Carbon dioxide emission coupled extracellular enzyme activity at land-water interface predict C-eutrophication and heavy metal contamination in Ganga River, India

Abstract

Ganga, the largest river system in India, has fundamental ecological and social values; and has a large number of monitoring and assessment programs, but is rarely considered to explore the ecological response indicator(s) to identify anthropogenic environmental impacts. This study was targeted to identify a sensitive response indicator that can be used to detect ecosystem changes, on appropriate spatial and temporal scales, as a result of anthropogenic impacts. Intensive sampling was conducted during summer low flow of two consecutive years (2017–2018) on a sub-set of eight study sites along with 518 km main stem; and thirty sites downstream two point sources (one at the mouth and 14 up to 1.4 km downstream each point source). The study shows that CO2 emission at the land-water interface (LWI) can be used as one of the most robust response predictors of C-eutrophication. This predictability, however, is masked at sites containing a high concentration of heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn). The total concentration of heavy metals (∑THM ranged from 161.12 to 887.04 μg g−1) and their total bioavailable fractions (∑TBF ranged from 49.96 to 611.71 μg g−1) indicated moderate to high degree of pollution load. The regulatory role of total organic carbon (TOC) to CO2 emission decreased as the metal concentrations tended to increase. The dynamic fit curve of main stem data showed that ∑THM > 347.44 μg g−1 is able to cause detrimental effects on microbial activity and CO2 emission, however, for the site with very high TOC concentration (Asdr mouth) this limit reached to 472.53 μg g−1 indicating the modulatory role of TOC in metal toxicity. Passing from low to high concentrations, a gradual reduction in FDAase, β-D-glucosidase, and protease was observed, and at ∑TBF of 611.71 μg g−1, the FDAase (a measure of overall microbial activity) declined by over 5.34 folds compared to those at ∑TBF of 49.96 μg g−1, the minimal level observed in this study. Our study gives a snapshot of a large river identifying, for the first time, the CO2 emission coupled extracellular enzyme activities at LWI as a ‘response’ predictor of C-eutrophication and potential consequences of heavy metal contamination.