The southeastern Mediterranean ecosystem revisited: Thirty years after the construction of the Aswan High Dam
by Sayed El-Sayed and Gert L. van Dijken
"Egypt is the gift of the Nile," wrote the Greek historian Herodotus in the fifth century B.C. No other country owes its very existence to a single lifeline. The annual cycle in which the Nile flood deposits layers of silt at the rate of several centimeters per century built the highly fertile Nile Delta in the north of Egypt and the Nile Valley in the south. The Nile flood normally begins in August and ends in October, originating as rainfall on the Ethiopian highlands and melting snow in the Mountains of the Moon, on the Uganda-Zaire border.
From time immemorial the eastern Mediterranean ecosystem has been relatively stable, and the annual flood of the Nile River has been the most important event regulating the fertility of the region. The great brown flood that came pouring out of the desert had a fertilizing effect on waters of the southeastern Mediterranean, or Levantine Basin, analogous to that which it had on land. During the past one hundred years, the Levantine Basin has been subjected to the effects of two important events, the opening of the Suez Canal in 1869 and the construction of the Aswan High Dam in 1964. The latter is considered the greatest public work to be undertaken in Egypt since the pyramids. Since 1965 when the High Dam became fully operational, the Nile flow to the Mediterranean has greatly diminished, while the effects of dangerous floods in 1964 and 1973 and threatening droughts in 1972-73 and 1983-84 were mitigated.
In marked contrast to more fertile, nutrient-rich seas such as the North Sea and the Arabian Sea, the Mediterranean Sea is noted for its nutrient-poor waters which contribute to its low level of primary productivity. Primary productivity is the synthesis of organic matter from inorganic substances through photosynthesis by unicellular organisms called phytoplankton. In the Mediterranean Sea low primary productivity is due to several unique physical features. The general circulation of the sea is lagoonal in character. A west to east surface current brings relatively nutrient-depleted water from the North Atlantic through the Strait of Gibraltar and allows nutrient-rich bottom water to exit the sea through the same opening. The arid climate of the region and the low levels of nutrient-rich river runoff also contribute to the low productivity of the Mediterranean.
The Levantine Basin is a relatively isolated ocean basin with moderate depths of approximately 2,000 meters. It is separated from the western basin by a submarine sill between Sicily and North Africa. The eastern continental shelf is very narrow, not more than eight kilometers wide, except off the Nile Delta where the 200-meter depth contour lies sixty kilometers offshore. Along the Egyptian coast there are a number of shallow coastal lagoons with water ranging from brackish to hypersaline. The extremely low organic production, or oligotrophy, of the offshore waters of the Levantine Basin is manifested in the extreme transparency of the water, measured by the disappearance of a Secchi disc (see footnote) at a world-record depth of fifty-three meters!
Before the High Dam was built, fifty percent of the Nile flow drained into the Mediterranean. During an average flood, the total discharge of nutrient salts was estimated to be approximately 5,500 tons of phosphate and 280,000 tons of silicate. The nutrient-rich flood water, or Nile Stream, was approximately fifteen kilometers wide and had sharp boundaries. It extended along the Egyptian coast and was detected off the Israeli coast and sometimes off southern Turkey.
Although the construction of the High Dam has been an unquestionably tremendous boon to Egyptian agriculture and has benefited industry by providing cheap electric power, it has also had far-reaching effects on the transport of fertile silt and sediments. These sediments are now trapped behind the dam, a situation which has led to severe erosion along the Egyptian coast. The dam also had great impact on the fertility of the coastal waters. The fertilizing effect of the inflow of the nutrient-rich water during the flood season once resulted in exceptionally dense blooms of phytoplankton off the Nile Delta. This "Nile bloom" provided sustenance to sardines and other pelagic fishes. It also constituted a large source of detrital material, the products of organic decay, which forms a vital source of food for commercially valuable organisms such as shrimp.
Total fish and sardine catches off the Egyptian coast between 1962 and 1992.
The decrease in fertility of the southeastern Mediterranean waters caused by the High Dam has had a catastrophic effect on marine fisheries. The average fish catch declined from nearly 35,000 tons in 1962 and 1963 to less than one-fourth of this catch in 1969. Hardest hit was the sardine fishery, primarily composed of Sardinella aurita, which is heavily dependent on increased phytoplankton during the flood season. Thus, from a total of 18,000 tons in 1962, a mere 460 and 600 tons of sardine were landed in 1968 and 1969, respectively. The shrimp fishery also took a heavy toll as the catch decreased from 8,300 tons in 1963 to 1,128 tons in 1969.
Although several studies have been conducted on the effects of the Nile discharge on the biological oceanography and marine fisheries off the Egyptian and Israeli coasts, the dramatic changes in the physical, chemical and biological conditions in the Levantine Basin have not yet been fully assessed or understood. For instance, we know very little of the seasonal fluctuations of primary productivity in this area, or the hydrographic and chemical factors which regulate this productivity. Furthermore, we do not know whether or not the reduced river outflow has caused changes in the species composition of the planktonic communities, which would lead to different food chains. We do know that sardines, which once migrated into the Nile Delta region to feed on the fall phytoplankton blooms, apparently altered their migration pattern after construction of the High Dam.
In recent years there has been a noticeable increase in the sardine catch along the Egyptian coast (8,590 tons in 1992) with most of the landings coinciding with the period of maximum discharge from the coastal lakes during winter. Since the late 1980s, the total fish catch (pelagic and bottom) off the Egyptian coast has grown to levels comparable to those that existed before construction of the dam. Whether this is due to increased fishing efforts or recovery of fish stocks is not clear.
URL=http://www-ocean.tamu.edu/Quarterdeck/QD3.1/Elsayed/elsayed.html Updated July 24, 1995