Intense anthropogenic activities have increased the frequency of benthic hypoxic patches in the Ganga River. We investigated the synergistic effects of elevated salinity and benthic hypoxia on sediment-metal release along a 150-km gradient of the Ganga–Hooghly River estuary. Also, we tested toxicological implications of metal pollutants in terms of changes in microbial extracellular enzyme activities (β-D-glucosidase, fluorescein diacetate hydrolase (FDAase) and alkaline phosphatase) and eco-toxicological indices. We found synergistic effects of salinity and hypoxia on sediment-metal release and sediment-P release. We found 3.5–7.73% (p < 0.001) increase in the concentrations of metal pollutants in the overlaying water at low levels of dissolved oxygen (DO ≤ 2 mgL−1) under controlled experiment. At high salinity and low DO, the sediment release of study metals enhanced by 5.8–20.25%. Total heavy metal concentration (⅀THM) above 350 µg g−1 decreased the activities of extracellular enzymes (18.6–47.8%; p < 0.001). Contamination factor, geo-accumulation index and ecological risk index showed concordance except at sites where salinity counterbalanced toxicological effects. Our study suggests that the synergism between hypoxia and salinity, which are expected to increase in future, will continue to increase the sediment-metal release with toxicological implications along the Ganga–Hooghly River estuarine gradient.
Gupta et al. (Mon,) studied this question.
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