Predicting the effects of sea level rise and salinity changes on west coast tidal marsh plant and avian communities
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Size and importance of San Francisco Bay-DeltaThe San Francisco Bay-Delta (hereafter referred to as the Bay-Delta) is the third largest estuary in the United States, covering approximately 4096 km2 of the central California coastal region and includes a broad mix of salt, brackish, and freshwater marsh ecosystems (Atwater et al. 1976, 1979; Josselyn 1983). The Bay-Delta is characterized by a Mediterranean climate, with precipitation limited to the winter and early spring seasons, and prolonged summer droughts. The wetland landscape is a complex mosaic of remaining historic wetlands, recently developed wetlands, restored wetlands, and potentially restorable diked bayland sites (farmland, former salt ponds, and seasonal and perennial wetlands), all situated within one of the country’s largest urban areas. Prior to 1850, tidal marshes in the Bay-Delta occupied 2200 km2, of which a substantial majority–1400 km2 –consisted of freshwater tidal marshes in the Delta region (Nichols et al. 1986; SFEP 1991). These extensive tidal marshes have now been reduced by more than 80% (95% in the Delta). Despite impacts from surrounding development, these remaining ecosystems are of critical regional importance for biodiversity, harboring a number of rare plant and animal species, including almost 50 special status species (Goals Project 1999; Olofson 2000). In addition to the ecological value of the Bay-Delta, the Delta’s freshwater storage and transport system is vital to California’s economy, providing water to meet agricultural, municipal, industrial, and environmental demands. The Bay-Delta is one of the most invaded aquatic ecosystems in the world (Cohen and Carlton 1998). Over 234 exotic species, including algae, plants, invertebrates, and vertebrates were introduced via a number of anthropogenic activities between 1850 and 1990, with most introductions having taken place in the latter part of the 20th century. Within tidal marshes, non-native cordgrass (Spartina alterniflora and hybrids with the native Spartina foliosa) (Callaway and Josselyn 1992; Ayres et al. 2004), as well as pepperweed (Lepidium latifolium) (Young et al. 1995) have been particularly effective at changing plant community composition and structure. Spartina alterniflora has invaded only the more saline portions of the San Francisco Bay, where native S. foliosa is also found, suggesting that an increase in salinity could increase invasibility in other areas of the Bay-Delta.
Climate change impacts on San Francisco Bay-DeltaMany studies have shown that the effects of a warmer global climate in this system would include reduced snowpack storage in the mountains, higher flood peaks during the winter rainy season, and reduced warm-season river flows after April (Gleick 1987a, 1987b; Roos 1989; Lettenmaier and Gan 1990; Gleick and Chalecki 1999; Knowles and Cayan 2002, 2004; Dettinger et al. 2004; Knowles et al. 2006). Even with some contention about which model might be the best and which direction certain parameters may shift, most models are in coarse agreement for California (Dettinger 2005). Dettinger (2005) compared multiple models and contingencies and determined that the most likely result of climate shift is a total precipitation regime similar to present, combined with warmer springs, reduced snowpack, and higher winter floods and lower summer flows. These hydrologic changes would propagate downstream to the estuary, resulting in an altered (i.e., increased in spring/summer, decreased in winter) salinity regime (Knowles and Cayan 2002). During the late spring and summer, the lower stream flows and increased salinities would affect many species that depend on the estuary and rivers. While several studies have examined current ecological conditions along the salinity gradient (Atwater et al. 1979, Pearcy and Ustin 1984), few have investigated how ecological systems in the estuary would respond to these changing conditions (Josselyn and Callaway 1988, Williams 1989). Another critical influence on estuarine conditions is SLR, which is projected to occur at a rate of up to 89 cm over the next 100 years (IPCC 2001; Cayan et al. 2005), an acceleration of the recent rate of 23 cm/century (Flick and Cayan 1984). Some recent predictions posit that future rates could be much greater due to more rapid melting of terrestrial ice sheets, primarily in Greenland and the Antarctic (Overpeck et al. 2006; Rignot and Kanagaratnam 2006). In response to increased rates of SLR, tidal marshes must either accumulate more sediment to keep pace with SLR, migrate inland to adjacent terrestrial areas, or face increased inundation (Donnelly and Bertness 2001, Morris et al. 2002). Most tidal marshes accumulate 2-8 mm of sediment per year (Stevenson et al. 1986; Reed 1995; Callaway et al. 1996), and this compensates for SLR and other processes. However, substantial data from Louisiana, Chesapeake Bay and modeling studies have shown that as increases in relative sea level get close to 10 to 12 mm/yr, most marshes can not keep pace and vegetation eventually may be inundated and converted to open water/mudflats (Baumann et al. 1984; Kearney and Stevenson 1991; Boesch et al. 1994; Morris et al. 2002, Rasse et al. 2005). Historic data from other systems has shown that slower increases in relative sea level (or loss in elevation) can lead to shifts in vegetation communities over time (Warren and Niering 1993). Although it may be possible for marsh accretion in the San Francisco Bay to keep up with SLR (Orr et al. 2003), bathymetric mapping studies have shown a decline in bay sediments over time (Foxgrover et al. 2004), and future large-scale tidal marsh restoration projects will further deplete existing bay sediments. Furthermore, in the heavily impacted Bay-Delta system, filled, diked, and developed baylands tidal systems are severely restricted in terms of adjacent terrestrial habitats for upslope migration in response to SLR. Thus there is a high level of uncertainty about tidal marsh responses to SLR. SLR contributes another significant stressor to the Bay-Delta system because most of the delta region is leveed and under agriculture. Thus, SLR further increases the pressure on these levees, adding to the probability of their failure (Ingebritsen et al. 2000; Mount and Twiss 2005). The increased possibility of levee failure that would result from higher wet-season flows and SLR could have additional impacts on the region’s ecosystems, particularly by drawing more saline water farther into the estuary. |