Comparison of replanting tactics after a simulated marine diesel fuel spill in estuarine mesocosms 2024 International Oil Spill Conference

Coastal marshlands are ecologically critical areas that provide essential food, refuge, and nursery habitat. They are highly sensitive to oil spills and exceedingly difficult to clean up. Many of the techniques used to clean oiled shorelines can cause additional damage in marshlands and are not viable treatment options in these sensitive environments. During the Deepwater Horizon (DWH) oil spill, a wide variety of clean-up and primary restoration tactics were investigated in the most heavily impacted coastal marshes of Louisiana, USA. Subsequent monitoring revealed that one of the most beneficial tactics employed was to replant native grasses in the impacted areas. The objective of this study is to determine what combination of marsh grass (Spartina alterniflora, smooth cordgrass) replanting tactics produces the best outcome for a marine diesel fuel oiled marsh in a controlled setting. The study consisted of 20 oiled mesocosms (approximately 450 L tanks with simulated tidal flux) with four treatments (control mesocosms with no oil and no replanting, oiled mesocosms with no replanting, oiled mesocosms replanted with local field transplants, and oiled mesocosms replanted with nursery stock) with 5 replicates each. Replanting tactics tested also included containerized/plug plantings vs. bare-root plantings. The oiled and dead vegetation was cut and removed from the mesocosms that later received replanted S. alterniflora. Marsh replanting success was followed over 9 months. Data presented include oil effects on the original marsh prior to replanting, hydrocarbon residues in water and sediments over time, and measurements of the structure and growth of the replanted grasses (stem density, shoot height, above-ground biomass, and below-ground biomass). An initial replanting effort was attempted 2 months after the oiling event. All the replanted S. alterniflora from the initial replanting event died, likely due to residual diesel that remained in the mesocosms and was still toxic. A successful replanting effort occurred 8 months after the oiling event and preliminary results indicate that plugs containing plants with sediment performed well and similarly between local field transplants and nursery-grown material, especially considering aboveground biomass at 9 months post-planting, although values were still well below reference conditions. Bare root nursery material failed overall, and bare root field transplants had intermediate results. Oiled marsh that was not replanted showed no aboveground recovery. This data will help inform future restoration efforts that are considering using replanting as a tactic for restoring an oil-contaminated saltmarsh.