Effect of pollution on the Western Harbour of Alexandria. II. Nutrient salts

Abstract

Variations of nutrient concentrations in the Western Harbour of Alexandria, a semi-closed Mediterranean coastal area under stress, were studied. The irregularity in the vertical distribution of nutrients is attributed mainly to the effects of sewage pollution and turbulence of the water column from navigation. The average nitrate concentrations inside the harbour were markedly low due to the influence of pollution. This is confirmed by the maximum regional average nitrate value found at the reference station in the open sea, where the water was well oxygenated. Also, the maximum regional average silicate value at the reference station reflects the effect of sewage wastes in diluting the silicate content inside the harbour. However, the minimum and low regional average concentrations of total and reactive phosphates at the reference station indicate the influence of sewage pollution in increasing the phosphate content inside the harbour. The minimum average nitrate value in June, accompanied by minimum average values of nitrite and dissolved oxygen, can be attributed to denitrification processes. The increase in the rate of uptake by phytoplankton in spring and summer must be also considered, as confirmed by the minimum average values of reactive and total phosphates and silicate in June. The nitrate:nitrite ratio indicates that most of the inorganic nitrogen is in the form of nitrate. Also, the nitrate:reactive phosphate ratio shows that nitrate values were generally higher than those of phosphate.