MAP Technical Reports Series No. 106 UNEP

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Since the '60s the inputs from the Nile have undergone drastic modification as a result
of the High Dam construction. While the reduction in the quantities of solids carried down has
affected the morphology of the delta area, the lower flow-rates have not only profoundly modified
the hydrology and biology of the South-eastern Mediterranean, but also the lagoon areas.
Further to the increase in fertilizer use and the rise in the human population of the delta area that
has brought about an increase of nutrients, and a significant increase in primary productivity in
the coastal sea-waters (GESAMP, 1988).
Lake Mariout. Mariout is not directly connected to the sea as the other lagoons; its
waters are discharged to the sea via the Mex pumping station west of Alexandria. Since the
study of Elster & Vollenweider, the lagoon area has been reduced due to land reclamation, and
in part by the construction of fish farms. The so-called Nusha Hydrodrome (5 km 2 ) is an
artificially impounded section of Lake Mariout. Originally, L. Mariout had a basin of quite different
biological properties, one part mostly covered by Potamogeton pectinatus, while another part
that received discharges from the city of Alexandria, was plankton dominated and of extremely
high productivity. Primary production measured ranged consistently over 1-2 g C/m 2 .day, and
reached peak values over 5 g/m 2 .day with Spirulina jenneri dominating. Also, vertical stratification
over only 1 m depth during calm periods could be very high with oxygen saturation in excess of
200% below surface, and severe reducing conditions over the bottom.
This in essence is still the situation of to-day, which is further aggravated by the huge
inputs of untreated sewage from the city of Alexandria and the effluents from industrial and
agricultural sources (Saad, 1973). Deterioration is so bad that the intense production and
release to the air of hydrogen sulphide makes breathing at times difficult. Of the cases of
eutrophication studied by Elster & Vollenweider, this is likely among the worst situation worldwide.
The Lagoon of Burullus (420 km 2 , average depth 1.25 m) shows the highest
concentrations of nutrients in its westernmost part since this is the area which receives 75% of
the inputs from drainage of the surrounding areas. It also has the longest water residence times,
since exchanges with the open sea take place mainly in the eastern part (Abdel-Moati et al.,
1988). Estimates made by Abdel-Moati et al. (1990) during 1987-8 show that the Burullus lagoon
receives nutrients equivalent to 558 tons/year of phosphorus and 2,318 of nitrogen. The
chlorophyll values are generally not particularly high (mean value 6.6 mg/m 3 ), although there are
recurrent peaks between 10 and 15 mg/m 3 .
The Lagoon of Menzalah (ca. 2000 km 2 between the Nile delta and the Suez Canal) is
in the same condition; due to the run-off from agricultural areas it reaches high trophic levels and
recurrent algal blooms occur (Halim, 1989). Mean chlorophyll-a of Manzalah are reported at 21
mg/m 3 .
As to nutrient limitation, Elster and Vollenweider (1961) found often N/P ratios low,
indicating prevailing nitrogen limitation. Increased use of artificial fertilizers over recent decades,
is therefore a possible main factor in promoting eutrophication.
5.2.12 Tunisia: Southern Mediterranean
Cases of eutrophication are reported above all in the Lakes of Tunis and Ichkeul.
Lake of Tunis. A lagoon of 48.6 km 2 with an average depth of 1 m. The only exchange
with the sea are the navigable canal (opening 500 m) and two other small channels (opening 40
m), one close to the town of Kherredine. The salinity varies between 25 in the