Post on 27-Jan-2016
description
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: