The present studies were carried out to know the occurrence, distribution, species richness and diversity of Cyanobacteria in relation to water chemistry in four different river systems such as Netravati of Dharmasthala, Kumaradhara of Subramanya, Sita and Shambhavi of Karkala of Western Ghat region for two consecutive years in the month of October 2012 and 2013 respectively. A total of 41 species belonging 16 genera were observed in these four rivers during the study period. The leaf/litter yielded maximum number of species. Maximum number of 21 species was identified in Netravati river followed by 18 species in Kumaradhara river 14 species in Sita river and 13 species in Shambhavi river throughout the study period. In the year 2012, 13 species each were identified both in Netravati river and in Kumaradhara river while nine and eight species were identified in Sita river and Shambhavi river respectively. While in the year 2013, maximum number of 18 species was identified in Netravati river followed by 16 species in Kumaradhara river, 11 species in Sita river and 09 species in Shambhavi river. Oscillatoria limosa was the most dominant species which was present in all the rivers in both the years, but abundant in Netravati and Kumaradhara river. Microcystis aruginosa was present in all the rivers except during first year in Netravati and Shambhavi rivers. Some species were particular to some rivers. Even though some species were rare in some rivers, however those were frequent in some other rivers. Microcystis scripta was one of the rare species identified in Sita river only during 2013. Occur rence and species r ichness was more in such condi t ions where pH was sl ight ly alkal ine and dissolved oxygen content was more. However bloom condi t ion was not reached.
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