Comparative study of phytoplankton production, composition and diversity index in the eutrophic eastern harbour of Alexandria, Egypt

ABSTRACT

The process of eutrophication, which has been in progress for some decades in theoastal waters of Alexandria, Eastern Harbour [E. H.], shows mainly cyclic seasonal features. In autumn and summer large blooms of diatoms usually occur, while in spring, blooms of dinoflagellates cause red tides. Low salinity and high nutrient loads carried with the Nile water and sewaqe outfalls favour the production of these blooms. The two episodes of red tides that Occurred during spring of both 1987 and 1992 taw caused fish, mortality and affected the Regional tourist industry Phytoplankton diversity indices are used to characterize the effect of eutrophication in the Eastern Harbour. Generally, the phytoplankton density was always higher in the surface water than the near bottom layer. Surface phytoplankton standing crop increased irom one year to the other. Thus, it ranged from 0.2x10[6] to 19x10[6] unit.l[-1] during, 1986-87, from 0.04x10[6] to 23.5x10[6] unit.l[-1] in 1990-91 and from 0.03x10[6] to 77x10[6] to unit.l[-1] during 1991-92 addition, changes in the dominant species were revealed. In 1986-87, Chaetoceros affinis, Cyclolella meneghiniana, Amphichrysis compressa [new record] and Alexandrium minutum dominated. During 1990-91, Anabaena sp., Skeletonema costatum and Rhizosolina spp. Prevailed, in 1991-92, different dominant species were recorded; namely, Prorocentrum triestinum and Skeletonema costatum. The Shannon diversity indices reflect an inverse relationship to the degree of dominance of the main species and the magnitude of phytoplankton standing crop rather than to the species richness. A simple mathematical regression equation describing the dependence of diversity on the species dominance variations for this area is given. The correlation between standing crop, diversity and the major environmental parameters and their regression equations are discussed