SEDIMENT ACCRETION RATES FROM 4 COASTAL WETLANDS ALONG THE GULF-OF-MEXICO

Our study of sediment accretion rates from four low tidal-range sites along the Gulf of Mexico does not support previous hypotheses concerni ng the relationship between tidal range and vertical accretion rates. The addition of our data to an earlier data set decreased the correlat ion between these variables, and all but one of our low-tidal range si tes had positive accretion rates, contradicting previous studies which have predicted that low tidal-range sites would have negative net acc retion rates. Additionally, in transects across the marsh, accretion r ates decreased from low- to high-marsh stations; however, this appeare d to be caused by changes in rates of organic matter accumulation, not mineral matter accumulation, as has been proposed in previous studies . Vertical accretion rates were more strongly correlated with organic matter accumulation rates than mineral matter accumulation rates, conf irming previous studies which indicated the important role of sediment organic matter in determining sediment structure. These results do no t imply that mineral matter is unimportant in maintaining the elevatio n of the marsh; mineral matter input affects organic matter production and sediment bulk density. There was little correlation between miner al and organic matter accumulation rates, with average organic matter accumulation rates for each site having little variation compared to t he variation in mineral matter accumulation rates. This result support s a previous hypothesis that there may be a limit to annual rates of o rganic matter accumulation. Finally, the study indicates that the nega tive net accretion rates documented in Louisiana are not typical of ot her Gulf coast wetlands.