Journal of Conference Abstracts

Nutrient Salts in the Damietta Estuary of the Nile,
an Area under Stress

Massoud A. H. Saad (fax: 203 4834381) & M. A. Abdel-Moati (fax: 203 4922918)

Oceanography Department, Faculty of Science, Alexandria University, Alexandria, Egypt.

Faraskour's Dam is constructed downstream of the Damietta branch of the Nile River, separated it from the Damietta estuary. The latter, which receives a very small amount of the Nile water, has an average depth of 7 m and its width at the mouth does not exceed 200 m. This estuary is under stress, principally from industrial sewage and agricultural wastes. This study deals with seasonal distribution of nutrients along the Damietta estuary to illustrate the influence of external factors on its water characteristics. Water samples were collected seasonally from the surface, middle and bottom waters at five selected stations.

General decreases in concentrations of NO₃^- with depth, which was more pronounced in certain months, coincided mainly with the increase in its reduction rate. Local variations in NO₃^- concentrations are due mainly to agricultural run-off (Saad, 1976) and discharge of sewage wastes (Casey & Newton, 1973). Decreases in NO₃^- levels upstream of the estuary (Table 1), are attributed to the effect of organic pollutants, which decomposed aerobically and then anaerobically utilising the oxygen of nitrate, as confirmed by appearance of hydrogen sulphide in this region (Saad and Abdel-Moati, 1984). This is also supported by the minimum average NO₃^- values in summer associated with maximum averages of organic matter and hydrogen sulphide.

The highest and lowest NO₃^- averages upstream and the mouth of the estuary, respectively (Table 1), reflect the effect of sewage wastes upstream. Such an effect is also supported by the highest averages of NH₄⁺ concentrations upstream, which generally decreased seaward. Minimum monthly average NO₃^- values in summer, accompanied by the maximum monthly average NH₄⁺ value coincided with the increased reduction rate of nitrite by elevation in temperature.

The increase in reactive and total PO₄^3- with depth is attributed mainly to the increase in the rate of decay of the descending organic material and the release of PO₄^3- from bottom sediments, accelerated by water currents (Saad, 1973). The highest averages of reactive and total PO₄^3- at the estuarine upstream (Table 1) also confirm the direct influence of the dumped wastes (Saad, 1973; Casey, 1975). Because a very limited amount of Nile water is allowed to discharge into the Damietta estuary after the construction of the High Dam, the high P levels in this estuary originate mainly from the discharged contaminants beside the PO₄^3- released from the estuarine sediments. Compared with the normal NO₃^-: PO₄^3- ratio for the sea water, the low ratio in Damietta estuary, which generally showed a gradual increase seaward (Table 1), indicates that NO₃^- was more critical for phytoplankton.

The increase in silicate content with depth coincided with decline of diatoms in the bottom waters and dissolution of diatom frustules in the bottom sediments (Bailey-Watts, 1976). The gradual decrease in silicate content seaward (Table 1) explains the effect of limited Nile water discharge at the estuarine upstream. This is confirmed from the significant negative correlation between silicate and chloride (r= -0.8265, P < 0.001).

The present data confirm that Damietta estuary was subjected to more pollution than the second estuary of the Nile (Rosetta estuary). The contaminated Damietta estuary affects markedly the adjoining Mediterranean waters.

References

Bailey-Watts, A.E., Freshwat. Biol. 6, 69-80 (1976).

Casey, H., Freshwat. Biol. 5, 507-514 (1975).

Casey, H. & Newton, P.V., Freshwat. Biol. 3, 317-333 (1973).

Saad, M.A.H., Water, Air and Soil Poll. 2, 512-522 (1973).

Saad, M.A.H., Arch. Hydrobiol. 77, 411-430 (1976).

Saad, M.A.H. & Abdel-Moati, M.A., Arch. Inst. Pasteur Tunis. 61, 453-462 (1984).

Table 1. Regional average values of nutrients (µg L^-1) and NO₃^-:PO₄^3- ration in the Damietta estuary.

Stations		Depth (m)	NO3-	NO2-	NH4+	RPO43-	TPO43-	SiO2	NO3-:PO43-
I		5.45	569-	45+	91	790+	942+	3520	0.72:1-
Upstream
II		12.50	588	28	93+	630	783	3200	0.93:1
III		5.85	586	26	51	212	263-	1770	2.76:1
IV		4.80	956+	25	33	186-	269	1490	5.14:1+
V		4.80	806	24-	29-	205	274	1310	3.93:1
Downstream

Maximum and minimum values are designated by (+) and (-), respectively.