10.2 Generalities on Tunisian coastal lagoons

Tunisia hosts five lagoons and one lake with the main following characteristics (Tables I and II; Fig. 1) located on a 1 000 km shore line (over 1 300 km of coastline) with a total surface of 50 000 ha (Table I).

 

Figure 1. Map of Tunisian lagoons

Table I: Surfaces of the Tunisian lagoons and lake (ha)

Table II: Physical characteristics of Tunisian lagoons: depth, environments and salinity

Idea Consult (2008)

The Monastir lagoon is situated on the east coast, very close to the INSTM (Institut national des sciences et technologies de la mer) experimental station. It has never been exploited for legal fish capture and no licence has been issued for that purpose. During the 1980s it was used by the former national aquaculture centre to process some trials on mussel and fish growing up.

Communication with the sea

The Bizerte, Ghar el Melh, Tunis, Monastir and El Bibane lagoons are connected to the Mediterranean Sea while the Ichkeul lake is communicating with the Bizerte lagoon through a 5 km small river locally named oued Tinja. The Bizerte lagoon is connected to the sea through an artificial channel dredged many years ago to link the dockyard of Menzel Bourguiba to the sea. In spite of their communication with the sea, some of these lagoons are suffering from the limitation of water flow from the sea which is deemed as not enough to refresh their entire water volume, especially in the case of the Tunis lake, in which water renewal needs more than 20 days (Ben Charada, 1992).

Climate

The Tunisian climate goes from humid and subhumid in the north to arid and Sahara climate in the south (Fig. 2).

 

Figure 2. Tunisia’s climatic zones

Freshwater input

The northern part of the country, where four lagoons and one lake are located, receives more than 500 mm of rain per year, whereas the lagoon of El Bibane (situated at the extreme south part of the country close to the Libyan border) receives less than 200 mm per year, with a predominating Sahara climate. The only lagoon that receives significant freshwater quantities is the Ichkeul lake, where several small size rivers (oueds) discharge large volumes of water especially during winter and spring. As a result, the water regime in this lake is highly fluctuating with salinity ranging from less than 4 ppt in winter to more than 30 ppt in summer. Salinity reaches more than 40 ppt in the other lagoons; bathymetry varies from one to more than six meters as a whole with different depths inside the lagoons. The Bizerte lagoon is drastically suffering from the decrease of freshwater flowing from lake Ichkeul (165 million m3 before the water management works: the construction of several dams and a lock gate settled a few years ago with the aim of regulating the water level inside the lake).
With the exception of the El Bibane lagoon, which does not receive significant organic pollutants (except for the agriculture run off), all other lagoons suffer from eutrophication due to the discharge of huge amounts of organic matter (Romdhane, 2002). In the Tunis lagoon, this phenomenon culminated before the restoring works undertaken in the early 1980s. For a long time, this lagoon received large volumes of sewage from the surrounding urban agglomerations and is currently going to become a marine waterbody following the restoring works that took place in the early 1980s. The mean depth of the lagoons never goes over 5 meters, the minimum being 1–1.5 and the maximum 6 meters, with a bathymetry varying inside the lagoon to reach more than 10 meters in dredged zones.

Main ecological features

The main ecological features derive from the nature of local systems, which go from semi-arid (average rain rate of 550 mm/year) to arid sub zones (200 mm/year). As a result, they are influenced by the surrounding inland and marine environment (the west Mediterranean basin in the north part and the south-east one for the El Bibane lagoon), which is under the direct impact of the Gulf of Gabes, characterized by an extended, smooth and biologically rich continental shelf.

Lagoon typology, ownership and management

The Tunisian lagoons are located along a littoral portion of about 1 000 km. The water exchanges of the lagoons with their surrounding environment (watershed and sea) highly influence their ecology. The Ichkeul lake water varies from fresh to brackish, while the other lagoons are known as marine bodies with high salinity, reaching more than 40 ppt. The lagoons belong to the state of Tunisia through the hydraulic and maritime domains, which are managed by the Ministry of Agriculture and Environment as far as capture fisheries aquaculture and the environment are concerned. The Ministry is represented at the regional level (governorates) by 26 regional Commissariats for agriculture development (CRDAs), which include fishery and aquaculture districts as well as water managing services (in the Ichkeul lagoon’s case).

Legal framework and constraints

The Tunisian littoral is protected by several legislative texts, among them those related to the law on public maritime domain (especially its provision that created the Agency for the Protection and Management of the Littoral [APAL]), the code of water (1974), the fishery code (1992) and the code of urbanism and territory management.
The main concerns related to pollution really exist in the lagoon of Bizerte, which is suffering from land-based outfalls (industrial and urban contamination), the North Tunis lake (eutrophication caused by sewages and petroleum pollution) and the Ichkeul lake (agriculture outfalls with high levels of organic loads and nutriments and growing salinity rates following the water managements works). The El Bibane lagoon is particularly suffering from overfishing and poor fishing practices, including non-selective gear and hazardous state of the fixed gear (bordigue).
Three marine protected areas were established in Tunisia, which are the Galite/Galiton archipelagos situated off the north coast, the Zembra/Zembretta Islands (northeast of the Tunis gulf) and the ornithological zone of Kneis (southeast of the country). Other zones of environmental interest are going to be established within the framework of PAM protocol of marines protected areas.
The Ichkeul lake is a Ramsar site as well as a MAB/ Unesco one; it belongs to a National park that encompasses the surrounding Djebel (a little mountain) and a set of marshes. The El Bibane lagoon is also a Ramsar site, specifically for its ornithological interest.