Legacy effects of Hurricane Katrina influenced marsh shoreline erosion following the Deepwater Horizon oil spill

Global hotspots of salt marsh change and carbon emissions

Salt marshes provide ecosystem services such as carbon sequestration1, coastal protection2, sea-level-rise (SLR) adaptation3 and recreation4. SLR5, storm events6, drainage7 and mangrove encroachment8 are known drivers of salt marsh loss. However, the global magnitude and location of changes in salt marsh extent remains uncertain. Here we conduct a global and systematic change analysis of Landsat satellite imagery from the years 2000-2019 to quantify the loss, gain and recovery of salt marsh ecosystems and then estimate the impact of these changes on blue carbon stocks. We show a net salt marsh loss globally, equivalent to an area double the size of Singapore (719 km2), with a loss rate of 0.28% year-1 from 2000 to 2019. Net global losses resulted in 16.3 (0.4-33.2, 90% confidence interval) Tg CO2e year-1 emissions from 2000 to 2019 and a 0.045 (-0.14-0.115) Tg CO2e year-1 reduction of carbon burial. Russia and the USA accounted for 64% of salt marsh losses, driven by hurricanes and coastal erosion. Our findings highlight the vulnerability of salt marsh systems to climatic changes such as SLR and intensification of storms and cyclones.

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

Multiple studies have examined the effects of the Deepwater Horizon oil spill on coastal marsh shoreline erosion. Most studies have concluded that the spill increased shoreline erosion (linear retreat) in oiled marshes by ~ 100–200% for at least 2–3 years. However, two studies have called much of this prior research into question, due to potential study design flaws and confounding factors, primarily tropical cyclone influences and differential wave exposure between oiled (impact) and unoiled (reference) sites. Here we confirm that marsh erosion in our field experiment was substantially increased (112–233%) for 2 years in heavily oiled marsh after the spill, likely due to vegetation impacts and reduced soil shear strength attributed to the spill, rather than the influences of hurricanes or wave exposure variation. We discuss how our findings reinforce prior studies, including a wider-scale remote sensing analysis with similar study approach. We also show differences in the degree of erosion among oil spill cleanup treatments. Most importantly, we show that marsh restoration planting can drastically reduce oiled marsh erosion, and that the positive influences of planting can extend beyond the immediate impact of the spill.

Meta-analysis of salt marsh vegetation impacts and recovery: a synthesis following the Deepwater Horizon oil spill

Marine oil spills continue to be a global issue, heightened by spill events such as the 2010 Deepwater Horizon spill in the Gulf of Mexico, the largest marine oil spill in US waters and among the largest worldwide, affecting over 1,000 km of sensitive wetland shorelines, primarily salt marshes supporting numerous ecosystem functions. To synthesize the effects of the oil spill on foundational vegetation species in the salt marsh ecosystem, Spartina alterniflora and Juncus roemerianus, we performed a meta-analysis using data from 10 studies and 255 sampling sites over seven years post-spill. We examined the hypotheses that the oil spill reduced plant cover, stem density, vegetation height, aboveground biomass, and belowground biomass, and tracked the degree of effects temporally to estimate recovery time frames. All plant metrics indicated impacts from oiling, with 20-100% maximum reductions depending on oiling level and marsh zone. Peak reductions of ~70-90% in total plant cover, total aboveground biomass, and belowground biomass were observed for heavily oiled sites at the marsh edge. Both Spartina and Juncus were impacted, with Juncus affected to a greater degree. Most plant metrics had recovery time frames of three years or longer, including multiple metrics with incomplete recovery over the duration of our data, at least seven years post-spill. Belowground biomass was particularly concerning, because it declined over time in contrast with recovery trends in most aboveground metrics, serving as a strong indicator of ongoing impact, limited recovery, and impaired resilience. We conclude that the Deepwater Horizon spill had multiyear impacts on salt marsh vegetation, with full recovery likely to exceed 10 years, particularly in heavily oiled marshes, where erosion may preclude full recovery. Vegetation impacts and delayed recovery is likely to have exerted substantial influences on ecosystem processes and associated species, especially along heavily oiled shorelines. Our synthesis affords a greater understanding of ecosystem impacts and recovery following the Deepwater Horizon oil spill, and informs environmental impact analysis, contingency planning, emergency response, damage assessment, and restoration efforts related to oil spills.

A review of Gulf of Mexico coastal marsh erosion studies following the 2010 Deepwater Horizon oil spill and comparison to over 4 years of shoreline loss data from Fall 2010 to Summer 2015

The 2010 Deepwater Horizon (DWH) oil spill affected nearly 1105 km of coastal marsh. Long-term shoreline loss in the northern Gulf of Mexico is an important question with far-reaching ecological and human-use implications. Numerous studies have examined potential exacerbated marsh shoreline retreat after the DWH using ground-level sampling and/or aerial/satellite imagery interpretation. This paper reviews previous DWH erosion studies, discusses their limitations and sometimes conflicting results, and provides a comprehensive analysis of a larger data set. Shoreline retreat measurements from multiple studies following the DWH incident were combined for 131 herbaceous marsh sample sites for the period from Fall 2010 to Summer 2015. Significant increases in shoreline loss were found only in the period from Fall 2010 to Fall 2011 for heavily oiled shorelines relative to other periods. The evidence does not suggest widespread long-term coastal marsh erosion from the DWH.

Effects of mangrove cover on coastal erosion during a hurricane in Texas, USA

We tested the hypothesis that mangroves provide better coastal protection than salt marsh vegetation using 10 1,008-m² plots in which we manipulated mangrove cover from 0 to 100%. Hurricane Harvey passed over the plots in 2017. Data from erosion stakes indicated up to 26 cm of vertical and 970 cm of horizontal erosion over 70 months in the plot with 0% mangrove cover, but relatively little erosion in other plots. The hurricane did not increase erosion, and erosion decreased after the hurricane passed. Data from drone images indicated 196 m² of erosion in the 0% mangrove plot, relatively little erosion in other plots, and little ongoing erosion after the hurricane. Transects through the plots indicated that the levee (near the front of the plot) and the bank (the front edge of the plot) retreated up to 9 m as a continuous function of decreasing mangrove cover. Soil strength was greater in areas vegetated with mangroves than in areas vegetated by marsh plants, or nonvegetated areas, and increased as a function of plot-level mangrove cover. Mangroves prevented erosion better than marsh plants did, but this service was nonlinear, with low mangrove cover providing most of the benefits.

Recovery of the salt marsh periwinkle (Littoraria irrorata) 9 years after the Deepwater Horizon oil spill: Size matters

Prior studies indicated salt marsh periwinkles (Littoraria irrorata) were strongly impacted in heavily oiled marshes for at least 5 years following the Deepwater Horizon oil spill. Here, we detail longer-term effects and recovery over nine years. Our analysis found that neither density nor population size structure recovered at heavily oiled sites where snails were smaller and variability in size structure and density was increased. Total aboveground live plant biomass and stem density remained lower over time in heavily oiled marshes, and we speculate that the resulting more open canopy stimulated benthic microalgal production contributing to high spring periwinkle densities or that the lower stem density reduced the ability of subadults and small adults to escape predation. Our data indicate that periwinkle population recovery may take one to two decades after the oil spill at moderately oiled and heavily oiled sites, respectively.

Biodegradation of MC252 polycyclic aromatic hydrocarbons and alkanes in two coastal wetlands

Complementary microbial and geochemical assessment techniques investigated the biodegradation of PAHs and alkanes in salt marshes impacted by crude oil following the Macondo spill. Contamination was observed in the top 10 cm of the marsh profile based on PAH analysis and measurement of the δ¹³C signature of impacted marsh soils. Measurement of evolution of ¹³C depleted CO₂ indicated mineralization of crude oil ranging from 2.7-12.1 mg CO₂-C/m²-hr. Changes in weathering ratios of alkylated phenanthrenes and dibenzothiophenes indicated loss of these 3-ring PAHs consistent with biodegradation. A diverse microbial population was observed at both locations dominated by Gammaproteobacteria and including known hydrocarbon degraders such as Marinobacter and Alcanivorax. There was shared richness between sites and across seasons but results suggested substantial turnover of phylotypes in space and time. Biodegradation of alkanes and alkylated PAHs occurred when oxygen was provided in laboratory reactors but not in the absence of oxygen.

High-Efficiency Determination of Coastline by Combination of Tidal Level and Coastal Zone DEM from UAV Tilt Photogrammetry

To meet the needs of coastline efficient extraction and dynamic monitoring, this paper proposes a new method for coastline extraction by combining the tidal level and the digital elevation model (DEM) of the coastal zone from tilt photography. Firstly, the DEM of coastal zone was obtained by using unmanned aerial vehicle (UAV) tilt photography; at the same time, the accuracy of aerial triangulation(AT) is improved referencing to the constraint of water boundary points, and then the mean high water spring tide was obtained by combining tidal harmonic analysis and Global Navigation Satellite System (GNSS) tidal level. Finally, the coastline and the dynamic water-surface line are extracted from the DEM of the coastal zone by tracking the contour lines with the elevation of the mean high water springs (MHWS) and the instantaneous sea-surface elevation, respectively. The experiments carried out in the coastal zones of Liaoning Province, China, proved the proposed method and achieved better than 0.2 m of horizontal position accuracy and 0.1 m of the vertical accuracy.

Macroinfauna responses and recovery trajectories after an oil spill differ from those following saltmarsh restoration

Given the severity of injuries to biota in coastal wetlands from the Deepwater Horizon oil spill (DWH) and the resulting availability of funding for restoration, information on impacted salt marshes and biotic development of restored marshes may both help inform marsh restoration planning in the near term and for future spills. Accordingly, we performed a meta-analysis to model a restoration trajectory of total macroinfauna density in constructed marshes (studied for ~30 y), and with a previously published restoration trajectory for amphipods, we compared these to recovery curves for total macroinfauna and amphipods from DWH impacted marshes (over 8.5 y). Total macroinfauna and amphipod densities in constructed marshes did not consistently reach equivalency with reference sites before 20 y, yet in heavily oiled marshes recovery occurred by 4.5 y post spill (although it is unlikely that macroinfaunal community composition fully recovered). These differences were probably due to initial conditions (e.g., higher initial levels of belowground organic matter in oiled marshes) that were more conducive to recovery as compared to constructed marshes. Furthermore, we found that amphipod trajectories were distinctly different in constructed and oiled marshes as densities at oiled sites exceeded that of reference sites by as much as 20x during much of the recovery period. Amphipods may have responded to the rapid increase and high biomass of benthic microalgae following the spill. These results indicate that biotic responses after an oil spill may be quantitatively different than those following restoration, even for heavily oiled marshes that were initially denuded of vegetation. Our dual trajectories for oil spill recovery and restoration development for macroinfauna should help guide restoration planning and assessment following the DWH as well as for restoration scaling for future spills.

Evaluation of UAV LiDAR for Mapping Coastal Environments

Unmanned Aerial Vehicle (UAV)-based remote sensing techniques have demonstrated great potential for monitoring rapid shoreline changes. With image-based approaches utilizing Structure from Motion (SfM), high-resolution Digital Surface Models (DSM), and orthophotos can be generated efficiently using UAV imagery. However, image-based mapping yields relatively poor results in low textured areas as compared to those from LiDAR. This study demonstrates the applicability of UAV LiDAR for mapping coastal environments. A custom-built UAV-based mobile mapping system is used to simultaneously collect LiDAR and imagery data. The quality of LiDAR, as well as image-based point clouds, are investigated and compared over different geomorphic environments in terms of their point density, relative and absolute accuracy, and area coverage. The results suggest that both UAV LiDAR and image-based techniques provide high-resolution and high-quality topographic data, and the point clouds generated by both techniques are compatible within a 5 to 10 cm range. UAV LiDAR has a clear advantage in terms of large and uniform ground coverage over different geomorphic environments, higher point density, and ability to penetrate through vegetation to capture points below the canopy. Furthermore, UAV LiDAR-based data acquisitions are assessed for their applicability in monitoring shoreline changes over two actively eroding sandy beaches along southern Lake Michigan, Dune Acres, and Beverly Shores, through repeated field surveys. The results indicate a considerable volume loss and ridge point retreat over an extended period of one year (May 2018 to May 2019) as well as a short storm-induced period of one month (November 2018 to December 2018). The foredune ridge recession ranges from 0 m to 9 m. The average volume loss at Dune Acres is 18.2 cubic meters per meter and 12.2 cubic meters per meter within the one-year period and storm-induced period, respectively, highlighting the importance of episodic events in coastline changes. The average volume loss at Beverly Shores is 2.8 cubic meters per meter and 2.6 cubic meters per meter within the survey period and storm-induced period, respectively.