PS 30-7
Estimating sediment carbon accumulation rates in the Plum Island Estuary (MA) salt marsh
Globally, sea level is now rising at approximately 3.2 mm per year and expected to accelerate (IPCC). Whether or not marshes are keeping up with this rate of sea-level rise is an important question. Salt marshes sequester a significant amount of carbon per unit area when compared to other ecosystems and provide other important ecosystem services such as providing nursery grounds for fish and removing nutrients. This study examined the resiliency salt marshes in the Plum Island estuary to sea level rise. Sediment cores were extracted from Spartina alterniflora tall, Spartina alterniflora short, Spartina patens, and a salt marsh and sectioned into 2 cm sections. Sedimentation rates were estimated by finding the 137Cs peak and assuming that this represented 1963. The amount of carbon in each section was measured using an elemental analyzer. Selected sections were analyzed for del 13C to determine past vegetation composition. Carbon accumulation rates averaged over the last 50 years were compared to annual net C exchange between the marsh and the atmosphere measured using eddy covariance techniques.
Results/Conclusions
The results showed that Plum Island estuary salt marsh is currently keeping up with rising sea levels having an average sediment accumulation rate of 3.3 mm/yr in all of the vegetated areas with slightly lower rates in ponds. C carbon accumulation rates were highest in short Spartina alterniflora and, Spartina patens, where they averaged over 75 g C m-2 y-1, and somewhat lower in tall Spartina alterniflora sediments. Lowest accumulation rates were in the pond which averaged about55 g C m-2 y-1. Carbon accumulation rates from this study were compared to net carbon exchange calculated from annual eddy flux data. While both the sediment C accumulation data and the eddy covariance data suggest that the marsh has been and is currently a sink to the atmosphere, carbon storage rates from eddy covariance were nearly double those of sediment C accumulation rates. This may be because the eddy covariance does not take into account losses of C from the system in water. At all locations 13C isotopes suggested that much of carbon being buried in the sediments likely came from C¬4 plants that are native to the marsh with a much smaller percentage attributed to algal or terrestrial material.