Interannual variability of a salt-marsh ecosystem

Data from Great Sippewissett Salt Marsh are used to gauge the extent of change of Spartina alterniflora marsh during periods of several to 22 yr. Our data all relate to plant production: from aboveground, maximum plant biomass measured over the entire period; from belowground, live root and rhizome biomass measured in 1974 and 1983, CO2 evolution from the sediments for 7 yr, and pore-water concentrations of dissolved sulfide for 6 yr. All the measures indicated a relatively low degree of interannual change in salt-marsh areas vegetated by tall- and short-form S. alterniflora. The similarity in both plant and biogeochemical parameters between years is likely related to the regular, high-frequency tidal flooding of grass stands. These interannual comparisons of each quantified parameter support the contention that measurements made in any single year can be applied to other years with suitable precautions to account for seasonal cycles. Our data also illustrate the importance of long time series for establishing sound correlations in environmental data.