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Vulnerability of intertropical littoral areas
Isabelle Manighetti, Rutger de Wit, Stéphanie Duvail, Patrick Seyler
To cite this version:
Isabelle Manighetti, Rutger de Wit, Stéphanie Duvail, Patrick Seyler. Vulnerability of intertrop-
ical littoral areas. Comptes Rendus Géoscience, Elsevier Masson, 2017, 349 (6-7), pp.235-237.
�10.1016/j.crte.2017.10.001�. �hal-02082790�
Foreword
Vulnerability of intertropical littoral areas
Thecoastalzoneisofveryhighimportanceforhuman development and human wellbeing. Half of the global urbanpopulationlivesin the coastal zone,whereit has accesstobothcontinentalandmarineecosystemservices andtomaritimetransport.Theseurbanpopulationscoexist with rural and traditional coastal populations, some of whichstillpossessgoodtraditionalecologicalknowledgeof thecoastalecosystems.Marinebiodiversityandfavourable environmental conditions sustain fisheries and aquacul- ture,representasourceofinspirationforhumankindand provide numerousopportunitiesforrecreationandtour- ism.Inaddition,coastalareasprovidenurseryfunctionsfor juvenilefishandinvertebrates,whichisimportantforthe fishandcrayfishstocksexploitedoffshore.Locatedatthe interfacebetweenmarineenergyandcontinentalprocess- es, the coastal landscapes are dynamic environments.
Nevertheless,thedestructionofhabitatsandtheincreasing exploitationofthecoastalzonerepresentseriousthreatsto theecosystems.Moreover,humanlanduseandmodifica- tionsinthewatershedshavestrongimpactsonthecoastal zone primarily by contributing to their pollution and nutrient over-enrichment. Damming and creation of reservoirsupstreamalsoheavilymodifythehydrologyof thewatershedsandoftendramaticallyreducethedelivery of sediments to the coastal zone. In addition to these regionalandlocalanthropogenicimpacts,thecoastalzone is vulnerable to global change among which sea level riseandclimatechangeareparticularlyimportantdrivers.
Manycoastalzonesextendalonggiantfaultsandsubduc- tion zones, which makes them particularly exposed to earthquakesandtsunamihazards.Otherformsofnatural hazardsarecausedbyhurricanesandcyclonesthatdevelop atseaandwhosetrajectoriesoftenhitthecoastlines.
Thequestiononhowtocombinetheconservationofthe highlyvaluablecoastalecosystemsandtheirsustainable exploitationwiththestrongsocio-economicdevelopment inthecoastalzoneisamajorchallengeforhumankindin the 21st century. This requires both a high degree of political will and appropriate governancetogetherwith
sound scientific knowledge. Hence, many international andnationalorganisationsforresearchanddevelopment workinthecoastalzonetocontributetotheobjectivesof sustainabledevelopment.TheFrenchIRDisoneofthem,as itdevelopsresearchintropicalandsubtropicalcountriesin partnershipwiththeirrespectivenationalinstitutions.
Thisthematicissueprovidesacollectionofresultsthat illustratesthe involvementofIRD,ofitspartners,and of othernationalandinternationalgroupsworkinginthefield oftropicalandsubtropicalcoastalenvironments.Overall, the objectivesof theresearchinclude: (i)toidentifythe major forcing factors and drivers of changes and to understandthemechanismsinvolved,(ii)toexaminethe possibility of a harmonious existence between socio- economicdevelopmentandbiodiversityconservationand tostudypotentialconflicts inthephysicalandbiological systems as well as in human societies, (iii) to develop observables, indicators and techniques to monitor the coastalzonesandtheprocessesatplayinordertoalleviate theimpactandthreatsofnaturalhazards.
Forthis purpose,thecoastalsystems’vulnerabilityis
‘‘instrumental’’.Vulnerabilityisaconceptthatrelatesto the extent to which perturbations, including hazards, threats,anddisturbances,canharmasystemandpushit into a different and often undesired state. Hence, vulnerability provides a measure of the propensity of thesystemtosustainchangeinducedbytheseperturba- tions. Assuch, it is related totheconceptof resilience, which describes the capacity of a system to absorb a perturbationwhile maintainingits integrity inthe long term,althoughitoftengoesthroughatemporaryperiodof deviation.Theseconcepts canbeappliedtogeophysical and ecological systems. Moreover, when the objective concerns the interdependence of people and their sur- roundingecosystems,itisappropriatetoadopttheconcept ofsocio-ecologicalsystemsforvulnerabilityassessment.
Three articles in this issue have a main focus on understandingtheprocessesthatdrivecoastalmorpholo- gy and how these can be impacted by natural forcing.
C.R.Geoscience349(2017)235–237
ContentslistsavailableatScienceDirect
Comptes Rendus Geoscience
w ww . sc i e nce d i re ct . co m
https://doi.org/10.1016/j.crte.2017.10.001
1631-0713/ C 2017Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Indeed,coastalerosionisaveryimportantissueforcoastal populations that may endanger their livelihood and survival.Besset etal. (this issue) analyse theimpactof the 2008 cyclone Nargis on the geomorphology of the Ayeryarwadydelta(Myanmar,formerlyBurma)byusinga combinationofdataincludingremotesensing,meteoro- logical (precipitation, wind speed and trajectory of the cyclone)andmeasuresofsuspendedparticles.Theyshow thatthecycloneinducedastrongerosionofthecoastline thatpersistedtwoyearsaftertheevent,thusdemonstrat- ingthelackofresilienceofthisdeltaicsystem.Aucanetal.
(thisissue)characterisethedifferenttypesofwavesthat occur in the coral-reef lagoon of New Caledonia. They discussthepossibleimpactofthreedifferentwavetypes onthelocalerosionandaccretionofsmallislandsinthe lagoon.Moreworkisneededtoestablishtheselinksmore precisely.However,theyinterestinglypointoutthatthe so-calledinfragravitywaves,whichformbybreakageof low-frequencywaveson thecoralreef,mayhaveahigh impact on the islet erosion, in addition to the more commonhigh-frequencywavesgeneratedbywind.Gratiot etal.(thisissue)focustheirstudyontheMekongdeltain Vietnamwhere damming acrossthe Mekong River has markedlyreducedthedeliveryofsedimentparticles.They arguethat,tounderstandthesedimentationinthedelta anditsabilitytocounteractthecoastalerosion,a sound understandingofthedynamicbehaviourofsedimentary flocsin suspensionisnecessary.Using field studiesand laboratoryexperiments,theauthorsdeterminetheinflu- enceofsedimentconcentration,turbulenceanddifferen- tialparticlesettlingontheflocculationprocess.Theyfind thatflocculationbydifferentialsettlingisapredominant factorforsiltationinquiescentenvironments.Thisprocess is evidenced in the inner mangrove fringe, where sedimentsareindeedcurrentlydepositingrapidly,modi- fyingtheequilibriumofthedeltazone.
Five articles have a major focus on the impacts of anthropogenicdrivers ofchangein thecoastalenviron- ments,among which three address theimpacts on the socio-ecosystems and two are more methodological.
Duvailetal.(thisissue)focusonthechangesinthedeltas ofthemainriversinEasternAfricaand Madagascar.By upstreamdamminginmanyofthoserivers,theseasonal flooding has decreased or disappeared with a negative impact on the productivity and delivery of ecosystem servicestothetraditionallocalpopulations.Theauthors adoptasocio-ecosystemapproachtoanalysethevarious management models, and they highlight contradictory approachesbetweencommercial hydroelectricalcompa- nies and conservation actors. They argue that these tensionscouldbeeasedbymaintainingorrestoringthe naturalhydrodynamicsthroughmanagedfloodreleases.
Mohamed et al. (this issue) analyse the water table in Nouakchott,thecoastalcapital ofMauritania,and show that,paradoxically,itswatertablehasrisendespiteitsarid climate.Thisisexplainedbylargewaterimportationfrom anexternalsource(theSenegalRiver)tothecityforhuman consumption,withthisexcesswatereventuallyreturning tothegroundand fillingthenaturalaquifer.Theriseis documentedbypiezometricobservationsanditsinterac- tionwithasalineintrusionfromtheAtlanticis studied
with chemical analyses and isotope data. The current increase in the water table causes regular flooding, damages the buildings and infrastructure, and creates humanhealthissues.Araujoetal.(thisissue)useLandsat satellite data to show the regression of the mangrove surfaceintheSepetibaBay,Brazil,overthelast30years, and theconcomitant increase in the urbanisedarea. In addition, theyanalyse the chemical composition of the intertidalsedimentflatsanddocumentthepollutionofthe sedimentsbyanancientelectricalplatingplant.However, themetalcontaminationisrelativelymodestinthebay and theauthors suggest that mangroves contribute, by absorption, totheabatement ofthecontaminantlevels.
The authors concludethat theongoing decrease in the mangrovecovermayenhancethepollutionofthebayin thenearfuture.
Two methodological articlesfocus on the problemof bacterialcontaminationinthecoastalzone.Rioetal.(this issue)developa ‘‘precipitationindex’’(API,basedon the precipitationaccumulatedduringthelast10days),anduse itasapredictorfortheamountoffaecalpathogenbacteria intheoutflowofthetributariesintothecoastalzone,and fortheirconcentrationinthecoastalwaters.Althoughthis toolhasbeendevelopedforaMediterraneancontext(NW Mediterranean,Nıˆmes–Montpellierregion),itisexpected thatitcaneasilybeadaptedandimplementedintropical and subtropicalenvironments.Almakkietal.(thisissue) developanewmethodtostudytheresistancetoantibiotics ofthebacteriacommunitiescontainedinfreshcontinental and brackish coastal waters. The method is based on culturing bacteria isolated from those environments, in different conditions of antibiotic concentrations (three antibioticstested).Theanalysedbacteria arefoundtobe resistant,evenwhengrowinginwaterswithhighconcen- trationsofantibiotics.Theresistanceismorepronouncedin brackishlagoonwaters.Theoccurrenceofresistantbacteria isproblematicastheresistantcommunitiesincludesome pathogensthatmayrepresentseverehealthproblemsfor humans.Again,whilethemethodwasheredevelopedina Mediterranean context (NW Mediterranean, Montpellier region),theauthorsarguethatitcaneasilybeadaptedto tropicalandsubtropicalenvironments.
Two articles together consider the impacts of both naturalandanthropogenicdriversofchange.Grenzetal.
(thisissue)reviewtheexistinginformationofthebenthic ecology(i.e.theecologyofthecommunitieslivingatthe seafloorandintheintertidalsediments)intheTerminos lagoon, Mexico.Most ofthis information is synthesized fromgreyliteratureinSpanish.ThelargeTerminoscoastal lagoon has been subjected to important anthropogenic forcingoverthelastdecades.Primarily,strongurbanand harbourdevelopmentinCiudaddelCarmenonthecoastal barrier is related to the development of offshore oil extractionintheadjacentGulfofMexico.Thisisreflected by the presence of contaminants (polycyclic aliphatic hydrocarbons,chlorinatedcompoundsandheavymetals) in the sediments, the water and the tissues of certain animals in the lagoon. Another important driver is deforestationofthewatershedthathasledtomodifica- tions inhydrologyandsedimentloadings tothelagoon.
Theauthorsarguethattheseenvironmentalchangeswill Foreword/C.R.Geoscience349(2017)235–237
236
interactstronglywithglobalchanges,particularlyseawa- ter rise, and advocate the importance of developing monitoring programs to anticipate the forthcoming ecosystemchangesintheTerminoslagoon.
Tounderstandandanticipatetherapidevolutionofthe coastal zones, it is necessary to monitor it through theacquisitionoflongtimeseriesofhigh-resolutiondata.
The most appropriate approach is to settle permanent observatoriesinpilotsitesrepresentativeofthezonesand evolutions under concern. Mahabot et al. (this issue) providerecommendationson what suchanobservatory shouldbetomonitorthemorphologicalchangesofcoastal beachesinanintertropicalreefenvironment.Therecom- mendations are illustrated on the Ermitage back-reef beachsiteinLaRe´unionIsland,wheretheauthorshave collected longseriesofdata.Theauthorspresentrecent technologies suchasTLS,Structure fromMotionphoto- grammetry and Lidarbathymetry to repeatedly acquire high-resolution topographic and bathymetric data over largeareasatalowtomoderatecost.Theacquireddatacan then be used to quantify the ongoing morphological changesofthecoastalareas.
Finally,threepapersexaminethecurrentparadoxesin themanagementofthelittoralareasandquestionhowthis management willallow thesustainabledevelopment of thesezones.Cormier-Salemetal.(thisissue)focusonthe Can Giomangrove intheMekong delta.Theypresent a pilotstudythatinvolvesbiologistsandsocialscientistsin definingtogethertheecosystemservicesdeliveredbythe mangrove. They compare the seafood and micro-orga- nismsproductioninseveralareasoftheCanGiomangrove biospherereserve,characterizedbydifferentmanagement policies.Theyconfronttheresultswiththelocalfishers’
perceptionofseafooddiversityandtheirknowledgeofthe protectionmeasures.Theinhabitantshaveagoodknowl- edgeoftheneedtoprotectthemangrove,andthismight favour the fair preservation of the biodiversity that is observed in the study. More work needs to be done however to validate this hypothesis, and the authors advocatetheneedforfurtherinterdisciplinarystudieson mangrove ecosystems services.Claeyset al. (this issue) giveaninsighton atopicinthecurrentfrontnews:the paperaimsatcharacterizingthedifferentialvulnerabilities to environmental hazards of two coastal towns of Guadeloupe.Thestudyexploresthehistorical, legaland
socio-economical context of the two cities to explain the current complexity of the coastal management in Guadeloupeandtounderstandthedriversofthepopula- tionvulnerability.Thestudyhighlightstheadverseeffects ofwell-intendedpublicpoliciesthat,insteadofreducing therisksforthecoastalpopulations,haveweakenedthe legalsituationofthemostvulnerable.Italsoshowsthat reducingvulnerabilityisaquestionofpoliticalandsocio- economic resources. The analysis of the socio-political frameworkisalsotheobjectiveofthepaperbyMaze´ etal.
(thisissue).Inthisconceptualpaper,theauthorsadvocate the needto combine social science (sociology, political anthropology,history)andnaturalsciencetoanalysethe currentcoastalmanagementissues.Theyshedlightonthe importanceoftakingthedetailsofthepowerrelationships betweenstakeholdersandtheirhistoryintoaccount,and not simply applying general globalized management modelsandconcepts.
Mostofthepapersinthisthematicissuedemonstrate thattheprocessesatworkinthecoastalzonesareboth natural and anthropogenic,both interactingstrongly to determine the vulnerability of coastal systems. This statementis truefor mostearth systemsbut especially relevant for the coastal zone where natural and social dynamics are deeply intertwined in the causes and responsestorisk and vulnerability. Aninterdisciplinary approachisgenerallyadvocated.Thisspecialissueisthusa calltostrengtheninterdisciplinaryapproachesanddevel- opnewtransdisciplinaryresearchtomonitoranddescribe thedynamicsatworkinthecoastalsocio-ecosystems.
IsabelleManighettia,*,RutgerDeWitb,Ste´phanieDuvailc, PatrickSeylerd
aUniversite´ Coˆted’Azur,OCA,IRD,CNRS,Ge´oazur,Sophia Antipolis,France
bUMRMARBEC(IRD,Ifremer,Universite´ deMontpellier, CNRS),34095Montpellier,France
cUMRPALOC,IRD–Muse´umnationald’histoirenaturelle, SorbonneUniversite´s,75005Paris,France
dUMRHydrosciences,IRD,UM,CNRS,34090Montpellier, France
*Correspondingauthor E-mailaddress:manighetti@geoazur.unice.fr (I.Manighetti)
Foreword/C.R.Geoscience349(2017)235–237 237
