Abstract: 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.
Keywords: Diatom, species composition , seasonal abundance, coastal waters
