By Alisha Renfro, Ph.D., National Wildlife Federation
The BP oil disaster introduced more than 200 million gallons of crude oil into the Gulf of Mexico, just off the southeastern Louisiana coast. An extensive effort is currently underway to assess the full damage of this catastrophic event on the rich and complex Gulf ecosystem.
A study lead by Deepak Mishra, Ph.D. and published in Remote Sensing of Environment (vol. 118) is the first look at short-term impacts the oil spill had on the salt marshes of southeastern Louisiana. The study, “Post-spill state of the marsh: Remote estimation of the ecological impact of the Gulf Of Mexico oil spill on Louisiana Salt Marshes,” examines the impact of the oil, dispersants and cleanup efforts on salt marshes in the months following the spill. Researchers used field measurements and satellite imagery from 2009 and 2010 to compare characteristics of vegetation related to health and productivity before and after the spill.
Oil can have a direct and immediate impact on salt marsh health by coating the vegetation and soil surface, leading to temperature stress and a reduction in photosynthesis. Compounds found in crude oil can also negatively affect the ability of salt marsh vegetation to tolerate salinity, leading to vegetation dieback and slowing down marsh recovery. And while oil can have a damaging affect on marshes, cleanup efforts such as skimming, oil collection and burning can cause additional damage.
For the study, the research group identified and sampled marsh plots ranging from no oil present to heavily-oiled during the post-spill growing season. Measurements of different vegetation characteristics were taken at the field sites and used to calibrate models that were then applied to satellite imagery from the 2009 and 2010 growing seasons (April-October). Fortunately, 2009 and 2010 were years with similar variations in temperature and precipitation, as well as no hurricanes or other disturbances that directly affected the area, except for the oil spill.
The research suggests that during the pre-spill growing season (2009), there were roughly 32 square miles of salt marsh that exhibited markers of vegetative stress and decreased productivity. After the spill, this area of stressed marsh increased dramatically to around 250 square miles. It is important to note that while the increase of stress and reduced productivity in a salt marsh is not directly related to land loss, loss can occur as a result of a weakened root system – increasing the vulnerability of salt marsh to erosion from waves, tides and storm events.
The long-term effects of the oil spill on the large and complex Gulf ecosystem are not yet known, but this first look at the short-term impact to the salt marshes of southeastern Louisiana indicates that the damage directly related to the oil spill may be significant. Some recovery of oiled marshes has been observed, but the residual oil on Gulf Coast salt marshes will have a long-term impact on overall marsh health and productivity for years to come.