Decomposition of Spartina alterniflora (smooth cordgrass) in coastal Louisiana brackish marshes influenced by a freshwater siphon

The combined effects of sea level rise, subsidence, and the leveeing of the Mississippi River have led to a land loss crisis in coastal Louisiana. The construction of tidal marshes and river diversions that reconnect the Mississippi River to adjacent estuaries have become common strategies to mitigate coastal land loss. This study aims to compare plant litter decomposition, an important ecological function in coastal wetlands, among marshes at varying distances from a freshwater siphon connected to the Mississippi River during periods of siphon openings and closures. We used a common litter bag technique at three sites in Plaquemine Parish, Louisiana to quantify decomposition. Replicate litter bags containing a standard amount of saltmarsh cordgrass Spartina alterniflora litter were placed in surface plots along three, 100-m transects at each site for two-month periods during the summers of 2018, 2019, and 2021. Litter loss and decomposition rate coefficients were compared between sites and years and to elevation, inundation, temperature, salinity, and precipitation. Our results indicate that wetting associated with precipitation, as well as the combined effects of precipitation and siphon openings on salinity contributed to differences in decomposition across years. Additionally, our results indicate that differences in elevation and inundation were likely drivers of differences in decomposition among sites. Therefore, while the reconnection of the Mississippi River to adjacent estuaries has the potential to offset land loss, it is important for future studies to further quantify the impact on litter decomposition and associated ecosystem services like carbon sequestration following implementation.