Human-driven eutrophy compounded by climate warming triggers ecosystem feedbacks to underscore the critical tipping point in the Ganga River, India.

Abstract

Eutrophy threatens aquatic ecosystems including rivers, and the adverse effects are compounded when it fosters synergy with the climatic temperature rise. Given the increasing states of eutrophy and climatic warming, through comprehensive field and controlled incubation studies, we investigated eutrophy-coupled temperature-driven dissolved oxygen deficit (DOD) and associated feedbacks-sediment-P and -metal releases, at sediment-water interface of the Ganga River. We selected seven study sites along a 520-km middle stretch of the river. We found a marked decline in the level of dissolved oxygen at sediment-water interface (DO_sw) of eutropied sites, and the site with the lowest DO_sw (Jjmu) showed 16.58% and 25.38% increase in the sediment-P release at 1 °C and 2 °C rise in the water temperature, respectively, whereas the respective metal release increased by 35.4% and 53.6%. Tukey's post hoc test revealed significant effect of DO_sw and temperature on sediment-P and -metal releases. Multiple linear regression analysis indicated DO_sw as the principal determinant (β_DO >  - 0.90; p < 0.001), whereas the temperature influenced these feedbacks by modulating the DO_sw. Our findings suggest that the eutrophied rivers will undergo a process of self-degradation under future climate warming and advocate for re-consideration of adaptive management practices because human-driven eutrophy and climatic warming both are continuing to rise.