Abstract:
Growth rate of diatoms were analyzed by 24 hours in situ incubation technique at coastal waters of Karachi, Pakistan (northern Arabian Sea). Sampling was done at two stations, station A located inside the channel and station B outside the channel during February, 2006 and at the mouth of the channel during May, 2007. The diatom species dominated in the community were Nitzschia closterium, Nitzschia logissima, Pleurosigma spp and Thalassiosira sp. Total community growth
rate was negative -0.11d-1 in summer and -0.7 d-1 at station B but positive 0.1d-1 at station A in winter. The individual species have given positive response of growth even in the presence of grazers. During the summer months the differences observed from the winter months may have been due to differences in microzooplankton composition. Heterotrophic dinoflagellates have a large impact on the growth rates. Since these dinoflagellates feed on the large diatoms, it suggests that this is a major factor determine the growth rate of diatoms. Total diatom abundance at initial T0 and final T24 incubation periods coincided with the chlorophyll a values at station B in winter and summer seasons, but they did not display similar pattern at station A, suggesting that when the large grazers removed from the sample through prescreening the diatom
growth increased. Presence of high concentration of nutrients near the coast could be one of the reasons of high growth rate at station A as compared to station B which was near off shore.
