Cyril Marchand

Mangrove production and carbon sinks: A revision of global budget estimates.

results in a conservative estimate of 218 ± 72 Tg C a 1 . When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at 112 ± 85 Tg C a 1 , equivalent in magnitude to 30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

The composition of sedimentary organic matter in relation to the dynamic features of a mangrove-fringed coast in French Guiana

The sedimentary organic matter content of a series of 2-m-deep cores was examined in relation to the evolution of mangrove forest, on the basis of geochemical analyses and optical observations. Avicennia-dominated forest deposits, developing along the highly dynamic coastline of French Guiana, were collected in five stations based on stage of forest evolution. The sedimentary organic matter in the upper sediment of the youngest mangrove swamp is mainly derived from algal mats with low carbon:nitrogen ratios (C:N ratio, from 6 to 8) and typical greyish amorphous organic flakes as observed in optical studies. Indeed, rare young Avicennia trees are present, and effectively, geochemical parameters do not give evidence of a litter made up of higher plant debris, these rare debris being probably exported by the tides. A slight increase with depth in the first decimetres of both total organic carbon (TOC) content and C:N ratio results from the development of the radial cable root-system of the pioneer Avicennia germinans. Early diagenetic conditions of this young forest are rather controlled by dominant suboxic processes, as suggested by high Eh values (range, 200–400 mV) and local anoxic processes (occurrence of pyrite) in micro-environments: this is mainly due to the oxygen available by roots and crab bioturbation. The organic content of the senescent mangrove sediment is mainly derived from higher plant debris in the uppermost 30 cm, as indicated by relatively high C:N ratios and the predominance of ligno-cellulosic debris. The strong decrease in hydrogen index values results from the degradation of the higher plant debris, losing hydrogen bounds through decay processes. Moderately acidic pH values, low Ehs and the presence of pyrite framboids point towards the reducing decay processes in surficial layers of the senescent mangrove mediated by sulphate-reducing bacteria. Whatever the stage of evolution of the forest, the geochemical characteristics of the sediment below 30 cm are those of the shoreface one, with opaque refractory debris derived from the Amazon river detritus discharge. The sediment collected from dead mangrove forests, subsequently recolonized by pioneer mangroves, contains organic markers, which predate recolonization, recording previous phases of erosion and accretion.

Multiple markers pyrosequencing reveals highly diverse and host-specific fungal communities on the mangrove trees Avicennia marina and Rhizophora stylosa

Fungi are important actors in ecological processes and trophic webs in mangroves. Although saprophytic fungi occurring in the intertidal part of mangrove have been well studied, little is known about the diversity and structure of the fungal communities in this ecosystem or about the importance of functional groups like pathogens and mutualists. Using tag-encoded 454 pyrosequencing of the ITS1, ITS2, nu-ssu-V5 and nu-ssu-V7 regions, we studied and compared the fungal communities found on the marine and aerial parts of Avicennia marina and Rhizophora stylosa trees in a mangrove in New Caledonia. A total of 209,544 reads were analysed, corresponding to several thousand molecular operational taxonomic units (OTU). There is a marked zonation in the species distribution, with most of the OTU being found specifically in one of the microhabitat studied. Ascomycetes are the dominant phylum (82%), Basidiomycetes are very rare (3%), and 15% of the sequences correspond to unknown taxa. Our results indicate that host specificity is a key factor in the distribution of the highly diverse fungal communities, in both the aerial and intertidal parts of the trees. This study also validates the usefulness of multiple markers in tag-encoded pyrosequencing to consolidate and refine the assessment of the taxonomic diversity.

Temporal variability of CO2 fluxes at the sediment-air interface in mangroves (New Caledonia)

Carbon budgets in mangrove forests are uncertain mainly due to the lack of data concerning carbon export in dissolved and gaseous forms. Temporal variability of in situ CO2 fluxes was investigated at the sediment-air interface in different seasons in different mangrove stands in a semi-arid climate. Fluxes were measured using dynamic closed incubation chambers (transparent and opaque) connected to an infra-red gas analyzer. Microclimatic conditions and chl-a contents of surface sediments were determined. Over all mangrove stands, CO2 fluxes on intact sediments were relatively low, ranging from -3.93 to 8.85 mmolCO₂·m(-2)·h(-1) in the light and in the dark, respectively. Changes in the fluxes over time appeared to depend to a great extent on the development of the biofilm at the sediment surface. We suggest that in intact sediments and in the dark, CO2 fluxes measured at the sediment-air interface rather reflect the metabolism of benthic organisms than sediment respiration (heterotrophic and autotrophic). However, without the biofilm, sediment water content and air temperature were main drivers of seasonal differences in CO2 fluxes, and their influence differed depending on the intertidal location of the stand. After removal of the biofilm, Q10 values in the Avicennia and the Rhizophora stands were 1.84 and 2.1, respectively, revealing the sensitivity of mangrove sediments to an increase in temperature. This study provides evidence that, if the influence of the biofilm is not taken into account, the in situ CO2 emission data currently used to calculate the budget will lead to underestimation of CO2 production linked to heterotrophic respiration fueled by organic matter detritus from the mangrove.

Trace metal geochemistry in mangrove sediments and their transfer to mangrove plants (New Caledonia)

Because of their physico-chemical inherent properties, mangrove sediments may act as a sink for pollutants coming from catchments. The main objective of this study was to assess the distribution of some trace metals in the tissues of various mangrove plants developing downstream highly weathered ferralsols, taking into account metals partitioning in the sediment. In New Caledonia, mangroves act as a buffer between open-cast mines and the worlds largest lagoon. As a result of the erosion of lateritic soils, Ni and Fe concentrations in the sediment were substantially higher than the world average. Whatever the mangrove stand and despite low bioaccumulation and translocations factors, Fe and Ni were also the most abundant metals in the different plant tissues. This low bioaccumulation may be explained by: i) the low availability of metals, which were mainly present in the form of oxides or sulfur minerals, and ii) the root systems acting as barriers towards the transfer of metals to the plant. Conversely, Cu and Zn metals had a greater mobility in the plant, and were characterized by high bioconcentration and translocation factors compared to the other metals. Cu and Zn were also more mobile in the sediment as a result of their association with organic matter. Whatever the metal, a strong decrease of trace metal stock was observed from the landside to the seaside of the mangrove, probably as a result of the increased reactivity of the sediment due to OM enrichment. This reactivity lead to higher dissolution of bearing phases, and thus to the export of dissolved trace metals trough the tidal action. Cu and Zn were the less concerned by the phenomenon probably as a result of higher plant uptake and their restitution to the sediment with litter fall in stands where tidal flushing is limited.

Changes in soil characteristics and C dynamics after mangrove clearing (Vietnam)

Of the blue carbon sinks, mangroves have one of the highest organic matter (OM) storage capacities in their soil due to low mineralization processes resulting from waterlogging. However, mangroves are disappearing worldwide because of demographic increases. In addition to the loss of CO2 fixation, mangrove clearing can strongly affect soil characteristics and C storage. The objectives of the present study were to quantify the evolution of soil quality, carbon stocks and carbon fluxes after mangrove clearing. Sediment cores to assess physico-chemical properties were collected and in situ CO2 fluxes were measured at the soil-air interface in a mangrove of Northern Vietnam. We compared a Kandelia candel mangrove forest with a nearby zone that had been cleared two years before the study. Significant decrease of clay content and an increase in bulk density for the upper 35cm in the cleared zone were observed. Soil organic carbon (OC) content in the upper 35cm decreased by >65% two years after clearing. The quantity and the quality of the carbon changed, with lower carbon to nitrogen ratios, indicating a more decomposed OM, a higher content of dissolved organic carbon, and a higher content of inorganic carbon (three times higher). This highlights the efficiency of mineralization processes following clearing. Due to the rapid decrease in the soil carbon content, CO2 fluxes at sediment interface were >50% lower in the cleared zone. Taking into account carbonate precipitation after OC mineralization, the mangrove soil lost ~10MgOCha-1yr-1 mostly as CO2 to the atmosphere and possibly as dissolved forms towards adjacent ecosystems. The impacts on the carbon cycle of mangrove clearing as shown by the switch from a C sink to a C source highlight the importance of maintaining these ecosystems, particularly in a context of climate change.

Seasonal Pattern of the Biogeochemical Properties of Mangrove Sediments Receiving Shrimp Farm Effluents (New Caledonia)

Coastal tropical shrimp farming may impact the adjacent ecosystems through the release of large quantities of effluents rich in nutrients. In New Caledonia, mangroves are considered as a natural biofilter to reduce impacts on the surrounding World Heritage listed lagoon. Our main objective was to understand the influence of effluent discharge on the biogeochemistry of mangrove sediments. A monitoring of the physico-chemical parameters of mangrove sediments was carried out during a whole year, including active and non active periods of the farm. The parameters studied were: i) benthic primary production (Chl-a concentrations), ii) physico-chemical parameters of sediments (redox potential, pH, salinity, TOC, TN, TS, δ13C and δ15N), iii) concentrations of dissolved nitrogen, iron and phosphorus. A mangrove developing in the same physiographic conditions, presenting the same zonation, and free of anthropogenic input was used as reference. The concentration of benthic Chl-a measured at sediment surface in the effluent receiving mangrove was twice to three times that measured in the control zone whatever the season. We thus suggest that nutrients inputs significantly increased the phytobenthic production in the effluent receiving mangrove during the whole year, even after the cessation of discharges and because of natural seasonal dynamic of phytobenthos. Although the flow of surface OM was increased, the OM content at depth was not higher than in the control mangrove. However, the contribution of mangrove detritus to the sedimentary organic pool was higher probably as a result of higher density and much greater individual size of the mangrove trees. Unlike the control mangrove sediment, the effluent receiving mangrove sediment was not stratified, redox potential values were high and presence of Fe3+ was detected down to 50 cm depth, probably as a result of a larger root system, allowing a better sediment oxygenation and accentuated OM decomposition processes, and thus limiting ecosystem saturation.

Meiofauna distribution in a mangrove forest exposed to shrimp farm effluents (New Caledonia)

Meiofauna abundance, biomass and individual size were studied in mangrove sediments subjected to shrimp farm effluents in New Caledonia. Two strategies were developed: i) meiofauna examination during the active (AP) and the non-active (NAP) periods of the farm in five mangrove stands characteristics of the mangrove zonation along this coastline, ii) meiofauna examination every two months during one year in the stand the closest to the pond (i.e. Avicennia marina). Thirteen taxonomic groups of meiofauna were identified, with nematodes and copepods being the most abundant ones. Meiofauna abundance and biomass increased from the land side to the sea side of the mangrove probably as a result of the increased length of tidal immersion. Abundance of total meiofauna was not significantly different before and after the rearing period. However, the effluent-receiving mangrove presented twice the meiofauna abundance and biomass than the control one. Among rare taxa, mites appeared extremely sensitive to this perturbation.

Tidal variability of CO 2 and CH 4 emissions from the water column within a Rhizophora mangrove forest (New Caledonia)

We performed a preliminary study to quantify CO2 and CH4 emissions from the water column within a Rhizophora spp. mangrove forest. Mean CO2 and CH4 emissions during the studied period were 3.35±3.62mmolCm-2h-1 and 18.30±27.72μmolCm-2h-1, respectively. CO2 and CH4 emissions were highly variable and mainly driven by tides (flow/ebb, water column thickness, neap/spring). Indeed, an inverse relationship between the magnitude of the emissions and the thickness of the water column above the mangrove soil was observed. δ13CO2 values ranged from -26.88‰ to -8.6‰, suggesting a mixing between CO2-enriched pore waters and lagoon incoming waters. In addition, CO2 and CH4 emissions were significantly higher during ebb tides, mainly due to the progressive enrichment of the water column by diffusive fluxes as its residence time over the forest floor increased. Eventually, we observed higher CO2 and CH4 emissions during spring tides than during neap tides, combined to depleted δ13CO2 values, suggesting a higher contribution of soil-produced gases to the emissions. These higher emissions may result from higher renewable of the electron acceptor and enhanced exchange surface between the soil and the water column. This study shows that CO2 and CH4 emissions from the water column were not negligible and must be considered in future carbon budgets in mangroves.

Biogeochemical Cycles: Global Approaches and Perspectives

This chapter uses a literature review to advance our understanding of the biogeochemical functioning of mangrove wetlands and to elucidate biogeographic differences. We identify potential sources of variation in biogeochemical processes among various locations and analyze current advances in evaluating transformations of carbon (C) and other critical elements (e.g., N, P, S, Fe, Mn) in the context of mangrove conservation and management priorities. We also identify knowledge gaps and research priorities across biogeographic regions and latitudes. Mangrove forests inhabit unique locations at the interface between marine and terrestrial environments and, as such, possess attributes that characterize both environments. Because of their transitional position in coastal regions, mangrove forests around the world are increasingly vulnerable to anthropogenic activities (e.g., deforestation and urban settlement) and associated environmental impacts. This threatens the critical ecosystem services provided by mangrove wetlands through their biogeochemical functions, including climate change mitigation, flood regulation, and water purification. Key differences in mangrove functionality among regions are currently difficult to explain due to limited data availability, further confounded by variations within and among mangrove forests depending on hydrological regime, mangrove ecotype, and local geomorphology. Rates and pathways of microbial C and nutrient transformation in mangrove sediments are dependent on a number of fundamental factors including organic matter input, electron acceptor availability, bioturbation activity, and presence/density of tree roots, as well as local hydroperiod. The spatial heterogeneity of redox processes caused by burrows and roots (oxygen pumping) is much more complex and variable in intertidal mangrove sediments than in adjacent coastal and oceanic sediments. Nitrogen and phosphorus are critical nutrients that regulate the magnitude and spatial distribution of both net primary productivity and structural properties of mangrove wetlands. The waterlogged and anoxic mangrove sediments promote slow decomposition rates allowing significant C sequestration and long-term organic C accumulation in the sediments. Accordingly, mangrove wetlands have a strong climate change mitigation function and thus act as sinks for atmospheric CO2. In addition, emissions of other greenhouse gases, methane and nitrous oxide, from mangrove sediments are usually low because of their oxidation before reaching the sediment–air interface. High rates have only been reported under excessive anthropogenic nutrient inputs.