Mercury Management Study: Prioritizing High-Leverage Sources & Pathways

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 >season 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 S&T • - 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?)