U.S. Department of the Interior

U.S. Geological Survey

Mercury and Selenium in the Colorado 

River Food Web, Grand Canyon 

D.M. Walters, E. Rosi-Marshall, T.A. Kennedy, W.F. Cross, 

and C.V. Baxter
Key Findings

· Mercury (Hg) and selenium (Se) concentrations are relatively high in the 

Colorado River food web throughout Grand Canyon. 

· Hg and Se in organic matter, algae, invertebrates, and fish exceed 

protective thresholds for fish, wildlife, and humans. 

· Hg concentrations were low in rainbow trout in Glen Canyon, so human 

health risks associated with this popular sport fishery are low. 

· However, Hg in trout was higher downstream. Some of these areas are 

targeted for trout removal, which are provided for human consumption. 

· Managing exposure risks in Grand Canyon will be challenging, because 

sources of Hg and Se are beyond Park boundaries. 
The Colorado River in Grand Canyon is an 

incredibly remote, but heavily altered ecosystem. 
Glen Canyon Dam fundamentally 

changed the river’s physical template

Water is 1000X 

clearer

Filamentous algae

Voichick and Topping 2014
Glen Canyon Dam fundamentally 

changed the river’s physical template

Cold and constant water temperatures

Voichick and Wright 2006
Altered Foodbase

Scuds

New Zealand 

mudsnails

Black flies

The dominant aquatic invertebrates in Glen and Grand Canyon

are non-native. 
?

Humpback Chub

Losses

Razorback Sucker

Roundtail Chub

Bonytail Chub

Colorado Pikeminnow

Remaining Native Fishes

Flannelmouth Sucker

Bluehead Sucker

Speckled Dace

Rainbow Trout

Brown Trout

Common Carp

Fathead Minnow

Plains Killifish

Our new friends

Many invasive fish species. 
On top of all this,

chemical alteration? 

Hg and Se data missing from Grand Canyon

piscivorous bird

piscivorous mammal

fish

piscivorous wildlife

larval fish
Why should we care about Hg and Se?

· Effects of Hg

· Neurotoxin (cognition and behavior)

· Lower reproductive success, growth, and survival

· Effects of Se

· Deformities of embryo/larvae

· Lower reproductive success, growth, and survival

· Hg –protective thresholds thoroughly vetted

· Se –protective thresholds are preliminary
Objectives

· Sample Hg and Se along upstream-downstream gradient and 

throughout entire food web

· Compare Hg and Se concentrations to protective threshold to 

determine risk to humans and wildlife

· Examine potential sources
Approach

· Six sites (river mile 0, 30, 63, 125, 165, 225)

· Hg and Se concentrations – 1 season (Summer 

2008) 

· ~4 replicate samples of dominant types of 

organic matter, macroinvertebrates, and fish per 

site

· Endangered humpback chub not sampled
Hg and Se concentrations are relatively high throughout Grand Canyon, 

with no clear downstream patterns.

Cladophora Epilithon Epiphyton FBOM Seston

Hg μg g-1

0.0

0.1

0.2

0.3

0.4

0.5 RKM 0 

RKM 48

RKM 100 

RKM 204 

RKM 264

RKM 362

Chironomid Gammarus mudsnail Simuliid Lumbricid

Hg μg g 0.000

0.025

0.050

0.075

0.500

1.000

1.500

2.000

2.500

BHS CRP FHM FMS RBT SPD

Hg μg g-1

0.0

1.0

2.0

3.0

4.0

Hg (μg g-1) Hg (μg g-1) Hg (μg g-1)

Cladophora Epilithon Epiphyton FBOM Seston

Se μg g-1

0

1

2

3

4 RKM 0

RKM 48

RKM 100

RKM 204

RKM 264

RKM 366

Chironomid Gammarus mudsnail Simuliid Lumbricid

Se μg g-1

0.00

0.05

4.00

8.00

12.00

16.00

20.00

BHS CRP FHM FMS RBT SPD

Se μg g-1

0

1

2

3

4

5

6

Se (μg g-1) Se (μg g-1) Se (μg g-1)

RM 0

RM 30

RM 63

RM 127

RM 166

RM 225

RM 0

RM 30

RM 63

RM 127

RM 166

RM 225

Walters et al. 2015
Hg ug/g

0 1 2 3 4

Cladophora (24)

epilithon (23)

FBOM (22)

epiphyton (19)

seston (23)

mudsnail (3)

chironomid (14)

lumbricid (3)

Gammarus (7)

Simuliid (24)

common carp (17)

rainbow trout (20)

bluehead sucker (13)

flannelmouth sucker (21)

fathead minnow (16)

speckled dace (16)

 

 

 

A

B Hg (μg g-1)

fish

piscivorous mammal

kingfisher

human

Dietary risk thresholds

Hg concentrations among sites

Hg concentrations commonly exceed protective thresholds

Walters et al. 2015
Se ug/g

0 2 4 6 8 10 12 14

Cladophora (19)

epilithon (24)

FBOM (22)

epiphyton (19)

seston (23)

mudsnail (3)

chironomid (13)

lumbricid (3)

Gammarus (8)

simuliid (23)

common carp (17)

rainbow trout (20)

bluehead sucker (13)

flannelmouth sucker (22)

fathead minnow (16)

speckled dace (16)

 

Se (μg g-1)

larval fish

piscivorous wildlife

Dietary and maternal risk thresholds

Se concentrations among sites

Se concentrations commonly exceed protective thresholds

Walters et al. 2015
Where does Hg come from?

Humans have greatly increased atmospheric Hg, 

contaminating remote & protected areas. 

“...human activities have increased 

atmospheric Hg concentrations 

3- to 5-fold during the past 150 years.”

“...[human activities] have resulted in some 

of the most remote and protected areas of 

the world becoming contaminated with 

Hg...”

Eagles-Smith and others 2014. 
Where does Hg in Grand Canyon come from?

Atmospheric sources of Hg to Grand 

Canyon are likely a mixture of distant 

and nearbye sources.

Emissions from Navajo Generating 

Station are deposited in Grand Canyon 

and Lake Powell.
Seiler et al. 1999

Where does Se come from?

• Se is high where:

– marine sedimentary 

geology

– high evaporation index

– irrigation

areas susceptible to contamination

irrigated land
Se data 1985-94: Engberg,1999

How much Se enters Glen Canyon?

Glen Canyon

83 kg d-1

29.9 MT y-1

Lake Powell

100 kg d-1

Green R. 

27%

Gunnison R. 31%

Grand Valley Pr. 

30%

San Juan R. 8%

irrigation 71%

natural weathering 21%

point source 8%
Hg and Se Transport Model

Se & Hg 

Hg

inorganic and organic

Se & Hg 

organic Se & Hg

(bioavailable and toxic) 

anaerobic conditions

microbial activity

inorganic Se & Hg 

Lake Powell likely plays a key role 

in Hg and Se cycling and transport

Hg/Se cycling
Key Findings

· Mercury (Hg) and selenium (Se) concentrations are relatively high in the 

Colorado River food web throughout Grand Canyon. 

· Hg and Se in organic matter, algae, invertebrates, and fish exceed 

protective thresholds for fish, wildlife, and humans. 

· Hg concentrations were low in rainbow trout in Glen Canyon, so human 

health risks associated with this popular sport fishery are low. 

· However, Hg in trout was higher downstream. Some of these areas are 

targeted for trout removal, which are provided for human consumption. 

· Managing exposure risks in Grand Canyon will be challenging, because 

sources of Hg and Se are beyond Park boundaries. 
Acknowledgements

· Funding

· US EPA 

· USGS