Dr. Irving Mendelssohn at Louisiana State University was awarded an RFP-II grant at $1,669,037.00 to lead the RFP-II project entitled “Accelerating Recovery after the Deepwater Horizon Oil Spill: Response of the Plant-Microbial-Benthic Ecosystem to Mitigation Strategies Promoting Wetland Remediation and Resilience” that consisted of 3 institutions and 21 research team members, with Drs. Kevin Carman, John Fleeger, Aixin Hou, Qianxin Lin, and Don Deis as co-investigators. Through this work, Dr. Mendelssohn and his team aimed to: 1) document longer-term impacts of the DWH event on the plant-microbial-benthic system; 2) quantify rates of, and controls on, plant-microbial-benthic recovery; and 3) evaluate the effectiveness of remediation treatments for accelerating rates of recovery and long-term sustainability of the plant-microbial-benthic system.
Dr. Mendelssohn’s research included 4 outreach products and activities and resulted in 10 peer-reviewed publications, 32 scientific presentations and 12 datasets that have been submitted to the GoMRI Information and Data Cooperative (GRIIDC), and are/will be available to the public. Dr. Mendelssohn also engaged 18 students in GoMRI research over the course of his award. Significant outcomes of his research are highlighted below.
Theme 3: Environmental Effects
- Heavily oiled marshes, which had highest soil TPH concentrations, showed recovery of Spartina alterniflora in 3 years, and no recovery of Juncus roemerianus. Moderately oiled marshes, with significantly less oil contamination, generally recovered vegetation within 2-3 years; heavily oiled marshes had not recovered after 5 years. Heavy oiling reduced live belowground biomass and soil shear strength, resulting in increased soil surface erosion compared to unoiled reference marshes.
- In contrast to soil surface erosion, post-spill shoreline retreat rates, although somewhat higher compared to immediately pre-spill, were not statistically different than pre-spill/post-Katrina rates within any of the oiling categories. As a result, we found no evidence that oiling, overall, accelerated shoreline retreat, although specific shorelines may show oil-induced accelerations.
- Bacterial communities were significantly altered by the spill, but they recovered over time with faster recovery in moderately oiled than heavily oiled marshes. However the relative abundances of certain species were still significantly different after five years post-spill. Evaluation of oil transformation products suggests heavy oil contamination hinders microbial activity.
- Although impacts to microalgal biomass and meiofaunal abundance and diversity were strongest in heavily oiled marshes, where almost complete mortality of S. alterniflora occurred, impacts were also apparent at lightly and moderately oiled sites. However, these metrics, as well as photosynthetic pigment composition and meiofauna community composition, indicated substantial recovery ~36 months post-spill, coincident with recovery of S. alterniflora stem density, even in heavily oiled marshes. Regardless, certain mostly rarer species with poor dispersal capabilities had not recovered four years after the spill.
- From the standpoint of macrofauna, neither fiddler crab burrow density nor diameter significantly differed among oiled and reference sites, possibly because recovery occurred before the first sampling event. Increases in density were observed in all oiling categories and shell-length groups during our sampling period; however, periwinkle densities over time were consistently highest at moderately oiled sites where Spartina alterniflora aboveground biomass was highest. Periwinkle mean shell length linearly increased over the study period at reference sites. In contrast, shell lengths at moderately oiled and heavily oiled sites increased through month 48 after the spill, but then decreased associated with a decline in the relative abundance of large adults at oiled sites which was likely caused by chronic hydrocarbon toxicity or oil-induced effects on habitat quality or food resources. Overall, the recovery of S. alterniflora facilitated the recovery of fiddler crabs and periwinkles.