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Methylmercury modulation in Amazon rivers linked to basin characteristics and seasonal flood-pulse Daniele Kasper, Bruce Rider Forsberg, João Henrique Fernandes Amaral, Sarah Sampaio Py-Daniel, Wanderley Rodrigues Bastos, and Olaf Malm Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04374 • Publication Date (Web): 25 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Environmental Science & Technology

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Methylmercury modulation in Amazon rivers linked to basin characteristics and seasonal

2

flood-pulse

3 a,b

a

a

a

4

Daniele Kasper *, Bruce R. Forsberg , João H. F. Amaral , Sarah S. Py-Daniel , Wanderley R.

5

Bastos , Olaf Malm

c

b

6 7

a

8

Ephigênio Salles, 2239, Manaus, AM, 69060-020, Brazil

9

b

Instituto Nacional de Pesquisas da Amazônia, Departamento de Dinâmica Ambiental, Av.

Universidade Federal do Rio de Janeiro, Centro de Ciências da Saúde, Ilha do Fundão, Rio de

10

Janeiro, RJ, 21941-902, Brazil

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c

12

Brazil

Universidade Federal de Rondônia, BR 364 km 9,6 sentido Acre, Porto Velho, RO, 76815-800,

13 14

* Corresponding author: [email protected]; Phone/Fax +55 92 3643 1904

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Laboratório de Ecossistemas Aquáticos, Instituto Nacional de Pesquisas da Amazônia, Av.

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Ephigênio Salles, 2239, Manaus, AM, 69060-020, Brazil

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Abstract Art

19 20 21

ACS Paragon Plus Environment


22 23

ABSTRACT

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We investigated the impact of the seasonal inundation of wetlands on methylmercury (MeHg)

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concentration dynamics in the Amazon river system. We sampled 38 sites along the

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Solimões/Amazon and Negro rivers and their tributaries during distinct phases of the annual flood-

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pulse. MeHg dynamics in both basins was contrasted to provide insight into the factors controlling

29

export of MeHg to the Amazon system. The export of MeHg by rivers was substantially higher

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during high-water in both basins since elevated MeHg concentrations and discharge occurred

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during this time. MeHg concentration was positively correlated to %flooded area upstream of the

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sampling site in the Solimões/Amazon Basin with the best correlation obtained using 100 km

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buffers instead of whole basin areas. The lower correlations obtained with the whole basin

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apparently reflected variable losses of MeHg exported from upstream wetlands due to

35

demethylation, absorption, deposition and degradation before reaching the sampling site. A similar

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correlation between %flooded area and MeHg concentrations was not observed in the Negro Basin

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probably due to the variable export of MeHg from poorly drained soils that are abundant in this

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basin but not consistently flood.

39 40

Keywords: Wetland, Inundation, Methylation, Negro River, Solimões River, Dissolved Oxygen

41 42 43 44 45 46 47 48 49


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1. INTRODUCTION

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Mercury (Hg) is a neurotoxic element which represents a significant health risk for many

55

organisms, especially those in aquatic environments where inorganic Hg can be transformed into

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methylmercury (MeHg), an organic Hg species which bioaccumulates and biomagnifies in the food

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chain. Since Hg bioaccumulation correlates with aqueous MeHg concentrations, it is important to

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understand the environmental factors that contribute to MeHg variation. Most Hg methylation is

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mediated by methanogens and iron- and sulfate-reducing bacteria, so the environmental conditions

60

that promote the activity of these microbes also increase MeHg formation.

1

2

3-5

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The combination of anoxia and low pH is known to promote Hg methylation. While relatively

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rare in river channels, these conditions are frequently encountered in the other sites within river

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basins, such as poorly drained soils, riparian and alluvial wetlands and in the deepest layers of

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stratified floodplain lakes and reservoirs, and high MeHg concentrations are often observed in these

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environments.

66

on the Hg dynamics of river systems.

67

important sources of MeHg in aquatic systems.

68

been shown to have higher methylation rates than those in the non-flooded soils or in the open

69

surface waters of lakes.

70

water and in the biota, such as zooplankton, invertebrates and fish.

6

7-9

The MeHg produced and exported from these habitats can have a strong influence 8,10,11

In particular, wetlands have been recognized as 2,9,12-15

9,16,17

Sediments and plant litter of these areas have

Consequently, wetlands tend to have high concentrations of MeHg in 2,7,14,15,17

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Mercury methylation and export from wetlands and floodplain lakes is expected to vary in

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response to seasonal flooding patterns, with the highest rates of methylation occurring during high-

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water periods when greater depth and thermal stratification lead to anoxic hypolimnetic conditions.

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9,11,18

75

along the Tapajós

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channel,

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the Amazon river system during this period.

7-

In the Amazon Basin, elevated levels of MeHg have been encountered in wetlands and lakes

19

18

8

and Solimões rivers at high-water and also in the Madeira River main

suggesting that floodplain wetlands and lakes may be an important source of MeHg to



78

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Positive correlations between aqueous MeHg concentrations and upstream wetland 2,10,15

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densities have been reported for river systems.

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small north temperate rivers with catchments of less than 10000 km .

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Amazon, where catchments can reach million km , these correlations may not be as clear. Since

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MeHg produced in fluvial wetlands can be demethylated, absorbed by biota, deposited in bottom

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sediments or degraded both before and after reaching a river channel, correlations between MeHg

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concentrations and wetland coverage are expected to weaken with distance downstream from the

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original source. Uncertainty regarding the downstream transformation of MeHg, derived from river

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catchments, can lead to confusion when assessing the impacts of deforestation and other

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anthropogenic changes on Hg dynamics. Forestry has been shown to impact Hg and MeHg levels

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in small streams and lakes, but it is difficult to extent these results to larger rivers since MeHg can

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be transformed to inorganic Hg along longer downstream reaches.

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zones between methylation sites affected by forestry and downstream surface waters is now a

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guideline recommended by the Swedish Forest Agency.

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areas and its subsequent transport to downstream aquatic environments are considered

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fundamental controls on the bioaccumulation of Hg.

However, most of these studies were done in 2 15,20,21

In large basins, like the

2

22

94

Mercury

methylation

and

export

from

22

poorly

The establishment of buffer

The production of MeHg in wetland

drained,

seasonally

waterlogged

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(hydromorphic) soils in the Amazon are highest during the rainy season when these soils become

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saturated with water and anoxic conditions develop.

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in hydromorphic Spodosols where both anoxic and acidic conditions prevail during this period.

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Elevated concentrations of MeHg have been found in small streams draining these soils with the

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highest levels occurring during the rainy season.

11

11

Methylation rates are expected to be highest

8

Brito et al. and Vasconcelos

11

11,23

attributed the 24

100

elevated export of total Hg and MeHg from Spodosols to their high total Hg content

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presence of favorable methylating conditions when they are waterlogged.

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have been observed between levels of Hg encountered in predatory fish and fish-eating human

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populations and the density of hydromorphic Spodosols and hydromorphic Entisols in the

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basin.

11,14,25


11

and the

Positive correlations

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Here we use data from six regional surveys of the Solimões/Amazon and Negro River

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systems and spatial analyses of wetland distributions to investigate the influence of hydromorphic

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soils and wetlands in upstream catchments, river chemistry and hydrological variability in the MeHg

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concentration dynamics of the Amazon River system.

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2. MATERIALS AND METHODS

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2.1. Study area

113 114

Sample sites for this study were located along the Negro and Solimões/Amazon River main

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channels and their main tributaries in the Central Amazon Basin (Figure 1). These two river systems

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drain over three million km , nearly half of the entire Amazon Basin, and have distinct

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geomorphological and biogeochemical properties.

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lowland forest covering extensive areas of hydromorphic Spodosols and sandy Entisols, together

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with a mixture of Oxisols and Ultisols.

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minerals, but high in quartz, kaolinite and insoluble metal complexes, including Hg. The rivers

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draining them are typically black waters,

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organic matter and total Hg and low in suspended sediments, dissolved ions and pH.

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contrast to the Negro, the part of the Solimões/Amazon River system studied (Figure 1) drains high

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relief areas in the Andes mountains and forested lowlands in the Central Amazon. The lowland

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areas are covered predominantly with highly weathered Ultisols, Oxisols and Entisols

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Andean uplands are covered with mineral-rich Inceptisols and Entisols of marine origin.

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Tributaries draining the Andean highlands, like the Solimões/Amazonas main channel, are called

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white waters.

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nutrients and pH and low in dissolved organic carbon (DOC), while those draining the lowlands tend

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to be black waters, low in suspended sediments, nutrients and pH and higher in DOC.

2

28,29

26

26,27

The Negro Basin drains predominantly

Most of these soils are highly weathered, low in soluble

that are olive-brown to coffee-brown, high in dissolved 24,26,30,31

29

In

while the 29,31

26

These waters are turbid, more or less ochre-colored, high in suspended sediments,

26,31,32

131

The climate in the region is tropical humid and its seasonality is reflected in river hydrology.

132

River levels tend to vary in response to seasonal variations in precipitation, a phenomenon referred



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to as the flood-pulse. Large rivers like the Solimões/Amazon and Negro main channels show a

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monomodal flood-pulse with four distinct hydrological seasons, rising-water, high-water, falling-

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water, and low-water (Figure S1, Supporting Information). During high-water, it is estimated that

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14% of the lowland Amazon Basin, predominantly river floodplains, is inundated.

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there is a large exchange of organic matter, nutrients, suspended solids and organisms between

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river channels and their floodplains.

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exchanged during this period and can alter the concentrations of these parameters in the river.

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Peak flooding in the central part of the Amazon Basin usually occurs in June, near the end of the

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sixth month rainy period, while the low-water minimum normally occurs in November. The highest

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precipitation occurs during the low-water and rising-water seasons, between November and May.

33

13

At this time,

Materials produced on the floodplain, such as MeHg, are also

34

143

Abundant hydromorphic Spodosols are present the lowland Negro basin that become

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waterlogged during the peak of the rainy season (February-March), and develop anoxic, acid

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ground waters, rich in dissolved organic carbon and MeHg at this time. The water table does not

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always rise to the surface of these soils when waterlogged, so they are not consistently classified as

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wetlands by L-band radar imagery. The Negro Basin also has extensive alluvial floodplains,

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covered with forested and open water habitats, that are inundated at peak high water (high-water

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season). At this time, these environments are thermally stratified and have anoxic acidic bottom

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waters that promote Hg methylation. Due to their different flooding patterns and environmental

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characteristics, hydromorphic Spodosols and floodplains are expected to have temporally distinct

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effects on Hg methylation and MeHg export, creating a complex pattern of MeHg dynamics in this

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river system.

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11

30

29

In contrast, hydromorphic Spodosols are rare in the Solimões Basin.

Hg methylation in

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this basin is expected to occur predominantly in floodplain environments that include open water,

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floodplain forests and extensive macrophyte beds which have been shown to have a high potential

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for Hg methylation. Since floodplains are the main methylation site in this system, Hg methylation

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and MeHg export to the river system are expected to be vary seasonally with the annual river flood

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pulse and be proportional to the density of fluvial wetlands. Contrasting the MeHg dynamics in the

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Solimões and Negro basins could therefore provide considerable insight into the factors controlling

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MeHg export to the Amazon River system.

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The area of the Amazon Basin studied is largely undisturbed. However, there is a history of

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gold mining in some regions that may have elevated Hg and MeHg concentrations in the river

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system above natural background levels.

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the Negro Basin, with the exception of the Branco sub-basin (discharge of 5000 m s , on average)

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where significant mining occurred during 1980s and 1990s. The Solimões Basin has also been

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relatively free of mining with the exception of the Traira River, a tributary of the upper Japura River

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(discharge of 16000 m s , on average), and the Napo River. The Madeira Basin (discharge of

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19000 m s , on average) was an important gold mining region in the 1980s and 1990s and

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significant levels of mining activity continue today in the Bolivian and Peruvian headwaters.

25,35-38

There is little documented history of gold mining in 3

3

3

-1

-1

-1

39,40

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2.2. Sampling and laboratory analyses

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River samples were collected during six expeditions between July 2011 and December

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2012, including one low and one high-water excursion in the Negro Basin and one low-water, one

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high-water, one early falling-water and one late falling-water excursion in the Solimões/Amazon

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Basin (Figure S1, Supporting Information). The Negro River was sampled at four sites along its

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main stem (comprising approximately 700 km) and at one site in each of its 21 main tributaries

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(Figure 1). The Solimões/Amazon River was sampled at seven sites along its main stem (along

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approximately 1200 km) and at one site in each of its six main tributaries (Figure 1). Subsurface

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water samples (0.3 m depth) were collected in the center of each river channel at a point

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equidistant from the river margins. Limnological measurements (pH, dissolved oxygen, electric

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conductivity, and temperature) were made in situ at the same sites and time of water sampling.

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41,42

Ultraclean techniques were used to collect samples for MeHg analysis.

250-500 mL of

®

185

water was collected directly in amber glass bottles with Teflon lids. Sample bottles were cleaned

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according to EPA 1630 ; briefly, the bottles were filled with sequential solutions of ultra-pure water

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(MilliQ Plus, Millipore) with HCl and heating by several hours. Between each cleaning step, the

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bottles were rinsed three times with ultra-pure water. Finally, the bottles were double-bagged in

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polyethylene zip-lock bags. Samples were examined for contamination by analyses of bottle and

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field blanks. The samples were preserved by adding HCl (0.4% v:v) a few hours after collection,

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and storing them in a cool dark environment.

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the precision of the field sampling.

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EPA 1630,

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tetraethylborate for ethylation, gas chromatography to separate ethylated MeHg and Cold Vapor

195

Atomic Fluorescence Spectrometer for Hg determination (MERX, Brooks Rand). We analyzed each

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water sample in duplicate and reanalyzed it when the coefficient of variation between duplicates

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was >24%, the acceptable value established by EPA 1630 . We checked accuracy by analyzing

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matrix spikes of MeHg. Samples were spiked with MeHg 2-30 h before analyses yielding a final

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concentration in the spiked samples (named as true concentration by EPA 1630 ) ranging between

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0.02 and 1.10 ng L (recovery: 98 ± 12%; n = 37). The detection limit of MeHg was 0.010 ng L ,

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which corresponded to the mean concentration of the analytical blanks plus three times the

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standard deviation of these blanks. An analytical intercalibration performed for MeHg analysis in

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water indicated satisfactory laboratory performance.

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43

43

43

43

Duplicates field samples were collected to assess

MeHg analyses were conducted on unfiltered water following

using 1% ammonium pyrrolidinedithiocarbamate for distillation, sodium

43

43

-1

-1

44

45

Water samples for DOC analyses were collected in polyethylene bottles and filtered through

205

pre-combusted (450 °C for 1 h) glass fiber filters (GF-1 Macherey Nagel; 0.7 µm of retention

206

capacity). Samples were stored cool in pre-combusted glass scintillation vials with Teflon lined

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caps. DOC concentrations were determined with a total carbon analyzer (TOC-VPN, Shimadzu)

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which had an analytical precision of 0.01 mg L . At least three replicate injections were made for

209

each sample, yielding average values with a coefficient of variation < 2%.

210

®

-1

Discharge measurements were made at the same times and sites using an Acoustic

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Current Doppler Profiler (ADCP, workhorse, RD Instruments). This was not possible in five sites at

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low-water in the Solimões/Amazon Basin. Discharge data for these sites and dates were obtained

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from the nearest gauging station maintained by the Brazilian Agência Nacional de Águas.

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Discharges of the Caurés River (Negro Basin) at low-water season, and of the Uatumã River


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(Solimões/Amazon Basin) at early and late falling-water were not measured and could not be

216

estimated from existing gauging stations.

217 218

2.3 Spatial analyses

219 220

The watershed above each sample site was determined from the digital elevation model 47

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developed by the Shutter Radar Topographic Mission (SRTM-DEM). The catchment area 100 km

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upstream from the sampling site, referred to as buffer, was also determined. These buffer

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catchment areas varied from 1.3 to 14.4 10 km . The upstream extension of this buffer (100 km)

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was chosen to optimize sensitivity of the measured concentrations/fluxes of MeHg in the rivers to

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methylation processes upstream and are based on the estimated half-life of MeHg in Amazonian

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rivers. The half-life of MeHg due to photodegradation in Amazon waters has been estimated as 4 to

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7 h (5.5 h in average

228

lowland Amazon rivers of 0.71 m s , MeHg levels would be reduced by 50% in 14 km. The MeHg

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half-life used in this calculation was estimated for surface waters with high incident radiation.

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However, only the first few centimeters of the water column in most Amazonian rivers water receive

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this much radiation. These experiments were also done with MeHg spikes that were more labile to

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chemical reactions than naturally occurring MeHg (e.g.; MeHg associated with organic and

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inorganic particles). Therefore, the half-life of MeHg in the studied rivers probably occurred along a

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reach much longer than 14 km. In addition to these experiments, variations in MeHg concentrations

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in the Uatumã River (Amazon Basin), downstream from Balbina dam, an anthropogenic source of

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MeHg, were used to evaluate in-situ MeHg degradations rates. Aqueous MeHg concentrations

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decreased on average 30% along the first 35 km below the dam, due to degradation, absorption by

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biota and deposition in the sediment. Considering these studies, we assumed that 100 km was an

239

approximate half-life distance for MeHg and a reasonable buffer for investigating the influence of

240

MeHg sources upstream of the sampling sites.

3

48,49

2

). Assuming this mean MeHg half-life and a mean of water velocity for -1

48,49

7

7

241

The percentage of flooded area (%Flooded) is the ratio of inundated area in relation to total

242

catchment area or buffer area. This parameter was calculated for high and low-water seasons using



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50

243

the wetland classifications developed by Hess et al. for these two periods. Flooded area was

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calculated as the sum of the areas of all inundated wetland classes within the basin or buffer area,

245

including: flooded herbaceous plants, shrubs, woodlands and forests areas and nonvegetated

246

flooded areas (e.g.; open area of the lakes). All spatial data analyses were performed using QGIS.

51

247 248

2.4. Data analysis

249 250

In all statistical analyses and figures, the samples of Negro and Solimões/Amazon basins

251

were evaluated and represented separately. The Negro River is a tributary that contributes about a

252

quarter of discharge of the Amazon River, and the remaining three quarters comes from Solimões

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River (based on discharge measured in the present study). Since the characteristics of the Negro

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Basin are distinct from those of the Solimões Basin (previously discussed), the furthest downstream

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Negro sampling site (N4 in Figure 1) was included in the Negro Basin analysis but not in the

256

Solimões/Amazon Basin analysis. The only exception is Figure 4, where results for this Negro River

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site were included in the results for the Solimões/Amazon Basin for comparison.

258

In order to compare MeHg concentrations between different hydrological seasons (high and

259

low-water) and sites (Negro and Solimões/Amazon basins), we performed a two-way ANOVA. This

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analysis detected the independent effect of both factors (season and site) as well as their

261

interaction. Within each basin, the seasonal differences of MeHg concentrations were assessed by

262

paired t test (two seasons collected in the Negro Basin) or Friedman test with Dunn’s multiple

263

comparisons test (four seasons collected in the Solimões/Amazon Basin; since data failed the

264

assumption of sphericity). The differences of MeHg concentrations between Negro and

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Solimões/Amazon basins within each of the two seasons, high and low-water, were assessed by t

266

test and Mann-Whitney test (since data were heterocedastic), respectively.

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For 28 sampling sites (15 in the Negro Basin and 13 in the Solimões/Amazon Basin) which

268

coincided with gauging stations maintained by the Brazilian National Water Agency (ANA - Agência

269

Nacional de Águas), we calculated the percent of peak water level of the river at the time of

270

sampling (%Peak water level), which corresponded to the percentage of water level on the


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sampling day in relation to the highest water level recorded at the local between July 2011 and

272

December 2012 (sampling period). This parameter allowed for comparisons within and between

273

rivers throughout the hydrological cycle. Linear regressions between independent variables (%Peak

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water level and %Flooded area) and the dependent variable (MeHg concentrations) were used to

275

investigate the influence of hydrological variations on MeHg dynamics. The transport of MeHg by

276

rivers was calculated by multiplying water discharge (m s ) by the MeHg concentration (ng L ).

277

This transport was compared between seasons in the Negro Basin with a Wilcoxon test (due to

278

absence of normality) and in the Solimões/Amazon Basin with a Friedman test with Dunn’s post-hoc

279

(due to absence of sphericity).

280

3

-1

-1

In order to compare the limnological parameters between the seasons, we used paired t

281

tests in the Negro Basin and Friedman tests with Dunn’s multiple comparisons tests (due to lack of

282

sphericity) in the Solimões/Amazon Basin. These parameters were compared between the two

283

basins with t tests (or Mann-Whitney tests when variances were not homogeneous). Multiple linear

284

regressions were used to investigate the influence of pH, dissolved oxygen, and DOC on MeHg

285

concentrations of each basin. We used Kolmogorov-Smirnov and Bartlett’s tests to test for the

286

normality and homocedasticity of data, respectively.

287 288

3. RESULTS

289 290

MeHg concentrations in water were influenced by the interaction between season and

291

hydrographic basin (F = 404.48, p < 0.0001). In both basins, MeHg concentrations were highest

292

during high-water season and lowest during low-water season (Negro Basin: t = 3.13, p = 0.005;

293

Solimões/Amazon Basin: Fr = 27.96, p < 0.0001; Figure 2). In the Negro Basin, abundantly covered

294

by hydromorphic Spodolos, MeHg concentrations at high-water were lower than in the

295

Solimões/Amazon, where hydromorphic Spodosols are rare (t = 3.09, p = 0.004). Conversely, at

296

low-water, the concentrations were higher in the Negro than in the Solimões/Amazon Basin (U =

297

46.00, p = 0.0004). Results for the Jutaí River (Solimões/Amazon Basin) at high-water were

298

eliminated as outliers from all statistical analyses, tables and figures, due to the exceptional levels



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of MeHg (six times higher than the mean for the Solimões/Amazon Basin) and dissolved oxygen

300

(five times lower than the mean for the Solimões/Amazon Basin) encountered (MeHg = 0.76 ng L ,

301

dissolved oxygen = 0.57 mg L ).

302

-1

-1

2

MeHg concentrations increased with %Peak water level of rivers (Negro Basin: r = 0.30, p 2

303

< 0.0001; Solimões/Amazon Basin: r = 0.60, p < 0.0001; Figure S2). The slope of the regression

304

for Negro Basin ([MeHg] = 0.0129 + 0.0008 x %Peak water level) was about half that for the

305

Solimões/Amazon Basin ([MeHg] = - 0.0243 + 0.0015 x %Peak water level). MeHg transport was

306

highest in both basins during the high-water season (Negro Basin: W = 283.00, p < 0.0001; Figure

307

3; Solimões/Amazon Basin: Fr = 21.90, p < 0.0001; High-water ≥ Early Falling-water ≥ Late Falling-

308

water ≥ Low-water; Figure 4). While MeHg varied consistently between hydrological periods, the

309

absolute fluxes differed considerably between tributaries.

310

Percent flooded area in relation to total basin area varied from 3 to 13% in the

311

Solimões/Amazon Basin and from 1 to 37% in the Negro Basin, and was positively correlated with

312

MeHg concentration in the Solimões/Amazon Basin (r = 0.45, p = 0.0009) but not in the Negro

313

Basin (r = 0.001, p = 0.79). The same pattern was observed when %Flooded area in relation to the

314

100 km buffer area was considered; MeHg concentrations increased with the %Flooded area in the

315

Solimões/Amazon Basin (r = 0.76, p < 0.0001) but not in the Negro Basin samples (r = 0.05, p =

316

0.13; Figure 5).

317

2

2

2

2

The limnological parameters varied seasonally within each basin and between the two

318

basins (Tables S1-S4). The Negro and Solimões/Amazon basins were more acid and less

319

oxygenated at high-water season (Tables S1 and S2). DOC varied between basins; concentrations

320

were higher at low-water season in the Negro Basin (Table S1) and in the late falling-water and

321

high-water seasons in the Solimões/Amazon Basin (Table S2). Comparing the two basins, we

322

observed lower pH and higher DOC concentrations in the Negro Basin when compared to the

323

Solimões/Amazon Basin at both high (pH: t = 8.82, p < 0.0001; DOC: t = 3.72, p = 0.001) and low-

324

water (pH: U = 7.50, p < 0.0001; DOC: U = 21.00, p < 0.0001). Dissolved oxygen concentrations

325

were similar between basins at both high (t = 0.24, p = 0.82) and low-water (U = 63.00, p = 0.30).

326

Independent of period in the Negro Basin (Figure S3), MeHg was negatively correlated with


Page 13 of 31


327

dissolved oxygen (r = -0.42, p = 0.02) but unrelated to pH (r = 0.06, p = 0.76) or DOC (r = 0.11, p =

328

0.59). Independent of period in the Solimões/Amazon Basin (Figure S3), MeHg concentrations

329

decreased with dissolved oxygen concentration (r = -0.85, p < 0.0001) but were unrelated to pH (r =

330

-0.32, p = 0.05) or DOC (r = -0.22, p = 0.15).

331 332

4. DISCUSSION

333 334

The highest MeHg concentrations and fluxes were observed at high-water in both basins,

335

despite the greater discharge and diluting capacity of the rivers at this time. This reflected a higher

336

level of MeHg loading from the upstream drainage basins that could have been due to a greater

337

availability of Hg due to changes in its partition coefficient, increased rates of Hg methylation or

338

both. The partition coefficient between sediment and water depend of a variety of environmental

339

factors. The higher amount of organic matter in sediment, lower oxygen levels and pH values at

340

high-water compared to low-water are associated to higher partition coefficients.

341

higher DOC levels, observed during low-water, increase Hg solubility and, therefore, the

342

displacement of Hg from sediment to the water.

343

higher release of MeHg to pore water from an anaerobic sediment due to degradation of organic

344

matter, a condition expected during the high-water in our study basins. Thus, literature data on the

345

partition coefficients vary significantly, and local conditions can be important, besides antagonist

346

and synergist effects that can modifying the distribution coefficient. Bisinoti et al.

347

water tributary of the Negro River and found the lowest concentrations of reactive Hg (operationally

348

defines as all Hg species that can be reduced by stannous chloride and easily transformed to

349

MeHg) at high-water, suggesting that methylation may contribute to the seasonal changes in MeHg

350

observed in the present study.

351

52-58

59-61

Moreover,

62

On the other hand, Muresan et al.

30

showed a

assessed a black

The extensive areas of flooded wetlands present on river floodplains at high-water are the

352

most likely source of MeHg to the river and high rates of methylation and export from these

353

environments would explain the higher concentrations and fluxes observed at this time. Flooded

354

forests, flooded macrophyte beds and the hypolimnion of the floodplain lakes have been shown to



Page 14 of 31

6,8,9,11

355

be important sites of MeHg production on the floodplains of Amazonian rivers.

356

MeHg production during high-water season apparently compensate the diluting effect of elevated

357

discharge resulting in higher MeHg concentrations at this time. Large seasonal variations in MeHg

358

concentrations have also been observed in small streams draining hydromorphic Spodosols in the

359

headwaters of the Negro Basin, but with a different seasonal pattern.

360

in these streams were observed in the middle of the rainy season (February-March), when

361

precipitation was highest and soils were saturated with acidic black water and conducive to

362

methylation.

363

11

Higher levels of

The highest levels of MeHg

11

Dissolved oxygen varied greatly between sampling seasons and was negatively correlated

364

with riverine MeHg concentrations throughout the year in both the Negro and Solimões/Amazon

365

basins. Dissolved oxygen has been shown to have an important influence on Hg dynamics in

366

aquatic environments. Negative relationships have been observed between oxygen concentrations

367

and MeHg contamination of the water

368

environments are commonly encountered in flooded habitats of the Amazon Basin, and are

369

especially conducive to MeHg production due to the presence of sulfate reducing bacteria, an

370

important methylator of Hg.

371

anaerobes, facultative anaerobes, and aerobes, but the potential for microbial methylation is

372

generally thought to be higher under anaerobic conditions.

373

64

7,8

and total Hg contamination in aquatic biota.

63

Anoxic

Organisms capable of Hg methylation have been found among

65

MeHg produced in the anoxic zones of fluvial wetlands can flow to rivers through

374

connecting channels and via over bank flow and this was presumably the principal source of MeHg

375

in the river channels sampled here. Dissolved oxygen levels in the river are expected to decline and

376

MeHg concentrations increase as the proportion of river discharge derived from anoxic wetland

377

environments increases. This proportion in presumably highest in both river systems at high-water,

378

when fluvial wetlands are deepest and large volumes of anoxic bottom waters rich in MeHg are

379

exchanged with the river, and lowest at low-water, when the presence of anoxic environments on

380

the floodplain and exchange with the river are reduced.

381 382

The exceptionally high MeHg concentration encountered in the Jutaí river during the highwater season could reflect additional processes linked to a local “friagem” event. In the 48 hours


Page 15 of 31


66

383

preceding this sampling, there was a drop in average of air temperature, from 28 °C to 23.5 °C.

384

This rapid but infrequent drop in temperature, locally called a “friagem” event, is known to promote

385

the rapid destratification and vertical mixing of floodplains waters, resulting in the introduction of

386

anoxic hypolimnetic waters rich in H2S, NH4, CH4 and MeHg to the surface.

387

low concentration of dissolved oxygen and high concentration of MeHg observed in the Jutaí river

388

at this time suggests that these anoxic floodplain waters were rapidly transported to the river

389

channel. The exceptionally high concentration of dissolved methane encountered in the Jutaí river

390

at the time of our sampling support the “friagem” hypothesis.

391

67-69

The exceptionally

70

While both basins showed clear seasonality, MeHg concentrations in rivers of the

392

Solimões/Amazon basin showed a greater increase from low to high-water and were better

393

correlated to both %Peak water level and %Flooded area than those of Negro Basin. These

394

differences may all reflect the strong influence of hydromorphic Spodosols on the MeHg dynamics

395

in the Negro basin, which is not directly linked to either river levels or surface flooding. MeHg

396

concentrations in the small black water streams draining hydromorphic Spodosols in the Negro

397

Basin are highest during the peak of the rainy season in the Central Amazon (February-March),

398

several months before peak water level along the Negro main channel.

399

coincides with peak water saturation in hydromorphic Spodosols and conditions favorable for

400

methylation. A comparison of MeHg dynamics in small streams draining predominantly Spodosols

401

versus predominantly Oxisols

402

draining Spodosols. It was not possible to correlate MeHg concentrations to the percentage of

403

Spodosol in the basins studied here due to a lack of reliable information on the distribution of these

404

soils. Existing wetland classifications based on the analysis of L-band radar imagery underestimate

405

Spodosol areas because the water table in these soils does not always rise the soil surface which is

406

necessary for L-band radar to detect inundation. Existing soil classifications for the Amazon based

407

on the analysis of soil profiles, such as those produced by the RADAM Project, Quesada et al.

408

and Batjes et al. , are also too coarse-scale and inaccurate to be use for this objective. In these

409

classifications data from a relatively small number of cores was interpolated using information on

410

topography and lithology to produce regional soil maps, of little use for defining the spatial

11

11

Maximum rainfall generally

also indicated much higher levels MeHg at low-water in the streams

71

29

72



Page 16 of 31

411

variations in soil hydrology necessary to classify hydromorphic soils. The percentage of Spodosols

412

in our study basins, derived from these maps, are listed in Table S5. While they do not correlate to

413

MeHg levels at individual sample points, they do indicate the major difference in Spodosol densities

414

between the Negro and Solimões/Amazon basins. While it is not clear how much of the MeHg

415

measured in tributaries of the Negro Basin was derived from hydromorphic Spodosols, significant

416

inputs from this source would explain the lower correlations observed with %Peak water level and

417

%Flooded area as well as the higher concentrations of MeHg observed at low-water when

418

compared to tributaries of the Solimões/Amazon Basin.

419

While these results suggest that Spodosols were an important source of MeHg in the Negro

420

Basin, the observed correlation of MeHg concentrations with %Peak water level indicates that the

421

extensive fluvial wetlands in this basin were also a major source of MeHg. Considering the

422

existence of two major sources of MeHg in the Negro Basin, it is surprising that the average

423

concentration of MeHg in this river system was lower that found in the Solimões/Amazonas Basin

424

during high-water. One possible explanation for this is that the fluvial wetlands in Solimões/Amazon

425

Basin could be more conducive to Hg methylation than those in the Negro Basin. This could be due

426

to differences in the composition, extension and quality of wetland habitats. The floodplains of the

427

Solimões/Amazon Basin are covered mainly by forest, floodplain lakes and macrophytes while the

428

Negro floodplain is covered mainly by forest. Floodplain forests in the Negro Basin have lower leaf

429

litter fall and slower leaf litter decomposition rates than those of the Solimões/Amazonas Basin

430

which may lead to higher methylation rates in the latter environments. The hypolimnia of the

431

Solimões floodplain lakes may also provide better environments for MeHg formation and potential

432

methylation rates in macrophyte roots have been shown to be an order of magnitude higher than

433

those in floodplain forest sediments. Therefore, the fluvial wetlands of Solimões/Amazon Basin

434

appear to provide a more favorable environment for methylation than those in the Negro Basin. The

435

Solimões/Amazon Basin also has a larger wetland area and thus, more areas favorable to

436

methylation. The MeHg produced in these wetlands also flows more efficiently to the river due to

437

improved distal wetland to river connectivity.

438

cannot create such redox conditions. In those high flow rivers with a narrow wetland, fluvial MeHg

73

8

9

71

20

In turn, the Negro Basin has narrow wetlands that


Page 17 of 31


439

concentrations can decrease since methylation sites are wipe out owing to high flow inside

440

wetland.

441

20

We found a better correlation between %Flooded area and aqueous MeHg concentrations

442

for rivers of the Solimões/Amazon Basin when we used buffers instead of whole basin areas. The

443

relationship for whole basin areas (Figure 5c) improves significantly if S1, S2 and the Madeira

444

(sampling points with highest residuals) are removed (r = 0.79, p < 0.0001). When buffered areas

445

are used, these outliers return to the regression line (Figure 5d). Part of the MeHg produced in

446

wetlands upstream of the buffer for these points was apparently demethylated, absorbed, deposited

447

or degradated before reaching the sampling site. We conclude that the assessment of MeHg

448

concentrations in large hydrographic basins is improved by using appropriate buffers. The size of

449

the buffer should consider the aquatic system studied, taking into account hydrological dynamics

450

and the time-course of processes that influence the flow and half-life of MeHg in the system.

2

451

This macro-scale assessment of MeHg dynamics in Amazonian rivers demonstrated the

452

strong influence of the seasonal flood-pulse on MerHg dynamics in the Solimões/Amazon Basin

453

and the influence of both the seasonal food-pulse and seasonal saturation of hydromorphic

454

Spodosols on MeHg dynamics in the Negro Basin. The higher levels of MeHg encountered in all

455

rivers at high-water suggest that aquatic organisms and human populations could also be more

456

exposed to MeHg contamination during this period. Direct exposure through water consumption is

457

not a concern since the observed concentrations are well below the upper limits recommended for

458

human consumption according to Brazilian law.

459

bioaccumulate and biomagnify MeHg, and that humans are the top consumers in the aquatic food

460

chain, the elevated levels of MeHg at high-water could have serious human health consequences.

461

The elevated fish consumption of the Amazon population, currently estimated at 10 million of

462

people, results in a chronic Hg exposure. Average Hg intake by riverine populations from the

463

Tapajós River Basin has been estimated at 0.9 µg kg day , considerably higher than maximum

464

dose of 0.3 µg kg day currently recommended by the US EPA.

465

fish is higher at high-water, but it is not clear whether this leads to higher fish bioaccumulation and

466

human exposition. On the other hand, the higher MeHg concentrations observed in Negro Basin,

74

However, considering that aquatic biota

-1

-1

-1

-1

75,76


The availability of MeHg to


Page 18 of 31

467

relative to the Solimões/Amazon Basin at low-water indicates greater potential exposure to MeHg

468

throught the year in the former basin.

469

24,25

The total amount of MeHg transported from rivers to ocean worldwide has been estimated -1

77

78

470

at 275±135 - 550±270 kg yr (assuming Hg input by Amos et al.

471

Considering the concentrations of MeHg and water discharge measured at our furthest downstream

472

sampling point in the Amazon River (S7 in the Figure 1) during the high- and low-waters seasons,

473

and assuming six months per season, the annual export of MeHg at this point was estimated to be

474

342 kg of MeHg yr , within the range of values estimated for all rivers. The discharge at this

475

sampling point (S7) represented approximately 60% of the total discharge of the Amazon River to

476

the ocean. If we assume similar MeHg concentrations at the Amazon mouth, total export of MeHg

477

from the entire Amazon Basin would be 570 kg yr , above the current range of values estimated for

478

all rivers. Based on this result, it is clear that the global riverine export of MeHg is currently

479

underestimated and that direct estimates of MeHg fluxes, like those presented here, will be required

480

from all major rivers to improve this value. If more Hg enters the system due to gold mining activities

481

or the hydrological and sedimentological characteristics of the rivers change due to infrastructural

482

development (e.g. the construction of dams and roads), land use change or future climate change,

483

the fluxes and patterns described here can be expected to change.

and that 5-10% is MeHg ).

-1

-1

484 485

ACKNOWLEDGMENTS

486 487

The authors thank the financial support of CNPq, FAPEAM, CAPES, INCT-INPeTAM/CNPq/MCT

488

and logistical support of INPA and ICM-BIO (license 27683-1). Thanks to J. Rocha, A. Santos, B.

489

Lima, G. Balassa and P. Barbosa for assistance with fieldwork; D. Macedo for help with spatial

490

analyses; R. Leitão for review the manuscript; C. Quesada and L. Anderson for help with soil

491

information; the staff of Laboratório de Limnologia/UFRJ, Biogeoquímica Ambiental/UNIR and

492

Radioisótopos/UFRJ for their help with analyses.

493 494

Supporting Information


Page 19 of 31


495

Figures and tables with detailed information on Spodsol occurrence in the basins, water level of the

496

rivers and its correlation with methylmercury and partial correlations between methylmercury and

497

limnological parameters (and these values).

498 499 500 501

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as mapped with JERS-1 Synthetic Aperture Radar. Wetlands 2015, 35, 745–756.

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sediment partition coefficients for mercury. Chemosphere 1997, 35, 791-808.

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chloride and methylmercuric hydroxide in pure water and salt solutions. Environ. Chem. 1991, 10,

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distribution of methylmercury in near-shore marine sediments. Environ. Sci. Technol. 2004, 38,

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(57) Bloom, N. S.; Gill, G. A.; Cappellino, S.; Dobbs, C.; McShea, L.; Driscoll, C.; Mason, R.; Rudd,

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water column and surface sediments in a contaminated area. Water Res. 2006, 40, 2893-2900.

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(59) Bone, S. E.; Charette, M. A.; Lamborg, C. H.; Gonneea, E. Has submarine groundwater

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discharge been overlooked as a source of mercury to coastal waters? Environ. Sci. Technol. 2007,

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(60) Marvin-Dipasquale, M.; Lutz, M. A.; Brigham, M. E.; Krabbenhoft, D. P.; Aiken, G. R.; Orem, W.

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H.; Hall, B. D. Mercury cycling in stream ecosystems. 2. Benthic methylmercury production and bed

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sediment-pore water partitioning. Environ. Sci. Technol. 2009, 43, 2726-2732.


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formation and sediment-water partitioning. Bull. Environ. Contam. Toxicol. 1991, 47, 743-750.

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at the sediment water interface in the Thau lagoon. 1. Partition and speciation. East. Coast. Shelf

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in various fish species from Orlík and Kamýr reservoirs in the Czech Republic. Ecotoxicol. Environ.

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Saf. 1999, 43, 231–240.

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estuarine sediment. Appl. Environ. Microbiol. 1985, 50, 498–502.

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of factors affecting methylation. Crit. Rev. Environ. Sci. Technol. 2001, 31, 241–293.

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S.; Hardy, E. R. Mixing patterns in Amazon lakes. Hydrobiologia 1984, 108, 3–15.

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conductivity and dissolved oxygen in an Amazon floodplain lake: description of a friagem

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phenomenon. Acta Limnol. Brasil. 2014, 26, 318–331.

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Diffusive methane fluxes from Negro, Solimões and Madeira rivers and fringing lakes in the Amazon

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mapping of wetland inundation and vegetation for the central Amazon basin. Rem. Sens. Environ.

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700

implications. Environ. Res. 2012, 119, 101-117.

701 702 703 704 705 706 707


Page 26 of 31

Page 27 of 31


708 709 710

Study area Brazil

South America

N 0

75

150

225

300 km

711 712

Figure 1. Sampling sites (n = 38) in the Amazon basin. N1-N4 represent points in the Negro River

713

main channel and S1-S7 represent points in the Solimões/Amazon River main channel. The

714

tributaries in the Negro Basin are: Marauia (1), Tea (2), Uneiuxi (3), Aiuanã (4), Urubaxi (5), Darahá

715

(6), Preto (7), Padauari (8), Arirahá (9), Aracá (10), Demeni (11), Cuiuni (12), Caurés (13), Jufari

716

(14), Branco (15), Unini (16), Jauperi (17), Jaú (18), Puduari (19), Apuaú (20) and Cuieiras (21).

717

The tributaries in the Solimões/Amazon Basin are: Jutaí (22), Juruá (23), Japurá (24), Purus (25),

718

Madeira (26) and Uatumã (27).

719 720 721 722 723



724 725 726 0.18 a

0.15

[MeHg] ng.L-1

ab

0.12 x

0.09 bc y

0.06

c

0.03 0.00

High

Beg_Fall

End_Fall

Low

Hydrological season

727

Hydrological season

728

Figure 2. Mean methylmercury concentrations ([MeHg]) in water of rivers from different hydrological

729

seasons from Negro (black circle with dashed line) and Solimões/Amazon (grey circle with

730

continuous line) basins. Different letters, within each basin, indicate statistical differences: a, b and

731

c for Solimões/Amazon (Friedman test with Dunn’s multiple comparisons test), and x and y for

732

Negro (Paired t test). Bars indicate 95% confidence intervals.

733 734 735 736 737 738 739 740


Page 28 of 31

Page 29 of 31


741 742

1 9 2

3

12

1

5

6

10 11

13

16 18 19

3 4

7 8

14 15 4

Low-water

17

2

1

5

6

9 12

water flow

Negro River distance (km 102)

3 4

2

13 * 16 18 19

7 8 10 11

14 15 17

Scale of tributaries 900 µg.s-1

High-water 0

5

6

743

20

20

21

21

Scale of Negro River 900 µg.s-1

744

Figure 3. Transport of methylmercury by the Negro River main stem and its main tributaries (1-21:

745

same numbers of tributaries in Figure 1) during the high-water and low-water. The width of the

746

rivers represents the amount of methylmercury transported. The scale of the Negro River main stem

747

and of the tributaries is horizontally and vertically oriented, respectively. The export was calculated

748

by multiplying of the water discharge by the methylmercury concentrations in unfiltered water. River

749

distance is related to the Negro River main stem and starts in the most upstream sampling point

750

(N1, see details in Figure 1). 13 *: Discharge not measured.

751 752 753 754 755



Page 30 of 31

756 757

22

22

23

23

23

22 *

23 3

Low-water

22

24

24

24

24

water flow

Solimões/Amazon River distance (km 102)

0

Late Falling-water

Scale of tributaries 900 µg.s-1

Early Falling-water

High-water

6

25

25

25

25

9 N

N

26

26

26

N

N

26

12 27

27 *

27 *

27

Scale of Solimões/Amazon River 900 µg.s-1

758 759

Figure 4. Transport of methylmercury by the Amazon/Solimões main stem and its main tributaries

760

(22-27: same numbers of tributaries in Figure 1, and N: Negro River corresponding to N4 sampling

761

site) during the high-water, early falling-water, late falling-water, and low-water. The width of the

762

rivers represents the amount of methylmercury transported. The scale of the Amazon/Solimões

763

main stem and of the tributaries is horizontally and vertically oriented, respectively. The export was

764

calculated by multiplying of the water discharge by the methylmercury concentrations in unfiltered

765

water. River distance is related to the Amazon/Solimões main stem and starts in the most upstream

766

sampling point (S1, see details in Figure 1). 22 *: Transport was 7,788 µg s , not plotted because it

767

is out of scale. 27 *: Discharge not measured.

-1

768 769 770 771 772


Page 31 of 31


773 774 775 0.24

a

0.21

0.21

0.18

0.18

[MeHg] ng.L-1

[MeHg] ng.L-1

0.24

0.15 0.12 0.09

0.15 0.12 0.09

0.06

0.06

0.03

0.03

0.00

0

10

20

30

0.00

40

b

0

10

%Flooded area (all basin) 0.20

0.20

c

S1

[MeHg] ng.L-1

[MeHg] ng.L-1

40

50

S2

S1

0.16

0.12

0.08

Mad

0.04

776

30

d

S2

0.16

0.00

20

%Flooded area (buffer of 100 km)

2

4

6

8

10

%Flooded area (all basin)

12

0.12

0.08

Mad

0.04

14

0.00

0

10

20

30

40

50

60

70

80

%Flooded area (buffer of 100 km)

777

Figure 5. Correlation between methylmercury concentrations ([MeHg]) in water and percentage of

778

flooded area (%Flooded area) in the total basin area (a and c) and in a 100 km buffer upstream

779

from sampling site (b and d) in the Negro (black circle with dashed line) and in the

780

Solimões/Amazon (grey circle with continuous line) basins. S1 and S2 are sampling points in the

781

Solimões main channel; Mad is the sampling point in the Madeira River (see details in Figure 1).

782


Methylmercury Modulation in Amazon Rivers ... - ACS Publications

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