MAP Technical Reports Series No. 106 UNEP

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winter and spring, and 45 in the summer. There is also a considerable variation in temperature
in the course of the year, ranging from 10EC in the winter to about 30EC in July-August. Cut in
two by a raised causeway running alongside the navigable canal, the lagoon has a southern part
and a northern part. The lagoon receives the only partially treated sewage of the city of Tunis
(population of 1,000,000), which flows mainly into the northern part, while the southern section
receives urban and industrial inputs (Stirn, 1968; Aubert and Aubert, 1986).
The high trophic state of the basin favours invasive proliferations of macroalgae and
frequent, intense algal blooms. It has been estimated that during summer and autumn the
macroalgal biomass consisting above all of algae of the Ulva may reach 1,479 g/m 2 with a total
mass assessed at 43,658 tons (UNESCO, 1984).
Belkhir et al. (1987) underline the presence of high nutrient values. The concentrations
observed at 15 stations in the period 11-15 July 1985 were between 1,852 and 11,064 mg/m 3
for soluble inorganic nitrogen, between 44 and 1,958 mg/m 3 for orthophosphate and between
8,700 and 15,400 mg/m 3 for silica. The presence of large amounts of phytoplankton biomass
consisted both of diatoms and of dinoflagellates (Gymnodinium sp. and Prorocentrum micans);
anoxia and hydrogen sulphide caused widespread fish kills.
Lagoon of Ichkeul. The lagoon of 95 km 2 and average depth of 1.2 m, located to the
North of Tunisia is described by Ben Rejeb and Lemoalle (1986). Phytoplankton biomass is
particularly abundant during the winter months, because of the hydrology and hydrodynamics
of the basin. During winter and spring the lagoon receives run-off rich in fertilizing substances
from the surrounding areas, while in the summer there is inflow of nutrient poor marine waters.
Chlorophyll and salinity show an inverse correlation; low salinity values in the winter correspond
to high chlorophyll values, and vice-versa.
5.2.13 Algeria: Southern Mediterranean
Cases of eutrophication have been reported in a number of lagoons and in the port
areas with heavy commercial traffic. Coastal sea waters generally show conditions of
oligotrophy. During the "Mediprod V" oceanographic survey of 27 May - 27 June 1986 the highest
chlorophyll-a values (0.6 mg/m 3 max.) were observed near the coast, and were associated with
the upwelling of deep waters (IFREMER, 1990).
The Lake of El-Mellah. Situated a few kilometres from the Tunisian border this brackish
lagoon of 680 hectares with a maximum depth of 5 m is affected by recurrent conditions of
dystrophy and in summer of anoxia in the bottom waters (De Casabianca-Chassany et al., 1986;
De Casabianca-Chassany et al., 1988). More recently, Samson- Kechacha and Touahria (1992)
have observed the presence of potentially toxic species such as Dinophysis acuminata and
others of the genus Gonyaulax and Gymnodinium. Conditions of environmental stress are
mostly manifested in the most confined part of the lagoon, which receives direct input from
rivers.
6. EFFECTS ON MARINE LIFE, RESOURCES AND AMENITIES
6.1 Phytoplankton blooms noxious for marine invertebrates and vertebrates
Massive and widespread fish and invertebrate mortality and damage to the marine ecosystems
associated with algal blooms and/or green, red and brown tides are phenomena