DOES WATERSHED SEDIMENT DELIVERY INCREASE THE RESILIENCE OF TIDAL FRESHWATER WETLANDS TO SEA LEVEL RISE?

Greater sediment availability is thought to increase surface elevation and resilience to rising sea level in tidal wetlands. We are testing the hypothesis that watershed sediment loads influence ecosystem processes in tidal freshwater wetlands. We are comparing rates of soil surface elevation change and their geomorphic and biogeochemical controls in tidal freshwater forested wetlands (TFFW) in two adjacent rivers that are similar except for a 6x greater average annual watershed sediment load in the Pamunkey River compared to the Mattaponi River, Virginia. Five sites along each river span a longitudinal river-estuary gradient from nontidal floodplain forest, upper TFFW, lower TFFW, salt-stressed TFFW, to oligohaline marsh. On each site, we are measuring soil surface elevation change as well as surface accretion of sediments, channel-floodplain bathymetry and turbidity, soil greenhouse gas fluxes, water level, salinity, plant community composition, and tree growth rates. We discovered similar sediment accretion rates along the two rivers, despite differing watershed sediment loads, with a strong monotonic increase from nontidal (average: 4 mm/yr) through TFFW to oligohaline marsh (19 mm/yr); a similar pattern of river turbidity was measured. Rates of surface elevation change also typically increased from nontidal through TFFW to oligohaline marsh. All tidal wetlands are increasing in elevation (minimum: +7 mm/yr), often at faster rates, along the Mattaponi than Pamunkey river due to greater retention of organic matter from smaller CO₂ soil efflux rates. Raised hummocks always had less surface elevation change and greater soil CO₂ efflux rates than adjacent hollows (especially along the Mattaponi River). In summary, watershed sediment load appears to have no influence on these tidal freshwater wetlands most likely due to sediment trapping by nontidal Coastal Plain floodplains upstream of tidal rivers. Oligohaline marsh had greater soil surface elevation change than TFFW due to greater availability of an estuarine sediment source. Finally, hummocks are disappearing ecogeomorphic features in these TFFW, possibly due to higher rates of soil respiration compared to hollows.