GROWTH RATE OF DIATOMS IN NATURAL ENVIRONMENT FROM THE COASTAL WATERS OF PAKISTAN

摘要

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 thechannel during February, 2006 and at the mouth of the channel during May, 2007. The diatom species dominated in thecommunity were Nitzschia closterium, Nitzschia logissima, Pleurosigma spp and Thalassiosira sp. Total community growthrate 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 individualspecies have given positive response of growth even in the presence of grazers. During the summer months the differencesobserved from the winter months may have been due to differences in microzooplankton composition. Heterotrophicdinoflagellates have a large impact on the growth rates. Since these dinoflagellates feed on the large diatoms, it suggests thatthis is a major factor determine the growth rate of diatoms. Total diatom abundance at initial T0 and final T24 incubationperiods coincided with the chlorophyll a values at station B in winter and summer seasons, but they did not display similarpattern at station A, suggesting that when the large grazers removed from the sample through prescreening the diatomgrowth increased. Presence of high concentration of nutrients near the coast could be one of the reasons of high growth rateat station A as compared to station B which was near off shore.