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ShareJuly 2, 2003 — After the closure of the Aswan high dam in 1965, the flow of nutrients from the Nile into Mediterranean coastal waters was reduced by more than 90 percent, and the once productive fishery collapsed. In the 1980s the fishery began a dramatic recovery, coincident with increasing fertilizer use, expanded agricultural drainage, increasing human population, and dramatic extensions of urban water supplies and sewage collection systems.
In a recent issue of Ambio, a Swedish scientific journal on the human environment, URI Graduate School of Oceanography biologist Scott Nixon discusses how human sewage and agricultural drainage now support the fertility once provided by the Nile, although the nature of the productive ecosystem now supporting the fishery appears to be quite different from the historical one.
Recent satellite images document the continuing fertility of the Nile valley and delta where continuous irrigation and intensive applications of fertilizer have replaced the traditional flood-based agriculture. Fish landings now greatly exceed those prior to the closure of the high dam, and the landings of prawn have reached 75% of their pre-dam value.
"The fish eaten today in Alexandria and Cairo may taste as good," said Nixon, " but I believe that they have been fed by sources far different from the rain and soils of Ethiopia that served as sources of the nutrients in the historic annual flood."
Nixon discusses three reasons for believing that the amount of phosphorus and nitrogen reaching the Egyptian Mediterranean coast in urban waste water increased dramatically during the 1980s, coincident with the recovery of the coastal fisheries. First, the population had increased substantially since 1965; second, the nutrition of the population had improved in terms of total per capita consumption; and third, there was a remarkable expansion of the public water and sewer systems of Greater Cairo, Alexandria, and other urban areas during the 1980s.
Based on population estimates, the potential release of man-made phosphorus from Cairo and Alexandria may now equal or exceed that of the historical Nile flood, and the excretion of dietary nitrogen is much larger than that delivered by the river. With Cairo and Alexandria accounting for only about 28% of the total Egyptian population, some of the human waste from the remaining population living in other urban areas and towns in the delta must also reach the coast.
The construction of adequate sewer systems since the late 1970s eliminated most of the health problems in Egypt, but the "water carriage" system of waste removal also insures that most of the phosphorus and nitrogen contained in the sewage will be eventually transported to the coastal ocean.
While the link between and increasing discharge of phosphorus and nitrogen from man-made sources and the recovery of the Egyptian Mediterranean Sea fishery remains hypothetical, the circumstantial evidence seems compelling.
The data available indicate that the productive engine of the Egyptian shelf ecosystem today is quite different from that which supported the pre-high dam fishery. The system has changed from one dominated by a large pulse of fresh water and nutrients held in a turbid surface layer to one exposed to a relatively constant water and nutrient input to a much deeper, well-mixed and very clear water column.
The loss of silica in the water as a result of the construction of the dam in relation to phosphorus and nitrogen have important consequences for lower food web structure, and changes in the seasonal distribution of production must have repercussions for larval fish.
"There may be many ways to produce large numbers of fish," said Nixon," but all must ultimately require an adequate supply of primary nutrients, regardless of the details of ecosystem structure.
"While there is some reason to be optimistic that similar results may be found on other coasts as the result of increased man-made nutrients, the story of the Nile also shows how quickly anthropogenic nutrient emissions can greatly exceed even those carried by one of the world's great rivers," continued Nixon. "The large number of coastal areas already manifesting serious effects of nutrient over-enrichment, including bottom water hypoxia or anoxia, undesirable algal blooms, and the loss of sea grasses and coral reefs make it clear that it is easy to deliver 'too much of a good thing' to the coastal ocean."
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The URI Graduate School of Oceanography is one of the country's largest marine science education programs, and one of the world's foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as acid rain, harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, and the National Sea Grant Library.
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