Seasonal variability of faecal indicator bacteria numbers and die-off rates in the Red River basin, North Viet Nam.

The Red River is the second largest river in Viet Nam and constitutes the main water source for a large percentage of the population of North Viet Nam. Here we present the results of an annual survey of Escherichia coli (EC) and Total Coliforms (TC) in the Red River basin, North Viet Nam. The objective of this work was to obtain information on faecal indicator bacteria (FIB) numbers over an annual cycle and, secondly, to determine the die-off rates of these bacterial indicators. Monthly observations at 10 stations from July 2013-June 2014 showed that TC and EC reached as high as 39100 cfu (colony forming units) 100 ml(-1) and 15300 colonies 100 ml(-1), respectively. We observed a significant seasonal difference for TC (p < 0.05) with numbers being higher during the wet season. In contrast, no significant seasonal difference was found for EC. The FIB die-off rates ranged from 0.01 d(-1) to a maximum of 1.13 d(-1) for EC and from 0.17 d(-1) to 1.33 d(-1) for TC. Die-off rates were significantly higher for free bacteria than for total (free + particle attached) bacteria, suggesting that particle attachment provided a certain level of protection to FIB in this system.

CDOM fluorescence as a proxy of DOC concentration in natural waters: a comparison of four contrasting tropical systems.

Chromophoric dissolved organic matter (CDOM) fluorescence or absorption is often proposed as a rapid alternative to chemical methods for the estimation of bulk dissolved organic carbon (DOC) concentration in natural waters. However, the robustness of this method across a wide range of systems remains to be shown. We measured CDOM fluorescence and DOC concentration in four tropical freshwater and coastal environments (estuary and coastal, tropical shallow lakes, water from the freshwater lens of two small islands, and soil leachates). We found that although this method can provide an estimation of DOC concentration in sites with low variability in DOC and CDOM sources in systems where the variability of DOC and CDOM sources are high, this method should not be used as it will lead to errors in the estimation of the bulk DOC concentration.

Seasonal effect on trace metal elements behaviour in a reservoir of northern Thailand.

Trace metal elements (TME) can be real threats for living organisms. However, few studies dealt with TME in reservoirs in rural areas where farming practises could induce negative effects. Mae Thang reservoir (northern Thailand) has been studied for 3 years to understand the seasonal behaviour of dissolved TME: Fe, Mn, Cd, Al, Pb, V, Cr, Co, Ni, Cu, Zn, Mo, U and As and associated physicochemical parameters. In situ measurements of these parameters were done during the dry and the wet seasons as well as water samples along the water column for further analyses and TME determination by inductively coupled plasma-mass spectrometry (ICP-MS). In the dry season, the water column was characterized by a strong stratification and anoxic conditions in the hypolimnion. High rain and water input from the watershed during the wet season induced mixing of the water. All TME, except Ni, Co and Cr were less concentrated in the wet season indicating a dilution effect by water input. There was thus no important dissolved pollution coming from the watershed. The anoxic conditions in the dry season enhanced the reduction of Fe and Mn and the desorption processes. Depth, and thus oxic-anoxic conditions were the main drivers of TME in the dry season, while in the wet season, dissolution processes from parent rocks of watershed were favoured. The average concentrations of TME in the reservoir were in the limit of the international and Thai standards. Only localized values in the bottom of the reservoir for Fe and Mn were higher than the limits.