Levels of silicate, the major nutrient for diatoms, in three Mediterranean coastal basins subjected to different pollution sources

The seasonal variations of silicate were investigated in the surface and bottom waters of three Mediterranean coastal areas to reflect the silicate sources and availability and consequently the biomass of diatoms inhabiting these marine basins subjected to different land based sources of pollution. The study areas are; Mex Bay, a semicircular basin west of Alexandria, has a mean depth of 9.8 m and receives several untreated effluents (agricultural, industrial and domestic wastes). The region in front of the Eastern Harbor (EH) of Alexandria has a maximum depth of 22 m with no distinct boundaries and receives only untreated domestic wastes. Abu-Kir Bay, a semi-enclosed basin east of Alexandria, has an average depth of 12 m and receives untreated industrial and domestic wastes from > 30 factories. The order of silicate abundance was: Mex Bay > Abu Kir Bay > in front of EH. This coincided with different industrial operations in Mex and Abu Kir regions, beside the continuous discharges of variable amounts of silicate enriched brackish waters in both bays and the effect of domestic wastes only on the area in front of EH. Accordingly, the allochthonous silicate sources dominated the autochthonous sources, resulting from decomposition of siliceous matter, as confirmed from the considerable silicate decrease with depth and with distance from land based sources of pollution. The present results are considered as data base for studying the distribution of diatoms, the dominant phytoplankton group.