Seasonal variations in the abundance of zooplankton groups in relation with physico-chemical parameters in three lotic ecosystems of Mysore

Zooplanktons are microscopic organisms, which are integral components of aquatic food web, as they contribute significantly to productivity of freshwater ecosystems. They are also very sensitive to environmental changes; therefore they make ideal indicator organisms. Total zooplankton (Rotifer, Cladocera, Copepod and Ostracod) abundance was significantly more in Cauvery river with the mean abundance of 18 Org/L compared to Kapila river (15 Org/L) and at their confluence site (14 Org/L). Rotifers were the dominant groups in all the three sampling sites followed by Cladocera, Copepod and Ostracod throughout the study period of two years. The highest fortnightly variation in the abundance was shown by Rotifers in Cauvery and Kapila rivers with CV 52% and 61% respectively. Whereas, at the confluence site, the Copepod showed more (CV=51%) fortnightly variation. The season (rainy, winter and summer) wise grouped data revealed that the abundance of zooplankton was highest in rainy season followed by winter and summer seasons. The mean abundance of zooplankton showed significant differences in all the three seasons in Cauvery and Kapila rivers. Whereas, at the confluence site, winter and summer seasons were significantly similar but rainy season showed significant difference. Overall, the results indicated that the abundance of zooplankton showed decreasing tendency in Cauvery river after it joined with Kapila river. It is interesting and noteworthy to note that Copepod-calanoid group was completely absent in all the three sampling sites throughout the study period. Copepodherpacticoids are present only in Cauvery river and were completely absent from the remaining ecosystems. The regression analysis revealed that in Cauvery River 49% of total zooplankton abundance was controlled by conductivity, in Kapila river, 52% of total zooplankton abundance was controlled by hardness and 43% of DO negatively controlled the total zooplankton abundance at the confluence site. Hence, this study suggests that the zooplankton may be good indicators of the variations in the water quality of these three lotic ecosystems.