Decay Rate of Escherichia coli in a Mountainous Tropical Headwater Wetland

Paty Nakhle,Norbert Silvera,Anne Pando-Bahuon,Nicolas Gallion,Alain Pierret,Chanthamousone Thammahacksa,Phabvilay Sounyafong,Bounsamay Soulileuth,Yoan Marcangeli,Emma J Rochelle-Newall,Laurie Boithias,Keooudone Latsachack,Olivier Ribolzi

doi:10.3390/w13152068

Abstract

Surface water contamination by pathogen bacteria remains a threat to public health in the rural areas of developing countries. Fecal indicator bacteria (FIB) like Escherichia coli (E. coli) are widely used to assess water contamination, but their behavior in tropical ecosystems is poorly documented. Our study focused on headwater wetlands which are likely to play a key role in stream water purification of fecal pollutants. Our main objectives were to: (i) evaluate decay rates (k) of the total, particle-attached and free-living E. coli; (ii) quantify the relative importance of solar radiation exposition and suspended particles deposition on k; and (iii) investigate E. coli survival in the deposited sediment. We installed and monitored 12 mesocosms, 4500 mL each, across the main headwater wetland of the Houay Pano catchment, northern Lao People’s Democratic Republic (Lao PDR), for 8 days. The four treatments with triplicates were: sediment deposition-light (DL); sediment deposition-dark (DD); sediment resuspension-light (RL); and sediment resuspension-dark (RD). Particle-attached bacteria predominated in all mesocosms (97 ± 6%). Decay rates ranged from 1.43 ± 0.15 to 1.17 ± 0.13 day−1 for DL and DD treatments, and from 0.50 ± 0.15 to −0.14 ± 0.37 day−1 for RL and RD treatments. Deposition processes accounted for an average of 92% of E. coli stock reduction, while solar radiation accounted for around 2% over the experiment duration. The sampling of E. coli by temporary resuspension of the deposited sediment showed k values close to zero, suggesting potential survival or even growth of bacteria in the sediment. The present findings may help parameterizing hydrological and water quality models in a tropical context.

Highlights

Solar radiation measured at 380 cm height, which is above the vegetation height (Figure S1), ranged between 40 and 1068 W m−2 while at 100 cm, which is inside the vegetation, it ranged between 3 and 170 W m−2 (Figure 2)
Solar radiations were highly attenuated inside the wetland vegetation, which blocks an important part of the radiation from reaching the water column and bottom sediment
Particle-attached bacteria prevailed in all mesocosms over the incubation period: over

Summary

Introduction

Surface water contamination by fecal pathogenic microorganisms is a threat for public health worldwide [2]. Mostly caused by water-borne pathogens, were responsible for the deaths of 1.6 million people in 2017 [3]. This major public health issue especially affects developing countries in the intertropical band, where populations rely directly on local resources and have poor access to clean water, sanitation, and hygiene [4]. Riparian zones in tropical systems are often characterized by wetlands and swampy areas [6]

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