The Aquatic Cycling of Mercury in the Everglades

(ACME) Project: Integrated Research Providing Information for

Management and Science

Authors:William Orem (USGS-Reston, VA), David Krabbenhoft (USGS-Middleton, WI),Cynthia Gilmour (Smithsonian Environmental Research Center-Edgewater, MD)George Aiken (USGS-Boulder, CO)

View of Everglades National Park

The Problem

•Very high levels of methylmercury (MeHg) have been found in fish, wading birds, alligators, and many mammals throughout the Everglades.•At least one Florida panther (an endagered species) has died due to MeHg intoxication. MeHg may act as an endocrine disruptor in fish, leading to declines in fish populations through effects on fecundity. Wading bird populations in the Everglades have declined by 90% since 1900, and MeHg may be one factor in this decline.

•Advisories limiting human consumption of fish have been posted throughout the ecosystem due to high MeHg concentrations, generally exceeding the 1.5 ppm criterion. MeHg poses a particular threat to unborn children.

Questions

•Why does the Everglades have such a large MeHg problem?Answer – This talk will address this question. A number of factors contribute to high levels of MeHg in the Everglades, including high Hg deposition, favorable environmental conditions, other contaminants (notably sulfur), and favorable food web dynamics.

•Was MeHg always high in the Everglades, or is this a recentphenomenon connected to anthropogenic activities?Answer – Studies have shown that current levels of MeHg in the Everglades are at least 5x as high as those in the late 1800’s. Increased anthropogenic inputs of Hg to the atmosphere, and changes to the Everglades ecosystem, both likely contribute to the MeHg problem in the Everglades.•What can be done to correct the MeHg Problem in the Everglades?Answer – Mitigation of local sources of Hg emissions may already behaving an impact on MeHg in Everglades’ fish, but because the problem is complex a multifaceted approach to restoration is likely needed. Management of sulfur contamination is also critical.

The Florida Everglades: Then and Now

Pre 1900’s Current

WCA 3A

WCA2A

WCA1

EvergladesNational

Park

EAA

Anthropogenic changes since 1900 have drastically alteredthe Everglades ecosystem. Concentrations of methylmercuryin fish and other biota have increased at least 5x. How willthe $8 billion restoration balance competing factors,including water quality issues like methylmercury, to achievea successful outcome?

The Mercury CycleHg(II) Hg0

Microbial methylation

Bacteria Bioaccumulation

Hg0, Hg(II)Local and

Long-DistanceEmissions

Hg deposition

Sulfate from Everglades

Agricultural Area (EAA) runoff

Sulfate

Sulfide MeHg

Hg

AnoxicSedimen

ts

Agricultural sulfur use

>1.5 ppmMeHg = 10-7 ppmin surface water

Summer averages

Hg

, n

g/g

dw

0

100

200

300

Me

Hg

, n

g/g

dw

0

3

6

% M

eH

g

0

3

6

9

me

thyl

atio

n,

pe

r d

0.00

0.03

0.06

EN

R F1

U3

2B

S

3A

15

TS

-7

TS

-9

Lox

me

thyl

atio

n,

ng

/g d

0

4

8

12

16

F1U3

Lox

3A15

2BS

TS9

TS7

ENR

MeHg distributionsin the Everglades are highest in the middle (GoldilocksArea).

>0.5 -2 mg/L

Sulfate Loading from the EAA

~50-150 mg/L

~2-10 mg/L

> 100’s mg/L

NS

~50~25

DOC gradient (mg.L)

North-Southgradient in sulfateand DOC. Sulfateoriginates fromagricultural runoff.

Sulfate-MeHg Response

Sulfate Loading

Meth

ylm

erc

ury

Sulfide Inhibition

Zone

SulfateLimitationZone

Sulfate stimulates MeHg production, but buildup of sulfide (a byproduct of sulfate reduction) inhibits MeHg production. Thus, maximum MeHg production occurs at intermediate levels of sulfate (Goldilocks Area) where things are “just right”.

Zone of OptimumMeHg Production(Goldilocks Area)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

12/1

3/95

03/2

7/96

06/0

7/96

08/1

6/96

12/0

4/96

04/2

1/97

07/11

/97

01/1

4/98

06/2

9/98

07/1

9/99

05/1

0/00

07/1

0/00

09/2

5/00

08/0

1/01

11/2

9/01

12/0

4/01

01/0

9/02

02/0

6/02

08/1

8/03

11/1

7/03

Tota

l Hg

(n

g/L

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Me

Hg

(n

g/L

)

MeHg

Total Hg

HgT and MeHg Time Series

‘95 MeHg/Hg = 0.50; ’03 = 0.27

Linked hydrologic and MeHg Production Cycle

Inundation:net methylation & bioaccumulation(June-February)

Dry down and internal SO4 & labile C production (March)

Rewetting,anoxic conditions reset, onset of methylation (April-May)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

F1 U3 2A 3A

Fil.

Me

Hg

(n

g/L

)

7/20/1999

8/23/1999

10/13/1999

11/18/1999

Persistence of MeHg Production After Rewet

(June 1999) Following 1998-1999 Drought

Long-term average

Driving factors for the summer spikes:

•SO4 liberation – oxidation of sedimentary organic sulfur and monosulfides

•Near surface (top 10 cm) Eh downward shift ~100 mV

•Hg increase ~1.5X, new atm. Hg or liberation from oxidized sediments

•No detectable changes in pH, dissolved organic carbon (DOC), or total Hg (could have been a DOC quality shift)

Hg (new),SO4,DOC

MeHg

Mesocosms used to test effects of sulfur, “new” mercury, and DOC on MeHg production and bioaccumulation

Over 88 mesocosms at primary site

spiked into mesos

•Hg isotopes added in increments of the average annual loading rate (22 ug/m2): 0.5x, 1.0x, 2.0x in successive annual experiments with differing isotopes

•SO42- added to achieve

both ends of the Goldilocks regime

•DOC added to increase ambient levels by 50% and 100% using native Hydrophobic Organic Acids

Surface Water Results

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Am

b.

Me

Hg

(n

g/L

)

0

0.2

0.4

0.6

0.8

1

1.2

Control

SO4 lo

w

SO4 m

ed

SO4 hig

h

Hg low

Hg med

Hg hig

hDOC

DOC-Hg lo

w

DOC-Hg h

igh

SO4+Hg lo

w

SO4+Hg m

ed

SO4+Hg h

igh

Me

20

1H

gT

(n

g/L

)

Ambient MeHg

Spike MeHg

Spike only

(net accumulated vs control)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Hg L Hg M Hg H

Hg

Bio

ac

cu

mu

late

d (

ng

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Sp

ike

/Am

b. R

ati

o

Ambient

Spike

Fxn Spike

Spike + DOC

(net accumulated vs control)

0

1

2

3

4

5

6

7

8

DOC DOC+Hg L DOC+Hg H

Hg

Bio

ac

cu

mu

late

d (

ng

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Sp

ike

/Am

b. R

ati

o

Ambient

Spike

Fxn Spike

Bioaccumulation results:

SO4 only

(net accumulated vs control)

0

5

10

15

20

25

30

35

40

45

SO4 L SO4 M SO4 H

Hg

Bio

ac

cu

mu

late

d (

ng

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Sp

ike

/Am

b. R

ati

o

Ambient

Spike

Fxn Spike

0

5

10

15

20

25

30

35

40

45

SO4 + Hg L SO4 + Hg M SO4 + Hg H

Hg

Bio

ac

cu

mu

late

d (

ng

)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Sp

ike

/Am

b. R

ati

o

Ambient

Spike

Fxn Spike

Spike + SO4

(net accumulated vs control)

Bioaccumulation results:

Sulfate Time Series

y = -1.0821Ln(x) + 3.6817

R2 = 0.7073

0

1

2

3

4

5

6

3/15

/1995

13-D

ec-9

5

27-M

ar-9

6

7-Ju

n-96

4-Dec

-96

21-A

pr-97

11-J

ul-97

14-J

an-9

8

29-J

un-98

19-J

ul-99

10-M

ay-0

0

10-J

ul-00

25-S

ep-0

0

1-Aug-0

1

29-N

ov-01

4-Dec

-01

9-Ja

n-02

6-Feb

-02

6/10

/2003

08/18

/03

9/15

/2003

11/17

/03

SO

4 (m

g/L

)

Everglades MeHg Cycle

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7 8 9 10

Month

Me

Hg

(n

g/L

)

dry down

rewet

acceleratedMethylation andRelease of new Hg, & SO4

(internal)relaxation

Fxn: external Hg, SO4 and DOC

Summary: MeHg and RestorationSummary: MeHg and Restoration

MeHg production in the Everglades is a function of small scale processes that are driven by large scale land-use and air-emission practices.

Ecosystem-scale and baseline-level MeHg distributions are driven by external driving factors of sulfate and DOC.

Strong spring time MeHg pulses are driven by natural and unnatural dry-down and rewet periods

Mesocosm results show that Hg, SO4, and DOC additions all lead to new MeHg production

0.5-2 year doses of Hg are undetectable in food webs 1 year after dosing.

Summary continued:

DOC and SO4 are equally efficient at producing new MeHg…however, DOC additions appeared to inhibit bioaccumulation of the new MeHg

Ecosystem scale changes in SO4 and DOC flows related to recent flow rerouting have lead to substantial changes in MeHg at long-term study sites.

Godilocks picked up and left, but where did she move? How will restoration impact magnitude and distribution of MeHg in the ecosystem?

Acknowledgements:

Funding provided by:•The USGS Priority Ecosystems Program•USEPA STAR Program•Florida DEP

Additional logistical support from: