summary Suisun Bay

The following is a brief synopsis of the National Wetlands Research Center Open File 94-04, The Distribution of Wetland and Deepwater Habitats in the San Francisco Bay Region, by Michael Josselyn, Lawrence Handley, Millicent Quammen, and Dennis Peters. Based on the study area designated by Josselyn et al., the South Bay Zone is south of the San Francisco-Oakland Bay Bridge (I-80) and includes the Foster City / Redwood Point (Fig. 1). For this workshop, however, San Francisco Bay and the South Bay Zone refer to the same area. Only those passages that specifically discuss the Foster City / Redwood Point area are summarized. For a more detailed explanation refer to Open file 94-04, which is an in-depth analysis and comparison of Suisun Bay, San Pablo Bay, and San Francisco Bay. This report is a time sequence trend-analysis that explains the complexity, change, and loss of wetland deepwater habitats within the San Francisco Bay area.

Wetlands of South San Francisco Bay

Josselyn et al. studied the distribution of wetland and deepwater habitats within the San Francisco Bay region by using aerial infrared photography and a geographic information system (GIS). They examined a broad range of wetland habitats from estuarine open water to nontidal seasonal wetlands on the bay margin also called the Bay Zone. Josselyn et al. defined the Bay Zone by cultural features and include historic tidal marshes and their contagious wetland and upland habitats.

Wetland change was calculated by comparing 1956-58 wetland maps with those developed for 1985. An overall 6% decline was noted over the 30-year period, with tidal marshes declining by over 1,200 acres during this period. Diked seasonal marshes declined by over 3,000 acres and seasonal wetlands not adjacent to dikes declined by over 9,000 acres. The loss of nontidal vegetated wetlands was 12, 492 acres since the 1950s. The amount of flooded habitats such as salt ponds, seasonal ponds, and lakes increased during the study period. Among upland habitats, agricultural land declined by over 30,000 acres as they were converted to residential and industrial land uses. The primary cause for wetland loss in the South Bay Zone has been residential development and indirect conversion to uses associated with housing such as flood-control projects, sewage-treatment facilities, and roadways.

Most estimates for San Francisco Bay area report that less that 38,000 acres of tidal marsh remain, a reduction of more than 81% since the late 19th century. Most of the tidal marsh was initially converted to salt ponds and agricultural uses; more recently, however, urbanization has claimed both agricultural lands and former wetlands.

Trend Analysis

Josselyn et al. compared 1956-58 NWI maps with 1985 NWI maps for the South San Francisco zone to develop trend data. They condensed the habitat types into a workable number and adjusted habitat designations between years so that they could detect the actual habitat changes verses changes in designation. Groupings were designated to examine wetland habitats that are of economic importance (e.g., salt ponds and flood-control channels) or habitats that are commonly considered in resource management such as tidal marshes, mud flats, and vegetated seasonal wetlands. They developed 13 categories for the trend analysis. Those that relate to this workshop are open water mud-flats, tidal marsh, channels, freshwater tidal marsh, seasonal vegetated wetlands, and seasonal ponds.

Open water refers to all estuarine deepwater habitat, generally with soft bottom. This habitat type is always covered by tidal waters. Some vegetation may occur such as eelgrass and various seaweeds such as Gracilaria, Polysiphonia, and Cryptopluera. Two federally endangered species, the California brown pelican and the California least tern, forage in the open water habitat.

Mud flats are soft-bottom habitats regularly exposed by the tides. In a few instances, mud flats occur behind dikes; however, these areas have at least partial tidal action. The tidal range over which mud flats are found are from about three feet below mean sea level (msl) to about one foot above msl. The tidal range is much greater in southern pat of San Francisco Bay; there, it increases to over four feet. Mud flats are largely barren of vegetation, where isolated patches of eelgrass and seaweeds occur, particularly in San Francisco and San Pablo Bays. In addition to the diverse benthic fauna, the mud flats provide foraging habitat for migratory bird populations.

Tidal marsh includes all estuarine, tidal, and partially tidal vegetated habitats. Partially tidal refers to those areas behind dikes, but which receive tidal action through partial breaches or culverts. Tidal marshes are generally found within the range of one foot above msl to about five feet above msl; however, this can extend to seven feet above msl in southern San Francisco Bay. Within San Francisco and San Pablo bays, pickleweed is the dominant vegetation of the tidal marshes. Tidal marshes are also home to a number of federally endangered species including the salt marsh harvest mouse and the California clapper rail.

Freshwater tidal marsh is also a limited habitat type that occurs in areas where a high freshwater discharge results in the conversion of salt marsh to fresh and brackish water vegetation. In some instances, this habitat is found in areas behind dikes where limited tidal action occurs. The dominant vegetation within this habitat type is bulrush and cattail. Many birds, particularly waterfowl, use this habitat for breeding and nesting.

Seasonal vegetated wetlands were divided into two categories of diked and undiked. Diked seasonal wetlands were once part of the estuary's tidal marshes while undiked seasonal wetlands are above the historic margin of the bay. Because diked and undiked seasonal wetlands are no longer connected to tidal waters, rainfall and surface runoff are their major sources of water. The high rate of evaporation during summer and fall removes surface water from these sites so that their primary period of apparent wetness occurs in winter and early spring. Both systems generally support halophytic vegetation such as pickleweed, salt grass, and alkali heath. Freshwater and brackish-water species also can flourish, especially during winter and spring. Alki bulrush is a preferred waterfowl food plant and specific management techniques have been developed to encourage its dominance. Wildlife values associated with seasonal wetlands vary considerably depending upon the location, density of vegetation, and time of year. Some seasonal wetlands do support shorebird and waterfowl populations, federally rare and endangered species, and fish. Seasonal wetlands have been the subject of a great deal of controversy in land development.

Seasonal ponds are primarily artificial habitats consisting of diked areas that serve as flood basins, managed waterfowl ponds, and dredge-spoil disposal areas that pond seasonal rainfall. Seasonal ponds may also include nonproducing salt evaporators or crystallizers. Vegetative cover is generally less than 30% although when ponded, these habitats may support extensive stands of widgeon weed. Soil salinities are often high during the dry season, thereby supporting pickleweed and other halophytic vegetation. Seasonal ponds are important habitat for waterfowl and are sometimes specifically managed for waterfowl production and hunting. Seasonal ponds, along with farmed wetlands and vegetated seasonal wetlands, provide additional habitat for migratory waterbirds during winter when their numbers are greater.

In 1985, farmed wetlands were identified as former wetlands that were used for crop production. Forage crops such as hay and alfalfa are the most common commodity although other row crops are grown in these areas. With the exception of corn and sorghum, which are grown by some game clubs for wildlife food, farmed wetlands have little fish or wildlife value.

Salt ponds consisted of a number of different habitats. Salt ponds vary in salinity from the initial salt evaporators that receive intake water from the Bay to crystallizers, near plant sites in Newark, Redwood City, and Napa. Bittern ponds, also found near plant sites, contain highly saline water that remains after salt crystallization has occurred. The South Bay Zone has a greater acreage of salt ponds. Most ponds are relatively shallow, approximately one to four feet in depth, and constantly circulate and move through the pond system. It takes about five to seven years from intake to salt production. The habitat value of salt ponds varies considerably on the salinity, depth, and location of the pond. Ponds of low to intermediate salinity support the greatest diversity of invertebrates, fish, and waterbirds. One invertebrate, the brine shrimp, is found in ponds with salinities as much as 150 parts per trillion or five times that of seawater. Salt ponds do attract a large number of wintering shorebirds such as marbled gogwits, dowitchers, western and least sandpiper, American avocets, and willets. Waterfowl, including northern shoveler, scaup, and canvasback are common in some ponds. Several species such as black-necked silt, American avocet, killdeer, Forester's tern, Caspian tern, snowy plover, and California gull nest on levees around ponds.

Lakes, ponds, and rivers are permanently flooded habitats that consist of natural features as well as artificial habitats such as residential lagoons, sewage treatment facilities, industrial waste ponds, and wildlife areas. Depending upon the water quality, there is a great variety of fish and wildlife resources associated with these habitats.

Distribution of Wetlands

Most of the nonsubtidal wetland acreage in the South Bay Zone (Fig. 1) occurs in the southernmost portion of the bay. Open water habitat is concentrated in the upper portions of the South Bay Zone with urbanized uplands directly adjacent to the water. The only significant nonsubtidal wetlands in the northern portion of the South Bay are those within the Oakland Airport and between San Leandro and Hayward. These areas are primarily diked seasonal vegetated wetlands or unvegetated seasonal ponds. Pickleweed and salt grass are the dominant vegetation in these diked areas. On the western side if the South Bay Zone, the largest contiguous seasonal vegetated wetland is at Bair Island. This wetland is also adjacent to one of the largest tidal marsh complexes (over 2,600 acres) in the South Bay Zone; this complex consists of Bird Island off Redwood Shores, Bair Island Wildlife Refuge, and Greco Island. Cordgrass and pickleweed are the dominant plants in tidal marshes.

The greatest concentration and diversity of wetland habitat is found in the southernmost portion of the South Bay Zone within the Mountain View, Niles, Newark, and Milpitas quadrangles. Salt ponds make up the largest percentage of wetland habitats followed by palustrine farmed and tidal marshes. Abandoned salt ponds and crystallizers near Union City and Newark cover extensive acreage of nonvegetated, nontidal habitat. Many of the wetlands in this region are highly fragmented because of the extensive levee construction for salt production and urbanization of the region. This region has the greatest diversity of wetland types when compared with other areas in the Bay.

Wetland Habitat Change

For the South Bay Zone, estuarine and palustrine acreage decreased by 15,261 acres during 1956 and 1985 while lacustrine habitat increased by 3,467 acres and upland habitats by 11,825 acres (Fig. 2). Considering all wetland types, total wetland acreage decreased from about 184,000 to 172,000, a six percent loss over a 30-year period.

Seasonal vegetated wetlands experienced the greatest loss of acreage between 1956 and 1985 (Fig. 3). Diked seasonal marshes declined from 7,140 acres to 3,987 for a loss of 3,153 acres. Seasonal vegetated wetlands not adjacent to dikes experienced in even greater loss, declining from 9,944 acres to 605 for a loss of 9,339.

Tidal mud flats and marshes also decreased, although presumably this decrease was before 1965 because stricter controls were enacted by the San Francisco Bay Development and Conservation Commission after 1965. The overall net loss ion the South Bay Zone since the 1950s exceeded 1,200 acres despite efforts to restore tidal marshes during the 1980s.

Figure 1: Wetland designated zones of the San Francisco Bay Area

(Source: NWRC Open File 94-4, Figure 2.3, page 6.)

Figure 2: Changes acreage of system level wetland habitats in the South Bay Zone from 1956 to 1985. (Source: NWRC Open File 94-04, Figure 3.5, page 33.)

Figure 3: Changes acreage of specific wetland groupings in the South Bay Zone from 1956 to 1985. (Source: NWRC Open File 94-04, Figure 3.6, page 34.)

Other wetlands types increased over time. Salt pond acreage increased by 700 acres because ponds drained in the 1950s were put back into production. Seasonal ponds increased by over 2,200 acres and artificial lakes by 1,045 acres. Seasonal ponds represent seasonal salt ponds that are now out of production or were vegetated wetlands that are now barren. Lakes represent either recreational areas or liquid waste storage facilities. The amount of farmed wetland increased by about 300 acres. Riparian and freshwater habitats also increased by slightly more than 300 acres.

Comparisons based on acreage alone may not reflect the total habitat modification occurring in the South San Francisco Bay area. The acreage in seasonal vegetated wetlands (diked and nondiked), channels, and seasonal ponds declined from 19,000 to 8,800 acres. Only 3,269 acres, however, had the same classification in 1985 as in 1996. About 5,500 acres of other habitat types were converted to seasonal wetland during this time period suggesting that hydrologic and vegetative modification of individual sites is a significant factor in the availability of specific habitat types over time.

Since 1956, in the Foster City area of San Mateo County, over 8,000 acres of seasonal wetlands were converted to upland uses (Fig. 4). Almost 5,000 acres were placed back into salt production. Almost 1,000 acres were converted to tidal marsh and subtidal habitat, largely the result of mitigation activities near Palo Alto. By 1985, most of the 19,000 acres of nontidal seasonal wetlands existing in 1956 had been converted to some other use.

Foster City Habitat Change

Development of Foster City and redwood shores along the western shoreline of the South Bay has been largely responsible for the conversion of salt ponds to residential uses (Fig. 5). Some of these former salt ponds, especially near Redwood shores, now function as seasonal ponds or lakes. This region had very little seasonal vegetated wetlands in the 1950s and little change was recorded in other wetland habitants. Mud flats increased as the alt ponds at Bair Island were converted to tidal habitats when the surrounding levees were breached, which restored tidal action to Bair Island Ecological Reserve.

Summary

Population pressures in the South Bay region, with has an average density of 1,000 people per square mile, is the most significant cause of wetland decline. Loss was not only due to conversion to residential uses, but to other indirect effects of human development. Flood-control facilities and sewage treatment plants have played a major role in the conversion of seasonal wetlands to other uses in the Bay region.

Figure 4: Acreage of 1985 wetlands in the South Bay Zone converted from seasonal wetlands in 1956. (Source: NWRC Open File 94-04, Figure 3.7, page 37.)

Figure 5: Changes in acreage of specific wetlands for the Redwood Point quadrangle between 1956-1985. (Source: NWRC Open File 94-04, Figure 3.12, page 45.)

Conversion between wetland types can lead to reduction in wildlife habitat values even though the total wetland acreage remain the same. For example, farmed wetlands do not provide diverse wildlife habitat. Sewage treatment "lakes" and salt ponds were built in former wetlands and, although these habitat may provide increased waterfowl habitat, they do not replace the endangered species habitat that was lost as a result of conversion.

Although wetland loss in the San Francisco Bay region is much less than other states, it is significant because of the rarity of wetlands remaining in California. Many rare and endangered species are endemic to the region's palustrine and estuarine wetlands. The Bay's wetlands also are a significant stopover for migratory waterfowl along the Pacific Flyway.

Seventeen percent of the estuary's wetland and deepwater habitat is protected through ownership or conservation easements for fish and wildlife resource. Forty-two percent of the protected lands are in the South Bay Zone. The US Fish and Wildlife Service and the California Department of Fish and Game are the largest landowners of protected lands. Regional park districts, open space districts, and local jurisdictions also maintain and protect natural resource in the Bay Zone.

The loss of nontidal seasonal wetlands is critical. Twenty-nine percent or about 15,000 acres, of the remaining diked baylands are slated for development over the next 20 years at a rate loss of 750 acres per year. Such declines will lead to further deterioration of fish and wildlife resources in the region and throughout the Pacific Flyway.

Source:

Josselyn, Michael, Lawrence Handley, Millicent Quammen, and Dennis Peters. NWRC Open File 94-04. The Distribution of Wetland and Deepwater Habitats in the San Francisco Bay Region. Washington D.C.: U.S. Department of the Interior, National Biological Survey.