Concurrent bloom of <i>Microcystis</i> spp. and <i>Aulacoseira</i> spp. in the River Ganga

Abstract The River Ganga is under increasing pressure from urbanization on both banks, coupled with ongoing anthropogenic influences. This present study assessed environmental parameters and phytoplankton community assemblages across a 96 km stretch of the middle stretch of River Ganga inter-connected with small rivers, based on observations from 11 sampling stations. Surface water samples were collected for in situ measurements of environmental parameters and phytoplankton during the onset of the pre-monsoon season (March, 2024). Along the studied stretch of the River Ganga, the pH ranged from 6.2 to 8.3, the euphotic zone depth varied between 1.56 m and 2.15 m and TDS ranged between 162 to 694 ppm. The high concentration of dissolved nutrients in studied stretch of River Ganga can be attributed to inflow from the Karamnasa River, Thora/Kao River and Tamsa River. The molar N:P ratio ranged between 12.3–34.9, often exceeding or aligning with the Redfield ratio (16:1), while the Si:N ratio was consistently lower (&lt;1) than the expected Redfield ratio (1:1) along the studied stretch of the River Ganga. Blooms of Microcystis spp. and Aulacoseira spp. were observed at all stations, despite variations in prevailing environmental and nutrient concentrations. Among the diatoms, Aulacoseira spp., Cyclotella spp., Synedra spp. and Nitzschia spp. were identified, along with green algae such as Pediastrum spp., Actinastrum spp., and Scenedesmus spp. Zooplankton taxa including Keratella spp., Brachionus spp., Notholca spp. and Bosmina spp. were consistently recorded in all the stations. Normalized Difference Chlorophyll Index (NDCI) and Floating Algae Index (FAI) analyses revealed elevated chlorophyll-a concentrations in the study area. The findings indicate that flow of nutrients may have triggered the simultaneous blooms of Microcystis spp. and Aulacoseira spp. in the studied stretch of the River Ganga.