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 recent phenomenon 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 be having 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 WCA 2A WCA 1 Everglades National Park EAA Anthropogenic changes since 1900 have drastically altered the Everglades ecosystem. Concentrations of methylmercury in fish and other biota have increased at least 5x. How will the $8 billion restoration balance competing factors, including water quality issues like methylmercury, to achieve a successful outcome?

The Mercury Cycle Hg(II) Hg 0 Microbial methylation Bacteria Bioaccumulation Hg 0, Hg(II) Local and Long-Distance Emissions Hg deposition Sulfate from Everglades Agricultural Area (EAA) runoff Sulfate Sulfide MeHg Hg Anoxic Sediments Agricultural sulfur use >1.5 ppm MeHg = ppm in surface water

F1 U3 Lox 3A15 2BS TS9 TS7 ENR MeHg distributions in the Everglades are highest in the middle (Goldilocks Area).

> mg/L Sulfate Loading from the EAA ~ mg/L ~2-10 mg/L > 100’s mg/L NS ~50 ~25 DOC gradient (mg.L) North-South gradient in sulfate and DOC. Sulfate originates from agricultural runoff.

Sulfate-MeHg Response Sulfate Loading  Methylmercury  Sulfide Inhibition Zone Sulfate Limitation Zone 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 Optimum MeHg Production (Goldilocks Area)

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)

Persistence of MeHg Production After Rewet (June 1999) Following Drought Long-term average

Driving factors for the summer spikes: SO 4 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), SO 4, 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 SO 4 2- 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 Ambient MeHg Spike MeHg

Spike only  (net accumulated vs control)  Spike + DOC (net accumulated vs control) Bioaccumulation results:

 SO 4 only (net accumulated vs control) Spike + SO 4  (net accumulated vs control) Bioaccumulation results:

Sulfate Time Series

Everglades MeHg Cycle dry down rewet accelerated Methylation and Release of new Hg, & SO 4 (internal) relaxation Fxn: external Hg, SO 4 and DOC

Summary: MeHg and Restoration vMeHg production in the Everglades is a function of small scale processes that are driven by large scale land-use and air-emission practices. vEcosystem-scale and baseline-level MeHg distributions are driven by external driving factors of sulfate and DOC. vStrong spring time MeHg pulses are driven by natural and unnatural dry-down and rewet periods vMesocosm results show that Hg, SO4, and DOC additions all lead to new MeHg production v0.5-2 year doses of Hg are undetectable in food webs 1 year after dosing.

Summary continued: vDOC and SO4 are equally efficient at producing new MeHg…however, DOC additions appeared to inhibit bioaccumulation of the new MeHg vEcosystem scale changes in SO4 and DOC flows related to recent flow rerouting have lead to substantial changes in MeHg at long-term study sites. vGodilocks 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: