Degradation and resilience in Louisiana salt marshes after the BP-Deepwater Horizon oil spill

Abstract

More than 2 y have passed since the BP-Deepwater Horizon oil spill in the Gulf of Mexico, yet we still have little understanding of its ecological impacts. Examining effects of this oil spill will generate much-needed insight into how shoreline habitats and the valuable ecological services they provide (e.g., shoreline protection) are affected by and recover from large-scale disturbance. Here we report on not only rapid salt-marsh recovery (high resilience) but also permanent marsh area loss after the BP-Deepwater Horizon oil spill. Field observations, experimental manipulations, and wave-propagation modeling reveal that (i) oil coverage was primarily concentrated on the seaward edge of marshes; (ii) there were thresholds of oil coverage that were associated with severity of salt-marsh damage, with heavy oiling leading to plant mortality; (iii) oil-driven plant death on the edges of these marshes more than doubled rates of shoreline erosion, further driving marsh platform loss that is likely to be permanent; and (iv) after 18 mo, marsh grasses have largely recovered into previously oiled, noneroded areas, and the elevated shoreline retreat rates observed at oiled sites have decreased to levels at reference marsh sites. This paper highlights that heavy oil coverage on the shorelines of Louisiana marshes, already experiencing elevated retreat because of intense human activities, induced a geomorphic feedback that amplified this erosion and thereby set limits to the recovery of otherwise resilient vegetation. It thus warns of the enhanced vulnerability of already degraded marshes to heavy oil coverage and provides a clear example of how multiple human-induced stressors can interact to hasten ecosystem decline.

Fig. 1.

Surveys of oil cover and cordgrass across transects at impacted (●) and reference (○) sites in October 2010. (A) Change in the proportion of plants oiled with distance from shoreline (defined as marsh platform edge). (B) Sediment PAH concentrations at three impacted and three reference sites at 3 m and 15 m from the shoreline. The more than 100-fold greater difference between concentrations at reference and impacted sites was not significant at inshore regions. (C) Change in the proportion of aboveground plants dead with distance from the marsh edge. A significantly greater proportion of the surface was oiled at impacted than at reference sites (P < 0.001), but oil coverage declined rapidly with distance reaching less than 50% coverage by 8.2 m from the marsh edge. There was also a greater proportion of plants dead at impacted sites (confirming our site characterizations) (P < 0.001). However, concomitant with the reduction in oil coverage with distance from shoreline, the proportion of dead plants at impacted sites also decreased with distance from the marsh edge, with the proportion of plants surviving exceeding 50% beyond 8.3 m from shore. Data illustrated in A and C are means from replicated surveys (n = 5) at three different reference and impacted sites. (D) Change in proportion of rhizomes dead at 3 m and 15 m distances from the shoreline at reference and impacted sites. There was a significant interaction between site type and distance from shore (P = 0.0003). This result is driven by the near-shore die-off zone where rhizome mortality was 63% higher at impacted than at reference sites. Data illustrated in B and D are means from three different reference and impacted sites (n = 3). Error bars are SEs.

Fig. 2.

Picture of (A) reference marsh (B) impacted marsh, (C), dead mussel at impacted site, (D) large pile of dead snails in impacted area, (E) clapper rail foraging on heavily oiled grasses at impacted site, and (F) typical covering of oil residue on the marsh surface at an impacted site.

Fig. 3.

Oil cover versus plant death as assessed from field observations and from manipulations. (A) Field observations of level of oil on individual plants and resultant plant death, indicated by blade browning. The proportion of plant stems that were green and alive is greater than the proportion dead (i.e., indicating improved health) when oil coverage dropped below 64%. (B) Observation of blade browning 30 d after a treatment of oil coverage of 40% or 80% of the plant’s height (n = 6 per oil-addition treatment). Superscript letters indicate treatments that were significantly different (P < 0.05) based on Tukey's HSD post hoc comparison. Error bars are SEs.

Fig. 4.

(A) There was a significant increase in average lateral shoreline erosion rate between reference and impacted sites (P = 0.007) based on measurements at each site type. Error bars are SEs, and unseen error bars are smaller than symbols. (B) Photo of erosion monitoring poles at an impacted site. Right-most PVC poles were installed to mark the marsh platform edge, and retreat of the marsh from this initial starting point is apparent. (C–F) Comparison between average percentage of plants alive at four times at impacted and reference sites from 0 to 15 m from the shoreline (n = 3). There was a significant effect of the presence of oil, the distance from shoreline, and time (P < 0.0001), with much lower plant coverage near shore for impacted sites than for reference sites during October 2010 and April 2011 but with similar levels of coverage near shore during October 2011 and January 2012 at these sites. Plant coverage was similar for all sites and times at greater distances from the shoreline beyond 10 m from the marsh edge.