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The Many Changes to the Coastline

More than any other habitat, coasts are places of constant evolution. And have always been so. Aside from the onslaught of human activity, the coasts lining the edges of continents confront, by definition, the twin influence of land and sea. In these parts, geography is a shifting concept, redrawn just as easily by the ocean swell as by sediments dragged over from thousands of kilometers away. The ecosystems that develop in these regions weather important and frequent variations in the parameters of their environment, such as temperature, salinity, or oxygen concentration in water and sediments. “Along the coast, all habitats are interdependent,” explains Jean-Pierre Féral, director of the DIMAR Laboratory1 in Marseille. “If the Rhône river were to flood, for example, it would have both short and long-term repercussions on fish populations in the Mediterranean. Some of these repercussions might be positive, others negative, but what is certain is that it would affect the economy of coastal fisheries in the region.”
Indeed, the coasts are what scientists refer to as complex systems: systems subject to several varying and contradictory influences. Pollution, erosion, global warming, resource exploitation... Due to the number of parameters at work, it is almost impossible to predict the future of one coastal segment over another. One thing is certain, however: As in the past, the shore will change. The breadth of this change, and its consequences for humans and ecosystems, can only be evaluated on a case-by-case basis.

Before looking to the future, we must first understand what happened since the last ice age, approximately 18,000 years ago. Since that time, the Earth has been continuously warming up. As a result, and due primarily to the melting of land-based ice masses, the sea level has risen by 120 meters. “The rise was at first very rapid, around 10 millimeters a year,” explains marine geophysicist Bernadette Tessier.2 “Then about 6000 years ago, the landmasses assumed the recognizable shape they have today, as the sea rise slowed down to the rate of one or two millimeters a year.”
To this history, we must now add the effects of global warming. Since scientists began tracking the sea level variations via satellite in 1993, a rise of three millimeters per year has been observed. For Alix Lombard, of the Legos3 in Toulouse, there is no doubt at all. “Since the beginning of the 20th century, the change of slope is clearly visible. The current sea level rise is due half to thermal expansion of the oceans and half to the melting of mountain glaciers and polar ice sheets. As for the future, projections predict a rise of 20 to 60 cm by 2100, with a large uncertainty regarding the stability of coastal glaciers in some regions of Antarctica and Greenland.” But scientists are clear on one fact: the coastline will change dramatically in the coming decades, resulting in the flooding of large coastal regions. According to geologist Frédéric Bouchette,4 “if there is a 40 cm rise, we can expect an increase in storms with variations in wave height of 1 to 2 meters, as well as a locking of the waters in some river estuaries. The consequences in terms of changes to the coastline go much further than the mean annual rises previously considered.”


One such phenomenon occurred during the storms which battered much of France and Western Europe in 1999. The sea breached the coastline and opened channels in the lagoon around Montpellier. “Ultimately, we can imagine the complete disappearance of the lagoon system, with a beach retreating tens or hundreds of meters,” says Bouchette. “This type of situation has already occurred several times in this region's lagoons over the past millennia.” And Tessier goes further, “what we should fear are the threshold effects, that prevent the stabilization of certain coastlines, especially since the volume of sedimentary deposits is lower today than when these coastlines were first established– approximately 6000 years ago. This natural slowdown in deposits is increased by dam construction on most rivers.”
Nevertheless, a coastline's evolution is the result of powerful local forces. Firstly, because the sea level rise is not uniform around the globe: The sea level has been rising on the Asian coast of the Pacific since 1993, yet it has been dropping on the American side, due to geographical variations in thermal water expansion.
In addition to variations in sea level, a myriad of other factors are at play–morphology being one example. A low-altitude sand bank is more affected by sea level rise than a rocky coast. Similarly, the movement of a coastline (advancing or receding from the water) largely depends on the way the surf redistributes sediments, sometimes hundreds of kilometers away from their source. As Bouchette indicates, “in the region around Montpellier, in southern France, two neighboring beaches are evolving in opposite ways. The Espiguette beach is gaining 2 to 3 meters a year on its beach head, a phenomenon known as accretion, while a few kilometers from there, the Beauduc beach suffers marked erosion.” The Mont Saint Michel, a rocky tidal island in France's northern Normandy region, is another example. It is located in a bay whose southern part is silting up, leading to a very rapid land gain over the sea. But simultaneously, the north of the bay is being severely eroded. Similarly, in the region southwest of the bay of Arcachon, there is a general accumulation of mud, yet the dunes that line the entrance to the site are eroding rapidly.

collect sediment

In short, tracking a coastline's evolution over a short time scale and a limited area is a matter of investing in local research projects. And that is what the Copter project is all about. In the framework of GLADYS,5 Bouchette and applied mathematicians Pascal Azérad, Damien Isèbe and Bijan Mohammadi developed numerical tools and digital models of new types of artificial structures immersed in the sea to optimize their size and location and thus limit the effect of storm swells on beach erosion. “It could replace other, more traditional methods, such as pouring concrete over the beaches, or hauling truckloads of sand before the start of each summer,” comments Yann Leredde, from the same lab.
The Memphys6 research partnership also aims to make more accurate predictions. “Our objective,” explains project leader Georges Chapalain,7 “is to develop tools to model hydrodynamics, the movement of sediments, and the morphological evolution of the bottom and coastline on any given site, as a function of different forcing parameters, such as tides, swell, wind, topographical effects, and the nature of the sea bed.” Like weather forecasting, the task is colossal, even on a small coastal domain. “For the time being, we have studied sites in the eastern English Channel and the bay of Douarnenez, in Brittany. And on small time scales, our prediction models are starting to match field observations. With time, we should be able to give useful predictions for coastal management, for example.”

wave numerical prediction

Extreme complexity and major social issues sum up the context of coastal research today. Indeed, what is true for morphology holds for the working and livelihood of ecosystems. Marine coastal ecosystems alone represent a third of biological and geological resources worldwide. As Féral explains, “80% of marine biodiversity evolves near the coast, where the habitat is most complex and most diverse. Yet this biodiversity is still relatively unknown today.” Predicting the future of these sites is therefore both crucial and difficult.
To accomplish this feat, the best approach is to explore the ecosystems at all levels, from the physiology of one member of a species (fauna or flora) to the organization of food chains and the evolution of populations under the effect of different natural or anthropological constraints. “At the CRELA8 in La Rochelle, we use experimental pools to analyze how certain fish react to danger, after their sensory organs have been destroyed by large doses of heavy metals,” explains Gérard Blanchard, director of the laboratory. “We also examine how pollution modifies pathogenic immune responses in oysters.” The real challenge lies in extrapolating to a natural environment the multifactor results obtained in controlled conditions. To do this, researchers from CRELA call on many fields: mathematical modeling, cataloging of population and species, population genetics, reconstitution of complex food chains, or follow-up of organic material flows.
cotes de la mancheUsing a multidisciplinary approach is indeed the current tendency in coastal research. It was placed at the heart of the National Coastal Environment Program (PNEC),9 initiated by CNRS and Ifremer in 1999. The program, created to better understand how global marine coastal ecosystems function, has held several workshops in France and abroad. One of these was held in the pond of Thau, the largest pond (75 km2) in France's southern Languedoc-Roussillon region (see Box). As Marc Troussellier10 explains, “this lagoon shelters a wide range of natural biodiversity, in addition to harboring important shellfish breeding activity. It's an extraordinary model, in the sense that it accumulates all the environmental crises possible: natural events amplified by human activity, overabundance of organic material, over-consumption of oxygen followed by hydrogen sulfide production events. It is also a site where exotic species represent a quarter of the macrophyte population, and which receives chemical and biological pollutants. In short, an ideal site to investigate these harsh environmental conditions and understand the factors that cause them.”
Eight years of research have allowed scientists to understand the subtle interactions that link the lagoon to the ocean, but also the Lagoon to the continent via the catchment area. They have thus been able to refine a three-dimensional hydrodynamic model for the lagoon waters, coupled with a model for the behavior and survival of enterobacteria (such as Salmonella), that lets them simulate and better manage bouts of contamination. This method should soon be replicated on other sites.

Demand for greater scientific knowledge of coastal ecosystems is on the rise. Beyond the fundamental scientific interest it represents for researchers, knowledge of coastal ecosystems is becoming increasingly necessary for controlled management of natural milieus. How depleted are the stocks? What will be the effects of global warming on species distribution? Can the environment survive the effects of various pollutants? So many questions call for answers that are neither simple nor clear-cut.


One such example is the prevailing idea that a storm has devastating consequences on the fauna that lives on the sandy bottoms of certain coastlines. As Jean-Michel Amouroux, from the Biological Oceanography Laboratory of Banyuls11 (on the Mediterranean coast) explains, the reality is far more complex: “The larvae of certain species of worm develop while attached to a grain of sand. For this meeting to happen, the seabed has to be sufficiently free of mud. In other words, the sand has to have been stirred up by storms. This is how we realized that some organisms benefited from a higher frequency of storms.” Pierre Chardy, director of the marine station of Arcachon,12 gives another example. “If today the scallop is ubiquitous to Brittany's bay of Saint Brieuc, it is because during a winter that was colder than average, the cephalopods–one of their predators, completely disappeared. This shows how difficult it is to predict global warming effects.”
Either way, coastal research specialists are now able to put forward models that recreate the manner in which ecosystems function. And sometimes, these have unexpected economic implications. Three researchers13 at Dimar have established a causal relationship between the rise in water levels of the Rhône River–which carries continental organic matter towards the sea–and the stocks of sole in the gulf of Lion, several years later. However, such useful tools for making long-term predictions, especially for fisheries, raise an essential question that remains unanswered: On what grounds can we assess the environmental quality of a coastal ecosystem? In other words, do we have reliable indicators as to its health status? Antoine Grémare, director of the Biological Oceanography Laboratory of Banyuls, is tackling this issue. He is responsible for the definition of indices used for the European directive on water, for the ecological well-being of all bodies of water by 2015.
“It is extremely important not to make hasty conclusions on the degradation of a milieu after only limited observations. By analyzing the composition of the fauna of the seabed as an indicator of the quality of the habitat, for example, we have noticed that this indicator fluctuated over a period of seven to eight years. But this is related to a climate phenomenon called the North Atlantic oscillation,”14 he explains.
Today, a growing number of laboratories work on identifying these types of biological markers. One is the ELICO laboratory,15 in the Pas-de-Calais department, where researchers analyze the way planktonic organisms called copepods, move. “This work allows us to develop ecotoxicology tools, in that the random displacement of copepods depends on the quality of the environment in which they are placed,” explains ELICO director François Schmitt,”
Complexity, diversity, uncertainty... Are we nonetheless capable of making a general diagnostic of the global health of our coasts? For Féral, “it's definitely not a complete disaster. For the Mediterranean, for example, it is 'average.' In certain locations, coastlines are even close to being in a 'natural' state.” This opinion is shared by Philippe Garrigues, director of the molecular sciences institute16 in Talence: “In certain aspects, like the quality of the water, the situation has even improved.”

So what about global warming? Might it radically upset the fragile equilibrium of the coasts? Its effects are already being felt. “During the periods of intense heat that we witnessed in the past few years,” details Dimar's Thierry Perez, “we have noticed a necrosis of up to 100% in certain colonies of sea fans, or gorgonians, on the rocky seabed near Marseille. And since 2003, in our area, we know that up to 45 species of sponges, corals, worms, and mollusks have been affected.” But if we look at the entire planet, marine ecosystems most threatened at the moment by the rise in water levels are the coral reefs. As Chardy explains, “they shelter species living in a specific temperature range and in a very precise depth of water, in a flora/fauna symbiosis. If the water level rises too quickly, the coral will not be able to keep up. This could well deplete coral reefs...”
poissonScientists have also noticed early changes in the areas of repartition of the species throughout the world, as well as their progressive replacements by other species. At Dimar, Pierre Chevaldonné and Christophe Lejeusne have shown that in certain areas of the Mediterranean, a difference in temperature tolerance of three degrees in two species of Mysidacea (a type of shrimp) had brought on the replacement of one by the other. “In the French region of Gironde,” explains Chardy, “over the last 10 years, we have observed a species of plankton that was never present there and the arrival of migratory fish usually associated with warmer waters.” In time, there is no doubt that there will be enormous consequences on the coastal ecosystems, but for Féral, “they are still unfortunately very difficult to predict.” The researcher offers one last piece of advice: “On a geological scale, the coastal ecosystems have constantly been evolving. We must not have an alarmist position when a species disappears. What is of capital importance is that the system continues to function.”

Mathieu Grousson


Simulating the effect of climate change on entire ecosystems is now possible, using the Medimeer1 platform. The Lagoon Ecosystems laboratory2 anchored this research platform on the banks of the Thau lagoon (Sète). Unique in Europe, this installation stocks up to 50,000 liters of water and facilitates the study of responses to simulated variations in sunrays and temperature, as well as community interaction, including the entire food web: bacteria, primary producers,3 fish, as well as oysters and worms. Financed by INSU, and CNRS' Environment and Sustainable Development department, Medimeer supports several projects for the French national coastal program (PNEC).

1. Mediterranean Platform for Marine Ecosystem Experimental Research.
2. Laboratoire Écosystèmes lagunaires (CNRS / Université Montpellier-II / Ifremer).
3. Primary producers (e.g., phytoplankton) are those organisms in an ecosystem that produce biomass from inorganic compounds.
CONTACT : Behzad Mostajir,



Tracking the displacement of sand banks, surveying the evolution of low tide plant life, or being able to quantify coastal erosion or the silting up of water channels are some of the observations that can be made using satellites. With this data, scientists are able to anticipate episodes of toxic algae proliferation, for example, simply by locating in advance the water retention structures where these organisms might develop. As explains Jean-Marie Froidefond, of the Epoc1 laboratory in Talence, “since SPOT's launch, about 15 years ago, [the satellite] has become a major element of our research.” So much so that in a few months, the bay of Arcachon will be at the heart of a project overseen by the CNES,2 which will aim to make satellite images that are relevant to coastal research available to scientists. “With our partners, we will attempt to validate these images by comparing them to data gathered on site,” continues Froidefond. In the long run, this could become a coastal phenomena databank.

1. Environnements et paléoenvironnements oéaniques (CNRS / Université Bordeaux-I).
2. Centre National d'Etudes Spatiales.

CONTACT : Jean-Marie Froidefond, 


Notes :

1. Diversité, évolution, écologie fonctionnelle marine (CNRS / Université Aix-Marseille-II).
2. Laboratoire Morphodynamique continentale et côtière (CNRS / Université Caen / Université Rouen).
3. Laboratoire d'étude en géophysique et océanographie spatiales (CNRS / Université Toulouse-III / Centre national d'études spatiales / IRD).
4. Laboratoire Géosciences Montpellier (CNRS / Université Montpellier-II).
5. Groupe Languedoc-Roussillon d'étude de l'hydrodynamique sédimentaire littorale (
6. Mesure et modélisation des processus hydrodynamiques et sédimentaires: This research partnership was developed in Brest between the Institut universitaire européen de la mer (IUEM) and the Centre d'études techniques maritimes et fluviales (CETMEF).
7. Laboratoire des sciences de l'environnement marin (CNRS / Université de Brest).
8. Centre de recherche sur les écosystèmes littoraux anthropisés (CNRS / Université La Rochelle / Ifremer).
9. Part of the program Écosphère continentale et côtière.
10. Laboratoire écosystèmes lagunaires (CNRS/ Université Montpellier II/ Ifremer).
11. Laboratoire d'océanographie biologique de Banyuls (CNRS / Université Paris-VI).
12. Station Marine d'Arcachon (CNRS / Université Bordeaux-I / École pratique des hautes études Paris).
13. Mireille Harmelin-Vivien, Chantal Salen, and Audrey Darnaude.
14. The North American oscillation is an atmospheric and oceanic phenomenon. Oscillation means that there is a wind movement, in a north-south direction, over the Arctic and Islandic regions, towards the subtropical belt near the Azores and the Iberian peninsula.
15. Écosystèmes littoraux et côtiers (CNRS / Université Lille-I / Université du littoral-Côte d'Opale).
16. Institut de sciences moléculaires (CNRS / Université Bordeaux-I / École pratique des hautes études Paris).

Contacts :

> Jean-Pierre Féral,
> Bernadette Tessier,
Alix Lombard,
> Frédéric Bouchette,
> Yann Leredde,
> Georges Chapalain,
> Gérard Blanchard,
> Marc Troussellier,
> Jean-Michel Amouroux,
> Pierre Chardy,
> Antoine Gremare
> Francois Schmitt
> Philippe Garrigues
> Thierry Perez,


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