Understanding Mechanisms for Coastal Marsh Sustainability in a Changing Climate

In the Chenier Plain of Louisiana and Texas, coastal marshes are subject to high rates of relative sea level rise (RSLR), restricted sedimentation, and declining elevation attributed to subsidence and decades of intensive management. These marshes were created via mineral sedimentation but now largely accrete via organic matter accumulation. This organic matter accumulation (OMA) – or the balance between plant productivity and decomposition – is essential for maintaining elevation and thus long-term sustainability. However, management for elevation is complicated by a poor understanding of how abiotic factors like flooding and salinity impact processes driving OMA and elevation. To better understand mechanisms driving elevation change in impounded Chenier Plain marshes, I performed a field and greenhouse study to examine the effects of abiotic conditions and plant species on the processes governing marsh surface elevation. I chose three perennial plant species for use in both studies: Phragmites australis, Schoenoplectus californicus, and Typha spp. My field study examined elevation change, accretion, belowground productivity, and belowground decomposition for all three species across two coastal marsh impoundments (Rockefeller Wildlife Refuge and JD Murphree Wildlife Management Area). I assessed elevation change and accretion using 23 rod-surface elevation tables (rSETs) and feldspar marker horizons. To quantify OMA among species in response to flooding and salinity, I performed a factorial greenhouse study in experimental mesocosms over a single growing season. I found that productivity was important for OMA and elevation change, and that productivity was influenced by flood depth and salinity, though these effects differed by plant species. The effects of hydroperiod and salinity on decomposition varied by species, with faster decomposition at intermediate flooding and salinities. Of my three target species, P. australis and S. californicus appeared to contribute more to OMA, while Typha spp. exhibited the fastest decomposition in my greenhouse study. Despite the contributions of OMA to elevation in Chenier Plain impounded marshes, this process was not sufficient to keep pace with RSLR under observed environmental conditions. Thus, as wetland vulnerability increases and losses accelerate, managers may consider increasing plant productivity or encouraging sediment influx to raise marsh elevation on a shorter time scale.