Nile River delta: Rosetta branch and Edku Lagoon

Hassan Awad and Nabiha A. Youssef

 Study area description

The Nile delta is situated almost in the middle of the Egyptian Mediterranean coastline (Figure 1), which extends about 1,000 km (16% of the total Mediterranean coast).  It connects to the Mediterranean Sea through its two branches surrounding the delta, the Damietta in the east and the Rosetta in the west.  Since 1964, when the High dam was built on the Egyptian Nile upstream in Aswan (~1200 km south of Alexandria), Nile water discharge through the Damietta Nile branch has almost stopped, but the Rosetta Nile branch (east of Alexandria) is still discharging into the Mediterranean Sea through four coastal lakes: Manzala, Burullus, Mariut and Edku.  These lakes could be considered as transitional sinks for the majority of anthropogenic wastes of Egypt.  The budget of the pollutant cocktail in these lakes is expected to be exposed to significant alteration in quantity and quality before reaching the sea.

The River Nile has a large discharge area of about 3x10⁶ km² with a high flow rate (up to 500 m³ sec^-1).  The flowing Nile water reaches the Mediterranean Sea directly or indirectly through the terminals of the drainage network effluents.  These effluents are continuously pumped to the sea carrying a complex of various wastes.  The quantity and quality of these wastes reflect the variation of human activities starting with those situated in the eight Upper Nile basin countries bordering its catchment.

The dramatic deterioration observed in the resources of the Egyptian coastal lakes and lagoons indicates that the impact of pollutant discharges in the upper Nile, as well as those in the Egyptian part, are being delivered to Mediterranean coastal waters through these lagoons.

The type, properties and persistence of discharged pollutants in Nile waters are determinant factors for their dispersal, residence, remobilization and impacts in their receiving aquatic environments (fresh, brackish or saline).

 Figure 1.  Location of the Nile River delta and mouth.

 Deterioration of the nearby marine environment in some areas, especially off the region of Alexandria metropolis, is a result of direct discharge of industrial pollutants and untreated domestic wastes into the coastal waters.  The zone of Mediterranean coastal waters off Alexandria is considered as one of the “hot spots” of the region (UNEP/MAP 1997).

 The present work utilised information on the inputs and dispersal of dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN) in a coastal embayment (Abu Qir Bay) subjected to the influence of terrigenous water discharge; freshwater from the Rosetta Nile distributary and brackish water from a coastal lagoons (Lake Edku) (Figure 2).

 Abu Qir Bay

 This shallow bay lies between 31.27-31.47�N and 30.07-30.33�E, with an average depth of less than 10 m and water volume of 4.3x10⁹ m�.  The area of the bay is about 430 km².  It receives a substantial load of pollution from the various land-based activities surrounding it and through River Nile drainage and Lake Edku.  The pollution inputs in the bay from these two main sources are as follows: 

a)       El Tabia Pumping Station (TPS), in the south-east part of the bay with about 2x10⁶ m� day^-1 discharging capacity.  It was estimated that about 730x10⁶ m³ of waste waters are discharged annually through this point source, in a channel (El Amia) of 200 m length.  These wastes are mainly industrial, with some agricultural and domestic contribution.

b)       Lake Edku (31.27�N, 30.15�E).  This coastal lagoon is almost an agricultural drain.  It covers an area of 126 km� with a mean depth of 1 m.   Through its connection with the bay (El Boghaz) about 389x10⁶ m³ of agricultural wastewater is discharged annually into Abu Qir Bay.

 Figure 2.  Satellite photograph of the Rosetta branch estuary, Lake Edku and Abu Qir Bay.

Historical data on the hydrography of Abu Qir Bay shows that the dispersal pattern of the land-based materials is governed by its water stratification and circulation regimes, which in turn depend on the seasonal variation in discharged fresh waters coming either from Lake Edku or through the Rosetta Nile branch.

 While the bay environment receives almost the same amounts of freshwater from Lake Edku all around the year (about 400x10⁶ m�), the contribution through Rosetta branch is enormously variable depending on the prevailing current direction.  During summer, when the current is mainly eastwards, a relatively small amount from the Rosetta branch reaches the bay (about 80x10⁶ m� month^-1).   This amount increases drastically to about 2,000x10⁶ m� month^-1 when the prevailing current alters towards the west.

 The Rosetta Nile estuary

 The Rosetta branch (31.50�N, 30.35�E; Figure 2) of the River Nile is about 220 km in length with an average width of 180 m (area of about 40 km²) and depth varying between 2 and 4 m (Abed el-Sattar an Elewa 2001).  The estuary is delimited by a barrage for controlling water discharge at Edfina City, 30 km before its connection with the sea.  It was estimated that the aquatic environment of this branch receives daily more than 0.5x10⁶ m³ of untreated or partially treated domestic and industrial wastes and huge amount of agricultural drain waters.

 From a biogeochemical viewpoint of the terrigenous substances in the marine environment, it is preferable to estimate the nutrient budgets for the Rosetta Nile estuary and Lake Edku separately.

 The data used in the present estimation is “secondary,” covering a wide timeframe and obtained mainly from historical references.

 Table 1.  Physical characteristics of the Rosetta Nile estuary and Lake Edku.

 Water and salt balance

 The results indicate a normal state where the V_R values are usually negative (Figure 3a).  The short water exchange time both in the winter and summer is significant.  This is in spite of the significance seasonal difference in the runoff rate (67x10⁶ m day^-1 in winter against 5x10⁶ m day^-1 in summer).  The water exchange time (t) is less than a day in the winter and about 3 days in the summer (Table 2).

 Table 2.  Runoff, precipitation and evaporation for the Rosetta Nile estuary.

DIP and DIN balance

Calculated DDIP in the winter (-271x10³) and summer (-13x10³) mol m^-3 day^-1 (Figure 3a ), indicated that the estuary seems to be a sink for DIP.  During the wet season (winter), the rate of phosphorus supply via the river reaches twenty times more than that during summer.  The same remark is noted for DDIN, as shown in the corresponding budget (Figure 4a).

Net system metabolism was not estimated from the nonconservative nutrient fluxes because of the very short water exchange time in the system in both seasons.

 Edku Lagoon

Water and salt balance

 The obtained values parallel the variation in the rate of water draining into the lagoon as shown in Figure 3b.  However, the water exchange time (t) during winter (42 days) is more than twice that during summer (16 days) which corresponds to the change in runoff during the two seasons, from 2x10⁶  m³ day^-1 and 4x10⁶  m³ day^-1, respectively (Table 3).

 Table 3.   Runoff, precipitation and evaporation for Lake Edku.

 DIP and DIN balance

The result for these two parameters indicate that, while the lagoon represents a sink for DIP during winter (DDIP = -2x10³ mol day^-1), it releases the DIP during summer (DDIP = +3x10³ mol day^-1).  Conversely, the lagoon supplies the marine environment with nitrogen during  winter (DDIN = +47x10³ mol day^-1) and takes up DIN (DDIN =-99x10³ mol day^-1 during summer.

 Stoichiometric calculations of aspects of net ecosystem metabolism

The lagoon changes from autotrophic ([p-r] = +2 mmol m^-2 day^-1) in the winter to heterotrophic conditions ([p-r] = -3 mmol m^-2 day^-1) in the summer.

 The results indicate that this lagoon seems to be net nitrogen fixing in the winter, (nfix-denit) = +0.7 mmol m^-2 day^-1, and net denitrifying, (nfix-denit) = -1.1 mmol m^-2 day^-1, in the summer.

Figure 3a.  Water and salt budgets for Rosetta estuary in the winter (a) and summer (b).  Water flux in 10⁶ m³ day^-1 and salt flux in 10⁶ psu-m³ day^-1. 

 Figure 3b.  Water and salt budgets for Lake Edku in the winter (a) and summer (b).  Water flux in 10⁶ m³ day^-1 and salt flux in 10⁶ psu-m³ day^-1. 

Figure 4a.  DIP budget for the Rosetta estuary in the winter (a) and summer (b).  Flux in 10³ mol day^-1.

Figure 4b.  DIP budget for Lake Edku in the winter (a) and summer (b).  Flux in 10³ mol day^-1.

Figure 5a.  DIN budget for the Rosetta estuary in the winter (a) and summer (b).  Flux in 10³ mol day^-1.

Figure 5b.  DIN budget for Lake Edku in the winter (a) and summer (b).  Flux in 10³ mol day^-1.

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Last Updated 21 May 2006 by DPS