Thursday 8 March 2012

How the Deepwater Horizon oil spill effected wildlife on the Gulf of Mexico.

On 20 April 2010 an explosion caused by a pressure build up destroyed the Deepwater Horizon oil rig in the Gulf of Mexico, killing eleven workers, and leaving oil pouring into the waters of the Gulf from the uncapped wellhead. This continued until 15 July, when oil-workers eventually managed to cap the well, by which time about 4.9 million barrels of oil had escaped. This resulted in a 210 km² 'kill zone' around the well, inside which everything was apparently dead, large amounts of oil being sighted at all levels in the water column, and oil slicks reaching coasts in 7 US states.

The Deepwater Horizon oil rig caught fire and sank after an explosion in April 2010, leading to the world's worst ever oil spill. New York Times.

This lead to a cleanup on an unprecedented scale and the creation of a US$1 billion fund to help restore the wildlife of the Gulf, on top of US$13.6 billion spent on trying to cap the well and mitigate the spill as it happened and the creation of a US$20 billion fund to help rebuild businesses effected by the spill, which may have some crossover with environmental projects since many of the worst effected businesses were in tourism & fisheries (the three main players in the oil spill, well owners BP, rig operators Transocean and maintenance contractors Halliburton are still involved in a legal battle over who will eventually foot the bill for this).

One positive outcome of all this is that there has been an unprecedented opportunity for scientists to study the outcome of a major oil spill, and the effect it has had on the biology of the Gulf.

In October 2011 a team led by William Montevecchi of the Psychology Department at Memorial University in Newfoundland published a paper in the journal Biology Letters detailing the results of a study into the oil spill's effects on Northern Gannets (Morus bassanus) which breed on the east Canadian coast, but overwinter in the southern US.

A Northern Gannet from a breeding colony on Bonaventure Island in Quebec. Wildlife North America.

Montevecchi et al. were able to compare results from radiotagging studies on birds, which were in process at the time of the disaster, to earlier studies based upon capture-tag-release methods using leg bands. This revealed that in recent years the population overwintering on the Gulf of Mexico has increased from 13.9% of the total in older studies using bands to 26.6% in the more recent, radiotagged studies. This is too big a jump to be accounted for by increased accuracy from a more reliable method, and probably reflects a long-term shift in migration patterns, away from the southern Atlantic coast of the US to the Gulf of Mexico. This implies that the birds were more vulnerable to pollution effects on the Gulf than had previously been realized.

Map of eastern North America showing the summer breeding sites and overwintering areas used by Northern Gannets (Morus bassanus), as recorded by banding, geolocators and Platform Terminal Transmitter (PTT) tags. From Montevecchi et al. (2012).

Montevecchi et al. report that bird rescue teams working on the Gulf recovered 225 dead, oil covered Northern Gannets during the rescue operation, and suspect that many more birds will have died and not been recovered. A further 189 live oiled birds were recovered, of which 117 subsequently died (the highest mortality rate of any birds studied). In addition 99 non-oiled birds were captured and moved to safety. This suggests that Northern Gannets were particularly vulnerable to the effects of the oil spill.

The team also noted that the majority of adult birds had returned to their breeding grounds by the time of the spill, so that the birds killed by the oil were overwhelmingly juveniles (Gannets do not breed till they are 5-7 years old). This means that the breeding colonies were not immediately effected by the oil spill, but that there may be more complex long-term effects as current adult birds die off and there are less juveniles to replace them; such effects would be inherently hard to predict.

Rescue workers attempting to clean an oiled Northern Gannet in Fort Jackson, Louisiana. National Geographic.

In a separate study Brittany McCall and Steven Pennings of the Department of Biology and Biochemistry at the University of Houston carried out research into arthropods in salt marshes around the Mississippi Delta, published in a paper in the journal PLoS ONE on 7 March 2012.

McCall and Pennings sampled populations of 100 species of insects in marshes dominated by Saltmarsh Cordgrass (Spartina alterniflora) both at the time of the spill and a year later. They also looked at the populations of Littoraria snails and burrowing intertidal crabs.

Sites used in the salt marsh insect study. Triangles represent sites where oil slicks occurred, circles sites included as a control. From McCall & Pennings (2012).

McCall and Pennings found that the Littoraria snails were apparently completely unaffected by the spill, with no discernible impact on their population either during or after the spill event. Burrowing crabs suffered a small decline in numbers during the spill, but had recovered a year later. The insects, despite living on plants above the water-line, were far more strongly effected by the spill, suffering a 50% decline in numbers across all species during the spill, but these also appeared to have completely recovered a year later. The Cordgrass itself was badly effected during the spill, dying back from the water's edge across areas of the marshes that were touched by the oil. However a year later this also appeared to have recovered, with the only sign that the spill had occurred being increased nitrogen levels in the Cordgrass leaves.

Cordgrass, Spartina alterniflora, was uneffected by the spill in the upper part of its range (A), but bellow a 'wrack line' (B) it was all dead (C). From McCall & Pennings (2012).

McCall and Pennings are cautious about implying that this suggests salt marshes are tolerant of oil spills. The note that this spill was unprecedented in the scale of the clean-up operation, in particular in the amount of dispersant chemicals used. The subtropical climate of the Mississippi Delta may also have helped bacteria in the salt marsh to break down the oil in a way that might not be reflected in a salt marsh in a cooler climate. Unfortunately there do not seem to have been any earlier studies of oil spills in salt marshes against which to compare this study.