Spatio-Temporal Variations of Diatom Community in the Coastal Waters off Karachi, Northern Arabian Sea

Spatio-temporal variations in the occurrence, abundance and distribution of diatoms (April 2008 to March 2010) have been studied from the coastal and near-shore waters along the Karachi coast, northern Arabian Sea. Pakistani coastal waters appear to receive nutrient-rich waters during winter (NEM) and thus showed diatom peaks during NE monsoon. A total of 32 species (out of 188 recorded species) occurring regularly during the study period were considered. The data depict that centric diatoms were more diverse and abundant compared to pennate species. Diatoms had a major contribution in the phytoplankton peak densities (> 90% at some instances). The frequently occurring species with high cell counts included Chaetoceros affinis, C. curvisetus, Coscinodiscus sp., Cylindrotheca closterium, Eucampia cornuta, E. zodiacus, Guinardia delicatula, G. striata, Hemiaulus membranaceus, Lauderia annulata, Lennoxia faveolata, Leptocylindrus minimus, L. danicus, Navicula sp., Nitzschia sp., Planktoniella sol, Pseudo-nitzschia sp., Rhizosolenia setigera, Thalassionema frauenfeldii and Thalassionema nitzschioides. The presence of eleven HAB species and one toxic Pseudo-nitzschia sp. in significant numbers is quite alarming with respect to the health and safety of human population, marine organisms and the environment. Therefore, regular monitoring of diatom population is recommended as the area in general is under the influence of climate change and increasing trend in bloom formation has been noted.