Bahiret el Bibane

About

Bahiret el Bibane (Arabic: بحيرة البيبان) is a large lagoon of around 33 km length by 10 km width near the Libyan border located 10 km North of the town of Ben Gardane and 20 km West of the city of Zarzis, in the Medenine governorate. It has been classified as a Ramsar Wetland since 2007. The lagoon itself is the second largest in Tunisia after the lagoon or gulf of Boughrara. In addition to the main lagoon, the Ramsar protected site of 39.266 ha also covers the coastal waters outside the lagoon, a smaller lagoon called Bahar Alouane (the sea of colors) along the coast towards the city of Zarzis, and two "sebkhets" or salt flats West of the main lagoon.

The main lagoon is separated from the sea by two peninsulas (Solb el-Gharbi and Solb ech-Charki) which almost meet. These two karstic peninsulas of ca 12 km length each are made of fossilized limestone slabs and nine small islets are located in the opening in between them. This unusual configuration where the open sea communicates with the inside of the lagoon is called "el bibane" (the doors in Tunisian Arabic), which gives the name of the site "the little sea (Bahiret) of the Doors" The lagoon has a maximal depth comprised between 5.5. and 6.5m. Despite being in permanent communication with the sea, it is hypersaline (on average 44 PSU but it varies depending on the location in the laguna and over time in the year). The lagoon is intermittently fed with fresh water from the wadi Fessi (average annual contribution of 10 million m3): The large-bed intermittent wadi originates in the Mount Matmata to the South. In the rare occurrence of flooding, the lagoon controls the floods and captures sediments, playing an important role in stabilizing the coast. Another small inflow of fresh water is also present along the southern edge of the lagoon. Bahiret el bibane is under the influence of the tide: Tidal sites are extremely rare in the Mediterranean and very important for birds, fish and shellfish. Bahiret el Bibane is located in one of the only two areas in the Mediterranean sea with considerable tide amplitude, in the one extending from Sfax to the Libyan border via the Farwa lagoon (The other one is in the Northern Adriatic Sea and includes the Lagoon of Venice). The average tidal range in the lagoon is 0.7 m, sometimes amplified by winds. Though this tide amplitude is inferior to the one near Gabes, the peculiar geography of the lagoon creates strong sea currents hydrodynamics at the opening passes where it connects with the sea. This attracts fish which naturally tends to favor swimming against strong currents.

Boujmel Sebakh (in the NW) and El Medeïna (at the SE end of the lagoon) dry up in the summer, with the ground turning salt-white. In winter, during the rainy season, they look in part like an extension of the lagoon. Their year-round vegetation consists mostly of Salicornia arabica and Salicornia fructicosa. In areas and during the drier season when a chloride or sulphate cust forms on a dry-looking ground, the Salicornias give way to Halocnemum strobilaceum. Where and when the water is deeper than 1m, these plants are replaced by Arthrocnemum indicum. The deeper center of the sebkha areas is generally devoid of vegetation except during the drier season when it is fleetingly dominated by Halopeplis amplexicaulis.

Dissolved oxygen is present in satisfactory quantities throughout the lagoon, due to the strength of the currents entering the lagoon and the presence of a dense underwater plant cover. Average oxygen saturation is 109 percent and can reach 133 percent. The importance of photosynthetic activity of marine Magnoliophyte beds is demonstrated by the fact that dissolved oxygen values are higher during the day that those measured at night. However, the lagoon retains a good water transparency (and chlorophyll concentrations no higher than 1.47 μg/L) due to the strength of the tidal currents. The number of benthic macrophyte species recorded in the lagoon is 22 species including 3 magnolithophytes and 19 macroalgae.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.