Summary:
Spartina alterniflora is a critical component of Louisiana's salt marsh-estuarine complexes, the health of which determines the sustainability of near-shore Gulf coast ecosystems and human uses thereof. The health and resilience of S. alterniflora marshes are strongly influenced by, and exert strong feedbacks on, hydrology, the physical matrix of the soil, soil stability and its associated animal and microbial communities (Howes et al. 2010). Stability of vegetated soils, which can be expressed as erosional resistance, is primarily affected by physical effects of plants on water flow regimes and belowground processes, which include interactions between plants and microbial communities. For example, aboveground vegetation and its root systems can protect soils from erosion by reducing current and wave energies and reinforcing the soil. In addition, microbial activities affect soil stability via formation of biofilms and extracellular polymeric substances (EPS). The erosional resistance of recently deposited sediment without mature microbial communities can be 4-6 orders of magnitude smaller than that of a sediment deposit with well-established microbial communities.
While much is known about general responses of salt marshes, and specific responses of S. alterniflora, to a variety of perturbations, including oil spills (DeLaune and Wright, 2011), several fundamental knowledge gaps preclude a predictive understanding of salt marsh and S. alterniflora recovery trajectories after oil spills, the extent of recovery, and interactions that affect sustainability in a context of likely future perturbations. Some of these fundamental knowledge gaps, which form the basis for research proposed here, include:
1. alteration of salt marsh plant above- and belowground allocation patterns and associated carbon dynamics in response to oiling and remediation efforts in (above-ground vegetation removal);
2. the interactions between the geomechanical properties of vegetated sediment and above- and belowground plant dynamics after oiling and remediation efforts;
3. impacts of oiling and mitigation efforts on interactions between rhizosphere bacterial communities and plant dynamics, including community composition and structure as well as critical activities;
4. relationships between surface sediment geomechanical properties and the composition and activities of surface sediment bacterial communities.
We will address these knowledge gaps and provide new, predictive insights about salt marsh sustainability in the context of hydrocarbon perturbations through a multi-disciplinary, multi-institution research program that will leverage state-of-the-science approaches and extensive past experience with Louisiana salt marshes. To accomplish this, we will exploit a set of field sites that were heavily oiled as a consequence of the Deepwater Horizon spill, a subset of which were treated using protocols that involved clipping and removing oiled vegetation. Additionally we will incorporate field sites with heavily oiled salt marshes in adjacent areas (the same hydrogeomorphic setting) that were not cleaned, as well as unoiled reference marshes. Our approaches will emphasize the connections between above- and belowground plant responses, microbial rhizosphere dynamics, and soil geomechanical properties (e.g., shear strength) investigated over time. We will also include a set of parallel analyses using greenhouse-based manipulations that will allow us to address initial responses to oiling as well as responses of existing oiled sites to controlled manipulations, including re-oiling. Our results will provide crucial new insights into the effects of oiling and oil cleanup on salt marsh plants, bacteria and soil geomechanical properties, and how interactions among them affect long-term salt marsh stability.