange, and estimate the CO2 evasion throughout tidal cycles contrasted by amplitude. Data from three 25-h time series showed a clear peak of DIC, DOC, pCO2, and ²²²Rn at low tide, particularly during tidal cycles of large amplitude, which directly relate to porewater discharge. Our mass balance models revealed that the tidal creek was a net exporter of dissolved carbon to coastal waters, with an important contribution (38%) coming from DIC in porewater discharge. Porewater exchange varied from 3.1 ± 1.6 to 7.1 ± 2.4 cm day⁻¹. DIC exchange ranged from 352 ± 34 to 678 ± 79 mmolC m⁻² day⁻¹; DOC exchange, 20.6 ± 1.9 to 67.7 ± 7.9 mmol C m⁻² day⁻¹; and CO2 evasion, 69.9 ± 10.5 to 173.7 ± 26.1 mmolC m⁻² day⁻¹. These estimates were in the high range of previous carbon assessments and were explained by (i) the monitoring station being located at equal distance from the head and the mouth of the creek, which minimized carbon degradation and losses associated to transport in water; and (ii) the site being a highly productive mangrove within South East Asia.
Article
Mangroves are known for exchanging organic and inorganic carbon with estuaries and oceans but studies that have estimated their contribution to the global budget are limited to a few mangrove ecosystems which exclude world's largest the Sundarbans. Here, we worked in the Indian Sundarbans and in the Hooghly river/estuary in May (pre-monsoon) and December (post-monsoon), 2014. Aims were, i) to quantify the riverine export of particulate organic carbon (POC) and dissolved organic and inorganic carbon (DOC, DIC)) of the Hooghly into the Bay of Bengal (BoB), ii) to estimate the C export (DOC, DIC, POC) from the Sundarbans into the BoB by using a simple mixing model, as well as iii) to revise the existing C budget constructed for the mangroves. The riverine exports of POC, DOC and DIC account for 0.07 Tg C yr− 1, 0.34 Tg C yr− 1 and 4.14 Tg C yr− 1, respectively, and were largest during the monsoon period. Results revealed that mangrove plant derived organic matter and its subsequent degradation is the primary source of DIC and DOC in the Hooghly estuary whereas POC is linked to soil erosion. Mangroves are identified as a major source of carbon (POC, DOC, DIC) transported from the Sundarbans into the BoB, with export rates of 0.58 Tg C yr− 1, 3.03 Tg C yr− 1, and 3.69 Tg C yr− 1 respectively, altogether amounting to 7.3 Tg C yr− 1. This C export from the Indian Sundarbans exceeds the ‘missing C’ of the previous budget, thus necessitating further research to finally resolve the mangrove C budget. However, these first baseline data on C exports from the world's largest deltaic mangrove improves limited global data inventory and signifies the need of acquiring more data from different mangrove settings to reduce uncertainties.
Article
Mangrove forests are important sinks and sources of carbon especially for connections to coral reefs and seagrass beds. However, they are increasing under threat from anthropogenic influences. We investigated correlations between carbon fluxes from the sediment and water column in deforested and intact mangroves. Our findings show that deforestation has a negative effect on sediment organic carbon storage and CO2 fluxes. However, species richness and density showed a positive correlation with sediment organic carbon storage and CO2 fluxes. An increased density of saplings showed a positive relationship with dissolved inorganic and organic carbon draining the mangrove forest at high tide. This research offers insights into the importance of the key forest characteristics influencing the storage and fluxes of carbon. Alterations in mangrove carbon stocks and retention may affect connected ecosystems.
Article
Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2 , and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic carbon (SOC) content and CO2 flux separately, but could not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, we used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10 cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (< 63 μm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994 ± 186 g m − 2 , mean ± SD) than in Rhizophora stylosa (2383 ± 209 g m − 2). SOC was significantly higher in winter (2771 ± 192 g m − 2) than in summer (2599 ± 211 g m − 2). SOC significantly increased from creek-side (865 ± 89 g m − 2) through mid (3298 ± 137 g m − 2) to landward (3933 ± 138 g m − 2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4 mmol m − 2 d − 1 in A. marina stands under dark conditions, lower than the global average dark flux (61 mmol m − 2 d − 1) for mangroves.
Article
In order to understand the processes that control organic matter preservation in tropical wetlands, we have evaluated the mineralogy, total organic carbon (TOC wt%), soluble organic matter (SOM wt%), bulk density (g/cm3), carbon stock (Mg/ha), FTIR to identify functional groups in SOM of soil in mangrove forest dominated by Rhizophora or Avicennia with different conditions (live, deteriorated and dead). Six locations along the Cuare Inlet and Morrocoy National Park were studied. Mineralogical analysis showed the presence of minerals, such as pyrite and rhodochrosite, from anoxic environments. Rhizophora mangrove soils have higher TOC compared with Avicennia, but we did not find significant differences in SOM. TOC/SOM ratios were lower for Avicennia soils. The carbon content ranges from 11.30 to 59.84 Mg/ha for the first 10 cm of soil. Regardless of stand condition, the TOC/SOM ratio was lower at a depth of 20−40 cm. The results of the TOC/SOM ratio are attributable to: (a) the association of TOC with clays and non crystalline minerals; (b) the leaching processes of soluble compounds of the OM; (c) a higher proportion of recalcitrant compounds; (d) a lower decomposition rate for recalcitrant or non recalcitrant compounds; and (e) physicochemical conditions that limit biological activity, such as high salinity, soil anoxia and hypoxia. The soils can be divided into three groups according to the presence and intensity of functional groups detected by FTIR. The functional groups identified could not be related to the sampling sites, to species composition or conditions. These differences may be due to other sources of organic matter, as well as to degree of preservation. Information of soil organic matter properties and their relationship with mangrove composition and conditions is important to understand carbon sequestration and storage potential in Venezuelan Caribbean mangrove systems.
Article
Mangrove forests are hotspots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia, and calculated the exchange of DIC, alkalinity and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m-2 d-1 across the six systems, ranging from import of 97 mmol m-2 d-1 to an export of 85 mmol m-2 d-1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr-1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m-2 day-1 and an export of 117 mmol m-2 day-1 with an estimated global export of 4.2 ± 1.3 Tmol yr-1. A net import of free CO2 was estimated (-11.4 ± 14.8 mmol m-2 d-1), and was equivalent to approximately one third of the air water CO2 flux (33.1 ± 6.3 mmol m-2 d-1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.
Article
The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon-climate interactions and land management.
Conference Paper
This paper reports the results of carbon stored in soil and aboveground biomass from the most important area of mangroves in Mexico with dominant vegetation of Red mangrove (Rhizophora mangle L.), Black mangrove (Avicennia germinans L.), white mangrove (Laguncularia racemosa Gaertn.) and button mangrove (Conocarpus erectus L.) in three sites located in the Atasta Peninsula, Campeche, Mexico. Samples were taken in 2009 and 2010 during the dry season from soils with high fertility. To determine tree biomass (AGB), allometric equations were used. Greater values of AGB were found for button mangrove (253.18 ± 32.17 t ha-1) and lower values were found for Black mangrove (161.93 ± 12.63), intermediate carbon storage were found in the other two species Red mangrove 181.70±16.58 t ha-1), and white mangrove (206.07±19.12 t ha-1). Carbon stored in soil at the three sites was measured in a range of 36.80 ± 10.27 to 235.77 ± 66.11 t C ha-1. The Tukey test (p
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Random forests are a combination of tree predictors such that each tree depends on the values of a random vector sampled independently and with the same distribution for all trees in the forest. The generalization error for forests converges a.s. to a limit as the number of trees in the forest becomes large. The generalization error of a forest of tree classifiers depends on the strength of the individual trees in the forest and the correlation between them. Using a random selection of features to split each node yields error rates that compare favorably to Adaboost (Y. Freund & R. Schapire, Machine Learning: Proceedings of the Thirteenth International conference, ∗∗∗, 148–156), but are more robust with respect to noise. Internal estimates monitor error, strength, and correlation and these are used to show the response to increasing the number of features used in the splitting. Internal estimates are also used to measure variable importance. These ideas are also applicable to regression.
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Automated in situ instrumentation captured high-resolution surface water pCO2, CH4 and 222Rn data at the creek mouth, and ~ 500m upstream in a sub-tropical mangrove ecosystem (Southern Moreton Bay, Australia, S27.78°, E153.38°) over a spring-neap-spring tidal cycle (~ 15 days) during November 2013. The partial pressure of CO2 (pCO2) ranged from 385 to 26106 µatm, CH4 from 1.8 to 889 nM, and 222Rn from 280 to 108172 dpm m-3. Average surface water pCO2, CH4 and 222Rn were 4-fold higher at the upstream station. Surface water fluxes of CO2 and CH4 ranged from 9.4 to 629.2 mmol CO2 m-2 d-1 and 13.1 to 632.9 μmol CH4 m-2 d-1 depending upon the gas transfer model used and station location. Creek pCO2, CH4 and 222Rn displayed changes over both semi-diurnal and spring-neap-spring tidal scales. Semi-diurnally, all gases had a significant inverse relationship with water depth. Over the spring-neap-spring cycle, all gases exhibited an inverse relationship with tidal amplitude, with higher values during neap tides than spring tides. Estimated fluxes, porewater observations, and the significant positive relationship between surface water pCO2 and CH4, and 222Rn suggests groundwater exchange (i.e. tidal pumping) drives pCO2 and CH4 within the mangrove creek. We hypothesize that a combination of hourly and weekly groundwater-surface water exchange processes drive surface water pCO2 and CH4 in mangrove creeks. Semi-diurnally, flushing of crab burrows leads to high pCO2 and CH4 concentrations at low tide. During the spring-neap-spring cycle, older groundwater enriched in CO2, CH4 and 222Rn seeps into the creek as tidal amplitude decreases, leading to higher concentrations at neap tides.
Article
Mangroves are recognized to possess a variety of ecosystem services including high rates of carbon sequestration and storage. Deforestation and conversion of these ecosystems continue to be high and have been predicted to result in significant carbon emissions to the atmosphere. Yet few studies have quantified the carbon stocks or losses associated with conversion of these ecosystems. In this study we quantified the ecosystem carbon stocks of three common mangrove types of the Caribbean as well as those of abandoned shrimp ponds in areas formerly occupied by mangrove-a common land-use conversion of mangroves throughout the world. In the mangroves of the Montecristi Province in Northwest Dominican Republic we found C stocks ranged from 706 to 1131 Mg/ha. The medium-statured mangroves (3-10 m in height) had the highest C stocks while the tall (> 10 m) mangroves had the lowest ecosystem carbon storage. Carbon stocks of the low mangrove (shrub) type (< 3 m) were relatively high due to the presence of carbon-rich soils as deep as 2 m. Carbon stocks of abandoned shrimp ponds were 95 Mg/ha or approximately 11% that of the mangroves. Using a stock-change approach, the potential emissions from the conversion of mangroves to shrimp ponds ranged from 2244 to 3799 Mg CO2e/ha (CO2 equivalents). This is among the largest measured C emissions from land use in the tropics. The 6260 ha of mangroves and converted mangroves in the Montecristi Province are estimated to contain 3,841,490 Mg of C. Mangroves represented 76% of this area but currently store 97% of the carbon in this coastal wetland (3,696,722 Mg C). Converted lands store only 4% of the total ecosystem C (144,778 Mg C) while they comprised 24% of the area. By these metrics the replacement of mangroves with shrimp and salt ponds has resulted in estimated emissions from this region totaling 3.8 million Mg CO2e or approximately 21% of the total C prior to conversion. Given the high C stocks of mangroves, the high emissions from their conversion, and the other important functions and services they provide, their inclusion in climate-change mitigation strategies is warranted.
Article
The exact size of the wetland area of South America is not known but may comprise as much as 20% of the sub-continent, with river floodplains and intermittent interfluvial wetlands as the most prominent types. A few wetland areas have been well studied, whereas little is known about others, including some that are very large. Despite the fact that most South American countries have signed the Ramsar convention, efforts to elaborate basic data have been insufficient, thereby hindering the formulation of a wetland-friendly policy allowing the sustainable management of these areas. Until now, the low population density in many wetland areas has provided a high level of protection; however, the pressure on wetland integrity is increasing, mainly as a result of land reclamation for agriculture and animal ranching, infrastructure building, pollution, mining activities, and the construction of hydroelectric power plants. The Intergovernmental Panel on Climate Change has predicted increasing temperatures, accelerated melting of the glaciers in Patagonia and the Andes, a rise in sea level of 20–60 cm, and an increase in extreme multiannual and short-term climate events (El Niño and La Niña, heavy rains and droughts, heat waves). Precipitation may decrease slightly near the Caribbean coast as well as over large parts of Brazil, Chile, and Patagonia, but increase in Colombia, Ecuador, and Peru, around the equator, and in southeastern South America. Of even greater impact may be a change in rainfall distribution, with precipitation increasing during the rainy season and decreasing during the dry season. There is no doubt that the predicted changes in global climate will strongly affect South American wetlands, mainly those with a low hydrologic buffer capacity. However, for the coming decades, wetland destruction by wetland-unfriendly development planning will by far outweigh the negative impacts of global climate change. South American governments must bear in mind that there are many benefits that wetlands bring about for the landscape and biodiversity as well as for humans. While water availability will be the key problem for the continent’s cities and agroindustries, intact wetlands can play a major role in storing water, buffering river and stream discharges, and recharging subterranean aquifers.
Article
Mangroves are the major ecosystems of tropical and subtropical coastlines. They are considered as a sink for atmospheric CO2 because they are characterized both by high net primary production, and by low rates of organic matter decomposition. However, a recent reassessment of the global mangrove budget suggests that organic carbon sinks have been underestimated, notably CO2 efflux from sediments and creek waters, and tidal export of dissolved inorganic carbon. Our objective was to understand the influence of mangrove zonation on the magnitude of CO2 fluxes at the sediment–air interface. Transparent and opaque dynamic closed chamber systems, coupled with an infra-red gas analyzer were used to measure CO2 fluxes. In addition, the physico-chemical properties (salinity, redox potential) of pore waters were determined, as well as the carbon content and the origin of surface sediments (Chlorophyll-a and δ13C). Depending on the type of measurement (in the dark with or without biofilm, in the light with biofilm) and mangrove stand (saltflat, Avicennia sp., or Rhizophora spp.), mean surface sediment CO2 fluxes ranged between 40 ± 56 and 199 ± 95 mmol·m− 2·d− 1. We suggest that these differences mainly result both from the organic content and the redox conditions of the sediments, which are influenced by the physiological activities of the root system, and by the position and the elevation of the stand in the intertidal zone. In addition, the quality and abundance of biofilm, which also vary with the mangrove stand, also appear to strongly affect sediment CO2 fluxes as a result of chemical (metabolism) and also physical (barrier) processes.
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1] Mangrove wetlands exist in the transition zone between terrestrial and marine environments and as such were historically overlooked in discussions of terrestrial and marine carbon cycling. In recent decades, mangroves have increasingly been credited with producing and burying large quantities of organic carbon (OC). The amount of available data regarding OC burial in mangrove soils has more than doubled since the last primary literature review (2003). This includes data from some of the largest, most developed mangrove forests in the world, providing an opportunity to strengthen the global estimate. First-time representation is now included for mangroves in and Thailand, along with additional data from Mexico and the United States. Our objective is to recalculate the centennial-scale burial rate of OC at both the local and global scales. Quantification of this rate enables better understanding of the current carbon sink capacity of mangroves as well as helps to quantify and/or validate the other aspects of the mangrove carbon budget such as import, export, and remineralization. Statistical analysis of the data supports use of the geometric mean as the most reliable central tendency measurement. Our estimate is that mangrove systems bury 163 (+40; À31) g OC m À2 yr À1 (95% C.I.). Globally, the 95% confidence interval for the annual burial rate is 26.1 (+6.3; À5.1) Tg OC. This equates to a burial fraction that is 42% larger than that of the most recent mangrove carbon budget (2008), and represents 10–15% of estimated annual mangrove production. This global rate supports previous conclusions that, on a centennial time scale, 8–15% of all OC burial in marine settings occurs in mangrove systems.
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Organic matter, which is dissolved in low concentrations in the vast waters of the oceans, contains a total amount of carbon similar to atmospheric carbon dioxide. To understand global biogeochemical cycles, it is crucial to quantify the sources of marine dissolved organic carbon (DOC). We investigated the impact of mangroves, the dominant intertidal vegetation of the tropics, on marine DOC inventories. Stable carbon isotopes and proton nuclear magnetic resonance spectroscopy showed that mangroves are the main source of terrigenous DOC in the open ocean off northern Brazil. Sunlight efficiently destroyed aromatic molecules during transport offshore, removing about one third of mangrove-derived DOC. The remainder was refractory and may thus be distributed over the oceans. On a global scale, we estimate that mangroves account for >10% of the terrestrially derived, refractory DOC transported to the ocean, while they cover only
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Single exponential decay models fitted to dry mass and carbon decomposition data for tree trunks in mixed Rhizophora mangrove forests in tropical Australia had decay constants of 0.083 and 0.108 yr-1 respectively. Decay of twigs and small branches was more rapid with decay constants of 0.276 (dry mass) and 0.310 yr-1 (carbon). Aboveground standing stocks of dead wood components in young and mature forests were (g C m-2): trunks, 15.3 and 221.9; prop roots, 5.7 and 86.8; branches, 3.5 and 32.2; twigs, 3.2 and 3.1. Combining decomposition and standing stock data gave flux estimates for wood detritus of 4 and 44 g C m-2 yr-1 in young and mature forests. Leaf burial by crabs, a major pathway by which mangrove detritus is retained in these forests, has previously been etimated at 62 g C m-2 yr-1. -Authors
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