LEDDCinEnglish

IV. Surface water
. Permanent environments
Running water: rivers and streams
 
In the World
 
The Amazon is the longest river in the world at 6,992 km and flows through Peru, Ecuador and Brazil.
The Nile, previously considered as the longest, comes in second place with a length of 6,852 km. Its huge watershed extends over 12 African countries: Burundi, Tanzania, Rwanda, Kenya, Congo, Uganda, Central African Republic, South Sudan, Ethiopia, Sudan, Eritrea and Egypt.
The Yangtze River in China and the Mississippi River in the United States follow with 6,380 and 6,275 km respectively.
 
In continental France ... and in Europe
 
The longest river in France is the Loire with 1,012 km. The Seine comes second with a length of 776 km. 
 
The Rhine flows for 1,320 km through Switzerland, Liechtenstein, Austria, Germany, France, and the Netherlands.
 
The Meuse is 950 km long and flows through France, Belgium and the Netherlands.
The Rhône (812 km) runs through Switzerland and France.
 
In Corsica
 
The island's hydrographic network comprises 3,000 linear kilometres of waterways. The longest are the Golu and the Tavignanu with 89 and 88 km respectively.
 
 
The main rivers of Corsica
 
 
Strahler Stream Order
 
A stream ordering system is a method for ranking portions of streams according to their position in the river system by assigning them a numerical code, based on a hierarchy of tributaries. The most common, and the easiest to use, is the Strahler stream Order (1957), which consists of assigning rank 1 to the rivers at the head of the basin, and then, progressing downstream, each river which receives a river of the same rank has its own rank increased by one unit.
 
 
Strahler Stream Order
 
In the Strahler stream order, rank 1 corresponds to spring outfalls. Ranks 2 and 3 are for very small and small streams respectively. Medium-sized streams are ranked 4 and large streams are ranked 5. Above rank 5, they are called very large rivers.
 
The application of the Strahler system to Corsican rivers shows that rank 5, for the Golu and the Tavignanu, is the highest rating given. In continental France, rank 8 is obtained for the Loire and the Rhône. On a global level, the Amazon and the Nile are rivers of 12 and 11 respectively.
 
The typology of Corsican watercourses
 
Six main types of watercourse can be distinguished in Corsica:
 
The Golu and the Tavignanu are the largest rivers on the island both in terms of length (89 and 88 km) and the surface area of their catchment areas (1,036 and 693 km²). These two rivers originate on the eastern slopes of hercynian Corsica; the altitude of the highest point of each catchment area is very high (2622 and 2707 m). These rivers then flow through the central depression, the shale and green rocks of alpine Corsica and finally through the eastern plain before flowing into the Tyrrhenian Sea. The average gradient is about 2 %; the low gradient of the lower course (0.3 to 0.4 %) reflects the presence of an alluvial plain on the sedimentary terrain of the eastern plain.
 
The Travu, Sulenzara, Osu and Cavu rivers originate on the eastern slopes of hercynian Corsica; the altitude of the highest point of the catchment area (50 to 128 km²) is high (1,377 to 2,134 m), and lengths of the rivers ranges from 22 to 32 km. The average slope is between 4.3 and 6.3 %, the dominant substratum is granite.
 
The Taravu, Rizzanese, Gravona, Prunelli and Liamone are the major rivers (41 to 65 km long and 276 to 482 km² in catchment area) of the western slopes of hercynian Corsica. The average slope varies from 2.2 to 4.4 %; the altitude of the highest point of the catchment area, where granites dominate, is high (2,041 m to 2,352 m).
 
The small rivers of the western slope of hercynian Corsica have lengths of between 19 and 32 km, the catchment area is between 130 and 150 km².
The Alisu, Ortolo, Ostriconi and Reginu rivers have the following characteristics: altitude of the highest point (1,314 to 1,680 m), average slope (3.1 to 5.6 %), slope of the lower course (0.3 to 0.8 %).
The Fangu, Figarella and Portu rivers are characterised by (i) a high altitude of the highest point of the catchment area (2,108 to 2,547 m), (ii) a high average slope (6.3 to 9.1 %), (iii) a very high headwater slope (10.4 to 25.3 %) and (iv) the presence of rhyolite in the catchment area.
 
The Alisgiani, Bevincu, Bravone and Fium'Altu are the biggest rivers of alpine Corsica. The main characteristics are: length (24 to 37 km), surface area of the catchment area (54 to 180 km²), altitude of the highest point (1,469 to 1,767 m), average gradient (4.1 to 5.2 %).
The Luri is a small river on Cap Corse, which is 11 km long, and characterised by a high upper slope.
 
Flow variations: the hydrological regime
 
The rivers of Corsica have a pluvio-nival-Mediterranean hydrological regime. This regime is influenced by rain and snow and has a severe summer low water level, characteristic of Mediterranean rivers.
 
The pluvio-nival-Mediterranean regime has two periods of low water and two periods of high water. The winter low water period, which is not very marked, corresponds to the storage of water in the form of snow; the summer low water period, which is more severe, coincides with the minimum rainfall.
 
The periods of high water correspond in spring to the melting of the snow and in autumn to heavy rainfall. This hydrological regime is marked by considerable and very sudden variations in flow, leading to sudden and violent floods.
 
 
 
Average daily flows (m³/s) of the Tavignanu in 2002
 
For flooding of the rivers in Corsica, the maximums known by the HYDRO database are presented in a later table.
The maximum instantaneous flow exceeds 800 m³/s for the Golu, Tavignanu, Solenzara, Liamone and Fium Altu.
The record is held by the Sulenzara (at Sari-Sulenzara) with 1,580 m³/s on 31 October 1993 at 23:59. In the classification of Corsican rivers, the Sulenzara is not one of the major rivers, in fact, it is in 21^st position for length, 18^th for catchment area surface and 11^th for average annual flow. The magnitude of this flood was due to the high intensity of precipitation localised in a particular catchment area.
 
Floods, maximums known by the HYDRO database
(Ministry of Ecology, Sustainable Development and Energy)
 
However, according to a document entitled: “L'Evaluation Préliminaire des Risques d'Inondation 2011, Bassin corse” (Ministry of Ecology, Sustainable Development, Transport and Housing), the Corsican record is held by the Tavignanu. In October 1976, the Tavignanu (at Caterraggiu) had an instantaneous flow of 3,500 m³/s. For the record, the average flow of the Seine is 480 m³/s, that of the Loire 900 m³/s and that of the Rhône 1,700 m³/s.
 
The maximum instantaneous height during flood periods (recorded by the HYDRO database) exceeds 8 metres in the Golu, Tavignanu, Sulenzara, Fium'Orbu and Fangu. The record is held by the Fangu (at Galéria) with 10 metres, on 21 October 1992 at 08:28. In the ranking of Corsican rivers, the Fangu is in 17^th position for the length and 12^th average annual flow.
 
But the record is held by the largest river in Corsica, the Golu. In September 1938, water levels reached 12.50 m at Barchetta (EPRI, 2011).
 
During the 20^th century, floods caused many victims and significant damage. In September 1974, 8 people were swept away by the Tavignanu at Baliri in Corti.
 
The major disaster on All Saints’ Day 1993 will be stay in people’s memories for a long time. Between 31 October and 2 November, the Sulenzara, Travu and Rizzanesi rivers bursting their banks caused 7 victims and significant damage (160 communes affected) estimated at a cost of some €420 million.
 
 
Longitudinal and cross-sectional profile (riverbed and floodplain)
 
The considerable and very abrupt variations in flow, leading to sudden and violent floods, come from the high gradient of the watercourses, resulting in a very short reaction time, particularly during autumn rains.
 
 
 
Longitudinal profile of some Corsican rivers
 
 
The bed of a river is the space covered by the water flowing in normal times before overflow; the floodplain corresponds to the flood expansion zone.
The floodplain is a buffer space that allows natural flood control and the slowing down of the speed of floods.
 
 
 
 
 
Cross-section of the Tavignanu downstream from Corti
 
 
 
 
The riverbed and floodplain of the Tavignanu downstream from Corti
 
 
The floodplain in flood (Tavignanu downstream from Corti)
 
 
 
 
 
Flow impacted by hydroelectric schemes
 
The hydrological regime of a river can be affected by the operation of a hydroelectric structure (dam). The impact is reflected in a significant reduction in flow in the instream flow section (short-circuited section downstream of the dam). But it is more marked in the part of the river subject to hydropeaking (downstream of the hydroelectric plant). Hydropeaks are daily variations in the flow, linked to the operation of the hydroelectric plant. This is referred to as a regulated flow.
 
 
 
Hydrograph of the Golu and Tavignanu rivers, from 9 to 13 January 2017.
 
The lower course of the Golu (downstream of Castirla) has a regulated hydraulic regime subject to locks, in relation to the operation of the Calacuccia-Corscia hydroelectric complex. This hydroelectric development has several units, in order from upstream to downstream: (i) the Sovenzia power station fed by the inflow from the Tavignanu, diverted through an intake located at 1084 NGF (French National Level reference) and with a capacity of 6 m³/s; (ii) the Calacuccia dam and its reservoir; (iii) the Corscia plant fed by the water from the Calacuccia reservoir; (iv) the Corscia dam and its reservoir and (v) the Castirla plant fed by the water from the Corscia reservoir.
 
Hydropeaks are artificial, sudden and frequent variations in flow rates, linked to the use of water from dams for hydroelectricity production. The Golu hydrograph shows variations between 4 and 11 m³/s, i.e. an increase of 275 %. At the same time, the natural flow of the Tavignanu oscillates between 6 and 7 m³/s.
 
In the Golu, changes in the hydrodynamic characteristics of the flow (water level, current speed) lead to changes in the habitat of aquatic organisms: fish, invertebrates, algae (see elsewhere).
 
In addition to the ecological impact, there is a potential risk to those who use the waterways (for fishing or swimming, for example) and who may be unaware of a potential water release.
Every summer, the EDF (the French national electricity company) carries out a communication campaign to warn the general public of risks associated with dam releases by recruiting hydroguides to raise public awareness.
 
 
Flow influenced by snowmelt
 
 
Hydrograph of the Restonica, at Corti, from 18 to 22 April 2018.
 
 
The Restonica, a major tributary of the Tavignanu, has its source at Lake Melu at an altitude of 1,711 metres. The highest point of its watershed is Monte Ritondu (2,622 m). The upper Restonica valley is covered by snow for a good part of the winter.
The hydrograph (graph representing the variations of flow in relation to time) of the Restonica, at the Rivisecu bridge, in Corti from 18 to 22 April 2018 shows daily cyclical variations of the flow. They are characterised by peaks between 17:00 and 18:00. These oscillations are between 5 and 6 m³/s, the amplitude of the intraday variation in flow is 20 %.
These cyclical variations reflect the daily periodicity of the liquid inflow in relation to the snow melting. The flow therefore varies in significant proportions between the sunniest hours of the day and the coldest hours of the night.
 
 
Solid transportation or sediment transit
 
Rivers, especially during floods, transport large quantities of sediment. There are two forms of transportation: bed load and suspension. 
Bed load takes place on the bottom of the riverbed and concerns hard substrates such as blocks, stones or pebbles which are coarse granulometric elements.
Suspension is the transport of loose substrates such as sand, silts, silt or clay, in other words fine granulometric elements.
 
For the rivers of Corsica we have estimated the level of solid transportation to be between 200 and 400 tonnes per km² per year.
For the Golu, with a catchment area of around 1,000 km², annual solid transportation is somewhere between 200,000 and 400,000 tonnes per year. During exceptional floods, values can exceed 1 million tonnes.
 
Under certain conditions (e.g. with very steep slopes between 30 and 40 %, and following fires), water runoff leads to significant quantities of ash. When the density of the mixture of water and matter is close to 2, the transported elements almost float. This is the phenomenon of hyper-concentrated bed load or debris flow.
In this type of situation, damage to both people and property can be significant. This has been the case in Corsica on a number of occasions: (i) August 1943, Ocana, 5 dead; (ii) November 1994, Canavaggia (4 houses washed away), damage in Santa Lucia di Mercuriu and Lentu; (iii) November 2000, damage in Restonica after the great fire.
 
In terms of the matter transported by rivers, sand with a diameter varying from 50 µm to 2 mm (fine to coarse sand), is at the origin of the beaches in the coastal areas.
 
The salinity of the sea and oceans is caused by dissolved salts transported by rivers. While the concentration of salts in freshwater is low (0.02 to 0.04 g/l), in the sea, evaporation of the water leads to a concentration (by a factor of 1,000) of these salts and therefore to an increase in salinity (Mediterranean: 38 g/l).
 
In the solid transportation or sediment transit discussed above, natural elements have been taken into account. Unfortunately, it is nowadays necessary to add anthropogenic elements, and particularly plastic waste.
This inorganic particulate waste is the source of chemical contaminants in water and in the food chain in the aquatic environment. Among these contaminants are PCBs (Polychlorinated biphenyls), PAHs (Polycyclic aromatic hydrocarbons), organochlorine pesticides, PBDEs (Polybrominated diphenyl ethers), Bisphenol A, Alkylphenols...
 
In rivers, plastic waste is visible in the form of macroplastics (bottles, etc.). Given that act a massive crushing machines, they not only break up stones But rivers, which are real crushing machines, not only break up stones, turning them into sand, but also macroplastics, fragmenting them into microplastics.
This plastic waste pollution has an impact on aquatic fauna. Invertebrates and fish ingest these microplastics and chemical contaminants, which are released in their digestive tract. These toxic substances concentrate in the organs including the muscles (fillets) of fish that is later consumer by humans.
If these microplastics remain in the intestines of the fish, without breaking the muscle tissue, they break down into nanoplastics and pass the intestinal barrier. They are therefore present in the fish we eat.
 
It should be remembered that the lifetime of plastic waste in the natural environment varies, depending on the type, from between 100 to over 1,000 years.
 
We are all aware today of plastic waste pollution affecting rivers, seas and oceans. Even though the pollution may be at sea, the solutions can be found on land. Saving water and aquatic ecosystems is also a question of protecting human health. We need to show solidarity with people living in the lower reaches of rivers and on the coasts.

 
Stagnant water: lakes
Natural lakes: mountain lakes
 
In the World
The largest lake is the Caspian Sea, wrongly named by geographers. Its volume is 78,000 km³ (1 km³ = 1 billion m³), its surface area is 374,000 km², i.e. 2/3 of France. The salinity of the water is 12.5 g/l. As a reminder, the Mediterranean Sea has a salinity of 38 g/l. The maximum depth of the lake is 1,025 m. The shores of the Caspian Sea are shared by Russia, Kazakhstan, Turkmenistan, Iran and Azerbaijan,
 
The deepest lakes are Baikal and Tanganyika.
Lake Baikal is located in Siberia, Russia, with a maximum depth of 1,740 m and a volume of 23,000 km³. With a surface area of 31,500 km², it is 3.5 times the size of Corsica. Salinity is 0.1 g/l.
Lake Tanganyika in Africa has a maximum depth of 1,471 m and a surface area of 32,900 km². Salinity is 0.4 g/l. The lake forms a natural border between four countries: Burundi, Tanzania, Zambia and the Democratic Republic of Congo.
 
 
In Europe
Lake Geneva is the largest lake in Western Europe with a surface area of 580 km² and a maximum depth of 310 m. It is located between France and Switzerland, where it is called Lac Léman.
Lakes Constance (Germany) and Maggiore (Italy) have surface areas of 538 km² and 213 km² respectively; their maximum depths are 252 m and 370 m respectively.
 
 
In mainland France
The Lac d’Hourtin et Carcans (which straddles two communes, hence the name) has a surface area of 57 km²but is shallow (maximum 10 m). With a surface area of 45 km², the Lac du Bourget is a little smaller, but its maximum depth is 147 m.
 
 
In Corsica
 
In the Corsican mountains there are about forty permanent bodies of water, but only 15 lakes have a maximum depth of more than 3 metres and a surface area of more than 0.5 hectares.
 
These mountain lakes are glacial. About 14,000 years ago, they were formed either by glacial erosion (Melu, Capitellu) or by the accumulation of moraines (Bastani).
 
Glaciers have often left traces such as striations (grooves) and gouges.
 
The deepest are Capitellu (44 m), Bellebone (or Ritondu, or Betaniella) (35 m) and Bastani (24 m). The largest are Bellebone (or Ritondu, or Betaniella) (7.4 ha), Ninu (6.5 ha), Melu (6.2 ha) and Capitellu (5.5 ha).
 
The water balance classifies these lakes as fluvial, i.e. inflows and outflows are mainly through tributaries and surface outfalls.
 
These lakes spend the winter under the ice, resulting in a slowdown of biological activity.
 
With two mixes a year (spring and autumn) these mountain lakes are dimictic; the stratification phase (thermal and chemical) takes place in summer.
 
 
Thermal stratification of mountain lakes
 
 
 
 
 
Alpine chough at Lake Melu
 
This image shows the bathymetry of the lake. Bathymetry (from the ancient Greek “bathys” meaning “deep”) is the science that deals with the relief and depth of lakes and oceans.
 
The alpine chough (a tacula) is a paleo-montane species, belonging to the crow family which lives in mountain ranges in Europe (Pyrenees, Alps, Corsica, Caucasus) and also Asia and North Africa. In the Alps it can be found at the summit of Mont Blanc (4,809 metres). It is a protected species throughout France. 
It is listed in Appendix II of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention). It is considered “in decline” by the Birds Directive Assessment (2013).
 
The vegetation of the catchment area depends on the altitude. The Corsican pine (pinus nigra corsicana) is present around the lakes below 1,700 m (Crenu); between 1,700 and 2,100 m altitude the Corsican sweet alder (alnus alnobetula suavolens) the dominant species of the shrub layer (Melu); above 2,100 m vegetation is rare and represented by a small number of thorny plants (Maggiore, Cintu).
 
These lakes sometimes have an original formation called a pozzines. These are hygrophilous and meso-hygrophilous grasslands located on substrates resulting from the complete or near-complete natural filling of glacial lakes. They are essentially formed by the subterranean organs of grasses, cyperaceae and dwarf juncaceae with sphagnum moss. Stream that meander through pozzines have a gentle slope, their substrate is sandy and gravelly, and the water flows without turbulence. This provides ideal conditions for trout spawning grounds. This hydrosystem plays an essential role in the reproduction of the endemic trout Salmo trutta macrostigma (species of community importance, European Habitats Directive 92/43EEC). The headwaters often constitute a nursery, on which a large part of the trout populations of many rivers depend.
 
The aquatic plantlife is mainly represented by microscopic algae and mosses. Macrophytes are rare, swimming pondweed is present in Ninu and Gialicatapiani lakes. The white waterlily is only present in Lake Crenu.
 
Aquatic invertebrates are dominated by insects, oligochaetes and crustacea. Vertebrates are represented by trout, brook salmon, Corsican mountain newts, salamanders and painted frogs. This fauna is characterised by a high level of endemism.
 
Typology of mountain lakes in Corsica
 
 
Typology of mountain lakes in Corsica
 
The typology proposed takes the following abiotic parameters into account: Altitude (m), Lake area (ha), Perimeter (m), Maximum depth (m), Distance from the sea (km), Distance from the Vaziu power plant (km), Watershed area (ha), Exposure to the Barrier, Duration of frost (months), Source of watercourse.
 
The projection of the lake points shows 3 groups of lakes:
Group 1: Bellebone (or Ritondu or Betaniella), Capitellu, Goria, Bastani, Ninu, Melu
This group includes large (lake area between 4.38 and 7.40 ha), deep (maximum depth between 7 and 42 m), east-facing lakes in relation to the barrier.
Group 2: Bracca, Crenu, Vitalaca
This group includes small lakes (lake area between 0.62 and 2.30 ha), shallow (maximum depth between 3.5 and 6.5 m), exposed to the west in relation to the barrier.
Group 3: Niellucciu, Oru, Gialicatapianu, Oriente, Cavacciole, Maggiore.
This group includes small lakes (lake area between 0.30 and 1.15 ha), shallow (maximum depth between 1.5 and 15.0 m), exposed to the east in relation to the barrier.
 
Exposure to the barrier may play an important role, particularly in considering the potential impact of particulate matter from the Vaziu power plant.
 
Groups 1a and 2a include the Ninu, Melu and Vitalaca lakes, which are the source of major rivers, the Tavignanu, Restonica and Prunelli respectively. These lakes are therefore an important challenge in the quantitative and qualitative management of these rivers.

 
Artificial lakes: dam reservoirs
 
In the World
 
Dams are built to store large quantities of water. The oldest dates back to 2600 BC, the Sadd-el-Kafara Dam, built in the Garawi Valley in Egypt. The dam was 14 metres high and 110 metres long, with a volume of 500,000 m³. The remains of this dam, now no longer in use, show a thickness of 98 metres at the base of the structure.
 
The oldest dam still in operation dates from 1594, the Tibi Dam in Alicante, Spain, has a height of 46 metres, a length of 65 metres and a base thickness of 30 metres.
 
The highest structure, 335 metres, is the Roghun Dam in Tajikistan. The dam is 70 km long and can store 11,600 million m³.
 
The highest in Europe (285 metres) is the Grande Dixence Dam in Switzerland. The dam’s volume is 400 million m³.
 
 
In mainland France
 
The highest in France (180 m) is the Chevril (or Tignes) Dam in Savoie. This structure can store 235 million m³. The downstream face of the dam has the largest fresco in the world (18,000 m²); it is the work of Pierret (1989) and represents Hercules.
 
 
In Corsica
 
Corsica has 48 dams, 12 of which are class A dams, with a height of over 50 m.
 
The Tolla, Calacuccia, Corscia, Sampolo, Trevadine and Rizzanèse dams are managed by EDF (Electricité De France), mainly for hydroelectricity production.
 
The dams of Teppe Rosse, Alzitone, Bacciana, Guazza, Peri, Alesani, Padula, Codule, Figari, Ortolo and Ospedale are managed by the OEHC (Office d'Equipement Hydraulique de la Corse) for irrigation and drinking water supplies.
 
Teppe Rosse, Alzitone, Bacciana, Guazza, Peri and Padula are hill reservoirs, i.e. artificial reserves, often located outside the hydrographic network, closed by a dam and fed either during the rainy period by run-off or by a permanent or temporary watercourse. This type of structure has no downstream release of water (no instream flow).
 

 
Dams and reservoirs managed by EDF (Electricité De France)
 
Dams and reservoirs managed by the OEHC (Office d'Equipement Hydraulique de la Corse)

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Fish zoning in the Tavignanu
1. Trout; 2. Eel; 3; River blenny
4. Stickleback; 5. Twait shad; 6. Sand smelt; 7. Bass; 8. Mullet.

Introduced species:
 
Whereas in 1900 there was only one introduced species (the Eastern mosquitofish, Gambusia holbrooki) in Corsica’s fresh waters, there are now 21 introduced species. The mosquitofish was introduced as part of a programme to combat malaria.
The cumulative number of introduced species per year is as follows: 1900 (1); 1970 (8); 1980 (12); 1990 (15); 2000 (20); 2010 (21).
 
There are 21 introduced species in Corsica’s fresh waters: the common sturgeon (Acipenser sturio), pike (Esox lucius), crucian carp (Carassius carassius), goldfish (Carassius auratus), European carp (Cyprinus carpio), Marmara chub (Leuciscus cephalus), common roach (Rutilus rutilus), gudgeon (Gobio gobio), topmouth gudgeon (Pseudorasbora parva), Eurasion ruffe (Gymnocephalus cernuus), minnow (Phoxinus phoxinus), catfish (Ictalurus punctatus), common rudd (Scardinius erythrophtalmus), pikeperch (Sander lucioperca), wels catfish (Silurus glanis), tench (Tinca tinca), common perch (Perca fluviatilis), largemouth bass (Micropterus salmoides).
 
These introduced species have been catalogued in the lower reaches of rivers impacted by the exploitation of aggregates (gravel pits) and in a few artificial reservoirs managed by Electricité de France or the Office d’Equipement Hydraulique de la Corse.
 
The charr or brook trout (Salvelinus fontinalis) was introduced in the early 1970s in some high-altitude lakes. The rainbow trout (Oncorhynchus mykiss) is regularly released into Corsica’s waterways before the opening of fishing season by the Fédération Interdépartementale de Pêche et de Pisciculture.
 
The consequences of introducing fish species:
These introductions are a threat to native fish, including the macrostigma trout (Corsican strain), as they bring predation, competition, pathogens (viruses and bacteria) and parasites.
 
At the beginning of the 1970s in Corsica, the brook trout (Salvelinus fontinalis) was introduced into mountain lakes. The consequences of this were catastrophic, resulting in the disappearance of the native trout (Salmo trutta), as the brook trout, which reaches almost 40 cm in length, is extremely voracious. This introduction is most probably responsible for the significant decline in the numbers of the Corsican brook salamander (Euproctus montanus).
 
The parasitic rosette agent (Sphareothecum destruens) has been identified in fish from the Calacuccia dam. The species concerned are the topmouth gudgeon and the minnow, two species introduced into Corsica. The topmouth gudgeon (Pseudorasbora parva) is a fish species originating from China. This infectious intracellular eukaryotic pathogen at the fungus-animal border is said to be the cause of mortality in the common rudd population present in this large artificial lake in Niolu.
This multi-host pathogen affects not only topmouth gudgeon and minnows but also other fish introduced into Corsican freshwater such as bream, common carp, gudgeon, roach, pond perch, rainbow trout and brook trout.
In continental France, the rosette agent also affects farmed sea bass, which benefits from the Label Rouge in Corsica. This infectious pathogen could, in the future, threaten the aquaculture sector in Corsica.
 
On the mainland, rainbow trout are involved in the spread of viral diseases such as viral haemorrhagic septicaemia and infectious haematopoietic necrosis. These are both rhabdoviruses, as the viruses belong to the genus Novirhabdovirus.
 
Carp are affected by spring viremia virus and Koi herpes virus. They can also carry monogenic (Diplozoon nipponicum) and cestode (Bothriocephalus acheilognathi) parasites. The latter also affects the Eastern mosquitofish.
 
The roach is the vector of a parasitic disease, ligulosis, caused by Ligula intestinalis. The pikeperch is the host of a trematode parasite (Bucephalus polymorphus) responsible for larval bucephalosis.
 
The safeguarding of our aquatic ecosystem requires a ban on the introduction of new species of fish and increased protection of the headwaters of the watershed where the Corsican trout live.
 
Some elements of trout and eel biology
 
Trout: Size / Weight Relationship
 
The relationship between the size and weight of trout is summarised by a mathematical equation that enables the weight of the fish to be determined by its length. A 10 cm trout weighs about 10 grams, a 30 cm trout 200 g and a 60 cm fish 1.6 kg.
 
 
Trout: Size / Age relationship (polygon of increasing cumulative frequencies)
 
The polygon of increasing cumulative frequencies enables a rapid determination of the age of the fish. The breaks in the slope are marked in order to determine the limits of the different age classes.

Size / Age relationship of trout (size class boundaries)

Size (mm)	Age (year)
65 to 83	0+ (fry of the year)
84 to 125	1+ (yearling trout)
126 to 152	2+
153 to 207	3+
208 to 266	4+
267 to 327	5+

 
 
 
Eel: Size/Weight Relationship
 
   
 
Eel: Size / Age relationship (polygon of increasing cumulative frequencies)
 
 
The polygon of increasing cumulative frequencies enables a quick determination of the age of the fish. The breaks in the slope are marked in order to determine the limits of the different age classes.
 

Size (mm)	Age (year)
55 to 149	0+ (elver of the year)
150 to 212	1+ (one-year-old eel)
213 to 309	2+
310 to 399	3+
400 to 500	4+
501 to 601	5+
602 to 800	6+

 
Fish in mountain lakes
 
At the beginning of the 20^th century, only the Ninu and Melu lakes were known for their trout populations. The other lakes were considered, probably wrongly, as apiscicultural, i.e. sterile for fish.
 
Lake Gialicatapianu, which has not been affected by breeding programmes, has a good population of brown trout. The connection with the Manganellu, a tributary stream of the Vecchiu (tributary of the Tavignanu) may explain this situation. Other mountain lakes in Corsica are possibly in the same situation.
 
At the end of the 1950s, a breeding programme of brown trout was carried out in the Crenu and Vitalaca lakes. At the beginning of the 1970s, breeding of brown trout and brook trout was carried out: (i) in the Ritondu massif in the lakes of Melu, Capitellu, Goria, Bellebone (or Ritondu, or Betaniella) and Oriente; (ii) in the Rinosu massif in the lakes of Bastani, Bracca and Vitalaca
 
In lakes where brook trout have been introduced and where there was a pre-existing population of brown trout, cohabitation between the two species lasted for a short time. The alien (introduced) species eliminated the native species, as is the case in Lake Melu.
 
Nowadays, in the lakes of Ninu, Vitalaca, Gialicatapianu, Oriente, Crenu and Braca, the fish population is only represented by the brown trout. The fish fauna of the Oriente, Crenu and Braca lakes is endangered.
Brook trout is the only species of fish present in the lakes of Melu, Bastani and Capitellu.
The lakes with the most fish are, in descending order: Melu, Ninu, Bastani, Capitellu, Vitalaca and Gialicatapianu.
 
The absence of fish in the lakes of Cavaciole, Maggiore, Niellucciu and Oru is probably due to the very low mineralisation of the waters and consequently to reduced primary production.
The conductivity value, which reflects the mineralisation of the water, is 42 μS/cm at Lake Ninu. Conductivity recorded at Cavaciole, Maggiore, Niellucciu and Oru (13 to 26 μS/cm) are lower and are characteristic of very weakly mineralised waters.
The Dissolved Organic Carbon content of water is an approach to the primary production of an aquatic ecosystem. At Lake Ninu, the value obtained is 6.7 mg/l, those found at Cavaciole, Maggiore, Niellucciu and Oru (0.6 to 0.8 mg/l) are ten times lower, they translate the low primary productivity of these mountain hydrosystems.
The failure of fry breeding in Goria and Bellebone (or Ritondu, or Betaniella) may have the same explanation.

B. Amphibians and Batrachians
 
In the freshwater ecosystems of Corsica, the two main groups of amphibians are represented. The Urodeles, which have a tail, and the Anurans, which do not have a tail in their adult stage.
 
Anurans:
Balearic green toad (Bufotes viridis balearicus): endemic to the Balearic Islands, Corsica and southern Italy
The Corsican painted frog (Discoglossus montalentii): endemic to Corsica
The Tyrrhenian painted frog (Discoglossus sardus): endemic to the Tyrrhenian
The Italian pool frog (Pelophylax lessonae bergeri)
The Sardinian tree frog (Hyla sarda): endemic to Corsica, Sardinia and the Tuscan archipelago
 
Urodeles:
The Corsican brook salamander (Euproctus montanus): endemic to Corsica
The Corsican fire salamander (Salamandra corsica): endemic to Corsica.


C. Reptiles (Turtles and aquatic snakes)
 
Turtles:
The European pond terrapin (Emys orbicularis)
 
Exotic species:
The red-eared terrapin (Trachemys scripta elegans)
The yellow-bellied slider (Trachemys scripta scripta)
The common snapping turtle (Chelydra serpentina)
 
 
Snakes (water snakes):
The Corsican garter snake (Natrix natrix corsa): endemic to Corsica
The viperine water snake (Natrix maura).

D. Invertebrates
 
The invertebrate population of Corsica’s fresh waters is characterised by the existence of numerous faunal gaps and a very high percentage of endemic species.
 
The specific composition of the fauna of Corsica’s fresh waters (650 species) is one of the poorest in Western Europe. The rivers of the Alps have nearly 2,200 species, and the Pyrenees nearly 1,000.
The faunal gaps vary depending on the order, and are very numerous in the Plecoptera, Diptera and Trichoptera, but less marked in the Ephemeroptera, Odonata, Coleoptera and Hydracaria.
 
The existence of these faunal gaps is explained by the fact that none of the continental species that are missing in Corsica have ever reached the island. 
It was during the Quaternary period that the fauna and flora of the Holarctic regions became most diverse. At that time, great climatic waves caused fauna and flora to move over vast areas, as the taxa followed the advance and retreat of the glaciers. These exchanges between different regions resulted in a great wealth of species in middle Europe. However, at the same time, Corsica remained outside these faunistic and floristic movements because it was already geographically isolated, the drift of the Corso-Sardinian block dating from the Oligocene-Miocene period (Tertiary).
 
The aquatic fauna of Corsica is characterised by a very high percentage of endemic species. In Europe there are three major centres of endemism for running water fauna: the Caucasus, the Tyrrhenian (which includes peninsular Italy, Sicily, Sardinia and Corsica) and the Iberian Peninsula.
With nearly 200 endemic species, the fauna of Corsica contains, given the small size of the island, the highest concentration of endemics in Europe. The originality of the entomo-hydro-fauna is unevenly marked in the various groups. In Corsica, the Plecoptera, Trichoptera and Diptera Blephariceridae contain the highest percentage of endemic species.
Endemism is mainly found in spring communities and in the upper reaches of rivers, where the endemism rate approaches 60%. The spring biotope provides a refuge and conditions for the survival of ancient species.
 
The majority of aquatic invertebrates have larval and pupal stages in the water and the adult stage (imago) in the air.
 
The fauna of rivers has two ways of adapting to the current: (i) ethological, by taking shelter under stones, and (ii) morphological (dorso-ventral flattening, claws, hooks, pseudopods, suction cups, reinforced envelopes).
 
In rivers, aquatic plants (algae and macrophytes) provide only a relatively small proportion of the organic substances needed by feeders. The ecosystem receives most of its primary energy from the surrounding vegetation cover (riparian and catchment vegetation) and possibly from various organic waste products.
Decomposing leaves are the staple diet of detritivorous aquatic invertebrates (fragmenters) that consume large plant debris, transforming it and breaking it down into finer debris. The presence of colonies of micro-organisms increases the nutritional value of this organic matter.
The fine elements resulting from this fragmentation serve as food for the collecting organisms, which obtain them either by filtering particles suspended in the water or by collecting fine elements deposited on the substrate.
A very specialised category of invertebrates known as bottom feeders, or grazers, feed on epilithic algae (Diatoms).
Invertebrate predators and parasites feed on fragmenters, collectors and grazers.
Salmonids, such as trout, are predators of aquatic invertebrates.

Curiositites
 
Peach blossom jellyfish (Craspedacusta sowerbii)
Originally from the Yangtze River in China, it was catalogued in 1880 in England in the pond of a botanical garden. It was transported with tropical water plants imported into England, including the water hyacinth. In 1928, it was present outside ponds in other bodies of water in England. It has now colonised all 5 continents through the aquarium plant market.
In France it appeared in 1962 in Lake Geneva, followed by Lake Annecy in 1990. In 2009 it appeared in Corsica in the Teppe Rosse and Coti-Chiavari dams.
 
 
Tardigrades
Tardigrades (water bears) are found in fresh, brackish and salt water, and also in humid terrestrial environments. The aquatic fauna of Corsica includes 19 species of tardigrades. The most represented families are the Echiniscidae and the Hypsibiidae. These very small organisms (between 0.05 mm and 1.2 mm) feed mainly on mosses and stringy algae, some rare species are carnivorous.
Tardigrades are resistant to extreme temperatures, from -272°C to +150°C. Some even survived a spacewalk during a Russian orbital mission, resisting both the vacuum of space and UV radiation.
These animals can enter into a slowed-down life, cryptobiosis, through extensive dehydration. In this biological phenomenon, water is replaced by a sugar-based “biological antifreeze”.
Tardigrades have the ability to repair DNA. Genome sequencing of this creature opens up interesting prospects, particularly in research programmes on DNA repair mechanisms and carcinogenesis in humans.
 
 
Molluscs
“On the abandoned beach, shellfish and crustaceans” is how the song “La Madrague”, sung by Brigitte Bardot in 1963, begins, words by Jean-Max Rivière and music by Gérard Bourgeois. Molluscs and crustaceans are clearly identified with the marine environment.
In the fresh waters of Corsica, 36 species of molluscs have been catalogued, 5 of which are endemic.
There is even a freshwater mussel (Unio elongatulus) in the lower Tavignanu.
 
 
Crustaceans
This faunal group is represented by 6 species, 4 of which are endemic. The presence of two introduced species should be noted: the white-clawed crayfish (Austropotamobius pallipes) and the spinycheek crayfish (Faxonius limosus). The latter is included in the list of invasive alien species of concern for the European Union. The spinycheek crayfish is a threat to native species either through competition or through the transmission of a type of water mould called Aphanomyces astaci, or crayfish plague, of which it is a healthy carrier. The spinycheek crayfish also weakens dykes and banks by digging deep burrows, thus threatening hydraulic structures.

Planaria, immortal worms
Planaria are aquatic flatworms (Plathelminths), they are represented in the fresh waters of Corsica by three endemic species: Crenobia alpina corsica endemic to Corsica, Dugesia benazzii endemic to Corsica, Sardinia and peninsular Italy, and Dugesia mediterranea endemic to Corsica, Sardinia, Sicily and Spain.
These organisms have the ability to regenerate any damaged part of their body and even regenerate a complete individual from a fragment of a body.
Recent studies have shown that this regeneration process is governed by stem cells called “neoblasts”. The result of this research opens up perspectives for humans on the regeneration of tissue and cells and the maintenance of vital functions despite ageing.
 
 
Gordionus cyrnensis, a manipulative parasite
The Corsican gordian worm (Gordionus cyrnensis) is a non-segmented parasitic worm with a cylindrical body (phylum Nematomorpha). It wraps around itself forming a living “Gordian knot”, hence its name.
In its larval stage, the parasite develops in the body of a terrestrial orthopteran insect (such as grasshoppers or crickets) or beetles. The gordian manipulates the behaviour of the host, causing it to “commit suicide” by drowning, so that it can return to its breeding environment.
In the aquatic environment, it first parasitises aquatic vertebrates such as amphibians or fish; it then parasitises the larvae of aquatic invertebrates such as wood-boring beetles. The parasitised caddisfly larva develops into an aquatic nymph and then into a terrestrial, aerial, adult that is eaten by a cricket or beetle. The life cycle of the Gordian beetle is thus completed.
 
 
Hydrachnidia, regulating mosquito populations
Aquatic mites are arachnids (best-known representatives of arachnids being spiders and scorpions) that are parasites or predators of aquatic insect larvae including diptera (mosquitoes and others).
In the fresh waters of the island, there are nearly 70 species, 41 of which are endemic to Corsica. The population is dominated by the genera Torrenticola and Hygrobates.
These aquatic organisms are present throughout the entire hydrographic network, from the spring to the mouth of a watercourse. They frequent not only running water (rivers) but also stagnant water in the plains (lower reaches of rivers, ponds, temporary pools) and in the mountains (high-altitude lakes).
It should be remembered that mosquitoes are vectors of malaria, chikungunya, dengue, zika, among other diseases.
 

E. Aquatic flora: macrophytes and microalgae
 
1) Macrophytes: are large aquatic plants. They are present in several plant phyla and groups. 
Among the Spermatophytes (seed plants) is pondweed (Potamogeton sp.), of which 13 species are present in Corsica.
Among the Pteridophytes (ferns, horsetails and lycopods), the pilularia minuta should be mentioned.
Among the Bryophytes (mosses, liverworts and anthocerotae), the genus Fontinalis is represented in Corsica by 3 species.
Among the macro-algae, i.e. those visible to and identifiable by the naked eye in the field, the genera Cladophora and Chara are noteworthy.
 
Macrophytes can be classified into two groups: helophytes and hydrophytes.
 
Helophytes are emergent plants that can withstand partial immersion. They are also called mud plants, amphibious plants or amphiphytes.
The most common species are: Phragmites australis (reed), Typha latifolia (broadleaf cattail), Scirpus lacustris (lakeshore bullrush), Juncus acutus (spiny rush), Osmunda regalis (royal fern), Nasturtium officinale (watercress), Mentha aquatica (water mint) and Carex nigra (black sedge).
 
Hydrophytes are submerged or floating plants.
This group of macrophytes consists of 4 types:

1. Submerged fixed and fully immersed hydrophytes
2. Fixed submerged hydrophytes with floating leaves
3. Free floating surface hydrophytes
4. Submerged free floating hydrophytes.

 
Submerged fixed and fully immersed 
Elodea canadensis (Canadian waterweed): present in the lower reaches of rivers, this alien (exotic species) has probably “escaped” from aquariums.
 
Fixed submerged hydrophytes with floating leaves
Nymphaea alba (White waterlily): present at the mouths of some rivers and at Lake Crenu, probably transported by air by the little grebe (Tachybaptus ruficollis) which breeds in the lake’s reed beds.
 
Pondweeds: 13 species are present in the hydrosystems of Corsica (rivers, ponds, canals, peat bogs). The best represented species are: Potamogeton natans (floating pondweed), Potamogeton pectinatus (sago pondweed), Potamogeton pusillus (lesser pondweed).
 
Free floating surface hydrophytes
Duckweed: Lemna minor (common duckweed) and Lemna gibba (gibbous duckweed).
 
Submerged free floating hydrophytes
Utricularia australis (southern bladderwort)
 
 
Macrophytes and popular naturalist knowledge
 
Aquatic plants were frequently used in traditional pharmacopoeias (Greeks, Romans, European herbalists, etc.).
Hippocrates (460-377) prescribed the rhizome of the sweet flag (Acorus calamus) to treat eye diseases, indigestion, toothache, colds, coughs and fevers.
The sweet flag essential oil is used in Egypt and India to treat skin diseases, coughs, asthma and haemorrhoids. The roots of this macrophyte have laxative and diuretic properties.
Traditional Chinese and Japanese medicine uses the dried rhizomes of two species of waterlilies (Nuphar japonicum and Nuphar pumila) for their tonic, haemostatic and diuretic properties.
In France, the rhizome of the yellow waterlily (Nuphar lutea) was used to treat skin ailments: insect bites, sunburn, superficial burns, etc.
In Sudan, the root and leaf of the Egyptian or tiger lotus (Nymphaea lotus) are used to treat dysentery and tumours. It is also used for its antibacterial properties.
 
In Corsica:
Watercress is eaten raw in salads or cooked in soups and herb pies. But it was also used to treat skin diseases.
Water mint can be used in the preparation of soups, herb pies, fritters, omelettes. 
Rushes are used to make cheese moulds and fishermen’s traps.
Wicker or purple willow is used to make baskets.

Ludwigia: invasive plants in France and in the Mediterranean region (including Corsica)
 
Ludwigia palustris is a species of evening primrose that has a wide geographical distribution. It is present in continental France and in Corsica in ponds and rivers. This species does not pose any particular problem.
But Ludwigia peploides is an invasive plant that causes serious damage to aquatic ecosystems. This species, originating from South America, was accidentally introduced around 1830 in the Montpellier region. It has since colonised the whole of France, the first observation in Corsica (Figari, South Corsica) dates from 2007.
Dense and extensive meadows lead to:

1. A decrease in biodiversity (competition with native flora);
2. A change in the physico-chemical nature of the water;
3. A hydraulic disturbance (flow).

The Conservatoire Botanique National de Corse (CBNC), a department of the Office de l’Environnement, is engaged in a strategy of fighting against invasive species in general and ludwigia in particular.
 
2) Microalgae
 
In aquatic environments, microalgae occupy two compartments: the water column and the sediment. The algae in the open water make phytoplankton, the algae attached to the bottom (benthic or epilithic algae) make up the periphyton.
 
Microalgae in waterways
 
In the waterways of Corsica nearly 280 species of microalgae have been catalogued. The Diatomophyceae are the best represented with nearly 70% of the species counted; the genera Navicula and Nitzschia dominate the diatomic population.
For many southern European rivers, the number of species is much lower.
 
Microalgae in high-altitude lakes
 
For all the lakes we studied, nearly 150 species of algae, phytoplanktonic and periphytic, were catalogued.
With nearly 130 species, Chlorophyceae and Diatomophyceae dominate the algal population. The development of a flora rich in Diatoms attests to the presence of silica in the water. The phytoplankton community is more diverse than the epilithic community.
In the lakes of Melu and Ninu, the proliferation of cyanobacteria during the summer period is evidence of pollution related to the overuse of the sites.
 
Microalgae in artificial reservoirs
 
In the reservoirs of the artificial lakes studied, more than 200 species were recorded, with Diatomophyceae and Chlorophyceae being the dominant groups during a very large part of the annual cycle.
During the summer period, there is a phytoplankton bloom resulting from the proliferation of cyanobacteria, which represents more than 90% of the microalgae sampled.

Cyanobacteria
 
Cyanobacteria, also known as cyanophyta, are autotrophic prokaryotes, systematically classified in the kingdom Eubacteria. These micro-organisms are distinguished from bacteria by the presence of chlorophyll and other pigments.
 
Cyanobacteria occur naturally in small numbers all over the world, and in virtually all environments, even the most extreme. Wherever there is water, there can be cyanobacteria.
 
The proliferation of cyanobacteria is the consequence of a combination of their adaptation systems and environmental factors. These factors are mainly:
            - a high concentration of phosphorus in the environment;
            - good stability of the water column;
            - high water temperature.
 
The growth of cyanobacteria can have an impact on water quality, as some cyanobacteria have the ability to produce toxins called cyanotoxins.
 
 

Evolution of the pozzines over time (reconstruction)
 
Core drilling of the pozzines of Lake Ninu
 
Ninu Lake pozzine core: blue clay (glacial moraine) at depth of 4 m 
 
 
Lake Ninu pozzine core: peat at depth of 3 m 

3D modelling of the north-western pozzine of Lake Ninu
 

Depth (m)	Estimated age (Ka cal.BP)
0.0				Pozzine
0.5	2.5
1.0	3.9
1.5	5.4
2.0	6.5			PEAT
2.5	7.4
3.0	8.0
3.5	8.6
4.0	9.0			Glacial moraine
4.5	10.0			BLUE CLAY
5.0	12.0
5.5	14.0
6.0
6.5
7.0	250 000			Hercynian base
7.5				GRANODIORITES
8.0
8.5
9.0
9.5

Variation in the structure of benthic invertebrate populations in a river impacted by urban discharges

Contamination
 
The impact of a former arsenic mine, located on the banks of the Presa, a tributary of the Bravona, has resulted in arsenic and antimony contamination of the physico-chemical and biological compartments of the water.
 
Downstream of the mine, the average arsenic concentration in the water is 1,000 times higher than that obtained in the reference station.
 
The contamination of the environment has led to the disappearance of 19 species of benthic invertebrates as well as a clear reduction in the number of about 10 species.
 
In the invertebrates present downstream of the mine, the larvae of Plecoptera show the highest concentrations of arsenic and antimony.
For Leuctra plecoptera, the value of the bioaccumulation factor (which is the ratio between the concentration of arsenic or antimony in the invertebrates and the average water contents) is 800 for arsenic and 1,300 for antimony.
 
At the most contaminated station, the average arsenic and antimony concentrations of the bryophyte Fontinalis antipyretica are 353 and 48 µg/g respectively; the arsenic and antimony contents in the mosses increase with those in the water. The bioaccumulation factor is 160 for arsenic and 370 for antimony.
 
Trout living downstream of the mine have an average concentration of 1.92 µg/g of arsenic and 0.45 µg/g of antimony. The phenomenon of organotropism has been highlighted, with classification in descending order of the various fish organs studied according to bioaccumulation is (i) for arsenic: operculum > liver > gills > axial skeleton > muscle and (ii) for antimony: operculum > gills > axial skeleton > liver > muscle. The arsenic and antimony concentrations for all organs correlate closely with those found in the water.
 
In the contaminated station, the study of the evolution of arsenic and antimony concentrations in the food chain shows that bioaccumulation decreases with increasing trophic levels.
 
 
3. Indirect anthropisation (climate change)
 
The consequences of global warming on the aquatic invertebrate populations of Corsica’s waterways are reflected in the modification of the altitudinal limits of the distribution area of certain species.
 
Climate change is reducing the ecological niche of many cold-water species, restricting their range to upper rivers and springs.

Altitudinal limits of some “cold” stenothermic organisms
 

Altitudinal limits of some “warm” stenothermic organisms

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Evolution of the annual average air temperature anomaly in France since 1900

Evolution of the intensity of extreme rainfall in the Mediterranean region

Evolution of the average annual temperature of 4 Corsican cities

 

∆ T (°C) = Increase in average air temperature over 40 years

In order to understand the evolution of the average annual temperature of the city of Aiacciu, we have noted the thermal anomalies, i.e. the deviation from a reference called "normal". In this case, the reference corresponds to the average annual temperature between 1971 and 2000.
From 1970 to 1986 (in 17 years), positive thermal anomalies occur 2 years out of 17 (11.7 %). Since 1986, average annual temperatures have almost always been above normal (93.9%). Since 2014, these thermal anomalies have been between +1 and +2 °C.

Average annual temperature (in °C) in Aiacciu, deviations from the reference average (1971 - 2000)

With climate change, the number of summer days, i.e. days when the temperature is above 25 °C, is increasing. In Bastia, the average number of summer days increases from 94 between 1970 and 1980 to 118 between 2010 and 2020. Aiacciu follows more or less the same evolution: 86 between 1970 and 1980 to 117 between 2010 and 2020.

Evolution of the number of summer days (Temperature above 25 °C)

 
For Aiacciu, the comparison of the number of monthly summer days between 1978 and 2018 shows:
(i) Little difference for August (almost 30 days)
(ii) October 2018 had a total of 10 summer days, whereas in 1978 there were none. The high water temperature of the Mediterranean (23 °C in October 2018) can explain this phenomenon. Hot autumns are often the cause of extreme precipitation events that lead to deadly floods.
(iii) For the month of June, there are three times as many summer days in 2018.
(iv) The number of summer days in May and September is twice as high in 2018.

Comparison of the number of monthly summer days between 1978 and 2018

 
For the cumulative annual rainfall in Bastia and Aiacciu, there is no trend (neither increasing nor decreasing). Nevertheless, the intensity of extreme rainfall continues to increase. The increase in the frequency and amplitude of extreme events is one of the consequences of climate change.
It should be noted that the cumulative annual rainfall is greater in Bastia (800 mm) than in Aiacciu (600 mm). This difference is related to the phenomenon of winds from the East.
Atmospheric disturbances, coming from the Atlantic Ocean, generally move from West to East. However, the atmospheric circulation can be reversed, i.e. coming from the East. Some fronts can bypass a high pressure system and come in from the East, this is called a blocking situation.

Changes in cumulative annual precipitation

 
Evapotranspiration is the amount of water returned to the atmosphere through evaporation from the soil surface and transpiration by plants. It depends on physical factors (solar radiation, soil type, water availability in the soil, etc.), meteorological factors (air temperature and humidity, wind, etc.) and biological factors (plant species, type of foliage, etc.).
The evolution of the evapotranspiration value in Bastia and Aiacciu shows a constant increase since the end of the 1980s. The current data for the two cities are close to 1,150 mm of water per year. They should be put into perspective with the average rainfall, 800 mm in Bastia and 600 mm in Aiacciu. This situation leads to an increase in the intensity and duration of soil drought.

Evolution of annual evapotranspiration

 
 
 
Impact on river flows
The evolution of the average annual flow of the Tavignanu shows a significant reduction in flows since the end of the 1980s. The average flow rate has dropped from 14.90 m³ /s (+/- 6.62) between 1973 and 1987 to 9.17 m³ /s (+/- 2.48) between 1997 and 2020. The rise in air temperature and the increase in evaporation, in connection with climate change, lead to a decrease in the water available in the rivers despite a more or less stable rainfall regime.

Evolution of the average annual flow of the Tavignanu at Pont du Faïu

 
The hydrological regime of the Tavignanu was, before 1985, of the pluvio-nival-Mediterranean type. It is influenced by rain and snow and has a summer low water level, characteristic of Mediterranean rivers, which lasted for one month
Since the 1990s, the hydrological regime of the Tavignanu has been much less influenced by snow because snow cover in the Corsican mountains is falling in surface area and thickness, due to global warming. The period of high water in spring, corresponding to the melting of the snow, is less marked.
Summer low water, corresponding to the minimum rainfall, is more severe today as it impacts the resource over 3 months.
Nowadays, the hydrological regime of the Tavignanu is more of a Mediterranean rainfall type.
Evolution of the average monthly flow of the Tavignanu at the Pont du Faïu
 
The evolution of the daily flow of the Tavignanu in 2017 suggests a bleak future for our surface water reserves. The duration of summer low water, which went from 1 month (1973-1984) to 3 months (1985-2020), is currently between 4 and 5 months.
The winter floods (January and December) of 2017 are due to liquid precipitation (rain) and not solid precipitation (snow) due to global warming. The increasing scarcity of snow at higher altitudes and the rise in air temperature explain the longer duration of summer low water. 

Evolution of the daily flow of the Tavignanu in 2017
 

Consequences on Human Health:

Extreme events are increasing in frequency and magnitude, leading to more deaths from heat waves, cold waves, floods, fires and storms.
The increase in the number of cases of cardiovascular disease is linked to air pollution, which increases with the heat.
Rising air temperatures lead to warmer water, not only stagnant water but also running water. The increase in water temperature favours the proliferation of microorganisms responsible for waterborne diseases. These diseases, which affect people living on the African continent, threaten to spread to southern European countries.
 
Indigenous urogenital bilharzia (schistosomiasis)
Corsica, France's southernmost region, is already affected by a parasitosis that is rife in tropical and intertropical areas of Africa, South America and Asia. More than 100 cases of indigenous urogenital bilharzia (schistosomiasis) have been diagnosed on the island since 2014. These are people who have been visiting bathing sites located in the Cavu and Sulenzara rivers in southern Corsica since 2013.
The parasite responsible for the disease, the schistosome, or blood fluke, is a non-segmented flatworm (Plathelminth), belonging to the class Trematodes. The parasite identified in Corsica is a hybrid strain between Schistosoma haematobium and Schistosoma bovis. S. haematobium is a parasite of primates (including humans) and S. bovis infects cattle, sheep and goats.
The schistosome thrives in waters with temperatures between 26 and 30 °C. The development cycle of the parasite includes humans (the final host) and a freshwater gastropod mollusc, Bulinus truncatus rivularis (the intermediate host).Infection can cause renal and/or genital problems. Recent studies have shown a significant association between urinary bilharzia and bladder cancer (squamous cell carcinoma).
In the context of global warming, the proliferation of pathogenic organisms makes it necessary to address health issues at the Human-Animal-Ecosystem interface.
 
The tiger mosquito, Aedes albopictus
Human health is threatened by emerging or re-emerging infectious and vector-borne diseases (malaria, chikungunya, dengue, zika, etc.).
The tiger mosquito, Aedes albopictus, a vector of formerly exotic diseases such as chikungunya, dengue and zika, is spreading its territory from the Mediterranean region to the north of Paris. In mainland France, this dipteran is now present in the Hauts-de-France region; in Corsica, it is found at an altitude of 800 m.


The tiger mosquito in mainland France
 
Malaria in Corsica
Malaria is a disease of the red blood cells caused by a parasite of the genus Plasmodium (mainly P. falciparum and P. vivax) transmitted by the female Anopheles mosquito (Culicidae Anophelinae), which occurs in tropical areas of Africa, Asia and Latin America. In 2020, malaria will affect approximately 200 million people worldwide, of whom 400,000 will die.
The presence of malaria in Corsica dates back to ancient times; several indigenous cases were recorded between 1970 and 1972 in the eastern plain and the Nebbiu. The last case occurred in 2006 in the Portu region.
Of the ten or so species of anopheles catalogued on the island, only Anopheles labranchiae and A. sacharovi are potential vectors for malaria transmission.
Although the potential for malaria in Corsica is low and its reappearance is unlikely, epidemiological vigilance is essential in view of the geographical displacement of tropical parasitic diseases, due to global warming.
Morbidity due to vector-borne diseases can affect humans (yellow fever, dengue, Chikungunya, filariasis...) and animals (plasmodium from birds and reptiles, Rift Valley fever, West Nile fever, filariasis...).
 
Ticks and other mites
Global warming is resulting in an increase in air temperature, especially in cities. The density of dust mites in house dust and bedding is increasing. Allergies to these microscopic arachnids are increasing in duration and intensity.
Asthmatics, whether allergic or not, are the first victims of this proliferation and should limit contact with dust mites.
Ticks, hematophagous mites, are now colonising the northern hemisphere thanks to global warming. They are capable of transmitting the infectious agents of viral meningoencephalitis or Lyme disease, the latter of which is currently on the increase.
 
Bluetongue
This viral disease affects domestic ruminants (sheep, cattle, goats) and wild animals; it is strictly animal and therefore not transmissible to humans. The pathogen is a segmented RNA virus of the genus Orbivirus (family Reoviridae); there are 24 known serotypes of this virus in the world.
The disease is transmitted by the bite of a blood-sucking midge (diptera) of the family Ceratopogonidae and the genus Culicoides. Hematophagy is restricted to the female, the male feeding on plant juices. The larvae of Culicoides midges develop in wetlands, at the interface between terrestrial and aquatic habitats.
Although bluetongue does not affect the sanitary quality of meat, it decimates flocks and has a heavy impact on the economic activity of farmers.
This exotic disease, which originated in Africa and Asia, has gradually spread to northern Europe due to global warming and globalisation.
Circulating virus strains from Africa affected flocks in Spain, Sardinia and Corsica in 2000. In 2004 the disease spread to Portugal. Epizootics of bluetongue are progressing in Northern European countries: 2006 (Belgium, Germany, Netherlands), 2007 (Denmark, United Kingdom), 2008 (Sweden).
The expansion of the disease is due less to the introduction of live animals, imported without control, than to heat waves and heavy rainfall, linked to climate change. Indigenous outbreaks are multiplying due to the proliferation of insect vectors (Culicoides) whose development cycle requires high temperatures.
In Corsica, bluetongue, which had not been recorded since 2017, reappeared in October 2021.
Vaccination is the only effective way to control this disease.
 
Throughout the world, many diseases are transmitted by Culicoides bites: African horse sickness, equine encephalosis, epizootic haemorrhagic disease and Oropouche fever. The latter is the only truly human pathogenic disease transmitted by Culicoides. It causes symptoms similar to those of dengue fever (fever, headache, muscle and joint pain, rash, nausea and vomiting). Neurological complications may occur.
 
Consequences on the energy sector: energy autonomy for Corsica in 2050... utopia or reality?

Background:
3 August 2009: Programming law relating to the implementation of the Grenelle Environment Forum, known as the Grenelle law.
12 July 2010: Law on national commitment to the environment, known as the Grenelle 2 law. This law provides for the development of a Regional Climate Air and Energy Plan (SRCAE) for each region.
20 December 2013: The Assembly of Corsica adopts the SRCAE and sets Corsica's energy autonomy for 2050.
17 August 2015: The law on Energy Transition for Green Growth. This law provides that energy guidelines are set by both the State and the Region, which co-validate a Pluriannual Energy Programme (PPE).
29 October 2015: The Assembly of Corsica validates its PPE (Pluriannual Energy Programme 2016 - 2023).
The main objectives are: (1) to improve energy efficiency, (2) to reduce electricity consumption and (3) to support renewable energy. The final goal of the PPE is in line with the SRCAE objective of energy autonomy by 2050. The PPE provides for measures that "should increase the share of renewable energies to 22% of final energy consumption in 2023, and 40% of electricity production. They are consistent with the trajectory of energy autonomy by 2050 as set by the SRCAE adopted by the Assembly of Corsica on 20 December 2013”.
The implementation of the Pluriannual Energy Programme for the period 2016 - 2023 provides for the investment of nearly 3.2 billion euros. But also the creation of nearly 4,500 jobs in energy renovation, more than 300 new jobs in the operation of RNE equipment. By 2023, nearly 5,000 permanent jobs will have been created and 3,500 non-permanent jobs linked to the construction of energy infrastructures.
 
The difficulties of implementing the PPE
Climatic difficulties
The consequences of climate change have an impact on the energy sector. The low filling rate of the reservoirs managed by EDF is changing the energy mix where hydroelectricity represents 22% of the island's electricity production. This leads to an increase in the share represented by thermal energy, particularly the use of oil.
Annual hydroelectricity production varies from 400 to 550 GWh but in the 15 years from 2003 to 2017 there were 6 years (almost one year in two) where hydroelectricity production was less than 300 GWh, due to a lack of water in the rivers. The low snowfall in our mountains in 2019 resulted in low river flows and limited hydro production.
Due to global warming and the consequent mild winters, energy savings for heating are real. But we must not forget the increase in energy consumption over the summer, due to tourism and the development of air conditioning.
The maximum power demand on the grid in winter varies from 400 to 450 MW. In summer, it approaches 350 MW, with a record of 379 MW reached in August 2017.
This increase in summer consumption has an impact on production costs. In Corsica, the average annual cost is €200/MWh, but it varies from €150/MWh in February to €250/MWh in August when hydraulic production is very low and oil represents 60% of the total electricity production.
As a reminder, the production costs of nuclear electricity vary from €40 to 50/MWh. The debate on nuclear power will obviously be reopened on the occasion of national elections.
 
Technical difficulties
1) The intermittency of random fatal energy production such as solar and wind power poses unresolved problems of storage and grid reliability. A scenario that relies mainly on non-controllable energies (intermittent and unavoidable) would be difficult to implement.
Even though the University of Corsica and its public and private partners (CEA, HELION, EDF) are developing an innovative concept, on the Vignola site near Aiacciu, associating RNE and electrical energy storage. The MYRTE and PAGLIA ORBA technical platforms consist of a photovoltaic power plant that feeds an electrolyser designed to produce the hydrogen needed to operate a fuel cell. These systems for producing energy from solar radiation combine mechanical and electrochemical storage means.
2) Is Corsica ready to do without interconnections with Italy (SARCO and SACOI cables), which are currently indispensable for the safety of the electricity system?
 
Environmental challenges
1) Impact of dams and micro-power stations on the aquatic ecosystem, particularly on endemic species: macrostigma trout, batrachians, invertebrates.
2) Impact of wind turbines on birdlife, especially birds of prey.
3) Installation of solar farms on land with high agricultural potential (Strategic Agricultural Areas of the PADDUC).
In the context of the fight against global change and adaptations to the consequences of climate change, the development of renewable energies is a necessity. However, sustainable development can only be achieved while respecting landscapes that are often emblematic of our identity. In terms of the landscape, solar and wind farms cannot be integrated into the landscape, they create it.
The impact of this type of installation must also take into account social acceptance.
 
Political difficulties
During his visit to the island in May 2018, Nicolas Hulot, then Minister of Ecological and Solidarity Transition, displayed a measured optimism for the island's energy autonomy.
Today, the 'special' relationship between the executive and the government does not help matters.
A symptomatic example. The PPE provides for the arrival of natural gas in Corsica to supply the Lucciana and Vazziu power stations. Firstly, the project to connect Corsica to natural gas from the planned GALSI gas pipeline, which is intended to link Algeria to Italy via Sardinia. Then the project of a gas supply barge off Lucciana, connected to a gas pipeline called Cyrénée which is to cross Corsica to the Vazziu and which will supply the two thermal power stations.
Today, the Energy Regulation Commission is talking about additional costs that it does not want to pay. The Directorate General for Energy and Climate, where representatives of the State sit, mentions a gas pipeline that is too expensive (600 million euros for investment and 10 million per year for operation).
A relatively "modest" investment compared to the 3.2 billion euros planned for the implementation of the PPE over the period 2016 - 2023.
In December 2016, a memorandum of understanding on the supply of natural gas to Corsica as part of the implementation of the PPE was nevertheless signed by Ségolène Royal, then Minister for the Environment, Energy and the Sea and Gilles Simeoni, President of the Executive Council of Corsica. Today, this agreement seems to be in question.
In April 2021, the Corsican Assembly unanimously adopted the revision of the PPE for the period 2019 - 2023 and the strategy until 2028. However, many elected representatives have expressed their concern about the delay in implementing the project to supply natural gas to Corsica's two power plants, in Lucciana and Aiacciu.
 
In conclusion, in view of the climate challenges, Corsica's energy autonomy in 2050 is an ambitious gamble, but the technical, financial and political difficulties are an additional burden.
But to conclude on a more optimistic note, it is necessary to point out the new tracks in the field of renewable energies: methanisation, electric biomass (wood furnaces), thermodynamic solar energy, solar and wind energy with storage, geothermal energy, tidal turbines, pumped energy transfer station (project on the Sampolu and Trevadine dams in the Fium'Orbu)
 
Resistance and resilience

We need to organise 'Resistance' to fight the causes of climate change and mitigate its consequences. By reducing the use of fossil fuels (oil, coal, gas, biomass). By using renewable energies (hydro, solar, wind, geothermal). Stopping deforestation, reducing the risk of fires and planting trees.
But we need to develop 'Resilience' now to adapt to the consequences of global warming. Saving water; reusing treated wastewater (for irrigation and groundwater recharge). By storing water in hillside reservoirs (not large dams). By selecting "Mediterranean" species in order to grow "dry" crops. A "revolution" in irrigated agriculture is needed.
The involvement of local populations in water governance is essential for the success of plans to adapt to the consequences of climate change.
But while there are technical solutions, the solution is essentially "political".
 
A Basin Plan for Adaptation to Climate Change in the water sector (Corsica Basin) was unanimously adopted by the Corsica Basin Committee in September 2018.
The "political" validation of the plan emphasises that one of the principles of action for effective sustainable adaptation is the fair sharing of water and solidarity between users.
The PBACC proposes a range of measures to reduce the vulnerability of territories to (i) resource scarcity, (ii) soil drying, (iii) eutrophication risks, (iv) natural hazards. She also presented measures to (v) maintain the capacity to host aquatic, wetland and coastal biodiversity as well as those to (vi) improve knowledge and (vii) organise action.
In Corsica, the most vulnerable territories are located at the extremities of the island: Capi Corsu Nebbiu, Bastia Bevinci and Balagna Agriate, in the North; South East, in the South. These territories experience periods of drought, the amplitude and frequency of which are constantly increasing due to climate change. Some of them are suffering the consequences of a significant increase in summer demand for water, in relation to the influx of tourists, particularly in Balagne and in the extreme south.
The territories located in the centre of the island have more water and are therefore less vulnerable. The great barrier, oriented North-West/South-East, is made up of 120 peaks that reach an altitude of over 2,000 m. In Corsica, precipitation is orographic in nature because these peaks catch the clouds laden with humidity which, coming from the Atlantic, crossing the Mediterranean from West to East.