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.
(ScienceDaily Magazine & University of Rhode Island, Kingston, RI 02881, USA)