After the Deepwater Horizon disaster unleashed more than 3 million barrels of crude oil into the northern Gulf of Mexico in 2010, many wondered how it would affect the area’s marine life.
Early on, scientists predicted the oil would kill large numbers of fish, leading to an overall decline in numbers. However, as authors of a recently published scientific study have learned, the ecological impact of the spill is more complicated than first imagined.
“During the period immediately following the spill, the total number of fish in our samples actually increased above levels we saw before the spill,” said Charles Martin, lead author of the study and assistant research professor of estuarine ecology at the University of Florida.
“During our sampling period, we found that the diversity of fish — how many different species we were finding — decreased, with some species far outnumbering others, and some not present at all,” Martin said.
These shifts in the abundance and diversity of sea life suggest that the oil spill’s many direct and indirect effects restructured the food web over the weeks, months and years following the disaster.
Martin was already doing research in the Gulf when the disaster occurred. As a postdoctoral researcher in the laboratory of John Valentine at the Dauphin Island Sea Lab, which sits off the coast of Alabama at the mouth of Mobile Bay, he was in the right place at the right time to study the effects of the oil making its way to the Alabama and Mississippi coastlines.
As oil began to cover the water and wash up on beaches, Martin and his team started trawling the area — literally.
“To collect fish, we used a trawl net attached to the back of our research boat, not unlike what you see on shrimping boats, except we were looking for more than shrimp,” Martin said. He and his team sampled a stretch of ocean just south of the barrier islands off the coasts of Alabama and Mississippi.
“This area was being impacted by the oil, and there was a lot being done in the area to manage it, so that made it a good location to sample,” said Martin, who is now based at the UF/IFAS Nature Coast Biological Station in Cedar Key, Florida.
He and his team trawled randomly selected areas of their research site for about 10 to15 minutes at a time. They then recorded the number of animals in the net, their species and size before releasing their catch back into the ocean. They did this nearly 200 times over two periods following the spill: 2010 to 2012 and 2014 to 2017.
“In the weeks and months after the spill we saw the biggest spike in the number of animals we were catching. At one time, we were pulling more than 100 fish onto our boat with each trawl,” Martin said.
They compared this post-spill data to trawl data gathered in the same area during two periods before the oil spill: 1997 to 2001 and 2007 to 2009. These data were collected by the National Oceanic and Atmospheric Administration (NOAA). Martin and his team used the NOAA data as baseline from which to compare their post-spill data.
The increase in the number of animals collected during the period immediately following the Deepwater Horizon disaster, though surprising at the time, makes more sense when you look at the many direct and indirect effects of the oil spill, Martin said.
“In our study, we identified several variables that, in combination, may have led to an increase in populations,” Martin said.
“Some of these variables were direct effects of the spill. They included things like behavioral avoidance: fish may have been avoiding the oil in the offshore area near the drill site and so moved toward the coast, where we were sampling. Also, we know many predators, such as birds and dolphins, were negatively impacted by the oil, and fewer predators would have led to an increase in fish and other animals lower down on the food chain,” he said.
At the same time, indirect effects of the disaster may have contributed to the increase in the number of animals sampled, he explained.
“Indirect effects include management actions meant to mitigate the impact of the oil on marine life, such as allowing more fresh water to flow out of diversions of the Mississippi River, as well as the temporary closure of the fishery to commercial and recreational fishing,” Martin said. “These actions may have offset losses we would have expected due to the spill.”
These direct and indirect effects may also be linked to the changes in species diversity documented in the study, he explained.
“Some species may have benefited more from these changes than others, which would have led to an increase in those species’ survival,” Martin said.
Martin says more research and monitoring are needed to tease out how different variables contribute to ecological changes in the Gulf.
“The Deepwater Horizon disaster really focused attention on the biodiversity of the Gulf. The more information we have on the ecosystem there, the better we’ll be able to quantify the impact of natural and man-made disasters, as well as all the ecosystem services the region provides,” he said.
Future research will aim to model the Gulf food web to better understand how disruptions like an oil spill can reverberate through the ecosystem.
Martin co-authored the study, which is published in Marine Pollution Bulletin, with Kristy Lewis of the University of Central Florida; Ashley McDonald, postdoctoral researcher in the UF/IFAS department of fisheries and aquatic sciences; Trey Spearman of the University of South Alabama; Scott Alford, a doctoral student in the UF/IFAS department of fisheries and aquatic sciences; Robert Christian of East Carolina University; and John Valentine of the Dauphin Island Sea Lab.
Both McDonald and Alford are part of Martin’s lab at the UF/IFAS Nature Coast Biological Station.
This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org (https://doi.org/10.1016/j.marpolbul.2020.111098).