Sedimentation in the Lake of Tunis: A lagoon strongly influenced by man

The Lake of Tunis in northern Tunisia is a eutrophic marine lagoon covering 45 km² to an average depth of 1 m. Exchange of water with the Mediterranean Sea is restricted to a few canals. Bottom sediment is mainly calcareous sandy mud with much organic material. Sand-size grains commonly include quartz, dolomite, gypsum, and pyrite, as well as calcareous skeletal material dominated by molluscan fragments. Pyrite is the only important authigenic mineral presently forming. Principal sources of sediment are sewage sludge and fill, calcareous marine organisms, including abundant worm reefs, and local intermittent streams. The concentrations of Hg, Pb, Cd, Cr, Zn, Cu, and Fe are significantly higher in Lake of Tunis sediments than in sediments from Bahiret el Bibane, an unpolluted lagoon in the south of Tunisia. Man has affected the Lake of Tunis in four important ways. (1) Cultivation by early settlers, particularly the Romans, caused increased regional erosion and probably provided the sediment that encloses the lagoon. (2) In the mid-1800's the French constructed a ship canal which reduced intralagoonal circulation as well as exchange with the Mediterranean. (3) Nutrient pollution from sewage has encouraged the growth of circulation-restricting worm reefs, caused eutrophication and fish kills, and produced H₂S odor which permeates the city of Tunis during the summer. (4) Finally, man has significantly reduced the size of the lagoon by filling the lagoon and operating salt pans.