Hg Process Study Options

1
Hg Process Study Options

• RMP CFWG
• September 14, 2007

2
RMP Mercury Management Questions

• Where is mercury entering the food web?
• Which sources, processes, and pathways contribute
  disproportionately to food web accumulation?
• Can we do anything about these high-leverage
  processes, sources, and pathways?
• What effects can be expected from management
  actions?
• Will total mercury reductions result in reduced
  food web accumulation?

3
Which sources, processes, and pathways contribute
disproportionately to food web accumulation?

• Sources- is all Hg ultimately bioavailable (for
  methylation, uptake)?
• Processes- what (co)factors affect transport,
  transformation, uptake (what spatial and temporal
  scales of interest)?
• Bioaccumulation- biggest jumps at lowest trophic
  levels (primary producer/consumer)

4
Can we do anything about these high-leverage
sources, processes, and pathways?

• Sources- Reduce tot Hg? Selected reactive
  souces? MeHg inputs?
• Processes- Adjust (co)factors (aeration,
  flushing, nutrient loading)
• Bioaccumulation- choose species, habitat design?
  (likely least flexibility?)

5
How to prioritize?

• MeHg mass budget as tool to ID gaps
• Large uncertainties, not very quantitative
• Illustrating sensitivity to uncertainties
• Large factors, large uncertainties first
• Portion of Hg convertible to MeHg
• MeHg sources- in situ production, sed-water
  exchange, tributary loads,
• Loss pathways- degradation, export

6
What Might We Try

• Sources- is all Hg ultimately bioavailable (for
  methylation, uptake)?
• Is availability more important than total load?
• Uptake- sufficiently established that MeHg
  concentration is most directly correlated to
  biota?
• Slotton pelagic fish and water column MeHg
• Petaluma high marsh sediment and inverts
• MeHg in all sources (needed for MeHg mass budget)
• Bioavailability for methylation
• Timescale of relevance- season/year/decade?

7
Hg Sources Availability
8
Hg Source Availability Approaches

• Reactive Hg measures (sequential extractions,
  SnCl2 reductions)
• straightforward, feasible
• - how relevant to field conditions, microbes?
• Incubation experiments with various sources
• uses microbes, can try to mirror field WQ
  measures
• - different strains, differences from field,
  difficulty in maintaining, interpreting,
  timescale gtseason hard
• Combination (correlational)
• Reactive Hg measures incubation kinetics
  correlation to MeHg (Marvin-DiPasquale approach)

9
Hg Source Availability Approaches

• Hg CM (conceptual model)- Field incubation
  experiments with lower Hg sediment sources
• mirrors possible mgmt option (capping)
• - preventing cap loss, mixing, controlling for
  temporal variation of various factors
• Point source near-field transects
• real world effect
• - might be other cofactors (TOC, hydrology)

10
Hg Transport Factors

• Sed water exchange large uncertainty
• Non turbulent exchange- bioturbation, diffusion
  (Kuwabara, Gill flux boxes)
• feasible
• - Likely site specific, but constrains bounds?
• Turbulent exchange- resuspension, advective
• - ?? Methods?? porewater measurements
  assumptions ??
• Also likely site specific (hydrology, sediment
  quality, porewater MeHg)

11
Hg Transformation Factors

• Axis of methylation C (N?) SO4 Hg ( O2)
• SF Bay, plenty of Hg (bioavailable?), SO4
• Hg CM priority- Characterize Bay Sediment
  Profiles of salinity, DO, TOC, SO4, S2, MeHg
• Organic matter supply limiting methylation?
• Measure correlate w/ MeHg
• Incubations with more bacterial food
• isolate limiting factor
• - relevance to field,
• ? usability as control option (nutrient
  controls?)

12
Hg Transformation Factors

• O2 limiting factor?
• Measure DO/ORP correlate w/ MeHg
• Already component of RMP ST
• - ORP a crude measure of net redox balance
• Incubations with more/less O2
• outcome likely predictable
• - usability as control option- only applicable on
  small scales- ?? Undesired consequences ??
• Control options ID/test already part of LFR/UCSC
  prop 13 grant

13
Hg Transformation Factors

• Hg CM suggests testing water column MeHg
  production via incubations
• might be needed to close bounds on MeHg mass
  balance
• ?? Is this necessary or high priority (few Bay
  surface waters anoxic, little stratification like
  in lakes) ?

14
Hg Uptake Factors

• Need to follow trophic link back to right
  starting matrix
• Field approaches, correlations
• field conditions
• - dynamic, episodic (time scale), confounding
  factors
• Incubations
• control of factors
• - relevance to field
• Time scale of interest
• Do we care about daily scale variation- week?

15
Hg Uptake Factors

• Low trophic resident biota integrators
• directly relevant to food web exposure
• - life cycle, growth dilution, depuration
  confounding
• SPMDs as integrator surrogates?
• more stable predictable response
• - still need to correlate back to biotic uptake

16
Hg Integrated Measures

• Hg stable isotopes- assumes distinct source
  signatures and predictable process fractionation
• not subject to incubation artifacts, may be
  able to eliminate/establish significance of some
  sources, processes.
• - isotope signature highly source specific, but
  not knowable until actually measured.
  Differences might be too small to track across a
  mix of sources

17
Hg Integrated Measures

• Other tracers?

18
Priorities?

• Find characterize methylation hotspots?
• Control methylation hotspots?
• Characterize range (random?) of methylation
  zones?
• Control range of methylation zones?
• Deduce contribution of individual sources (to
  methylation/accumulation)?
• (assign of effort?)