Title

Carbon Sequestration in Mediterranean Tidal Wetlands: San Francisco Bay and the Ebro River Delta

Document Type

Article

Publication Date

2013

Abstract

Tidal wetlands accumulate soil carbon at relatively rapid rates, in large part because they build soil to counteract increases in sea-level rise. Because of the rapid rates of carbon sequestration, there is growing interest in evaluating carbon dynamics in tidal wetlands around the world; however, few measurements have been completed for mediterranean-type tidal wetlands, which tend to have relatively high levels of soil salinity, likely affecting both plant productivity and decomposition rates. We measured sediment accretion and carbon sequestration rates at tidal wetlands in two mediterranean regions: the San Francisco Bay Estuary (California, USA) and the Ebro River Delta (Catalonia, Spain). Sampling sites within each region represented a range of conditions in terms of soil salinity and plant communities, and these sites serve as potential analogs for long-term carbon sequestration in restored wetlands, which could receive credits under emerging policies for carbon management. Within San Francisco Bay, we collected six sediment cores per site at four salt marshes and two brackish tidal wetlands (two transects with three stations per transect at each site) in order to identify spatial variation both within and among wetlands in the Estuary. At the Ebro Delta, individual sediment cores were collected across 14 tidal wetland sites, including salt and brackish marshes from impounded areas, river mouths, coastal lagoon, and open bay settings. Cores were collected to 50 cm, and cores were dated using 137Cs and 210Pb. Most sites within San Francisco accreted 0.3-0.5 cm/yr, with slightly higher rates of accretion at low marsh stations; accretions rates based on 137Cs were slightly higher than those based on 210Pb, likely because of the shorter time frame covered by 137Cs dating. Accretion rates from the Ebro Delta sites were similar although more variable, with rates based on 137Cs ranging from 0.1 to 0.9 cm/yr and reflecting the wide range of conditions and management history across sites within the Delta. At the San Francisco Bay sites, carbon sequestration rates averaged approximately 80 g/m2/yr over the 100-year time span of 210Pb and as with accretion, were slightly higher for 137Cs-based rates. Variation in long-term carbon sequestration rates across sites and stations within San Francisco Bay was much smaller than the variation in mineral inputs, and there was little difference in carbon sequestration rates among sites, or across stations within San Francisco Bay sites. Carbon sequestration rates from the Ebro Delta were more variable and higher than San Francisco Bay sites, with 137Cs-based rates ranging from 20 to 500 g/m2/yr. Low salinity sites had slightly higher rates of sequestration than salt marsh locations; however, even some salt marsh locations within the Delta had high rates of sequestration (> 200g/m2/yr). As expected high rates of sequestration were associated with locations that had high rates of sediment accretion.

Journal

American Geophysical Union

Volume

46th Annual Fall Meeting