Accelerated marsh erosion following the Deepwater Horizon oil spill confirmed, ameliorated by planting

We start by looking at our field erosion data, collected through 6 years post-spill (7 years total), to determine the duration of oiling effects, and to look for longer-term cleanup treatment influences. We previously observed increased erosion in our heavily oiled marsh sites for a 2-year period after the spill, the duration of our study at that time⁷. We also observed no major differences in erosion among oiled sites that were untreated versus those with manual cleanup treatments. In a separate but related experiment, using slightly different methods, there were indirect indications that mechanical cleanup treatments may have further worsened erosion and direct evidence that planting limited erosion in mechanically treated sites over a 1 year study period⁷.

In the present study, our field-based comparisons of oiling/treatment categories included reference, oiled and untreated, oiled and manually treated, and oiled and mechanically treated sites. Untreated sites had no active cleanup (i.e., natural recovery), an approach which is commonly prescribed for oiled marshes (see Zengel et al.⁷ for background on the trade-offs of typical oiled marsh treatment tactics). Manual cleanup treatment involved raking, cutting, and removal of oiled wrack, oiled vegetation mats (laid over oiled and dead vegetation that was still rooted), and underlying thick oil on the substrate by small crews using hand tools, to remove surface oiling to the extent possible and to better expose remaining oil to natural weathering and degradation processes⁷. Hand crews used walking boards to minimize foot traffic on the marsh surface. Mechanical cleanup treatment involved mechanized grappling to remove oiled wrack and mechanized raking, cutting, and scraping to remove or reduce oiled vegetation mats and oil on the substrate. The mechanical treatments were applied using long-reach hydraulic arms mounted on shallow-draft barges and large airboats stationed just seaward of the marsh shoreline^7,23. Mechanical treatment was aimed toward the same goals as manual cleanup but with anticipated increases in speed and scale; however, mechanical treatment can also be less precise, resulting in removal of soils, mixing of oil into the substrate, etc. Oiling conditions were highly consistent across all the oiled sites, characterized as heavy oiling using Shoreline Cleanup Assessment Technique (SCAT) methods⁷, although we agree that oiling in our sites could be considered “very heavy” as proposed by others¹⁹. Oiling conditions consisted of a continuous 6–13 meters (m) wide oiling band along the marsh shoreline, with heavily oiled wrack and vegetation mats overlying a 2–3 centimeter (cm) layer of emulsified oil on the marsh surface with ~ 90–100% oil cover⁷. The heavily oiled sites all experienced complete or near complete vegetation die-off, with vegetation recovery spanning multiple years^7,23. Nearby reference sites on the same shoreline had lighter to no oiling, intact vegetation structure, and no cleanup treatments⁷. Further details on oiling conditions, cleanup treatments, and vegetation response are included in our prior papers (including photographs)^7,23.

Annual shoreline erosion rates were determined each year by ground surveys using tape measures and differentially corrected GPS (± 10 cm horizontal accuracy) to measure shoreline position along established transects. In the present study, we observed 147–198% greater marsh shoreline erosion for the oiled versus reference sites over 2 years (2010–2011 and 2011–2012), with no clear differences among oiled sites with or without cleanup treatments (Fig. 2, Supplementary Table S1). There was some indication that mechanical treatment may possibly have worsened erosion in some sites in 2011–2012, which matched our field observations and prior experiments⁷, though our sample size was small and highly variable in this case.

Potential causes for increased shoreline erosion in oiled marshes may lie most directly with the die-off of marsh vegetation, which baffles wave energy and binds marsh soils. Die-off of vegetation at the marsh edge was likely caused by several related factors, including thick persistent oiling covering all or most of the aboveground vegetation and soil surface, repetitive oiling, and penetration and mixing of oil into the soils, resulting in fouling and smothering effects on the plants such as interference with photosynthesis, gas exchange, thermal regulation, etc., leading to plant death. Our prior work showed substantial reductions (77–100%) in aboveground total plant cover (all species) and Spartina alterniflora cover (the dominant marsh species) in our oiled sites versus reference over 2010–2011 (and 45–99% reductions over 2011–2012)⁷. Belowground plant biomass was likely also reduced, although this was not measured in our study. However, belowground biomass was reduced in similarly oiled sites in studies by others^5,9,14,24. Significant relationships have been established between marsh belowground biomass and soil shear strength (a measure of erosion potential)²⁵, including in oiled marshes⁹, and belowground biomass is the main vegetation trait that resists erosion in coastal marshes²⁶. Working in collaboration with us, Lin et al.⁹ conducted ancillary sampling in a subset of our study sites over the 2011–2012 period and provided their unpublished soil shear strength data (0–6 cm) for our use (their sampling did not include our planted sites). There was a 42% reduction in soil shear strength in the oiled sites relative to the reference sites (Fig. 3, Supplementary Table S2). Our reference site mean values are very similar to marsh soil shear strength values reported by others in our study region^9,27, whereas our oiled site values are much lower. Thus, there is evidence that oiling affected both the marsh vegetation and the erodibility of the marsh soils, which likely led to the observed differences in marsh erosion between the reference and oiled sites. Oyster beds were not present near the marsh edge in our study area; thus, oyster cover impacts did not contribute to observed erosion differences between our oiled and reference sites (see Powers et al.¹⁷).

Source: Ecology - nature.com