Ecosystem recovery at hypersaline salt marsh remediation projects impacted by the Gulf War Oil Spill

The Gulf War Oil Spill was the largest marine oil spill in history, spilling an estimated 6–11 million barrels of crude oil into the Arabian Gulf and impacting approximately 800 kilometers (km) of shoreline in Saudi Arabia between the border with Kuwait and Abu Ali Island (Tawfiq and Olsen 1993, Barth 2008). Many intertidal areas were heavily oiled, with oil penetrating upwards of 40 centimeters (cm) into sediments in sheltered areas (Hayes et al. 1993). The affected area includes large expanses of hypersaline salt marsh, which were severely impacted by the spill. These marshes are very sheltered, with low relief and limited sediment input. Summer temperatures and salinities are extreme, often exceeding 40°C and 45 parts per thousand (ppt), respectively. Marsh topography is dominated by mounds of sediment created by the burrowing activity of the crab Nasima dotilliformis and naturally occurring channels that flood and drain the marsh surface. Vegetation is dominated by two species of annuals and three species of perennial halophytes (salt-tolerant plants). Thin layers of cyanobacterial algal mats are naturally present in the upper elevations of marshes. Many of the marshes impacted are part of the Jubail Marine Wildlife Sanctuary and are habitat for fish and wildlife.

Oiling of sheltered salt marshes caused high mortality of marsh invertebrates and vegetation, which subsequently facilitated the overgrowth of algae into thick, laminate mats. The proliferation of algal mat prevented recolonization of substrates by flora and fauna and altered marsh hydrology by restricting or blocking water flow through marsh channels, preventing marshes from draining fully during the low tide. Although some habitats recovered naturally (e.g. some exposed beaches), recovery in heavily impacted sheltered habitats, such as salt marshes and tidal flats, was limited, even 15 years after the spill (Höpner and Al-Shaikh 2008). This slow rate of recovery by natural processes led Jones et al. (2008) to predict that severely impaired sheltered intertidal habitats such as salt marshes would take at least 50–100 years to recover without intervention.

The Coastal Environment Restoration and Remediation Program (CE-RRP) was established in 2009 by the United Nations Compensation Commission (UNCC) as part of the Kingdom of Saudi Arabia's awards for damages resulting from the 1990 Gulf War Oil Spill, with the aim of restoring selected areas that were not recovering naturally. The goals established by the UNCC for the CE-RRP are to return the sites to a well-functioning ecosystem, as defined by: (1) a characteristic assemblage of native species, (2) the presence of key functional groups of organisms necessary for development of stability of the restored ecosystem; (3) the ability of the system to reproduce and sustain itself over time; (4) the demonstrated resiliency of the system to stress; and (5) the integration of the restored ecosystem into the larger ecological and social matrix of the landscape (UNCC Decision 258 2005).

CE-RRP remediation projects have been ongoing since early 2010 at 19 project areas (‘CRC’ or ‘MRC’) throughout the area of impact (Figure 1). The objective of CE-RRP activities is to accelerate natural recovery by restoring hydrology, providing suitable habitat for recolonization, and removing contaminated sediments (Langman et al. 2012). Primary remediation activities include removal of heavily oiled sediments; excavation of channels to restore hydrology; and substrate tilling, which disturbs laminate algal mats, exposes sediment for recolonization by flora and fauna, and enhances oil degradation in moderately oiled sediments. Additionally, vegetation (primarily mangroves) was planted in selected areas, and unoiled or lightly oiled sediment that was excavated during remediation works was re-formed into “marsh mounds” to provide topographic diversity.

In order to track progress towards restoration goals, long-term monitoring sites were established prior to remediation work. Monitoring sites represent three different ‘treatments’ with respect to remediation activities:

1. Remediation sites - Degraded sites where remediation activities were conducted commencing in 2010;

2. Set-aside sites - Degraded sites that were excluded from remediation activities and are used to provide a contemporaneous baseline indicating natural recovery; and

3. Comparison sites – Reference sites which exhibit indicators of ecological health and represent the goals of the restoration project.

Initial monitoring and assessment of these sites took place biannually (in spring and fall) from 2009–2013. Results were used to evaluate the relationship of environmental variables to project restoration goals (Hale et al. 2011) and develop methods to track overall progress at a site using multimetric indices (MMIs) (Langman et al. 2012). MMIs provide a robust and widely accepted method for combining multiple types of field data to compare ecosystem health at a site to the level of impact or degradation, and can be designed to reflect variation in specific processes (Karr and Chu 1997). Langman et al. (2012) developed a restoration monitoring model for CE-RRP projects, oriented towards including metrics representative of each of the goals of the CE-RRP described above. MMI results from initial monitoring in 2010 and 2011 showed separation between comparison sites and impacted sites (both remediation and set-asides; Figure 2). It was hypothesized that remediation activities would reduce stressors, accelerating natural recovery at a site that can be detected by increases in the ecosystem health MMI. Therefore, by plotting scores for stressors against ecosystem health, recovery at remediation sites can be visualized relative to comparison and set-aside sites, and site-specific progress can be visualized by plotting MMI scores over time.

A long-term biannual monitoring and assessment program began in 2018 to provide information on long-term effects of remediation projects, assess remediation progress, and provide information for adaptive management. Here, we evaluate restoration progress in salt marsh habitats using paired MMIs applied to data from fall 2018 and spring 2019 to provide an initial snapshot of recovery at 18 salt marsh remediation sites.

CE-RRP remediation projects have been ongoing since early 2010 at 19 project areas throughout the area of impact (Figure 1). Projects are designed to remediate salt marshes and/or tidal flat habitats within their boundaries. Methods and results presented here are specific to salt marshes, which are found in 13 of the 19 project areas. Two projects (CRC-2 and CRC-3) have three sets of monitoring plots each, initially established to test variations in remediation methods (e.g. channel excavation and tilling design), bringing the total number of salt marsh remediation sites to 18. There are four set-aside and five comparison sites.

Each salt marsh site consists of nine 10-meter (m) × 10-m monitoring plots, arranged so that three plots are in each of three intertidal elevations (lower, middle, and upper). Plots are also arranged within each elevation, such that one plot is adjacent to a main channel, one adjacent to a smaller channel, and the third is on the marsh plain. This arrangement allows for inference with respect to tidal elevation and proximity to tidal channels, in addition to site-wide progress toward restoration goals and overall trends among comparison, remediation, and set-aside sites.

Monitoring occurs twice a year in the fall and spring. Fall 2018 monitoring was conducted between 12 September and 28 October 2018 and spring 2019 monitoring was conducted between 2 March and 29 April 2019. Sampling is done on a low tide, while the surface of the plots is exposed. Field data are either recorded based on observations of the entire plot or subsampled using 10 representative 0.5 × 0.5 m quadrats. Table 1 summarizes the sampling methods for metrics included in the MMI. The complete sampling methodology can be found in Pandion Saudia (2019) and is also summarized in Langman et al. (2012).

Metrics included in the stressor MMI are direct indicators of ecosystem stressors mitigated by the remediation activities. Stressor metrics include the percentage of unnatural ponding on the marsh surface, percent distribution of thick algal mat, and subsurface oiling condition. Metrics included in the ecosystem health MMI capture the response of natural processes to remediation activities, including increased abundance and distribution of natural flora and fauna and presence of key species. Ecosystem health metrics include snail abundance, Nasima burrow abundance, percent cover of perennial halophytes, species richness, abundance of burrows along channel banks, and abundance of annual plants.

Scaled values for ponding and thick algal mat percentage were reversed, so that all stressor values ranged from degraded (0) to ideal (100). In other words, a low MMI score (whether stressor or ecosystem health) indicates a degraded or impaired condition, while a high score indicates a healthy condition. Ecosystem health and stressor index scores were generated by averaging values of S for component metrics and rescaling so that values ranged from zero to 100. Paired MMI plots were generated for salt marsh sites using data from fall 2018 and spring 2019 and used to compare natural recovery at remediation sites to comparison sites and set- asides. Additionally, sites were grouped by their relative age to facilitate comparisons in progress relative to the time passed since restoration.

Evaluation of individual metrics shows trends that indicate signs of recovery at restoration sites attributable to remediation methods (Table 1; Figure 3). Channel bank burrows, Nasima burrows, perennial cover, and species richness are all higher at comparison sites than remediation or set-asides. Snail and annual abundance are both higher in remediation sites than comparison or set-aside sites. Additionally, several metrics show trends in elevation that differ between remediation sites and set-aside sites (Figure 4). Values for ponding and thick algal mat cover are similar between remediation and set-asides in lower elevation plots but reduced in remediation sites relative to set-asides in middle elevation plots. Nasima burrows are more abundant at set-asides than remediation sites in lower elevation plots but that trend is reversed in middle elevation plots. Snail abundance is higher in middle and high elevation plots at remediation sites compared to comparison and set-aside sites.

Multimetric indices generated from 2018–2019 data show variation in recovery progress across sites (Figure 5). Comparison sites are in the upper right corner, indicating low levels of stressors and high levels of ecosystem health. Set-asides have relatively low stressor values compared to remediation sites. Stressor MMI values of most remediation sites are higher than three of the four set-asides, suggesting that for most sites, remediation measures have reduced stressors relative to sites that were not remediated. MMI values for SA-1 are high compared to other set-asides, which may be because it is less sheltered than the other set-aside sites and situated between two restored areas, which could be sources of recruitment for crabs and vegetation. Three restoration sites stressor MMI values are comparable to the set-aside sites, largely due to the presence of thick algal mat in the middle and upper plots at those locations. There is a wide range of values for the ecosystem health MMIs across all sites, which indicates that the response of natural processes (as defined by indicators of ecosystem health) to remediation activities is variable and possibly site-specific. This variability may be attributed to several factors such as exposure, tidal elevation, inundation, time since restoration, or differences in ambient conditions. Remediation projects completed in fall 2011 or spring 2012 all have ecosystem health scores higher than the mean score for set-asides, and one of the sites from this group, CRC-3 T3, shows scores that are similar to comparison sites (Figure 6). Remediation projects completed in 2013 show a wider range of ecosystem health scores relative to set-asides.

Most remediation sites show some level of recovery, indicated by reduced stressors and the return of annual vegetation and resident marsh invertebrates in lower elevation plots, and presence of thinner, less continuous algal mat types at the middle and upper elevation plots (Figure 7). While lower elevations of untreated sites (set-asides) are beginning to show recruitment of annual vegetation and burrowing crabs, thick, continuous algal mat cover still occurs in the middle and upper elevations at untreated sites creating unnatural ponding and preventing colonization by vegetation and crabs (Figure 8). Additionally, several metrics show trends in elevation that differ between remediation sites and set-aside sites that appear to be indicators of: 1) the effects of refreshing/creating channels at remediation plots, such as improved draining of the marsh surface leading to decreased ponding and preventing regrowth of thick algal mat; and 2) increased tidal flushing, which transports crabs and vegetation to middle plots and provides inundation levels suitable for recolonization. In combination with channel excavation, tilling appears to have created suitable substrate for colonization by annuals and snails. It is possible that there are aspects of recovering sites that enhance habitat for these species, including lack of competition from species that are slower to recruit to an area, relative uniformity of the topography across these plots, and, for mud snails, increased algal cover and refuge from desiccation provided by the structural complexity of algal mats present in recovering habitats.

The status of ecosystem restoration as defined by the MMI indicates that stressors have been reduced in most of the remediation sites; however, ecosystem recovery is variable across sites. There is a relationship between restoration progress and time, illustrated by the cluster of older remediation closest to the region of comparison sites in the upper right corner of Figure 6. Remediation activities concluded at these sites in 2011–2012, suggesting that remediation activities in salt marsh habitats may lead to conditions near fully functional ecosystems in about 10 years. This is in contrast to the observed difference in positions occupied in the same figure by set-aside sites and comparison sites, which indicate that, almost 30 years after the Gulf War Oil Spill, there are still clear differences in stressors and ecosystem health between degraded and healthy salt marshes. Incorporating additional years of monitoring data will allow the project to continue to measure recovery over time in conjunction with historical data and provide information about seasonal variation in these sites. Examining sites that are under- or over-performing sites of the same relative age may also give some insight into site-specific factors that promote or inhibit recovery, such as exposure.

MMIs provide a robust framework for combining data and a method for visualizing restoration progress that is easily interpretable by scientists, managers, and decision-makers alike. The differences illustrated in Figure 2 and Figure 6 clearly justify the value of conducting CE-RRP remediation activities at degraded salt marsh sites. Long-term monitoring data may be applied to improve the design of projects that are currently in progress, or to suggest remedial action for sites which show low levels of recovery. For example, CRC-11 has the lowest ecosystem health score and ranks low on the stressor MMI. The main channel shows signs of recovery, including changes to the channel morphology and the presence of amphipods in the channel bottom; however, secondary channels appear stagnant and are still matted over, vegetation is sparse, and much of the site is still covered with thick algal mat. Remedial actions could include excavation of additional channels, seasonal tilling to encourage the recruitment of annuals, and planting perennials in appropriate habitat.

MMI plots may also be used to evaluate potential sites for new remediation projects, by comparing MMI values observed at candidate sites to the long-term monitoring sites. Sites that score low on the stressor MMI would be expected to show the most improvement from restoration activities. Data presented here are from year one of a four-year program. The incorporation of subsequent monitoring data will allow for the continued refinement of evaluation metrics and application to adaptive management of remediation projects, including the ability to adjust for seasonal variation.