Influence of salinity variations on zooplankton community in el-mex bay, alexandria, egypt
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El-Mex Bay receives a heavy load of waste waters (7×109 m3/year) both directly from industrial outfalls and indirectly from Lake Maryuit via El-Mex Pumping Station. Zooplankton samples were collected bimonthly from seven stations during the period March 2005 to January 2006 to illustrate the influence of salinity variations on the abundance and community structure of zooplankton in El-Mex Bay waters. Based on the salinity values, four water types are identified: the mixed land drainage (L) of salinity < 10ppt, mixed water (M) of salinity range 10 to 30ppt, diluted sea water (D) of salinity range 30 to 38.5 ppt and Mediterranean Sea water (S) of salinity > 38.5. The highest zooplankton standing crop (106.6×103 ind/m3) was recorded in the mixed land drainage water type (L); while the lowest counts (5.9×103 ind/m3) was found in the Mediterranean Sea water type (S). Rotifera were the most dominant zooplankton groups in water type (L) constituting about 86% to the total zooplankton and represented by 19 species belonging to 10 genera. Brachionus urceolaris and Filinia longiseta dominate Rotifera population. Protozoa was the second important group in this water type contributing 9.23% to the total zooplankton crop. In water type (M), Rotifera, Protozoa and Copepoda were the most dominant groups constituting 57.87%, 21.32% and 13.45% to the total zooplankton counts respectively. In water type (D), Copepoda and their larval stages were the most dominant zooplankton groups constituting about 51% to the total zooplankton. Protozoa was the second important group constituting 37.20%, while Rotifera represented only 4.20%. In the Mediterranean water type (S), Copepoda and their larvae were the most dominant zooplankton group, forming 49.46% of the total zooplankton. Oithona nana, Acartia clausi and Paracalanus parvus dominated copepod population. Cirriped larvae occupied the second order of abundance at this water type with a percentage frequency of 19.17% to the total zooplankton crop.