Abstract
Present concentrations of mercury in large portions of San Francisco Bay (Bay), the Sacramento-San Joaquin Delta (Delta), and the Sacramento and San Joaquin rivers are high enough to warrant concern for the health of humans and wildlife. Large scale tidal wetland restoration is currently under consideration as a means of increasing populations of fish species of concern. Tidal wetland restoration activities may lead to increased concentrations of mercury in the estuarine food web and exacerbate the existing mercury problem. This paper evaluates our present ability to predict the local and regional effects of restoration actions on mercury accumulation in aquatic food webs. A sport fish consumption advisory is in place for the Bay, and an advisory is under consideration for the Delta and lower Sacramento and San Joaquin rivers. Mercury concentrations in eggs of several water bird species from the Bay have exceeded the lowest observed effect level. A variety of mercury sources, largely related to historic mercury and gold mining, is present in the watershed and has created a spatially heterogeneous distribution of mercury in the Bay-Delta Estuary. Mercury exists in the environment in a variety of forms and has a complex biogeochemical cycle. The most hazardous form, methylmercury, is produced at a relatively high rate in wetlands and newly flooded aquatic habitats. It is likely that distinct spatial variation on multiple spatial scales exists in net methylmercury production in Bay-Delta tidal wetlands, including variation within each tidal wetland, among tidal wetlands in the same region, and among tidal wetlands in different regions. Understanding this spatial variation and its underlying causes will allow environmental managers to minimize the negative effects of mercury bioaccumulation as a result of restoration activities. Actions needed to reduce the uncertainty associated with this issue include a long term, multifaceted research effort, long term monitoring on local and regional scales, and careful evaluation of individual restoration projects with regard to potential increase of food web mercury.
Highlights
Present concentrations of mercury in the San Francisco Estuary and the Sacramento and San Joaquin rivers are high enough to warrant concern for the health of humans and wildlife
Tidal wetland restoration on a large scale is being considered by CALFED and other agencies as a means of increasing populations of fish species of concern (Brown 2003)
Restoration actions that affect mercury cycling in tidal wetlands or that increase the amount of tidal wetlands in the Estuary can result in changes in methylmercury production and accumulation on a regional scale
Summary
Present concentrations of mercury in the San Francisco Estuary and the Sacramento and San Joaquin rivers are high enough to warrant concern for the health of humans and wildlife. Regional changes in water or sediment quality in the Estuary caused by large scale restoration actions will affect water and sediment quality in tidal wetlands. Restoration actions that affect mercury cycling in tidal wetlands or that increase the amount of tidal wetlands in the Estuary can result in changes in methylmercury production and accumulation on a regional scale. Past and present sources of mercury and the mercury cycle are described to provide context for understanding the possible impacts of restoration actions. This is followed by a discussion of factors influencing mercury accumulation in tidal wetlands.
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