Seasonal phytoplankton succession in Netravathi – Gurupura estuary, Karnataka, India: Study on a three tier hydrographic platform

Abstract

The Netravathi-Gurupura estuary is a tropical, micro to meso-tdal estuary situated at Mangalore, southwest coast of India. The estuary outlets into the Arabian Sea and it is regarded as one of the major upwelling systems of India. Our study focusses on the seasonal phytoplankton succession in Netravathi-Gurupura estuary through studying the hydrochemical and hydrobiological physiognomies of riverine, estuarine and marine components. Hypoxic conditions prevailed at the estuarine zone during monsoon seasons (D.O.< 3 mg L−1). Salinity was very low in the estuarine and marine zone during the 2018 monsoon season and hefty silicate loadings (>100 μmol L−1) contributed to the proliferation and blooming of diatoms like Skeletonema costatum in the estuary. A strong negative correlation between phytoplankton composition and silicate was observed during the 2018 monsoon season (r = −0.977). The substantial entrainment of TDS (>10000 ppm), relatively higher salinity (33–35 psμ) and high values of TN: TP (42.0–67.0 μmol L−1) and proliferation of dinoflagellates were the characteristics of non-monsoon seasons. A significant positive association of phytoplankton was observed with NO3 (r = 0.989) and TN: TP (r = −0.997). In terms of phytoplankton species diversity, our study classifies estuarine zone as least diverse zone, particularly during the 2018 monsoon season (D > 0.55, 1/D < 1.82). Phytoplankton succession from diatoms (monsoon) to dinoflagellate (non-monsoon) is attributed to the alteration in physic-chemical and nutrient parameters as a function of seasonal variation.