MINERAL AND ORGANIC MATTER ACCUMULATION RATES IN DELTAIC COASTAL MARSHES AND THEIR IMPORTANCE TO LANDSCAPE STABILITY

Abstract

ABSTRACT Marsh deposits may occupy little of a profile, but the environments they represent may be long lived. Marsh longevity depends partly on the deposition of mineral and organic matter to counter relative sea level rise, or submergence. Understanding soil formation in Louisiana’s rapidly subsiding coastal marshes may provide insights relevant to other coastal marshes if global sea-level rise increases as expected. Mineral and organic matter accumulation rates required to prevent flooding stress on marsh vegetation in a submerging environment are partially dependent on whether the marsh is fresh, brackish, or saline. Estimates of the rates of mineral matter accumulation required for a marsh surface submerging at 1 cm yr-1 range from 424 gr m-2 yr-1 in fresh, to 1,798 gr m-2 yr-1 in saline marsh. Mineral requirements likely vary in response to the availability of SO4-2, which reduces to a plant toxin, but which is buffered by soil iron. Estimates of the rates of organic matter accumulation required range from 1,969 gr m-2 yr-1 in fresh to 1,136 gr m-2 yr-1 in brackish marsh. Organic requirements vary because of differences in the decomposition rates of plants in the different marsh types. Another aspect of increasing rates of global sea-level rise is increased marine influences in tidal fresh and brackish marshes, i.e., increased tidal flushing and availability of sea salts such as NaCl and SO4-2, which cause shifts in plant communities. This is important because mineral matter accumulation must almost double for Spartina alterniflora to occupy brackish marsh areas experiencing increased marine influences. Thus even though vertical accretion may initially keep pace with submergence, we hypothesize that if increasing marine influences accompanies submergence, brackish marsh may be more likely to convert to open water than to saline marsh. This may be a factor in the rapid conversion of Louisiana’s brackish marsh to open water. If the rate of global sea-level rise increases, other brackish marshes with low soil bulk density may be similarly threatened.