Mercury in the San Francisco Estuary - Reviews of Environmental Contamination and Toxicology - page 46

Some of the management questions identified by the conceptual model have
highlighted additional field studies required to fill current gaps in our knowledge
and understanding (Table 5). Although extensive monitoring programs in the estu-
ary provide a synoptic view of conditions (Hoenicke et al. 2003; Thompson et al.
2000), detailed information on areas where methylation rates and MMHg flux are
highest is not currently available. If these could be identified, then they could
either be targeted for restoration directly or managed such that they do not expand.
Additionally, this information is needed to guide and evaluate the relative effec-
tiveness of alternative corrective actions. Some of this information is becoming
available as several large multi-year field projects are nearing completion in the
region (e.g., Marvin-DiPasquale et al. 2005; Schwarzbach et al. 2005; Yee et al.
2005).
The implementation plan for mercury management developed by the
SFRWQCB (2006) must also address the anticipated changes in and around the
estuary. For example, the planned conversion of salt ponds to wetlands surrounding
the estuary will restore unique habitat for biota, particularly for wading birds, and
nursery grounds for many species (Goals Project 1999). However, as wetlands are
hotspots of mercury methylation, there is a trade-off between the beneficial
functions of wetlands and the environmental risk of increased MMHg production
and export to the estuary. It is estimated that the response time of concentrations
to changes in mercury loading to the estuary is several decades (MacLeod et al.
2005). To determine whether restoration actions over this time frame are achiev-
ing any benefits, new and effective monitoring strategies are required to identify
locations and processes where changes are anticipated to occur over much shorter
time frames.
Summary
This review presents some of the published and other important literature on
mercury contamination in San Francisco Estuary. Studies on human consumption
of contaminated sportfish and on detecting ecological impacts of this contamina-
tion in wetland areas validate concerns regarding mercury's toxicity in this sys-
tem. Mining, industrial, and environmental uses of mercury have occurred for
more than a century, resulting in its large historic and continuing transport to the
estuary. Consequently, there is a widespread distribution in the estuary, but more
work is needed to show its relative chemical and biological availability from
these sources. The uptake of mercury in the estuary has been shown in phyto-
plankton, but studies on biomagnification in local food webs have yet to draw a
clear path to impairment in sportfish and waterbirds. In light of these concerns of
impairment and the need for further information, large restoration activities
planned for the estuary will require new technical approaches to solve important
management questions, such as the location of key areas of methylmercury
production.