Human-driven changes in sediment-water interactions may increase the degradation of ecosystem functioning in the Ganga River

Abstract

While it is widely accepted that the magnitude of river water quality degradation depends upon the proportion of human interventions, the overall changes are ultimately the consequence of interconnected biogeochemical processes with poorly understood role of ecosystem feedbacks. Here, we conducted in situ and incubation experiments, considering a 620 km Ganga River main stem, two tributaries and two point source downstream locations for trajectory studies to analyze the human-driven changes in ecosystem feedback associated changes in ecosystem functioning of the Ganga River and its tributaries. The main stem coupled trajectory analyses show that benthic hypoxia/anoxia resulting from intensive human releases generates positive feedbacks (sediment-P and –metal release) to exacerbate the degradation of ecosystem functioning in the Ganga River and tributaries. We found 1.9 to 4.6 times higher rates of sediment-P release and about 1.1 to 3.7 times higher rates of sediment-metal releases at sites with DOsw < 2.0 mg/L. Excess release of phosphorus from sediment enhanced the eutrophy whereas sediment-metal release and bioavailability led to a sharp decline in microbial biomass and FDAase activity. The Carlson’s index, ecological response index, Dodds’s trophic state classification, and risk index support these results because the sites with benthic hypoxic/anoxic condition did show trophic state in eutrophic to hypereutrophic range and metal pollution in very high to extremely polluted and high risk category indicating significant effect of these drivers. The study, for the first time, showed that positive feedbacks exacerbate the degradation of ecosystem functioning in human-impacted large rivers. We suggest the need for increased efforts considering the magnitude and connectivity of positive feedbacks and associated repercussions for improving mechanistic understanding of their contributions to overall structural and functional shifts in the large rivers.