How will wetlands respond to an accelerating sea-level rise and warmer conditions? Wetlands in Mediterranean-climates experience rainless summers which increases salinity and can constrain productivity. San Francisco Bay-Delta estuary is the largest estuary on the Pacific Coast. Until recently, over 90% of the tidal wetlands had been diked and converted to agriculture, grazing or urban development. Considerable areas are in the process of being restored, although many of these sites have subsided considerably. The future of these sites as well as existing tidal wetlands depends on suspended sediment and organic productivity in the root zone, as well as areas uphill for retreat. Trends of suspended sediment, plant productivity and the potential for wetlands to maintain themselves with sea level rise are assessed for this estuary. Because of the Mediterranean-climate, and recent trends toward highly reduced regional precipitation, increases in wetland salinity has been occurring in saline, brackish, and the western edges of freshwater tidal wetlands.
Results/Conclusions
The potential for suspended sediment differs between the North Bay and South Bay. While impacts will occur in the South Bay, the system is already saline and has adjacent areas for retreat. Currently suspended sediment may be sufficient to enable wetland elevation rise. In the North Bay, suspended sediment levels have been declining due to upriver hydrological modifications. Soil salinity increases greatly in the less frequently inundated marsh plain during rainless summer months, and brackish wetlands display a within-wetland disjunction in salinity patterns that correlates with the vegetation relationships. Future trends are likely an increasingly saline system, especially with human diversions of freshwater for agricultural and urban demands. Accelerating sea-level rise, declines in freshwater runoff, and warmer summer temperatures will have an immediate impact on the brackish wetland systems in the North Bay. Salinity shifts likely will modify brackish wetlands toward salt marshes at a faster rate than sea level rise will affect vegetation composition. Plant productivity and elevation distributions have been used to calibrate the Marsh Equilibrium Model (MEM); predictions indicate considerable wetland losses.