Scott W Nixon. American Scientist. Volume 92, Issue 2. Mar/Apr 2004.
The White Nile, from Lake Victoria in Uganda, and the Blue Nile, from Lake Tana in Ethiopia, merge in Sudan to form the Nile-the longest river in the world. Fifty-five hundred kilometers from its source, the Nile pours into the Mediterranean Sea along the coast of Egypt. Each summer before 1964, waters from rains in Ethiopia gathered and sent a flood into the Nile Valley. The river picked up sediment over hundreds of kilometers and then deposited it-rich nourishment-in the floodplain and the Nile Delta. Given the flow of sediment, the ancient historian Herodotus called Egypt “the gift of the Nile.”
If not for the Nile, Egypt would be nothing more than desert, but even with the river, most of the country remains barren. Farmers can raise crops on only about 4 percent of Egyptian land, and most of that stretches along the banks of the river. The historic Nile, though, also fertilized another desert-one in a sea of water. The circulation in the Mediterranean Sea keeps nutrients from flowing to the east. In fact, most of the eastern Mediterranean is less hospitable to life than the Sargasso Sea or the North Central Pacific Gyre, two famous oceanic deserts. In the past, the August-October floods generated a Nile bloom, or upsurge of phytoplankton, which fed a valuable fishery.
The bloom and the fishery both collapsed after the Aswan High Dam closed in 1964. Few scientists were surprised that the dam damaged the fishery. The surprise came 15 years later, when the fishery revived. Catches began to recover around 1980, and the reasons have been a source of considerable scientific curiosity. Some think that improved fishing technique and more determined efforts are responsible. I, on the other hand, think that increases in nutrients triggered enhanced primary production, by organisms such as phytoplankton. With more food to eat, the fisheries could rebuild. As I shall show, human activities-including the use of fertilizers for agriculture and increased sewage outfalls-provide more than enough nutrients to replace those captured by the Aswan High Dam.
A Duo of Dams
For many millennia, the Nile’s penchant for flooding has been both boon and bane, bringing nutrients to the soil but threatening crops with inundation. By the late 1890s, with both population and agriculture expanding rapidly, the situation worsened. In high-water years, entire fields could be swamped and the plants destroyed. To control the floods, officials designed a dam to be built at a settlement called Aswan, which lies along the first cataract of the Nile as it enters Egypt. Construction started in 1899. By 1902, a 1,900-meter dam spanned the river. At just 54 meters tall, though, that dam failed, because flood waters could run over it. So construction teams raised the height of the dam in a project that lasted from 1907 till 1912, and then again from 1929 through 1933.
Despite these efforts, the flood of 1946 nearly breached the structure. Instead of making the dam even higher, engineers decided to build a new, larger dam upriver. The Russian Zu k Hydroproject Institute designed the so-called Aswan High Dam, or El Sand al Aali in Arabic, and construction began in 1960. When the new dam was finished in 1970, it stretched 3,600 meters long, 111 meters tall, 980 meters wide at its base and 40 meters wide on top.
The Aswan High Dam began to impound water in 1964, long before it was completed. The rising water created Lake Nasser, named after Gamal Abdel Nasser, president of Egypt from 1954 to 1970. When Lake Nasser filled completely in 1976, it was 480 kilometers long and 16 kilometers across at its widest point. In the end, the Aswan High Dam cost $1 billion dollars.
Besides controlling floods, the dam also includes a dozen generators. At peak capacity, the dam produces 2.1 gigawatts of power. Getting that power, though, cost more than dollars. First, Lake Nasser flooded much of lower Nubia. More than 90,000 Nubians were forced to relocate as the lake filled. Worse still, long-lasting problems lay ahead for residents downstream from the dam.
Feast, Starve, Feast
Before the era of the Aswan High Dam, fishing off the Nile Delta produced bountiful catches. But soon after the dam closed, catches-especially of sardines-plummeted. Between 1962 and 1965, an average of 37,000 tons of sardines were caught per year. After the dam closed, the average annual catch dropped to just 6,500 tons-a decrease of more than 80 percent. But the decline in fish extended far beyond sardines. Data from trawling surveys off the western part of the Nile Delta also revealed a nearly 80 percent decrease in fish and shrimp when comparing 1960-61 with 1969-70. The damage to fish populations continued for 15 years.
By the early 1980s, however, more fish-particularly the major groups of finfish, such as pollock and grouper-and shrimp started showing up in nets. In fact, the catches soon exceeded levels prior to the closing of the dam. The available data go beyond fishing catch alone. Although a few species really dropped in numbers, M. Bebars of the National Institute of Oceanography in Egypt and his colleagues, for instance, studied the species richness-basically the number of species in the ecosystem-in the Nile Delta, and found no difference from 1962 to 1989. That research team, however, did find decreases in the relative abundance of virtually all species after 1966, with the effects lingering until 1979, when the numbers returned to pre-dam values or higher. Bebars and his team also examined the ratio of pelagic (near-surface) fish to demersal (deep-sea) fish. Before closing the dam, that ratio was about 1, showing essentially equal numbers of fish near the surface and near the bottom. In 1966, though, that ratio dropped below 1 and remained at around 0.5 until 1979-that is, demersal fish outnumbered the pelagic ones about 2:1. Before the dam closed, sardines-a pelagic species-made up much of the catch, but these fish dropped dramatically in numbers when the dam went into operation.
The numbers show clearly enough that the decline in fish populations happened soon after the closing of the Aswan High Dam. Was the dam responsible? Even more intriguing, what caused the numbers to climb around 1979? No one collected data on nutrients, chlorophyll or primary production in Egyptian coastal waters during those years. Nonetheless, the return of the fish populations suggests that primary production-the things that fish eat-increased. But for primary production to go up in the eastern Mediterranean, the nutrient supply must increase, as well.
Not everyone, however, has agreed that the increased catch resulted from improved nutrient supply-or even that entrapment of nutrients in Lake Nasser was key to fishing declines. Yousef Halim of the University of Alexandria, Egypt, found no evidence of increases in nutrients, phytoplankton or productivity in the late 1970s. Instead, Halim concluded that bigger catches after 1980 were achieved by covering more area. He also suggested that more powerful boats with more efficient equipment were in part responsible.
Halim made another point. He said that the drop in catches after 1965 might be overestimated. According to him, overfishing before 1965 was already pushing down the size of the take, and data support that position. A study by W. Wadie of the National Institute of Oceanography and Fisheries in Alexandria, Egypt, showed annual catches of fish and shrimp of around 10,000 tons a year from 1928 to 1932. That number increased to 41,000 tons between 1958 and 1962. According to the database of the Food and Agriculture Organization (FAO) of the United Nations, the catches of fish and shrimp peaked at about 36,000 tons in 1962 and dropped to about 25,000 tons by 1965.
It is true that the data are not conclusive, but I find them highly suggestive. The volume of the Mediterranean affected by flow from the Nile is far too large to accurately monitor nutrients, phytoplankton or productivity. Yet the data that are available cannot exclude the possibility of increased primary productivity after the dam closed. Further, the degree of improvement in fisheries appears too large to be just the result of more and better fishing. I am not alone in looking to primary productivity as a factor in increased catches. John Caddy of the FAO Fisheries Department wrote that biological productivity must play, at the least, some role in the improved catches.
The numbers supplied by the FAO for catches in 1962 and 1965 also suggest something more. Apparently, the waters just off Egypt could support a catch of about 35,000 tons a year in the early ’60s, but the take has been greater than that every year since the late ’80s. To catch more than an environment can ordinarily support, something must increase the primary production. Let us look more closely at the available data to see what else we might conclude.
The Flow of Phosphorus
Here I focus primarily on phosphorus, because its abundance usually limits the level of primary production by phytoplankton in the eastern Mediterranean. Nitrogen levels also merit examination, as they can also run quite low in the Mediterranean, which could limit primary production. To look for changes in the availability of these nutrients, we first need to establish the levels before the Aswan High Dam closed.
Fortunately, good data exist for nutrients near the middle of the 20th century. In 1941 and 1942, G. Abdin of Fouad I University, now known as Cairo University, recorded the concentrations of nutrients in the Nile at Cairo, which is about 160 kilometers from the coast. During a flood, Abdin recorded an average level of 2.9 millimoles of dissolved phosphate per cubic meter of water. In addition, he said that figure matched measurements made by others during the flood of 1908. During the rest of the year, however, Abdin measured an average of 1.9 millimoles of dissolved phosphate per cubic meter of water.
To find out how much dissolved phosphate reached the Mediterranean, the concentration must be combined with output. From 1959 to 1963, the Nile poured an average of 42.9 billion cubic meters of water per year into the Mediterranean, and about half of that came during the flood. Combining the concentration of 1941-42 and output figures from 1959-63 suggests that about 100 million moles, or 3,200 tons, of dissolved phosphorus ran out of the Nile each year.
Halim, on the other hand, reported a significantly higher output of phosphorus-5,800 tons per year. He based that figure on one concentration measurement of 6.4 millimoles per cubic meter of water. During late August 1957, he collected that sample three kilometers upstream from the mouth of the Damietta branch-one of the two branches that carry the Nile through the delta and into the Mediterranean. He also recorded an output of 30 billion cubic meters per year. Two years later, in mid-October 1959, Halim measured just 2.5 millimoles of phosphate at the same spot.
More data support a figure lower than Halim’s 1957 reading. After 15 separate measurements, Abdin recorded a maximum phosphate concentration of 3.2 millimoles. In addition, the measurements in 1908 and 1959-the latter by Halim himself-also back up the argument for a lower concentration. Consequently, I trust the lower phosphorus output of 3,200 tons.
Halim made the first post-dam-closing measurements of phosphate in 1972 and 1973. He collected samples 40 kilometers upstream from the coast in the Rosetta branch, which carries virtually all of the Nile’s output now, because another dam blocks the mouth of the Damietta. Working from Halim’s data, I calculated that about 27 tons of phosphorus would flow out of the Rosetta every year, or less than 1 percent of the amount before the Aswan High Dam closed. Presumably, most of the dissolved phosphate now accumulates in the sediments of Lake Nasser or is diverted to agricultural fields in irrigation water.
Looking at nitrate concentrations before the dam closed, Abdin found an average of about 15 millimoles per cubic meter of water during the flood, and only 7.4 millimoles during the rest of the year. Those figures indicate an annual nitrate output of 480 million moles, or 6,700 tons, of nitrogen. In 1995, Saleh Ismail and A. Ramadan of the Vienna University of Technology measured concentrations of nitrates at Cairo that indicate an annual output of 180 tons of nitrogen per year, or about 2.5 percent of the pre-dam output. Since fishers took a large catch in 1995, Ismail and Ramadan’s data suggest that either the fishery is not limited by nitrogen from the Nile, or human factors increase the Nile’s nitrogen concentration downstream from Cairo. Either way, the data and calculations on phosphorus and nitrogen in the Nile’s outflow show dramatic decreases since the Aswan High Dam closed.
Settling on Sediment
In addition to phosphate dissolved in water, fine-grained sediment can carry inorganic phosphorus. When the sediment reaches the Mediterranean, some of the phosphate gets released, which makes it available for the phytoplankton and, thus, the fishery. Thus one of the main questions is, how much sediment from the Nile reaches the Mediterranean?
Many investigators measured the sediment levels in the Nile before the Aswan High Dam, but they did so in different places and in different seasons. Consequently, their results covered a wide range. Halim, for instance, reported that 130 to 140 million tons of Nile sediment reached the Mediterranean every year from 1903 through 1963.
There is, however, reason to doubt those figures. In an abstract to an article published in 1982, S. Shalash noted that the Nile carried an average of 124 million tons of sediment a year. Then, in the body of that article, Shalash mentioned studies that put the sediment load at 134 million tons per year. The crucial information is that the 134 million tons came from measurements at Kajnarty, which is 400 kilometers upstream from Aswan. The 124 million tons, on the other hand, were measured at Gaafra, which is 35 kilometers downstream from Aswan. The paper attributes the 10-ton difference to sediment captured by the low dam. Keep in mind, however, that the 124 million tons measured at Gaafra still faced 1,000 kilometers of river and delta before reaching the Mediterranean. Clearly, all of that sediment would not reach the sea. In fact, data collected at Cairo-only about 160 kilometers from the coast-during the floods of 1919 through 1926 revealed only half as much sediment in the water as that recorded at Gaafra. In other words, lots of sediment settled between Gaafra and Cairo.
To get an idea of the overall amount of sediment actually discharged to the Mediterranean, I did a few simple calculations. I took the sediment concentration measured at Cairo during the floods of 1919 through 1926 and multiplied that by the average volume of flood water. The result is 61 million tons of sediment, which happens to be very close to the 57.6 million tons that A. Mitkees and his colleagues cited for Cairo in 1939. I took that roughly 60 million tons and subtracted the amount necessary to spread 0.9 millimeters of sediment-a figure endorsed by the Hydrological Department of Egypt and even one of Napoleon’s engineers-across the 22,000 square kilometers of the Nile Delta. That leaves about 25 million tons of sediment to enter the sea.
How much of the phosphate gets released from the sediment? Philip Nissen Froelich of the Georgia Institute of Technology examined a variety of techniques for estimating the amount of phosphorus that escapes from river sediment, and he concluded that from 5 to 10 moles of phosphate would be released from every gram of sediment. From the 60 million tons of Nile sediment, that translates to between 3,875 and 7,750 tons of phosphorus released from the sediment every year. Other calculations run as high as 10,400 tons per year. So the phosphorus dissolved in the water entering the sea could be just a third of what entered as adsorbed to the sediment, at least before the closing of the dam. Once the dam closed, essentially zero Nile sediment reached the sea. Thus we can ignore the river sediment as a source of phosphate since 1965.
Fertilizing the Fishery
Before looking at nutrients added after the dam closed, we should sum up the pre-dam inputs. Dissolved nutrients added 3,200 tons and 6,700 tons of phosphorus and nitrogen, respectively. Nutrients trapped in sediments added roughly 4,000 to 8,000 more tons of phosphorus and an unknown amount of nitrogen. So before the dam closed, the Mediterranean received from 7,000 to 11,000 tons of phosphorus and 7,000 tons or more of nitrogen from water flowing out of the Nile. With the Aswan High Dam closed, could agriculture and urban waste be replacing that nutrient flow? (Industrial releases of phosphate and nitrogen really should also be included, but I lack the data for even a crude estimate of that contribution.)
First, let’s look at agriculture. Although Egypt imports large amounts of food, agriculture remains a major component of the country’s economy. About a third of the land gets planted in birsim, a clover that animals eat. In addition, Egyptian farmers raise cotton, dates, peanuts, rice, sugar cane, vegetables and wheat.
Before completion of the dam, farmers vised relatively little synthetic fertilizer-about 260,000 and 19,000 tons of nitrogen and phosphate, respectively, each year. Since the dam closed, nitrogen fertilizer application has increased about linearly, climbing to more than 800,000 tons annually by the mid-1990s. The use of phosphate fertilizer, on the other hand, remained constant until about 1970, when it began to rise-to about 200,000 tons a year in the mid-1980s. Since then, phosphate fertilizer application has declined to about twice the level of the 1960s.
By around 1980, if just 4 or 5 percent of the phosphate fertilizer got to the Mediterranean, it would make up for the dissolved phosphate removed after the dam closed. Likewise, by the mid-1980s, only about 1 percent of the nitrogen fertilizer would offset the dissolved nitrogen lost by closing the dam. As agricultural runoff, however, more nitrogen would escape than phosphate, because the latter nutrient binds more tightly to sediment. In fact, as early as 1967, measurements from the Nile Delta showed an increased ratio of nitrogen to phosphate.
Where’s the Waste Go?
Three changes in Egypt during the 20th century support the hypothesis that Egypt’s Mediterranean coastal waters have seen dramatic increases in phosphate and nitrogen inputs from human waste. First, the population grew significantly. Although the informal settlements and many transients along the Nile in the greater Cairo area make for poor estimates of the population, both greater Cairo and Alexandria, which lies on the Nile Delta, grew dramatically during the 20th century. From 1930 to 2000, greater Cairo’s population increased from less than 2 million to more than 16 million people. During the same period, Alexandria’s population jumped from less than half a million to more than 3 million.
Second, Egyptians consumed more calories-especially more animal protein-during the second half of the 20th century than they did previously. In 1965, an average Egyptian consumed only 23 percent as many calories as an average American, but that figure rose to 91 percent by 1989, despite increased consumption in the U.S. As food is metabolized, phosphorus and nitrogen are released as waste products in feces and urine.
For phosphorus, the U.S. Department of Agriculture’s data indicate daily intakes of 1.0 and 1.5 grams for females and males, respectively, between the ages of 9 and 50. In addition, detergents in Egypt still contain phosphates, and-according to studies in the United States-that adds 0.8 grams of phosphorus each day from each person. Putting all of that data together gives an average daily production of 2.1 grams of phosphate per person. From 1965 to 1995, that translates to an increase from 4,400 to 12,600 tons of phosphorus per year from Cairo and Alexandria combined.
An estimate of nitrogen excreted depends on protein consumption. The typical Egyptian consumed 63 grams of protein per day in 1965 and almost 89 grams by 1995. On average, nitrogen makes up about 16 percent of protein. So, the average Egyptian could release about 10 grams of nitrogen per day in 1965 and 14.2 grams in 1995. For Cairo and Alexandria combined, that leads to 21,000 tons of nitrogen waste in 1965 and 87,000 tons in 1995.
Those figures for phosphorus and nitrogen swamp what the Nile provided to the Mediterranean before the Aswan High Dam closed. Nonetheless, just because humans excrete those levels of nutrients does not mean that it all reaches the sea. The third crucial change-an increase in sewer systems-could deliver more of those nutrients to the Mediterranean. Without running water and sewer connections, most human waste ends up on or in the ground, and little of the phosphorus or nitrogen reaches surface waters. Even in the 1960s and early ’70s, much human waste ended up in the streets of Cairo. Reports into the early ’80s note up to 300 raw-sewage overflows in Cairo in a day. Between 1981 and 1998, however, Egypt’s daily sewer capacity increased from 2,200,000 to 10,700,000 cubic meters of waste, and nearly one-third of that growth took place in Alexandria and greater Cairo. Further, more than 70 percent of the houses were connected to the sewer system by 1991.
By combining the dietary-consumption and sewage data, I estimated how much phosphorus and nitrogen could end up in the discharged wastewater. In 1965, the total urban population could have discharged 2,400 and 12,000 tons of phosphorus and nitrogen a year, respectively. By 1995, those figures probably climbed to about 16,000 and 108,000 tons. Even using the outputs of just Cairo and Alexandria, the wastewater should have included 9,500 and 65,000 tons of phosphorus and nitrogen, respectively, in 1995. So the nutrients in the sewage discharged from these two cities alone could replace all-and considerably more-of the phosphorus and nitrogen that the Nile carried before the Aswan High Dam closed.
Trouble in River City
My proposal-that nutrients resulting from human activities now feed the plankton that feed the fish off the coast of Egypt-remains a hypothesis. Still, the circumstantial evidence makes a compelling case. In any event, my largely conservative calculations show that anthropogenic nutrient emissions surpassed those carried naturally by the Nile soon after the closing of the Aswan High Dam. Moreover, many coastal areas already display signs of too many nutrients, including unwanted algal blooms and the loss of sea grasses and coral reefs. The fish may have returned at the mouth of the Nile, but-to an ecologist-they may never taste quite the same.