Diatom Species Composition and Seasonal Abundance in a Polluted and Non-polluted Environment from Coast of Pakistan

Diatoms species composition and their seasonal abundance was analyzed at two stations (St. A, 24°49.77'N 66°57.85'E and St. B, 24°47.93'N 66°58.87'E) of Manora Channel, coastal waters of Karachi, northern Arabian Sea bordering Pakistan. Samples were collected bimonthly from May 2002 to July 2003 and examined using inverted microscope. There was no significant variation observed between species composition at both A and B stations. Distribution pattern and seasonal variations in cell abundance of dominant, abundant, frequent and rare species were observed in which seven dominant species were recorded from station B and six dominant species from station A. Among both centric and pennate diatoms, the genera Chaetocerose affine, Pleurosigma sp 1, Thalassiosira sp, Navicula directa and Nitzschia longissima, Rhizosolenia setigera, Thalassionema nitzschiodes remain dominated all year although seasonal variations in their cell abundance were observed for these species. Statistical analysis showed that most of the dominant species were positively correlated with the salinity and temperature at both stations A and B. It suggests that temperature and salinity are the most significant factors that give shape to the diatom community and control the community changes. Dominance of pennate species over centric types with high abundance of these pennate species showed that they have better tolerance against pollution and other environmental variables.