San Francisco Bay Wetlands Priority Ecosystem Studies

Science Center Objects

The U.S. Fish & Wildlife Service's (USFWS) San Francisco Bay National Wildlife Refuge manages wildlife at a number of salt ponds that the Cargill Corporation currently operates for salt production. A number of these ponds probably will be purchased by the USFWS. With acquisition, the USFWS will be responsible for maintenance or restoration of thousands of hectares of wetlands, an extremely complex and expensive task.

Map of the San Francisco Estuary

Map of the San Francisco Bay

The USFWS and conservation organizations have supported conversion of salt ponds and other bay lands to tidal wetlands to benefit species of concern. However, no guidelines, model, or management strategies for such conversions exist. Also, the USFWS recognizes that artificial salt evaporation pond systems have become integral habitat for wildlife in the estuary during the past century and currently support diverse and unique communities of migratory birds, invertebrates, and fishes. Projections for wetland restoration from a multi-agency team suggest that only a few hundred hectares of the more than ten thousand hectares of salt ponds throughout the estuary will likely remain during the next century. The remaining ponds probably will be converted or will return to tidal marsh once salt production is terminated. The potential implications of changes to the existing structure of ponds to the thousands of migratory birds that currently use them are unknown. Presently, we have a limited understanding of obligatory versus opportunistic use of the ponds by migratory birds. Historical use of natural ponded areas (for example, vernal pools) by migratory water birds probably was similar to current use of commercial salt ponds. The ponds provide for roosting during high winds or tides on the open Bay, and blooms of invertebrates that supplement the open Bay avian prey base, for example, brine shrimp that are not found in Bay waters. Most natural ponds or pools have been destroyed by urban development or agriculture.

The prevailing consensus is to convert available land to tidal marsh to replace that lost to human encroachment. This consensus is driven largely by the concern for endangered species, but does not account for the possible obligatory use of salt ponds by migratory birds. A number of concerns face resource agencies responsible for managing or converting salt ponds to tidal marsh, for example, lack of scientific guidelines for conversion or management, deteriorating water-exchange capability, toxic hyper-saline water, levee integrity, invasive species, and so on.

These continuing studies will provide resource managers with a comprehensive assessment of the ecology of the San Francisco Bay salt ponds. This information will assist our client agency, the Fish and Wildlife Service, in choosing optimal management strategies for salt pond restoration and management that will maximize benefits to wildlife in south San Francisco Bay.

We are studying specific North Bay salt ponds, also called the Napa-Sonoma salt ponds, to develop background and guidance for possible conversion. Our initial findings of these ponds indicate significant avian use and conditions (for example, habitat quality, and prey abundance) that benefit migratory birds as well as unique invertebrate populations that are important forage for migratory birds. Salinity and depth seem to play an important role in invertebrate assemblage structure and subsequently avian use at different ponds. Pond 2A has breached levees that influenced heavily vegetated habitat. This pond supports far fewer migratory birds than the other ponds studied. A shortcoming of the Napa-Sonoma pond work was the inability to replicate sampling, that is, each pond studied was physically and biologically unique. Study of salt ponds in other regions of the Bay might allow replication of North Bay research, and facilitate interpretation of results and inference derived from the Napa-Sonoma study. Salinity and water surface elevation data collected monthly (February 1999 ? Present) on the Napa-Sonoma salt ponds are being used to calibrate a hydrological Salt Pond Box Model, known as SPOOM. SPOOM uses individual pond bathymetry, rainfall, evaporation, and water transfers to calculate daily pond volume and salinity values using the conservation of mass principle. Preliminary model output for ponds 3, 4, and 7 match the observed data reasonably well.

The other ponds in the Napa-Sonoma salt pond complex are either tidally influenced, or were not sampled and are not being calibrated by the model. The effects of vertical mixing by wind waves on mixing and water quality of the ponds also are being evaluated. Our continuing field efforts will focus primarily on 8 salt ponds in the South Bay that are or under USFWS management. The USFWS has very limited understanding of the ecology and physical dynamics of these ponds, and as a DOI client agency, requested assistance from USGS. Pt. Reyes Bird Observatory biologists conducted bird surveys on some of these ponds and studied bio-physical characteristics of ponds A9 - A15. Ponds A9 and A10 appear important for waterfowl, A9 and A14 important for shorebirds, and A11 - A13 apparently not as important.

Trophic, geomorphic, and hydrologic study components are combined to develop a conceptual model to provide a foundation for management or mitigation of these ponds and future wetland restoration in lieu of commercial salt pond operations. The SPOOM model provides water and salt budgets for the ponds that substantially aid interpretation of the ecological data, and eventual development of the conceptual model. These models also are useful tools for other agencies planning restoration of the Napa and South Bay ponds.

We use existing bird surveys and past and current data augmented with the proposed objectives to provide information needed by the USFWS to develop the best management decisions for South Bay salt ponds under their jurisdiction. We help to identify those ponds and key habitat qualities that support highly diverse (abundance and species) avian communities with attention to avian species of concern, balanced with those ponds that likely might be converted to tidal marsh with the least impact of existing natural biological communities. We continue limited monitoring of avian utilization and prey dynamics in the North Bay ponds that are subject to uncertain management regimes; such knowledge increases our capability to predict changes at the San Francisco Bay salt ponds. The goal of the continuing studies is to provide resource managers with a comprehensive assessment of the ecology of the San Francisco Bay salt ponds, such that optimal management strategies can be exercised that maximize benefits to wildlife.