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Published bySabrina Veiga Zagalo Modified over 6 years ago
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Methylmercury and Mercury in SF Bay & Wetlands
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What- MeHg Worry? Most bioaccumulative form of mercury…
Formed by sulfate reducing bacteria under anoxic conditions But is subject to degradation- Microbial Abiotic (photolytic)
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More MeHg Worries? Horvat et al., MeHg higher collected under N2- why? Less degradation? Or more production? (~4 hrs field to lab)
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Random Number Generator v1
Puckett & Bloom 2000 Calfed study Samples frozen in field vs overnight chilled shipping No correlation in MeHg analyses of samples generate MeHg degrade MeHg
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SF Bay MeHg (Calfed) Heim et al (2003)
Samples collected October to December 1999 Frozen in field 0-0.5cm sediments Primarily bay margins MeHg mostly < 1ng/g dry weight
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What (Not) to Do? Yes No Freeze, ASAP Minimal thawing before analysis
repeated freeze-thaw (Horvat et al 1992) Separate small samples as replicates processing in air (but maybe not an issue if frozen quickly)?
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RMP Methodology (& Mistakes?)
RMP ~50 stratified random sites van Veen grab, top 5 cm Composite of 2-3 grabs Homogenized on board (In air) ~40 minutes, 1st grab to last jar in freezer Analyzed months to years later
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MeHg for RMP July & August 2002- 2004 (2002-3 shown here)
0-5cm sediments Mix of shallow & deep sites MeHg range ~0-2ng/g dry weight Hg, TOC, TN, grainsize, Redox starting 2003
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Hg & MeHg – Random Number Generator v2?
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Hg Necessary, Not Sufficient
Literature correlations over many orders of magnitude 95% of RMP Hg within 10x ( mg/kg) [MeHg] = +production –degradation -uptake ±transport Need reducing conditions for methylating bacteria Other important parameters? TOC, TN, grainsize, Eh (redox potential)
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Dominant Factors: TN, Redox
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Redox Does Matter
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TOC and MeHg
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(Total) Nitrogen TOC?
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Why TN and ORP? TN = biologically active organic matter (TOC includes refractory material) Low ORP = anoxia, measure of net status Metal sulfides (black) visually apparent Field meter measurement
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Conclusions Surface (0-5cm) sediment MeHg similar range to previously found 0-0.5cm concentrations MeHg poorly/un-correlated to total Hg, TOC MeHg better correlated to TN, Redox Consistent w/ indicators of active microbes Relationships match expectations despite possibly compromised handling
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What Can Be Done? Reduce reducing environments? Decrease sediment Hg?
Prevent eutrophication & anoxia May run counter to other ecosystem goals (Bay & wetland productivity & habitat) Decrease sediment Hg? May need total Hg <0.2 mg/kg to see impact on MeHg
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Future Directions RMP Sampling Petaluma River Wetlands CBDA
Schollenberger Park Toy Marsh Greenpoint Water treatment plant Petaluma Marsh South Petaluma Marsh North Carl’s Marsh Black John Slough Future Directions RMP Sampling Unmixed site replicates Rapidly homogenized splits Sediment Hg speciation Petaluma River Wetlands CBDA MeHg processes in fresh/brackish to saline tidal marshes Food web studies in channel/low marsh, and high marsh plain SFEI, Avocet Assoc., USGS (MP, Wi, BRD)
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Hg ~Constant Among Wetlands
Similar sources- air deposition, tidally mixed waters
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Sediment MeHg Varies Among Wetlands
MeHg produced within wetlands- mid salinity maximum?
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Black Rail MeHg Varies Among Wetlands
Sediment invertebrates birds
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