Increased erosion of salt marshes after Deepwater Horizon oil spill

26-06-2012   The edges of the salt marshes in the Gulf of Mexico have been eroding more quickly since the Deepwater Horizon oil spill, and parts are lost permanently. This is the outcome of research that is published by the American journal Proceedings of the National Academy of Sciences and that was carried out by an American-Dutch research team. This erosion is caused by a combination of marsh compaction and acute oil contamination.

Salt marsh vegetation is largely resilient to currents and waves. The plants break the waves and they slow down the current, because the water cannot flow freely between the stems; sediment is deposited in the sheltered zones in the vegetated area. In this way, the marshes grow and keep up with the rising sea level. These proved to be effective characteristics to protect the marshes against oil contamination.

During the Deepwater Horizon oil spill in April 2010, large amounts of oil were released into the Gulf of Mexico. A considerable quantity was washed ashore. Where oil was washed into the marshes, almost all oil was blocked by the plants growing on the first ten to twenty metres of the marshes. The remainder of the marshes were thus largely protected against contamination, but on the edges where the oil was left behind, most plants died.

Marsh plant roots hold the sediment, and thus they protect the marsh against erosion by currents and waves. When the plants die, the marsh is no longer protected. The scientists found that the edges of the marshes eroded more quickly after the oil spill. This part of the marshes is lost permanently. There is only little tidal fluctuation and the layer of sediment is too low after erosion to allow for new vegetation to grow. The relative rise of the sea level in this area, which is caused by marsh compaction, increases the pressures on the marsh vegetation. Oil pollution enhances this, and it causes larger and permanent loss of vegetation on the edges of the marshes. This may well occur more often in the future. Climate change and human activities increase the vulnerability of the coastal ecosystem, so that it gets harder to deal with additional threats, which may cause natural losses.

This research project was carried out by an American research team collaborating with Prof. Johan van de Koppel, a Dutch scientist at NIOZ in Yerseke, the Netherlands. He said “We have previously studied erosion at the edges of the Dutch salt marshes. Erosion is a cyclic process here. Vegetation holds sediment, the marshes are elevated and they form steep, cliffed edges that are eroded by waves. In front of the cliff, new vegetation sprouts, and this is the beginning of a new cycle of rise and decay. The principle is the same in the American salt marshes, but the circumstances are different because of the salt-marsh compaction. It is remarkable that the additional threat of the oil spill immediately caused a loss of salt marsh. It could be that the Dutch coast will also get more vulnerable when the sea level rises.”