Lead Author: Brigitte Poulin (TdV)
Hydrology is a prime factor influencing the functions, biodiversity and services provided by wetlands (Coops and Hosper, 2002; Janssen et al., 2005). In semi-permanent and brackish environments, seasonal variations in water levels are particularly crucial for the maintenance of emerged and submerged macrophytes and their associated fauna (Bolduc and Afton, 2004; Osland et al., 2011). As a result of urban and agricultural development, palustrine wetlands (shallow lakes, marshes, ponds) are often disconnected from most of their catchment area, requiring active water management to maintain or enhance their functions (Tamisier and Grillas, 1994; Janssen et al., 2005, Lefebvre et al., 2015). Further reduction in freshwater availability and quality either due to changes in land use, water allocation, or climate is increasingly threatening the functions, biodiversity and human uses of wetlands (Lyons et al., 2008). Preserving wetlands functions and services is a priority of the European Water Framework Directive, with management of water scarcity being recognized as a major future challenge in southern Europe (European Commission, 2010).
The Camargue Biosphere Reserve in the Rhône delta covers 193.000 ha, including natural habitats such as lagoons, brackish/freshwater marshes with emergent or aquatic vegetation, wet meadows, temporary pools, salt pans, as well as halophilous scrubs and steppes. These ecosystems are intermingled with agro-systems dominated by rice, an irrigated crop. Through a complex network of irrigation and drainage channels, 730 millions of cubic meters of water are pumped from the Rhône on average each year to compensate for river embankment, to avoid soil salinization, enhance primary production (overcome summer drought) and create suitable breeding habitat for waterbirds either for sustaining commercial hunting activity or conversely enhancement of bird populations for conservation. Half of the excess water is returned to the Rhône through drainage channels, the other half being evacuated to the Vaccarès lagoon. This water, primarily pumped for rice farming, is also used for flooding marshes used for nature conservation, wildfowl hunting and reed harvest, as well as for irrigation of pasture meadows. Pesticides used for rice farming have been shown to affect biodiversity (Mesléard et al. 2005, 2016), but management options to reduce pesticide use (organic farming, dry sowing, winter flooding) with positive impacts on wildlife are gaining in popularity (Pernollet et al. 2015, Delmotte et al. 2016).
Wetland ecosystems of the Camargue are important for a range of regulating ecosystem services such as climate regulation, flood mitigation, water purification and nutrient cycling. They also provide important provisioning and cultural services through agriculture, salt production, fishing, cattle grazing, wildfowl hunting and bird watching. Each ecosystem type has a different dependency on water management, and resilience to fluctuations in water levels and salinity. External factors influencing agricultural land use (eg., CAP reform, global market evolution for rice or biofuels) or hydrological conditions (increased salinity in the Rhône estuary, reduced rainfall due to climate change) will affect the functions, biodiversity and services of these ecosystems, with potential tradeoffs among provisioning, regulating and cultural services. For instance, wetland management for wildfowl hunting has resulted in the imposition of hydrological conditions opposed to natural cycles. This affects the native Mediterranean flora and fauna adapted to seasonal brackish wetlands, and increases vulnerability to invasive plant species. Flooding of hunting marshes and pasture meadows in summer further increases mosquito abundance with deleterious effects on ecotourism. Inadequate grazing pressure can lead to proliferation of unpalatable species, but also to habitat shift when in combination with hydrological management (Mesléard et al. 1999).
This storyline aims at making the best use of remote-sensing tools to document the evolution in the state of wetlands and the services they can deliver within a context of global changes, integrating feedback processes occurring at local scale through stakeholder management.
(1) A new monitoring tool to track water presence in wetlands: from 35-year back to the future!
Robust monitoring tools are necessary to quantify the flooding duration of various types of wetlands found in the Camargue with the multiple objectives of measuring their functions, services & biodiversity, conducting prospective and companion modeling related to climate changes, and elaborating a global delta management framework complying with European and national policies. A major challenge consists of detecting water presence in wetland having a dense cover of emergent vegetation such as reed beds or rice paddies, many of which are located on large private estates where accessibility is difficult.
Camargue bull in a brackish marsh partially covered with bulrush (Bolboschoenus maritimus)
Photo credit: Tour du Valat
The ECOPOTENTIAL project, in synergy with SWOS (another H2020 European project under grant agreement No 642088), has provided the opportunity to develop such a water index through a data mining approach combining in-situ water level measures, optical-space generated maps and satellite data (Landsat & Sentinel). In particular, the spectral (8 bands), spatial (10-20 m) and temporal (every 3-4 days) resolutions of Sentinel 2 allowed us to develop a most performing model predicting water presence using the near (NIR) and shortwave (SWIR2) wavelengths. According to this work, flooded areas (irrespective of vegetation cover) can be distinguished from dry areas when pixels have a reflectance value equal or above 0.1804 in B8A and 0.1131 in B12. This predictive model provides an overall accuracy of 94% and a Kappa coefficient of 0.82 for all habitat types combined. Similar models have been developed with Landsat 5, 7, and 8 for which the NIR and SWIR2 wavelengths were selected as well, so that the evolution of hydrology for the natural and man-made ecosystems of the Camargue can be traced back to 1985.
Annual water map of the Camargue obtained by applying the NIR/SWIR2 thresholds to 35 Sentinel-2 scenes from 24 December 2017 to 24 December 2018.
Reference: Lefebvre G., Davranche A., Willm L., Campagna J., Redmond, L., Merle C., Guelmami A., Poulin B. A water index to track past and future changes in seasonal water of wetlands using Landsat and Sentinel data. Submitted to Remote Sensing of Environment.
(2) A simulation tool to estimate the vulnerability of Mediterranean wetlands to climate change in the Camargue and beyond!
Thanks to a collaboration with experts from the Institute of Atmospheric Sciences and Climate from the National Research Council of Italy (ISAC-CNR), the climate projections at the 2050 and 2100 horizons were integrated into Mar-O-Sel, a free online software (mar-o-sel.net/) developed by Tour du Valat to promote rational use of Mediterranean seasonally-flooded wetlands (Lefebvre et al. 2015). Climate projections were calculated based on two greenhouse gas emission scenarios, RCP4.5 and RCP8.5 for 229 Mediterranean localities. We first parameterized a seasonally-flooded wetland under current climate conditions by setting iteratively catchment area, overflow level and water table depth at each of the 229 localities. Second, we simulated the future state of these parameterized wetlands based on the precipitations and evapotranspiration projected in 2050 and 2100. Wetland evolution (maintenance, degradation or habitat shift) was estimated based on ecological thresholds in terms of flooding duration and frequency defined by wetland experts from Tour du Valat. For instance, it was considered that a Mediterranean marsh with emergent vegetation needs to be flooded for a minimum of 6 months and dry for at least 2 months to be in good condition. The different habitat outcomes under climate change are shown in the figure below. It appears that only 27% of the 229 localities will still offer adequate hydrological condition to seasonally-flooded wetlands at the horizon 2100 under the business as usual scenario of greenhouse gas emission. The water input needed to preserve wetland functions would vary from 1055 m3/ha for slightly degraded sites to 3537 m3/ha for a wetland collapsing into dry land. We hope that this work will contribute to orient adaptive management and territorial planning in order to save as many wetlands as possible for future generations.
Graphical abstract from:
Lefebvre G., Redmond L., Germain C., Palazzi E., Terzago S., Willm L., Poulin B Predicting the vulnerability of seasonally-flooded wetlands to climate change across the Mediterranean basin. Submitted for publication in Science of the Total Environment.
(3) A joint review paper by SWOS and ECOPOTENTIAL scientists on the challenges of mapping wetlands by remotely-sensed data
Wetlands are a key habitat within the Mediterranean area for their biodiversity but also because they provide important ecosystem services for human well-being. Remote sensing (RS) has significantly boosted our ability to monitor changes in Mediterranean wetlands, especially in areas where little information is being collected. However, its application to wetlands has sometimes been flawed with uncertainties and unrecognized errors, to a large extent due to the inherent and specific ecological characteristics of Mediterranean wetlands. In this paper we present an overview of the state of the art on RS techniques for mapping and monitoring Mediterranean wetlands, and the remaining challenges: delineating and separating wetland habitat types; mapping water dynamics inside wetlands; and detecting actual wetland trends over time in a context of high, natural variability. The most important lessons learned are that knowledge by ecologists needs to be integrated with RS expertise if we ought to achieve a valuable monitoring approach of these ecosystems.
Reference: Perennou, C., Guelmami, A., Paganini, M., Philipson, P., Poulin, B., Strauch, A., Tottrup, C., Truckenbrodt, J. and I.R. Geijzendorffer. Mapping Mediterranean Wetlands with Remote Sensing: A Good-Looking Map Is Not Always a Good Map. In Advances in Ecological Research, vol. 58, pp. 243-277. Academic Press, 2018. https://doi.org/10.1016/bs.aecr.2017.12.002
A temporary grazed marsh at the Petit Saint-Jean estate owned by Tour du Valat in the Camargue.
Photo credit: Gwen Wasse, Tour du Valat.
(4) A review paper on the dynamic of saltworks since 1950 along the southern coast of France: what happens after salt production?
Some 70 years ago, 29 saltworks (122 km2) were exploited along the Mediterrean coast of France. Today, only five saltworks are still producing salt but they occupy a much larger area (175 km2). Two of these saltworks are located in the Camargue, where 5000 ha of lagoons formerly used as preconcentration ponds are currently being restored. This situation brought Ecopotential scientists based in Montpellier to work on a review paper with Tour du Valat researchers on the landscape changes that occurred with the creation/exploitation of the saltworks and on the conservation perspectives of these sites after salt production. Landscape changes were documented based on chronological GIS mapping. This task required validation from local knowledge to integrate not only the easily recognizable, rectangular man-made crystallizer ponds, but also the more extensive and irregularly shaped ponds used for salt pre-concentration. It appears that most (97%) of the abandoned saltworks are today integrated to the Natura 2000 network. Preservation of the natural and cultural heritages values of these sites is a real challenge in the context of sea-level rise and limited economic outcomes. In some cases, such as in the Camargue, natural processes have been privileged by restoring hydrobiological exchanges between land and sea with re-establishment of hydroperiods respecting natural cycles. In other areas, former saltworks continue to be filled artificially by water pumping in summer, which in combination with the creation of artificial islets provides nesting ground for shorebird colonies.
A shorebird colony nesting in the Camargue saltworks
Photo credit: Damien Cohez, Tour du Valat.
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Delmotte, S, Barbier, J.-M., Mouret, J.-C., Le Page C., Wery J., Chauvelon P., Sandoz A.,Lopez-Ridaura, S. 2016. Participatory integrated assessment of scenarios for organic farming at different scales in Camargue, France. Agricultural Systems 143:147-158
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Janssen, R., Goosen, H., Verhoeven, M.L., Verhoeven, J.T.A., Omtzigt, A.Q.A., Maltby, E., 2005. Decision support for integrated wetland management. Environ.Modell. Softw. 20, 215-229.
Lefebvre,G., Germain, C., Poulin B. 2015. Contribution of rainfall vs. water management to Mediterranean wetland hydrology: Development of an interactive simulation tool to foster adaptation to climate variability. Environmental Modelling & Software 74 (2015) 39-47.
Lyons, J.E., Runge, M.C., Laskowski, H.P., Kendall, W.L., 2008. Monitoring in the context of structured decision-making and adaptive management. J. Wild. Manage. 72, 1683-1692.
Mesléard, F.; Garnero, S.; Beck, N.; Rosecchi, É. 2005. Uselessness and indirect effects of an insecticide on rice field invertebrates. Comptes Rendus Biologies, 328: 955-962.
Mesléard, F., Gauthier-Clerc M., P. Lambreta. 2016. Impact of the insecticide Alphacypermetrine and herbicide Oxadiazon, used singly or in combination, on the most abundant frogi n French rice fields, Pelophylax perezi. Aquatic Toxicology 176 2429.
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Pernollet, C.A., Guelmami A., Green, A.J , Masip A.C., Dies, B., Bogliani G., Tesio F., Brogi, A., Gauthier-Clerc M., Guillemain, M. 2015. A comparison of wintering duck numbers among European rice production areas with contrasting flooding regimes. Biological Conservation 186 (2015) 214224.
Tamisier, A., Grillas, P., 1994. A review of habitat changes in the Camargue: an assessment of the effects of the loss of biological diversity on the wintering waterfowl community. Biol. Conserv. 70, 39-47.
De Wit R., Vincent A., Foulc L., Klesczewski M., Scher O., Loste C., Thibault M., Poulin B., Ernoul L., Boutron O. (2019). Seventy-year chronology of Salinas in southern France: Coastal surfaces managed for salt production and conservation issues for abandoned sites. Journal for Nature Conservation. doi: 10.1016/j.jnc.2019.03.003
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Last update: May, 2019