Phytoplankton Dynamics of Tidal Influenced Stretch of River Nethravati, Dakshina Kannada, Karnataka

Knowledge on the phytoplankton dynamics is vital for the ecological management of rivers. River ecosystems are widespread across the landscape through series of channels ultimately draining into the sea. The present exploration embarks on comprehending the temporal variations in the salinity, chlorophyll a and phytoplankton dynamics of water from tidal influenced stretch of Nethravati River at Farangipet. Surface water samples and plankton (Meso/Net plankton) were collected at monthly intervals for a period of 16 months covering post-monsoon, pre-monsoon, monsoon and post-monsoon of subsequent year. Plankton community composition revealed the order of diversity as Chlorophyta (28 genera) > Chrysophyta (16 genera), Cyanophyta (16 genera) > Rhodophyta (1 genera) and the order of abundance as Cyanophyta (20.25% to 91.39%) > Chlorophyta (5.25% to 74.58%) > Chrysophyta (1.48% to 30.75%) > Rhodophyta (0% to 0.22%). Alpha diversity indices of plankton were estimated and are as follows: Maegalef’s Richness Index (1.15 to 2.29); Pielou’s Evenness Index (0.36 to 0.71); Shannon’s Diversity Index (1.17 to 2.52). Salinity varied from 0.03 ppt (monsoon) to 19.85 ppt (pre-monsoon). Based on salinity distribution river seemed to be mostly limnetic (<0.5 ppt) during post-monsoon and monsoon seasons as does with all other rivers, but unusually mesohaline (5.0 – 18 ppt) during the pre-monsoon time. Chlorophyll-a content fluctuated between 2.45 µg/L and 4.85 µg/L, thereby indicated the oligotrophic nature.