Disturbance legacies and shifting trajectories: Marsh soil strength and shoreline erosion a decade after the Deepwater Horizon oil spill

Abstract

Marsh resilience post disturbance is strongly dependent on the belowground dynamics affecting the emergent plants aboveground. We investigated the long-term impacts at the marsh-water interface in coastal wetlands of south Louisiana after the 2010 Deepwater Horizon oil spill with a combination of fieldwork (2010-2018) and spatial analysis (1998-2021). Data were collected on shoreline erosion rates, marsh platform elevation heights and cantilever overhang widths, and soil strength up to 1 m depth. Oil concentration in the top 5 cm of the marsh soil were determined using gas chromatography/mass spectrometry and were 1000 times higher than before the spill and remained 10 times higher eight years post-oiling. The oiling initially caused the marsh edge to subside, and chronic effects lowered soil strength, creating a faster erosion rate and deeper water within 150 cm of the shoreline. Soil strength declined by 50% throughout the 1 m soil profile after oiling and has not recovered. The mean erosion rate for 11 years post-spill was double that before oiling and there was an additive impact on erosion rates after Hurricane Isaac. Erosion appeared to have recovered to pre-spill rates by 2019, however from 2019 to 2021, the rate increased by 118% above the pre-spill rate. The continuing loss of soil strength indicates that the belowground biomass was seriously compromised by oiling. The perpetuation of oil in the remaining marsh may have set a new baseline for soil strength and subsequent storm induced erosional events. The remaining marsh soils retain chronic physical and biological legacies compromising recovery for more than a decade that may be evident in other marsh habitats subject to oiling and other stressors.

Keywords: Deepwater Horizon oil spill; Resilience; Restoration; Salt marsh; Shoreline erosion; Soil strength.