SEASONAL ABUNDANCE OF DIATOMS IN CORRELATION WITH THE PHYSICO- CHEMICAL PARAMETERS FROM COASTAL WATERS OF PAKISTAN

The seasonal abundance of diatom (20-200µm) and variations in physicochemical properties were studied using bimonthly collected samples at Manora Channel coastal waters of Karachi, north Arabian Sea bordering Pakistan through the period of May 2002 to July 2003 for two stations. Triplicate samples were settled for 24 hours in a settling chamber and were observed using inverted microscope and number of diatoms counted. Seasonal abundance was clearly evident throughout the study period. This variation was due to the influence of monsoon system existing in the region. Primary productivity and hydrographical parameters in the northern Arabian Sea are greatly affected by the monsoon system that prevails from May to October for southwest monsoon and between November and March for northeast monsoon. The total phytoplankton community was dominated by diatoms and the mean proportion was 69% at station A and 56% at station B. Diatom abundance showed a peak in September, 2002 at station B and in February, 2003 at station A. There were about 0.847-13.093×10 3 average cells l -l in station A and 0.667-30.067×10 3 cells l -l in station B waters showing polluted station A had low abundance compared to non-polluted station B. Seasonal change in chlorophyll a has shown a strong negative correlation with diatom abundance. Highest total average abundance of pennate diatoms was observed at both stations as compared to centric diatoms. Statistical analysis shows overall lower species diversity from 0.1 to 3.6 at both stations. It suggests that organic loads decreased the abundance and diversity of diatom communities in the region.