Understanding the governing dynamics and trade-offs between heavy metals and nutrients in heavily contaminated wetlands

Protecting wetlands from various human activities requires a deep understanding of their aquatic limnology. This calls for continuous monitoring, which generates extensive and complex datasets. By applying statistical analyses and modelling techniques, these datasets can be effectively interpreted to uncover, define, and gain critical insights into the functions and processes that drive aquatic ecosystems. The present study aims to integrate water quality, sedimentology, aquatic toxicology and modelling techniques to present a detailed and comprehensive assessment of different components of Deepor Beel's (a Ramsar site) ecosystem. Deepor Beel is situated on the banks of the Brahmaputra River in the northeastern region of India and holds immense significance to the city of Guwahati. Originally spanning across more than 40 sq. km area, rampant encroachment and anthropogenic disturbances have not only degraded the wetland ecosystem but also reduced its effective area to now a meagre four sq. km. Large-scale eutrophication due to the discharge of untreated municipal wastewater has played a significant role in the wetland's deterioration. Although several restoration measures were undertaken in the past, they could have been more effective as they lacked prognosis. Hence, we carried out systematic monitoring (the first such extensive monitoring was undertaken) of four components of Deepor Beel's ecosystem, i.e., water, sediment, fish, and aquatic weeds, to understand the governing factors responsible for the wetland's deterioration. We employed different multivariate statistical techniques to understand the sampling site's characterization and behaviour under various environmental and climatic stresses and identify and quantify latent pollution sources contributing to wetland pollution. In addition, a novel water quality index was developed employing Shannon Entropy, which encompasses all essential variables for a comprehensive understanding of the wetland's water quality. We assessed sediment contamination from heavy metals—including chromium (Cr), cadmium (Cd), iron (Fe), manganese (Mn), copper (Cu), lead (Pb), and mercury (Hg)—and conducted fractionation studies, revealing important insights into how these metals interact within the ecosystem. Fish samples from three indigenous species that are locally consumed were collected, and we analyzed the bioaccumulation of heavy metals in various tissues and organs. Our findings indicated significant amounts of heavy metals in the fish organs, making their consumption potentially carcinogenic for humans. Finally, a eutrophication-based ecological model was developed to understand the nutrient dynamics within the wetland. The model was calibrated, and sensitivity analyses were performed and validated using the dataset generated through the laboratory analyses. The model was then simulated for two scenarios: 1) harvesting of aquatic weeds reflecting the current practices, and 2) establishing a treatment unit handling the nitrogen and phosphorus loadings. The results demonstrated that treating the inflow is a more sustainable approach to reducing eutrophication, and this strategy should be implemented promptly. Given the gravity of the situation for Deepor Beel, the findings of this study are significant and call for immediate attention and action.