Mercury Strategy for the Bay-Delta Ecosystem - CALFED Bay-Delta ...

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(4) To estimate the cumulative contribution of restored wetland and floodplain habitats to the
total methylmercury budget for the Delta and Bay. The internal, systemic production of
methylmercury in all restored areas should be estimated and compared to the external
methylmercury budget for the Bay-Delta System. Wetlands may be important sites of
methylation, but do they contribute significantly to methylmercury budgets at the scale of the
whole ecosystem? This effort should be an iterative process, and estimates should be refined as
quantification of external and internal methylmercury production improves.
Some restoration activities (channel reconstruction and dam removal) and remedial efforts
(reduction in loadings of mercury and fine sediment) are expected to affect the transport and
distribution of sediment and associated (mostly inorganic) mercury. This core component,
however, does not emphasize the effects of restoration on the distribution and transport of total
mercury in the ecosystem. Load reduction is addressed in core components 1 and 2, and
potential effects of dam removal are being addressed in CALFED investigations in the Upper
Yuba River Studies Program.
With regard to evaluating potential effects of ecosystem restoration on mercury cycling, we
recommend that highest priority be given to investigations examining effects of restoration on
(1) the bioavailability of inorganic mercury for methylation and (2) the microbial production of
methylmercury. Mercury contamination of aquatic environments is widespread in the Bay-Delta
ecosystem. We believe that changes in bioavailability or methylation rates have much greater
potential to significantly increase methylmercury exposure in this ecosystem than do changes in
the spatial distribution of total (mostly inorganic) mercury. Studies in other aquatic ecosystems
show that experimental stimulation of methylation can increase the abundance of methylmercury
and its uptake in biota by 10- to 20-fold, even in lightly contaminated environments where no
mercury was added (Kelly et al 1997, Bodaly et al. 2002).
4. Monitoring of Mercury in Fish, Health-Risk Assessment, and Risk Communication
The consumption of fish and other aquatic organisms is the primary pathway for human exposure
to methylmercury. A regional program for monitoring mercury concentrations in fish should,
therefore, be in effect during ecosystem restoration. The first management goal of the
monitoring program would be to protect human health by providing informed guidance for
reducing dietary exposure to methylmercury, the dominant form of mercury in fish. The second
management goal of the monitoring program would be to provide a “performance measure” to
gage methylmercury contamination of the Bay-Delta ecosystem.
Goal 1, the protection of human health, should include the following objectives.
(1) To monitor concentrations of total mercury (present largely as methylmercury) in sport fish
eaten by humans. Monitoring should identify fish, shellfish, and other aquatic biota consumed
by humans that contain mercury concentrations exceeding criteria for protection of human
health. Monitoring should also identify fish with low concentrations that can be safely eaten.
(2) To assess health risks of fish consumption to humans. This objective would be facilitated by
the development of an effective data management system for storage and retrieval of data on
mercury in fish, shellfish, and other edible aquatic biota.
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