Linking landscape development intensity within watersheds to methyl-mercury accumulation in river sediments

Understanding Food Web Mercury Accumulation Through Trophic Transfer and Carbon Processing along a River Affected by Recent Run-of-river Dams

Unlike large dams which favor methylation of Hg in flooded soils over long periods, run-of-river dams are designed to flood a limited area of soils and are therefore not expected to significantly affect mercury (Hg) cycling or carbon processing. We studied the Hg and carbon cycles within food webs from several sectors along the Saint-Maurice River, Quebec, Canada, that differ in how they are influenced by two run-of-river dams and other watershed disturbances. We observed peak Hg concentrations in fish five-year postimpoundment, but these levels were reduced three years after this peak. Methylmercury concentrations in low trophic level fish and invertebrates were related to their carbon source (δ¹³C) rather than their trophic positions (δ¹⁵N). Biomagnification, measured by trophic magnification slopes, was driven mainly by methylmercury concentrations in low-trophic level organisms and environmental factors related to organic matter degradation and Hg-methylation. River sectors, δ¹³C and δ¹⁵N, predicted up to 80% of the variability in food web methylmercury concentrations. The installation of run-of-river dams and the related pondages, in association with other watershed disturbances, altered carbon processing, promoted Hg-methylation and its accumulation at the base of the food web, and led to a temporary increase in Hg levels in fish.

Historic contamination alters mercury sources and cycling in temperate estuaries relative to uncontaminated sites

Mercury (Hg) is a global and persistent pollutant which can be methylated to more toxic forms (methylmercury; MeHg) in natural systems. Both forms pose a health risk to humans and wildlife, and exposure often begins in aquatic environments. Therefore, quantifying aquatic concentrations and identifying source pathways is important for understanding biotic exposure. In this study, data from estuaries in the Northeast United States were combined to evaluate how point source contamination impacts the concentration and source dynamics of water column total and MeHg with an emphasis on sediment versus non-sediment sources. Partial least squares regression models were implemented to identify a set of variables most related to water column MeHg and total Hg (HgT) across the estuaries. The main findings suggest that contaminated sites have strong internal recycling of HgT that dominates over external inputs, and this leads to elevated concentrations of HgT and MeHg in the local water columns. However, HgT sources in uncontaminated estuarine systems have a strong connection to the local watershed with dissolved HgT linked to dissolved organic carbon, and particulate HgT linked to watershed land use and estuarine mixing. There was little correlative evidence that water column MeHg concentrations were linked to sediment in such systems, but unlike HgT, the concentrations were also not clearly linked to the watershed. Instead, in situ methylation of dissolved water column HgT appeared to dominate the MeHg source pathway. The results suggest that Hg point-source contaminated sites should be considered independently from non-contaminated sites in terms of management, and that land use plays an important indirect role in coastal MeHg dynamics.

The influence of legacy contamination on the transport and bioaccumulation of mercury within the Mobile River Basin

Past industrial use and subsequent release of mercury (Hg) into the environment have resulted in severe cases of legacy contamination that still influence contemporary Hg levels in biota. While the bioaccumulation of legacy Hg is commonly assessed via concentration measurements within fish tissue, this practice becomes difficult in regions of high productivity and methylmercury (MeHg) production, like the Mobile River Basin, Alabama in the southeastern United States. This study applied Hg stable isotope tracers to distinguish legacy Hg from regional deposition sources in sediments, waters, and fish within the Mobile River. Sediments and waters displayed differences in δ²⁰²Hg between industrial and background sites, which corresponded to drastic differences in Hg concentration. Sites that were affected by legacy Hg, as defined by δ²⁰²Hg, produced largemouth bass with lower MeHg content (59-70%) than those captured in the main rivers (>85%). Direct measurements of Hg isotopes and mathematical estimates of MeHg isotope pools in fish displayed similar distinctions between legacy and watershed sources as observed in other matrices. These results indicate that legacy Hg can accumulate directly into fish tissue as the inorganic species and may also be available for methylation within contaminated zones decades after the initial release.

Effects of Landscape Development Intensity on River Water Quality in Urbanized Areas

Urban development and human activities have greatly changed the appearance of urban landscapes, and also affect urban river water environments. Rapidly urbanized regions in China face particularly severe pressures and challenges in alleviating degradation of river water quality. Information is needed on which indexes of landscape development intensity in rapidly-urbanized areas are the key factors affecting the quality of river water environments, and how these factors affect water quality. In order to answer these questions, this research selected six indexes belonging to three dimensions for landscape development intensity evaluation. Based on five water quality parameters of 20 rivers and the land use data of 20 small watersheds of Liangjiang New Area, Chongqing, China in 2014, this research explored the correlation between the landscape development intensity indexes and river water quality through redundancy analysis. We found that the impervious surface rate and the land average fixed asset investment are the key indexes to affect river water quality. Regulating the corresponding indexes at the urban planning and design level, as well as the decision making level, can effectively achieve the goal of improving urban river water quality. The conclusions inspire strategies in planning and design, and are helpful for government decision making to effectively protect river water environment in rapidly urbanized areas in the developing countries.

Spatio-Temporal Variation in Mountainous Landscape Changes: A Case Study of Shizhu County

The study of dynamic changes and spatial variation of landscape patterns is important to deeply understand the relationship between human activities and the natural environment. We selected a typical mountain area, Shizhu County, as the study area and analyzed the landscape’s dynamic changes and spatial variation in that area from 2000–2015. The results showed that cropland and forestland were the dominant landscape types in the study area. Cropland and grassland areas decreased, being mainly converted to forestland. Forestland and built-up land areas were increasing; the increase in built-up land was mainly due to the invasion into cropland areas, and the increase in forestland was mainly due to the conversion of cropland and grassland. Water bodies were affected by factors such as water storage in the Three Gorges Reservoir, and their area continued to increase. The change in landscape was most dramatic from 2005–2010, mainly due to the rapid increase in the areas of built-up land and water bodies and the rapid decrease in grassland area. There were apparent spatial variations in landscape distribution, patterns, and dynamic changes. Although water bodies were mainly distributed in the relatively gentle slope areas with an elevation of less than 200 m and a slope of 0°–6°, other landscapes were concentrated at an elevation higher than 500 m, a slope of 15°–35°, with a westerly or northwesterly aspect. These areas also had the most drastic landscape changes. At the type-level and the landscape-level, landscape indices showed greater variation with elevation and slope than with aspect. Finally, the variations with elevation, slope, and aspect differed among different landscape types.

Co-contamination of antibiotics and metals in peri-urban agricultural soils and source identification

To identify the dominant sources of contamination in peri-urban land, this study investigated the concentrations and distributions of antibiotics and metals in agricultural soil of this area. An index of landscape development intensity (LDI) was used to characterize the distribution of human disturbance-related land use. The results showed that total antibiotic concentration in the soil reached 395.55 μg/kg and that chlortetracycline was the predominant antibiotic compound, with a relatively high mean concentration of 30.62 μg/kg. In soils, the mean concentrations of Cu, Zn, and Pb were 38.41, 127.88, and 56.61 mg/kg and those of Al, Fe, and K were 83.73, 24.17, and 23.42 g/kg, respectively. A redundancy analysis showed that the landscape pattern in a 300-m buffer zone can well explain the variation in the concentrations of antibiotics and metals (24%, p < 0.05). The LDI in the 300-m buffer zone significantly correlated with the concentrations of total antibiotics and total amounts of Cu and Zn in the soil, suggesting that the risk of soil contamination increases with the intensity of anthropogenic activities. A structural equation modeling analysis indicated that Al, Cu, and Zn could significantly aggravate accumulation of tetracycline antibiotics in the soil, whereas there were only significantly direct paths from Cu to ciprofloxacin and norfloxacin. Overall, the results showed that aggravated co-contamination of antibiotics and metals occurs in agricultural soil under intensive human disturbance.

Impact of Land Use on the Mobility of Hg Species in Different Compartments of a Tropical Watershed in Brazil

This study evaluated the levels of total Hg and CH₃Hg⁺ from a comprehensive perspective, considering the retention, leaching, and deposition of these contaminants in the main compartments (soil, plant litter, and sediment) of three landscapes (Atlantic Forest, pasture, and agricultural area) in a watershed in northern Rio de Janeiro State, Brazil. Variables analyzed were total Hg, CH₃Hg⁺, organic carbon, total nitrogen, grain size, and surface area. In soil samples, total Hg levels were the highest in agricultural soil followed by forest soil and pasture (97.3, 87.6, and 77.1 ng g^-1, respectively), and CH₃Hg⁺ was lower than 1.7%. Total Hg levels in leaf litter varied between 22.6 and 34.2 ng g^-1, and CH₃Hg⁺ was 4.37%. In sediment, Hg (60-180 ng g^-1) and CH₃Hg⁺ (<1%) indicate the transport of these contaminants from soils to this compartment and may be associated with soil use and cover. Multiple regressions were used to understand the dispersion of Hg species, and the effect of each variable varied with the landscape, showing that plant cover should not be ignored in investigations related to Hg species retention in a watershed. The landscapes surveyed in the present study clearly influence the quantitative and qualitative distribution of Hg species. On the other hand, anthropic processes associated with changes in soil use did not have any critical effects on the absolute levels of total Hg and CH₃Hg⁺, meaning that the landscapes evaluated seem to represent the background concentration of these chemical species for the evaluated watershed.