Oil spills can wreak havoc on ocean ecosystems, but ocean waters aren’t the only places where oozing oil and its cleanup compounds may cause harm. Spills can creep into coastal estuaries, where fresh and salty waters mix, and into the bodies of animals that dwell there.
New experiments have now exposed a common shrimp important to the health of estuaries to dispersants, the chemicals used to break up spilled oil. The study found a deadly impact from two dispersants approved by the U.S. Environmental Protection Agency (EPA), one of which was heavily used by British Petroleum (BP) in response to the 2010 Deepwater Horizon spill.
What’s more, the harm to shrimp in the experiments grew worse in fresher water. Because an estuary’s salinity can shift quickly, animals that regularly swim between salty and fresher waters could be caught in a suddenly toxic environment.
In the study, marine scientist Charlotte Eckmann, of the University of South Carolina in Columbia, tested the dispersants on Palaemonetes pugio, a species known commonly as the daggerblade grass shrimp. Eckmann did the research as a Hollings Scholar at the National Oceanic and Atmospheric Administration’s (NOAA) Center for Coastal Environmental Health and Biomolecular Research in South Carolina.
She exposed the shrimp to varying concentrations of two EPA-approved dispersants: Corexit 9500, of which Eckmann said BP dumped 7 million liters into the Gulf of Mexico during the 2010 Deepwater Horizon oil spill, and Finasol OSR 52, which she said is being stockpiled in the United States for future spills.
Because estuary waters range from heavily salty near the sea to fresher toward land, Eckmann ran her tests in waters of three different salinities. Over 4 days, she examined the shrimp for signs of stress and recorded how many of them died at each level of salinity.
Eckmann said grass shrimp heavily influence other estuarine wildlife. “Grass shrimp are an important prey item,” so contamination and loss in their population would harm their predators and other species higher up in the food chain, she noted. Also, because grass shrimp free up nutrients locked inside decaying matter, a drop in shrimp could mean a drop in nutrient levels that other animals and plants rely on.
Eckmann presented the findings by her and marine scientist Marie DeLorenzo of the College of Charleston, also in South Carolina, and the NOAA center on Wednesday at the 2015 American Geophysical Union Fall Meeting in San Francisco, Calif.
From her shrimp body counts, Eckmann assigned each of the tested compounds what’s known as an LC50 toxicity score. It tells how little of a chemical is needed to kill half the animals in a closed system. A lower score means greater toxicity, so highly toxic compounds have single-digit scores.
The EPA lists Finasol’s and Corexit’s LC50 scores as 11.66 and 25.2, respectively, from tests on the inland silverside, a small neotropical fish. In Eckmann’s study, Finasol and Corexit scored 8.21 and 35.44, respectively, on larval shrimp in the least salty water.
Eckmann noted that larval shrimp were most sensitive. According to her LC50 scores, Finasol can kill twice as many larval shrimp as adults. A large portion of the shrimp’s population could be killed if dispersant levels were sufficiently high. At all salinity levels, Finasol ranged from 2 to 25 times more toxic than Corexit, which is banned in the U.K.
Even when the shrimp survived exposure to high concentrations of the dispersants, the chemicals tinted their gills a deep red. Exposed shrimp also showed internal signs of stress; Eckmann found high levels of malondialdehyde, a chemical that signals cellular stress, in the tissues of exposed shrimp. “They weren’t normal healthy shrimp,” she said.
Less Salty Means More Toxic
The dispersants became more toxic in less salty water, said Eckmann. Because weather can significantly shift an estuary’s salinity in as short a time as 1 day, said Eckmann, less toxic water could quickly become dangerous to shrimp. In high-salinity water, it took 64 milligrams per liter for Finasol to kill half the adult shrimp. However, when Eckmann did the same test in low-salinity water, it took less than a quarter of that amount to kill the same number of shrimp.
Eckmann said she doesn’t know if the concentrations of dispersants used in the experiments were comparable to the amounts in the environment from responses to oil spills such as Deepwater Horizon. It’s difficult to tell what concentrations of dispersants might be in the wild, she noted. The complex fluids contain many different chemicals. “Measuring concentrations of dispersant in the wild is pretty much impossible at this point, so, as of now, there’s no protocol I know of to determine the concentration from a seawater sample,” Eckmann said.
Biologist Pamela Betancourt, who didn’t participate in the study, said the research made her curious about what long-term changes may come from exposure to dispersants. “We don’t know how the dispersants will affect [shrimp] development, so, in generations, are we going to have smaller shrimp? Are we going to have longer development?”
“The biggest thing I got out of it was that we have to be more careful when we use Finasol or Corexit,” said Betancourt.
—Brendan Bane, Science Communication Program Graduate Student, University of California, Santa Cruz, email: [email protected]
Citation: Bane, B. (2015), Oil dispersants deadly to a common estuary species, Eos, 96, doi:10.1029/2015EO042227. Published on 18 December 2015.