Human Pollutants Enrich Egyptian Coastal Fishery
NARRAGANSETT, Rhode Island, July 1, 2003 (ENS) - When the Aswan high dam was finished in 1965, the flow of nutrients from the Nile River into Mediterranean coastal waters was cut by more than 90 percent, and the once productive Egyptian coastal fishery collapsed.
In the 1980s the fishery began a strong recovery, at the same time as fertilizer use increased, agricultural drainage expanded, and the human population increased along with extensions of urban water supplies and sewage collection systems.
A study by University of Rhode Island Graduate School of Oceanography biologist Scott Nixon published in "Ambio," a Swedish scientific journal on the human environment, shows how human sewage and agricultural drainage now support the fertility once provided by the Nile.
The two mile long Aswan high dam provides irrigation and electricity for all of Egypt from Lake Nassar, the 500 mile long reservoir created when it was built.
"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."
Fish landings now exceed those before the dam was erected, and the landings of prawn have reached 75 percent of their pre-dam value.
Nixon says the phosphorus and nitrogen reaching the Egyptian coast in urban waste water increased because the population had grown since 1965. In addition, more people were eating and excreting, and there was an 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 phosphorus from Cairo and Alexandria may now equal or exceed that of the historical annual Nile floods, and the excretion of dietary nitrogen is much larger than that delivered by the river.
With Cairo and Alexandria accounting for some 28 percent of the total Egyptian population, some of the human waste from the people living in other urban areas and towns in the Nile delta also reaches the coast.
Nixon's study shows that the productive engine of the Egyptian shelf ecosystem today is different from that which supported the fishery before the dam was closed.
The system has changed from one dominated by a large pulse of fresh water and nutrients held in a turbid surface layer, says Nixon, to one exposed to a relatively constant water and nutrient input to a deeper, well mixed, clear water column.
"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.
"The story of the Nile shows how quickly anthropogenic nutrient emissions can greatly exceed even those carried by one of the world's great rivers," Nixon said.
"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," said Nixon, "make it clear that it is easy to deliver too much of a good thing to the coastal ocean."