Hooghly River, a ~460 km long distributary of the Ganga River, passes through a highly industrialized Metropolis-Kolkata in West Bengal, India, and eventually empties into the Bay of Bengal at Gangasagar. To determine the patterns and drivers of planktonic community, spatiotemporal variations in water quality and micronutrient content and planktic prokaryotic and microeukaryotic abundance and diversity across the salinity gradient (0.1 to 24.6 PSU) in the Hooghly River estuary (HRE) were studied. Plankton and water samples were collected at six sites during October 2017, February 2018, and June 2018. The biotic parameters—phytoplankton (Chlorophyll a), total bacterial abundance (cfu), and copepods—were significantly higher in the downstream estuarine sites than in the upstream riparian sites; conversely, rotifer and cladoceran abundances were significantly higher at upstream stations. The most culturable bacterial strains were isolated from the two freshwater sites and one at the confluence (estuarine) and are characterized as Bacillus subtilis, Pseudomonas songnenesis, and Exiguobacterium aurantiacum. Among zooplankton, rotifers (0.09 ± 0.14 ind L−1) and cladocerans (5.4 ± 8.87 ind L−1) were recorded in higher abundance and negatively correlated with bacterial concentrations at upstream stations. On the temporal scale, February samples recorded lower proportions of bacterivorous zooplankton at the three upstream stations. Cluster analysis separated samples on the basis of seasons and water mass movement. The February samples showed distinct spatial characteristics, as three freshwater (FW) stations grouped together and segregated at second 2nd hierarchical level, whereas the three estuarine stations formed a separate cluster at the 50% similarity level. Samples collected in October 2017 and June 2018 exhibited mixed attributes. June samples recorded higher influence of freshwater discharge. The zooplankton abundance showed significant negative correlation with Chl a. Our results demonstrate the relative role of river continuum, land-driven lateral discharge, and seawater intrusion in shaping community structure, which needs to be considered in management and conservation planning of aquatic ecosystems, especially in highly productive and overexploited HRE.

The increasing amount of sewage has become a serious concern globally, demanding sustainable solutions. The constructed wetland system (CWS) can be installed at the wastewater discharge site and properly screened bio-purifiers can be used for efficient wastewater treatment. Filter-feeding zooplankton have the potential to graze on bacteria and reduce biological oxygen demand. However, higher suspended particles choke the zooplankton filtering appendages. An integrated application of zooplankton and macrophytes can solve the problems by reducing contaminants and providing the surface area for zooplankton to take refuge. Using three different approaches: (a) screening of natural bio-purifiers through time series study, (b) assessment of sewage treatment potential of screened bio-purifiers in the CWS and, (c) evaluation of the growth potential of screened bio-purifiers in untreated wastewater, this study explored the combined use of macrophyte and zooplankton in the CWS. The 3-year time series study in the vicinity of the wastewater discharge area recorded the zooplankton, Brachionus angularis and Moina macrocopa as highly indicative zooplankton. The top two highly indicative macrophytes are Typha and Phragmites. Under laboratory conditions, the population growth rates of B. angularis and M. macrocopa were significantly higher in wastewater without any external nutrient source than those in control with algae as a food source. The integrated application of zooplankton (B. angularis and M. macrocopa) macrophytes (Typha and Phragmites) in CWS yielded upto 56% reduction in the total bacterial count and upto 92% reduction in BOD along with substantial increase in the DO level. The present results strongly suggested the use of CWS planted with macrophytes and inoculated by zooplankton. The usefulness of this system is further supported by the natural occurrence of selected species, as it is easy to maintain, has low installation cost, and excellent efficiency in treating wastewater. As both the zooplankton species are preferred live feed for rearing of fish larvae, the present results suggested the use of zooplankton and macrophytes for treatment of wastewater, reduction of sludge, and harvest of live feeds for the aquaculture industry to augment circularity and promote decentralized wastewater treatment.

The spread of Covid-19 has reduced human intervention in aquatic ecosystems, which has shown a discernible improvement in air and water quality. Ganges River, being a historical, economic and cultural icon of India providing multiple ecosystem services as industrial, provisioning, regulatory and cultural services, constitutes an important habitat for Gangetic Dolphin (Platanista gangetica). The surfacing of dolphins in Ganga river is patchy and unpredictable. The present study unravels 25 different chemical, microbiological, biological and meta-genomics of most abundant bacteria at dolphin surfacing sites in Ganges water during October 2017 February 2018 and January 2021 to elicit the impact of Covid-19 lock down at the Garhmukteshwar (28.7601°N, 78.1437°E) stretch in Hapur district of Uttar Pradesh, India. The most abundant bacteria at dolphin surfacing site was Bacillus subtilis in February 2018 which was replaced by Aeromonas sp. in January 2021. The zooplankton community was dominated by rotifers during pre-Covid period however the dominance pattern recorded a shift towards larger herbivorous crustacean species during Covid −19 lock down. Overall bacterial count decreased whereas Chl a level increased during the lock down. The total zooplankton abundance did not show a significant difference, but relative proportion of larger herbivorous crustacean zooplankton increased in January 2021 samples. The present results convincingly establish effects of shutdown on chemical, metagenomics and biological components of the riparian ecosystem and the information of micro to mesoplanktonic community may be useful for characterizing dolphin surfacing sites, enhancing existing water quality monitoring efforts and also for human epidemiological studies associated with recreational use of Ganga water.