Project Overview

Overview

In January 2016, Dr. Qianxin Lin at Louisiana State University was awarded an RFP- V grant of $1,847,459 to lead the GoMRI project entitled, “Long-Term Impact, Recovery and Resilience: Wetland Plant-Microbial-Benthic Ecosystem Responses to the Deepwater Horizon Oil Spill and Mitigation Strategies Promoting Sustainability” consisted of 3 collaborative institutions and approximately 13 research team members (including students).   The Deepwater Horizon (DWH) oil spill exposed the nation's largest and most productive wetland-estuarine environment, the Mississippi River Delta coastal wetland ecosystem, to an unprecedented potential for environmental damage. The coastal marshes are of special concern because of the suite of environmentally and economically important services they support, all of which depend on a healthy, well-functioning plant- microbial-benthic complex that drives the food web base. Over the last nine years, the research team monitored DWH oil spill effects on coastal salt marshes along northern Barataria Bay in Louisiana, making 26 field-based data collections that have quantified both the impacts on, and recovery of, a broad array of flora and fauna. The overall goals of this research were to (1) document the long-term impacts of the DWH oil spill on the coastal marsh plant-microbial-benthic complex, (2) quantify rates of, and controls on, long-term recovery, and (3) evaluate the potential and effectiveness of a restoration and remediation strategy for promoting and accelerating long-term sustainability.  

Research Highlights

As of December 31, 2019, this project’s research resulted in 11 peer-reviewed publications , 42 scientific presentations and 15 datasets being submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are/will be made available to the public. The project also engaged 2 Master’s and 3 PhD level students over its award period. Significant outcomes of this project’s research according to GoMRI Research Theme 3 are highlighted below. 

The research team conducted a series of field studies to assess oil impacts to coastal salt marshes and their long-term recovery of ecological structure and function of the plant-microbial-benthic system for more than nine years. In the salt marshes of northern Barataria Bay, one of the most heavily oiled areas, the research team established replicated field stations that received heavy (HV), moderate (MD) and no oiling, the latter serving as reference (RF) sites. Impacts of the Deepwater Horizon oil spill to coastal salt marshes along northern Barataria Bay were severe in some areas and moderate in others. Average concentrations of total petroleum hydrocarbons (TPH) in the surface sediment of heavily oiled marshes were > 500 mg g-1 dry soil nine months after the spill and approximately 50 mg g-1 nine years after the spill. Although there has been variable recovery in oiled shoreline marshes after the DWH oil spill, total live plant aboveground biomass was still significantly lower in heavily oiled marshes than that of reference marshes nine years after the spill, however, recovered in moderately oiled marshes within 2-4 years. In addition, impact and recovery were species-specific; in moderately oiled marshes, Spartina alterniflora recovered as early as our first sampling event, 9 months after the spill; however, Juncus roemerianus did not recover until 2-3 years. In heavily oiled marshes, little recovery of Juncus occurred even nine years after the spill although Spartina was able to recover in 2-3 years. Therefore, plant community structure in heavily oiled marsh changed from the Spartina-Juncus mixed community to mostly Spartina community. Total live plant belowground biomass of heavily oiled marshes was also significantly lower than that of reference marshes. The results indicate that heavy oiling still severely affected coastal salt marsh plant community structure and function even more than nine years after the DWH oil spill.

Previously documented impacts of Deepwater Horizon (DWH) oiling on salt marsh plant growth have potentially important implications for ecosystem stability by reducing the rate of soil carbon accumulation, which directly influences the capacity to keep pace with sea level rise. Soil carbon results spanning an 8-yr timeframe from 2011 to 2018 show that both moderately and heavily oiled marshes had significantly lower soil carbon content (mg C g-1 soil) compared to reference marshes. Reduced soil carbon caused by oiling also corresponded with increased soil bulk density (g soil cm-3), likely due to reduced belowground biomass contributions to soil volume. However, when soil carbon was converted to a volume-basis (taking into account soil bulk density), the main effect of oil exposure on oiling category was masked and instead revealed the initial intensity of the DWH spill at heavily oiled marshes. These results indicate that the DWH oil spill compromised salt marsh stability over the long-term, and recovery of ecosystem function is not yet complete.

We tracked the macroinfaunal communities (polychaetes, amphipods, >0.5 mm) until October 2018. Preliminary analysis reveals that overall species richness and Shannon diversity increased significantly in both the heavily oiled and moderately oiled sites, reaching the richness and diversity levels in the reference marshes 8 years after the spill. However, the overall species composition—that is, the relative contribution of different species to the total species pool at any site—did not change significantly over time, suggesting that the oil spill had impacted macroinvertebrate species indiscriminately and that the order of recolonization of these sites by macroinvertebrates was likely not species-specific. The mean density of invertebrates increased in all sites regardless of the impact from the oil spill, with amphipods higher in heavily oiled versus reference marshes. Thus, while the densities of many, but not all, species appear to have recovered, the community structure is different between oiling levels.

No significant differences have been found to exist between either the sampling period or oiling levels and the number or size of crab burrows. This could indicate that the effects of the spill on fiddler crabs are short term. The dominant fiddler crab in the marshes is Uca longisignalis and that was the only species found in the marshes except for the first sampling period in November 2012 when three specimens of Uca spinicarpa were identified from one heavily oiled station and one moderately oiled station. All of this information would indicate that recovery of fiddler crabs at our oiled sites occurred at or before 2.5 years after the spill coinciding with the recovery of vegetation. After 2016, we have started to see a high density of periwinkles at HV stations in the spring and a lower density in the fall in opposition of the growth of Spartina alterniflora. This trend is not evident in the RF and MD sites. Statistically significant trends for snail shell size were found with the oiling levels. All oiling levels including reference sites were affected by the spill and the mean body size was reduced at the beginning of the study. The mean body size at the reference sites has demonstrated a gradually increasing trend to about 19 mm. Both the moderately oiled sites show a gradual increase in average shell length to 18 mm. The heavily oiled increased in average shell length to about 15.5 mm by approximately 5 years after the spill, but gradually decreased to below 14 mm by 8 years after the spill. The relative frequency of subadults and smaller and larger adults at reference and moderately oiled sites remained nearly equally high through Year 3 after the spill. A large proportion of smaller adults occurred at all of the oiling categories in Years 4 through 8 after the spill. However, the relative frequency of larger adults at moderately and heavily oiled sites decreased after Year 4 after the spill. The differences in the proportion of larger adults became more apparent in Year 5 after the spill when reference sites had a higher frequency than moderately oiled sites and moderately oiled had a higher frequency than heavily oiled sites. The proportion of larger adults at the heavily oiled sites remained nearly the same between 10 and 20% through 2015, 2016, 2017, and 2018. This effect would not have been captured without long-term monitoring and indicates that recovery has yet to occur at HV sites.