Far below the Gulf's surface, an ecological collapse has been unfolding largely out of sight over the past 16 years.
A new study published in PNAS found populations of deep-water fish species, crustaceans and squid declined significantly following the 2010 Deepwater Horizon oil spill. More than 16 years later, they have yet to recover. The study combines more than 2,000 net samples, acoustic surveys and genetic analyses to provide the most comprehensive long-term assessment yet of post-oil-spill life in the Gulf of America's deep, open waters. The research team was led by Tracey T. Sutton from Nova Southeastern University and includes FIU marine scientists Kevin Boswell, Heather Bracken-Grissom, Marta D'Elia and Pedro A. Peres, all researchers in FIU's Institute of Environment.
"Genetic diversity is nature's insurance policy," said Bracken-Grissom. "It is the foundation for species to adapt and survive in a rapidly changing world. As genetic diversity is lost, so too is the evolutionary potential that enables populations to respond to environmental change."
The losses occurred from the surface to the deep-pelagic ecosystem including the vast expanse of dark ocean between 600 and 5,000 feet. This includes the deep scattering layer, a zone teeming with fish, squid and other small animals that migrate toward the surface at night to feed and return to deeper waters during the day. This habitat accounts for more than 90 percent of the Gulf's volume and supports an extraordinarily rich assemblage of marine life.
"The community of organisms that comprise the deep scattering layer is a diverse complex of animals," Boswell said. "To be able to measure changes in the ecosystem across independent methodologies really drives home the point that the midwater ecosystem has undergone a significant change."
The researchers analyzed the net samples collected between 2011 and 2023, paired those findings with acoustic surveys and examined changes in the genetic diversity of three lanternfish species. The results revealed declines of 64 percent in fish species, 62 percent in large crustaceans and 60 percent in small squid and other cephalopods. Some of the most ecologically important animals were hit even harder. Krill declined by nearly 73 percent, lanternfish by 63 percent and bristlemouth fish by 69 percent.
Dense patches of prey that were common in 2011 also virtually disappeared. These concentrations are essential for whales, dolphins, tuna, sharks, seabirds and other predators that must find enough food to make the energy spent hunting worthwhile.
"Our findings are particularly concerning because these are species that are abundant, widespread, and historically might not have been considered vulnerable," Peres said. "What we are seeing suggests that even species that appear abundant can experience substantial changes that may go unnoticed without monitoring."
The researchers considered a variety of potential culprits and ruled out several possible explanations. They used consistent sampling equipment and methods, accounted for differences among Gulf water masses and found no corresponding changes in deep-water temperature, oxygen or food availability.
While the study does not establish direct causation, the authors identify the Deepwater Horizon spill and its associated response efforts as the most plausible explanation for the timing and scale of the decline. The disaster released between 4 million and 4.9 million barrels of oil, while approximately 770,000 gallons of chemical dispersant were injected near the wellhead.
Oil-related compounds have since been detected in deep-water fish eggs and cephalopod tissue. Such contamination can cause mortality, developmental damage and reduced fitness. It may also interfere with smell — potentially preventing widely dispersed deep-sea fish species from locating mates in the darkness.
The consequences could extend throughout the Gulf. These animals provide food for its largest predators and transport carbon from surface waters into the deep ocean. Their loss may weaken marine food webs and reduce the ocean's ability to store carbon.
The researchers conclude that the Gulf may have entered a persistently depleted ecological state with no clear timeline for recovery — revealing that even the planet's largest and seemingly most stable ecosystems can fall victim to human impacts.
