Mercury Strategy for the Bay-Delta Ecosystem: A Unifying Framework for Science, Adaptive Management, and Ecological Restoration

This document outlines a strategy for integrated mercury investigations linked to restoration and adaptive management of the San Francisco Bay and Sacramento-San Joaquin Delta ecosystem (termed the Bay-Delta ecosystem and defined as the combined watershed, Delta, and Bay). Ecosystem restoration and management of the Bay-Delta ecosystem are complicated by mercury contamination from historic mining sites in the Sacramento and San Joaquin river watersheds, the principal sources of fresh water for the Bay-Delta System. Mercury-enriched sediment now contaminates extensive downstream reaches of streams and rivers, adjoining floodplains, and the Bay-Delta Estuary. Concentrations of methylmercury in some resident fishes exceed 0.3 mg/kg (parts per million) wet weight, the U.S. Environmental Protection Agency's fish-tissue criterion for protecting the health of humans who consume noncommercial freshwater or estuarine fish. A challenge to scientists and managers involved with restoration of this ecosystem is to avoid increasing exposure of biota to methylmercury, the highly toxic form that readily accumulates in exposed organisms and biomagnifies to high concentrations in fish and wildlife atop aquatic food webs. It would be desirable to eventually decrease methylmercury exposure in this ecosystem to levels where fishery resources, wildlife, and human health are unaffected; however, the development of an effective approach for achieving such a goal is presently hampered by our very limited knowledge of mercury cycling in this ecosystem. The production of methylmercury via the microbial methylation of inorganic divalent mercury in the environment is a key process affecting methylmercury concentrations in biota at all trophic levels. Natural processes and human activities – possibly including ecosystem restoration projects – that alter the net production of methylmercury (i.e., methylation minus demethylation) can influence the abundance of methylmercury in the ecosystem and the associated exposure of resident biota and humans who consume fish and other aquatic biota from the ecosystem. The strategy provides guidance to the California Bay Delta Authority's Ecosystem Restoration Program, which is supporting ecological restoration of the mercury-contaminated Bay-Delta ecosystem. The overall goals outlined in the strategic plan for the Ecosystem Restoration Program for the Bay-Delta System are (1) to assist and recover at-risk native species, (2) to rehabilitate the Bay-Delta to support native aquatic and terrestrial biotic communities, (3) to maintain or enhance selected species for harvest, (4) to protect and restore functional habitat for both ecological and public values, (5) to prevent the establishment of additional non-native species, and (6) to improve or maintain water and sediment quality. …