RECENT ACCRETION IN 2 MANAGED MARSH IMPOUNDMENTS IN COASTAL LOUISIANA

Recent accretion was measured by the feldspar marker horizon method in two gravity-drained, managed, marsh impoundments and unmanaged refere nce marshes located on the rapidly subsiding coast of Louisiana. Water level management was designed to limit hydrologic exchange to the man aged marsh by regulating the direction and rate of water flows. During a drawdown-flooding water management cycle, the unmanaged reference m arshes had significantly higher vertical accretion rates, higher soil bulk density and soil mineral matter content, lower soil organic matte r content, and higher rates of organic matter accumulation than the ma naged marsh. The rate of mineral matter accumulation was higher in bot h reference marshes, but was significantly higher in only one. Spatial variability in accumulation rates was low when analyzed in one manage d marsh site, suggesting a primarily autochthonous source of matter. I n contrast, the associated reference marsh apparently received allocht honous material that settled out in a distinct spatial pattern as wate r velocity decreased. The impoundment marshes experienced an accretion deficit of one full order of magnitude (0.1 vs. 1.0 cm/yr) based on c omparison of accretion and sea level rise data, while the unmanaged re ference marshes experienced a five-fold smaller deficit or no deficit. These data suggest that the gravity-drained impoundments likely have a shorter life expectancy than the reference marshes in the rapidly su bsiding Louisiana coast.