Researchers at the University of Southern Mississippi, US, have now assessed how plants in this wetland area have been affected by the oil spill by using a novel multi-scale hierarchical Bayesian (HB) model. The results are reported in Environmental Research Letters (ERL) as part of the ERL Focus on the Deepwater Horizon Oil Spill.

Coastal wetlands are one of the most important and productive ecosystems in the world but are constantly at risk from natural and man-made disasters such as flooding, erosion, rising sea levels and pollution.

The way coastal wetland vegetation responds to weathered crude oil is complex – the plants can either rapidly recover or die over a longer period. To study the impact of the Deepwater Horizon oil spill on the photosynthesis of coastal salt-marsh plants (an important marker for wetland health), a group of ecologists from the Department of Coastal Sciences at the University of Southern Mississippi visited wetland at an estuarine environment in Mississippi 13 days after weathered oil had arrived at the site. The researchers sampled Spartina alterniflora (smooth cordgrass), a dominant marsh species in the northern Gulf of Mexico, at three locations that had been affected by the oil spill. They measured the rate of photosynthesis in randomly chosen individual plants using an LI-6400 XT portable photosynthetic gas exchange and chlorophyll fluorescence system.

The group continued with the measurements every month until September 2012. Using field data from July 2010 to November 2011, a multi-scale HB model was developed to determine how plants in different areas had been affected by the spill, and when they recovered.

The results showed that photosynthesis rates at locations heavily impacted by the spill recovered to the level of a control location about three to four months after the initial impact. The quick recovery might be because the oil was present only in small concentrations in these locations and patchily distributed. Other explanations include the fact that the oil quickly degraded thanks to high ambient temperatures, or may have been naturally removed by waves and tides on the eroded shoreline.

The scientists are currently testing the likely mechanisms behind this quick recovery by analysing the photosynthesis data collected at other wetland shorelines on the Mississippi Gulf Coast. The team is also trying to understand how the resilience of wetlands to oil varies from area to area along the coast.

The study improves understanding of the impacts of the Deepwater Horizon oil spill on coastal wetlands in Mississippi and is complementary to previous studies that have focused on regions like Louisiana, a large marsh area. It also shows that the HB modelling approach is a new and promising tool to assess ecological recovery.