Effects of damming on the distribution and methylation of mercury in Wujiang River, Southwest China

Mobilization and Transformation of Mercury Across a Dammed Boreal River Are Linked to Carbon Processing and Hydrology

“…In particular, there was a significant, about twofold increase along the reservoir sequence in August, and an about fivefold increase in June. This trend agrees with previous studies reporting systematic increases in MeHg concentrations upon flooding in inundated areas (Brigham et al, 2002; Coquer et al, 2003; Kelly et al, 1997; St. Louis et al, 2004; Zhao et al, 2017) and increases from upstream to downstream in a dammed river (Zhao et al, 2017). For example, Kasper et al (2014) observed concentrations of 0.11 – 0.18 ng L −1 , 0.5 km downstream from the reservoir, comparable to our range of measures from the river stretch, which collects water from the bottom waters of R1 (0.06 – 0.18 ng L −1 ).…”

“…The contrasting spatial patterns for MeHg versus THg may further be explained by the fact that, in addition to promoting Hg methylation, reservoirs may contribute to enhanced THg sedimentation as it is mostly bound to particles, which settle close to dams, counterbalancing potential increase in THg remobilization. For example, in a recent study, THg levels diminished along a series of cascade reservoirs, whereas MeHg increased with every consecutive reservoir (Zhao et al, 2017). Therefore, the fact that THg was less variable in the Romaine reservoir continuum could be due to an equilibrium between sedimentation, inputs and its transformations, as seen by the increase in %MeHg in the reservoirs.…”

“…As opposed to THg, concentrations of MeHg showed unexpectedly weak relationships with environmental and DOM properties, as well as counterintuitive seasonal patterns, being significantly higher in the colder June month of 2017 along the reservoir sequence. First, this is surprising since warmer temperature typically enhance the conditions that favor methylation (i.e., anoxia; Ullrich et al, 2001; Paranjape & Hall, 2017) and export of MeHg by hydroelectric reservoirs in other regions typically peak during summer (Canavan et al, 2000; Zhao et al, 2017). Moreover, we expected DOM pools associated to recent production (e.g., protein‐like C3 and C5; see Lapierre & Del Giorgio, 2014) to be related to concentrations of MeHg, as has been shown in recent studies (Bravo et al, 2017; Bravo, Kothawala, et al, 2018; Herrero Ortega et al, 2018), because high abundance or proportion of these DOM pools are often found in warm, productive sites and would represent high biological activity in the water column or in the sediments that may co‐occur with Hg methylation.…”

Vertical Distribution of Bacterial Community in Water Columns of Reservoirs With Different Trophic Conditions During Thermal Stratification

“…HJD Reservoir was built in 2004, with a total reservoir capacity of 49.3 × 10 8 m 3 , the highest water level of 1140 m, the normal water level area of 80.5 km 2 , and the retention time of the water body is 360 days, which suggests it is a regulating reservoir in the year of oligotrophic nutrition (Zhao et al, 2017); WJD Reservoir was built in 1979, with a drainage area of 27,790 km 2 , a total reservoir capacity of 21.4 × 10 8 m 3 , and the normal water level of 47.8 km 2 . The reservoir is a seasonal regulating reservoir with eutrophic nutrition (Zhao et al, 2017) and appreciably affected by human development and use. The samples were collected from the lacustrine zones of the two reservoirs of the Wujiang River in April 2017.…”

“…The spatial change in bacterial abundance in HJD Reservoir was relatively slight. HJD Reservoir was reported as an oligotrophic reservoir (Zhao et al, 2017) with long hydraulic residence time and high light intensity. Bacteria selectively grew in the water layer with appropriate light, and displayed increased abundance with increasing water depth in the epilimnion layer.…”