This paper in the Journal of Earth System Science deals with a study by National Institute of Oceanography (NIO), Goa to investigate the influence of monsoonal rainfall on hydrographic conditions in the Mandovi River of India. The study was undertaken at a location approximately 2 km upstream of the mouth of the estuary.

The findings reveal the evolution of phytoplankton communities and their biomass as the southwest monsoon rainfall progressed and freshwater influx changed conditions in the Mandovi estuary. The datasets revealed rapid changes in near-surface salinity in the Mandovi river following the onset of the south west monsoon and progress of the rainy season. The present study represents the first detailed investigation on the effects of freshwater discharge on phytoplankton populations in a tropical monsoon-influenced estuary.

The study suggests the following –

• During the pre-monsoon period in May 2007, when circulation in the estuary was dominated by tidal activity, phytoplankton communities in the high saline (35–37 psu) waters at the study site were largely made up of the coastal neritic species Fragilaria oceanica, Ditylum brightwellii and Trichodesmium erythraeum.
• During the later part of the intermonsoon phase, an abrupt decline in salinity led to a surge in phytoplankton biomass (Chlorophyll a ∼14 mg m−3), of a population that was dominated by Thalassiosira eccentricus. As the southwest monsoon (SWM) progressed and the estuary freshened salinity and Chlorophyll a (Chl a) concentrations decreased during the MoN, Skeletonema costatum established itself as the dominant form. Despite the low biomass (Chl a <2 mg m−3), the phytoplankton community of the MoN was the most diverse of the entire study.
• During the post-monsoon the increase in salinity was marked by a surge in dinoflagellate populations comprising of Ceratium furca, Akashiwo sanguinea, and Pyrophacus horologium.

Download the paper here -