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Hfli thao khoa hpc - Hpp tic Qu6c tg digu tra nghign ctm tai nguygn va mfli trufmg bi^n HEAVY METALS AND CARBON DYNAMICS IN MANGROVE ECOSYSTEM: PREVIOUS RESULTS FROM NEW CALEDONIA, AND RESEARCH PERSPECTIVE IN HALONG BAY Cyril Marchand Institut de Recherche pour le Developpement (IRD), UR 206/ UMR 7590 IMPMCThe mangrove is a specific ecosystem of the intertidal zone, having de, 98848 Noumea, New Caledonia, France; cyril.marchand@ird.fr Introduction The mangrove is a specific ecosystem of the intertidal zone, having developed adaptation capacities to the extremely selective environment, and which the distribution of different plant species is based on parameters such as salinity (Walsh, 1974), duration of flooding (McKee, 1993), sedimentation rate (Ellison, 1998) The mangrove is, with Rainforest, one of the most productive ecosystems in land area, i.e 30.0 Tmol C y"' (Twilley et al 1992; Jennerjahn and Ittekkot, 2002, Along! et al., 2005, Kristensen and al., 2008) The mangrove is crucial at botii ecologically and economically levels The mangrove has \ key role in the conservation of tropical coastiines First, it stabilizes the shoreline, and serves as barrier against erosion caused by waves, by reducing wave energy, and changing hydrocirculations Moreover, because of its high productivity, the mangrove is the basis of nutrient cycling in coastal environments Coastal waters bordering mangroves are generally rich in shrimps and fishes; mangrove has thus a vital importance to the fishing industry Moreover, the mangrove acts as a home to a large animal biodiversity, notably endangered species live there Some fishes come to reproduce, others ensure their growth It serves as protection for juveniles before migrating into coastal waters Predators come to feed Crabs are numerous In monetary terms, income related to fisheries undertaken by the presence of mangroves is estimated at 10 000 $ per hectare per year, with a high variability between areas and market value The total value of goods and services paid by the mangroves can reach up to 200 000 $ per hectare per year Currently, the mangrove occupies about 75% of tropical coastlines, on neariy 200 000 km' , and its vegetation is composed of ± 19 families, made up to ± 27 genera and ± 70 species (Ellison and Farnsworth, 2001) However, due to population grovrth, increased urbanization, expansion of industrial activities, exploration and exploitation of natural resources, the mangrove is now fading at a rate of I to 2% per year This rate is equivalent or even higher than that of threatened ecosystems such as coral reefs or primary rainforest (UNEP-WCMC, 2006, Valiela et al 2001; Wilkie and Fortuna, 2003) The destruction of the ecosystem takes place all-around the world, Cyril Marchand 365 Woriishop -Inteinanonal Coopeiation on Investigation and Research of Marine Natural Resouice and Envimm^g^ especially in emerging countries, where 90% of mangroves are Within this context, the French Research Institiite for Development (IRD) intents to develop research programs on the evolution of mangroves in relation with nahiral and anthropogenic pressures On the one hand, the programs developed focus on carbon cycling in mangroves, understanding its origin and fate, paying special attention to CO2 fluxes in different compartment (soil, water and canopy) On the other hand, these programs seek to describe the ability of mangroves to act as a sink for trace metals coming from different kind of watersheds (urban, aquaculture, mining areas) Heavy metals dynamic in the mangrove of New Caledonia Due to their persistence, potential toxicity, and bioavailability, heavy metals represent a major threat for mangrove biodiversity and also for human health Additionally to their anthropogenic origin (e.g mining activities, industries; etc.), natural processes, such as geologic weathering of soils and rocks, increase their occurrence Heavy metals are transported by water or wind to coastal areas, where they can be deposited as sediments Because of the capacity of mangroves to efficiently trap suspended material from the water column (Furukawa et al., 1997), and the high affinity of organic matter (OM) for hea'vy metals (Nissenbaum and Swaine, 1976), mangrove sediments have a large capacity to accumulate these pollutants (Lacerda et al., 1988; Tam and Wong, 2000) Ramos et al (2006) suggested that mangrove trees can be considered as a biochemical reactors, not only because of their physiological and biochemical processes but also due to their active role in organic matter decomposition within the sediments that greatly influence the mobility of heavy metals Mangroves can act as a long term sink for heavy metals because of their precipitation with sulfides during diagenetic reactions and the relative stability of these minerals (Huerta-Diaz and Morse, 1992) However, mangrove organic-rich sediments are subjected to various diagenetic processes Sulfate reductjon is thought to be the dominant process, but aerobic respiration and also Fe and Mn respiration can be important OM decomposition pathways in mangrove sediments (Alongi et al., 1998; 2000; Kristensen, 2000) Decay processes vary spatially and temporally as a result of various factors, such as the ability of root system to diffuse air into the sediment, intense faunal bioturbation, and seasonal alternation of water logging (Scholander et al., 1955; Clark et al., 1998; Marchand et al., 2004; Marchand et al., 2006) This variability may induce changes in heavy metals distribution between dissolved and particulate phases, and thus changes of their toxicity As an example, metals in pore-waters are more bioavailable than ones adsorbed on mineral surfaces, and can be easier uptake by organisms, thus entering foodwebs As a consequence, mangrove sediments may shift from a heavy metals sink to a heavy metals source for adjacent systems (Harbison, 1986) In New-Caledonia, mangroves are associated with a lagoon of more than 20,000 Km^ delimitated by an almost continuous coral barrier reef of over 1500 km in total length Extensive mangrove are fringing 80% of the western coastiine of the Island and 20% of the eastern's As 3*6 Cyril Marchand HQi thao khoa hpc - Hpp tac Ougc lg uong digu mi, nghign ciru tai nguygn va mfli trudng bign a consequence of their distribution, mangroves represent a major source of nutrients for the lagoon Nevertheless, the economic development of the Island increases the pressure on mangrove environment New-Caledonia is currently the 3' nickel producing countiies in the world Anthropogenic pressure can be expressed following two main ways The former corresponds to the construction of road and buildings, i.e urbanization, and the latter is the mining activities (Ni-ore) occurring in lateritic soils located upstream mangrove areas Processes of erosion and sedimentation along the coastiine, and which occur naturally, are strongly amplified by mining activities and urbanization (Femandez et al., 2006) The purpose of this study was to understand the distribution of some heavy metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) in sediments and pore-waters in mangroves, one for which the catchment not contain significant Ni occurrence, and the second, which had suffered from nickel mining In this context, we were interested in the relationships between mangrove stands, sedimentary organic matter, pore-water parameters and heavy metals concentrations To reach our goal, quantitative analyses, e.g Rock-Eval and HR-ICP-OES, were carried out on cores collected beneath living mangrove stands, Avicennia marina and Rhizophora stylosa, and in an unvegetated coastal area Our conclusions can be summarized as follow : - Organic content is higher beneath the Rhizophora stand than beneath the Avicennia stand We suggest that this can result from the more developed rootsystem of Rhizophora compared to the radial cable root system of Avicermia that develop only in the subsurface The difference of extension of their root system may also explain that the OC enrichment is deeper beneath Rhizophora than Avicennia - Organic sources also differ beneath the two species, and as a result of their distribution Beneath Rhizophora, the sedimentary organic pool derives mainly from woody tissues, whereas beneath Avicennia, the pool can be a mixture of decomposing leaves and wood Actually, Rhizophora stand develops on the seaside of the mangrove, and is continuously subjected to tidal flushing, and thus leaf litter cannot accumulate compared to the Avicennia stand that develops on the landside of the mangrove Mangrove zonation also induces differences in the redox conditions that occurred beneath the species We suggest that the suboxic cdfiaitidns that prevailed in the upper sediment beneath the Rhizophora stand result from air diffusion from the atmosphere This layer is not very deep because Rhizophora stand is frequently submerged by tides Below this layer, and as a result of the high organic content, the conditions were anoxic and sulfidic Conversely beneath Avicennia, suboxic conditions prevailed deeper We suggest that oxygen difftision from atmosphere is more important because of its higher position in the intertidal zone, and that this diffusion add its effect to the one via the root system, which is well known to create oxidized rhizosphere Consequently, the present study Cyril Marchand 367 Woricshop "Intemational Coopeiation on Investigation and Research of Marine Natural Resource and Environing confirms that the length of waterlogging of a mangrove stand, depending on its topographic situation in the intertidal zone, seems to be a main factor contiolling redox conditions in the upper sediment, as well as the activity of the root system - Despite its high organic content, the pristine mangrove is not enriched in heavy metals, except Ni (Tab 1) We suggest that even if the catchment of the present study is not enriched in Ni, coastal sediments of the bay present relatively high Ni concentrations, probably as a result of the current that bring sediments originated from lateritic watershed Cr (pmol g" Mean (n=45) S.D Max Min V 1.22 0.66 3.11 0.36 Ni (pmol g Cu (pmol g"') ') 0.22 0.12 0.51 0.08 l.ll 0.82 3.55 0.03 (pmol g' Mn (Hmol g" Co (pmol g ') ') ') 331.85 175.66 721.69 22.64 2.90 1.05 5.00 1.13 0.14 0.08 0.38 0.01 Fe • Tab 1: Concentrations of heavy metals in the pristine mangrove - Mangrove sediments developing downstream Ni mining area are highly enriched in Ni and Cr (Tab 2) Concentrations in Ni are 10 to 100 times higher than in mangrove developing downstream a watershed that is not ultramafic - Heavy metals distribution within sediments and pore-water appear to result from diagenetic processes linked to OM decomposition The higher the organic content, the higher the sulphur content too As a consequence, with our data set, it was not possible to decipher between heavy metals complexation with humic acids or incorporation into sulphide minerals Cu Co (pmol g" (pmolg'") ') Mean (n=39) S.D Max Min 3.38 1.13 6.83 1.34 0.207 0.06 0.371 0.093 Ni (pmol g" 44.19 10.29 65.37 21.89 Cr (pmol g' 31.91 6.06 40.22 20.18 Fe (pmol g Mn (pmol g' ') ') 1997.5 359.94 2682.86 1003.34 8.81 4.41 20.99 2.21 Tab 2: Heavy metals concentrations in a mangrove developing downstream an ultrabasic area - Concerning heavy metals in the dissolved phase, the higher concentrations were measured in the upper suboxic conditions of the Avicennia forets We suggest that because of the specificity of the Avicennia root system and its position in the 368 Qyjj^ Marchand Hgi thao khoa hpc - Hop tac Ou6c tl digu tra nghien cuu tai nguyen va moi truong bign intertidal zone, heavy metals in the dissolved phase present higher concenfrations than beneath Rhizophora stand, and thus that heavy metals are more bioavailable and potentially more mobile beneath Avicennia stand This specificity enhances the potential of some metals entering into the food chain beneath Avicennia Project concerning carbon dynamic in the mangrove of Halong Bay Processes of carbon storing and transfer, between the various reservoirs of its biogeochemical cycle, plays a key role on the pressure of CO2 and CH4 in the atmosphere, and were the subject of numerous studies (see e.g reviews of Bemer in 1989, Hedges 1992, Forster Rigby in 2007 or in 2008) The recent increase pressure, continuous and rapid, of CO2 in the atmosphere (280 ppm in 1750,367 in 1999 ppm, 379 ppm in 2005, IPCC 2007) and CH4 (0.7 ppmv in 1750 to 1.8 ppmv 2007) arose from ,the use of fossil fuels, but also from change of land use by humans Their radiative effect, today testified on climate change across the globe, attracted the attention of many researchers, on the one hand, oh the quantification of CO2 and CH4 emissions in the atmosphere and, secondly on ecosystems that are able to fix and store carbon Understanding the factors influencing the fluxes of CO; and CH4 between the various reservoirs of its cycle has become a research priority at the global level Because of its high productivity, its global disfribution, and its position at the interface between land and sea, mangroves are considered important ecosystem in carbon cycling It has dual functions, sink for atmospheric CO2, and source of organic and inorganic carbon to the coastal areas 70% of CH4 natural emissions are located in wetlands It is now estimated from the limited data available that the tropical environment is responsible for 60% of natural emissions to an area representing only 35% of the total area of wetlands However recent estimates of carbon balance in mangroves contain many uncertainties When one combines the various carbon sinks within the mangrove, that is to say, export, burial, and mineralization, the latter represent only 50% of carbon fixed by mangroves during the photosynthesis (Bouillon et al., 2008) The physico-chemical and biological processes involved in the fluxes of CO2 and CH4 are sensitive to biotic and abiotic conditions of the environment Fluxes are likely to vary in both space and time, both daily (tides, light) and seasonally Thus, trees release CO2 during photorespiration associated with photosynthesis, and during respiration linked with diurnal and nocturnal metabolic acti-vities Furthermore, breathing consttuction starts with the beginning of the growing season and remains very active as the plant grows In turn, a higher atmospheric concentration of CO2 will reduce photorespiration, because the increase in the ratio CO2/O2 ensuing promotes CO2 fixation by the enzyme Rubisco at the expense of O2 Monitoring high frequency and large spatial coverage of the fluxes of CO2 and CH4 will allow a better estimate of the contribution of mangroves in the overall Cyril Marchand 369 Workshop: -Intemational Cooperation on Investigation and Research of Manne Natural Resource and Environiiienf^' balance of carbon; an intemational issue which has now become crucial in the context of global warming The site chosen will be inshiimented to determine the continuous fluxes of COj and CHi from the ecosystem by an eddy covariance system These measurements will be linked to ad-hoc analysis effluxes of CO2 and CH4 from mangroves soils and tidal channels draining sites Among the terrestrial carbon pools, soil pool represents one of the largest reservoirs (Lai, 2004) with the wetiands as a major component (Chmura et al 2003; Krankina & Dixon, 1995) Soil is also the main source of CO2 from land (Schimel, 1995) and it is generally accepted that mangrove sediments behave as a net source of carbon to the atmosphere (Borges et al., 2003) The soil CO2 is produced primarily by decomposers (heterotiophic respiration and nonrhizosphere) and living roots (autotrophic respiration and rhizosphere) In order to estimate the contribution of each sources in tlie total CO2 efflux, a transect will be left as it is (autotrophic and heterotrophic respiration), and the other will be subject to an exclusion protocol to inhibh autotrophic respiration, while the flux will be related solely to heterotrophic respiration CO2 fluxes exhibit temporal variability (daily and seasonal) Each transect will be monitored once every months Furthermore, measures will be implemented over 24h to assess the significance of daily cycles The flux of CO2 from the sediment will be measured using a closed chamber system coupled to a gas analyzer to Infra Red (IRGA) The system provides a measure of the rate of change (accumulation or decrease) of COj concentrations in tiie chamber per unit of time, for a surface and a known sample volume To estimate the contiibution to CO2 fluxes of different compartments present in the chamber (biofilm chlorophyll, sediment respiration ), two types of chambers will be used, an opaque and a transparent Alongside these measures, and in order to study the factors contiolling the variation of these fluxes, each point will be characterized in terms of sediment and air temperatures, relative humidity of the sediment and photosynthetically active radiation (PAR) The leaf area index (LAI) of each facies and chlorophyll a in the biofilm will also be studied Rivers and estuaries have frequently marked supersaturation of CO2 partial pressure (Borges & Frankignoulle, 2003, Cai & Wang, 1998; Frankignoulle et al., 1996a, 1998, Guerin et al 2007; Zhai et al 2007 ) and may represent a net source of CO2 to the atmosphere Thus, the aquatic compartment of mangroves is considered as net heterotrophic, considering the additional input of organic carbon that is decomposed (Borges et al, 2003, Kong & Borges, 2008, Gatusso et al., 1998) As a result, and although significant developments and occasional phytoplankton blooms that can affect the direction of exchange of CO2 between the water surface and atinosphere (Mukhopadhyay et al., 2002), the water column in the mangrove is a source of carbon to the atmosphere However, these fluxes are poorly evaluated and the impact of mangroves as a source of organic and inorganic 370 Cyril Marchand HOi thao khoa hpc - Hop tac Qu6c tg trpng digu tra nghign ctru tai nguygn va mfli uvfmg bign carbon to the river has been little studied Accordingly, the protocol established will study spatial and temporal variability of these fluxes, as well as the physical and chemical parameters of the water An upstieam-downstream transect of several fixed stations (GPS tracking) along the rivers traversing the selected mangrove will be followed regularly for the study of CO2 fluxes at the air-water interface Thus, flie measurement campaigns on this transect will be conducted over years, once per season (wet vs dry seasons) and twice a tidal cycle (low vs high tide) Fluxes will be measured directly through a system of floating chamber coupled with IRGA (Frankignoulle, 1988; Frankignoulle et al 1996a; Guerin et al., 2007) The system allows a measure of the change in pressure of CO2 in the air trapped in the chamber situated at the water surface (Frankignoulle, 1988) Six measurements per station will be conducted during each campaign To study the daily dynamics of CO2 fluxes at the air-water interface, continuous measurements will be performed during 24 hours (1 measurement per minute) Because of the difficulties of this protocol, these measures will be conducted once per season over a year and on two stations (upstream vs Downstieam, Figure 5.1) In this case study, fluxes are not measured directly but are calculated from direct and continuous measurements of pC02 in the water To this, an equilibrator is coupled to an IRGA (Borges et al 2003; Frankignoulle & Borges, 2003; Frankignoulle & Borges, 2001; Frankignoulle et al 2003; Zhai et al., 2007) The technique is based on measuring the pressure of C02 in the air balanced with the sample water through equilibration REFERENCES Alongi, D.M., Clough, B.F., Robertson, A.I., 2005 Nutrient-use efficiency In arid-zone forests of the mangroves Rhizophora stylosa and 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" ! ' ' '•:, ^°^'"- •';'- ^ ° * ' '•^- 2'""- Mangrove forests: one of the world's threathened major tropical environments BioScience 51 -807-815 ^^'Z1lL°fu t"!"- '^^"Sroves, a review In: Relmold, R.J., Queens, W.H (Eds) EcologyofHalophytes.AcademlcPress.pp 51-174 "• li^us.j, ''ouTckBM t r / f ã"ã' ' 'ããã ^'^'"ô °- '•• ^°°'- Comparison of IKONOS and £ o ? e l r ? f n M T ) ! ' r p « T r " ^ " " ' ^ °" '"^ ^ " " " ' ^ » ""^ " " - ^ ^ 50 Wilkie, M.L., Fortuna, S., 2003 Status and trends In mangrove area extent worldwide Forest Resources Assessment Working Paper No 63 Fore'st R e s o u r c e s " " ; " f ninner n e ; eestuary « i l ' oof f ' ^Chanjiang L ° i ° ' ''•^v'""? river, * ™ ' 'China ' ''''""Marine '"' chemistry ^02 degassing fluxes in the (Yangtze) 107: 342-256 Cyril Marchand ... entering into the food chain beneath Avicennia Project concerning carbon dynamic in the mangrove of Halong Bay Processes of carbon storing and transfer, between the various reservoirs of its... ecosystems and the carbon cycle Global chanw Blologgy 1:77-91 ^ 45 Twilley, R.R., Chen, R.H., Hargis, T, 1992 Carbon sinks in mangrove forests and their implications to the carbon budget of tropical... the total area of wetlands However recent estimates of carbon balance in mangroves contain many uncertainties When one combines the various carbon sinks within the mangrove, that is to say, export,
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