Experiments Probe How Copepods Cope with Oil

The seabirds and harp seals that grace Dawn dish soap bottles are the posterchildren of oil spill clean-up, but spills may profoundly impact a vital animal that's harder to see – the tiny, ubiquitous copepod.

The size of a single rice grain, copepods are a critical part of the ocean food web in the Arctic, a region whose rich resources and remote location makes it especially vulnerable to oil spills. Bigelow Laboratory Senior Research Scientists Christoph Aeppli and David Fields are uniting their expertise in oil spill chemistry and copepod biology to clarify how spills impact copepods in the Arctic and beyond.

"This project is so powerful because we are applying the understanding from our complimentary research specialties to address a complex, real-world problem," said Aeppli, an environmental chemist and the project lead. "We are taking a new experimental approach to bridge the gap between the spills that occur in nature and the studies that take place in the lab, and what we find will help responders make the best possible choice in a bad situation."

Oil spills impact life at every level of the ocean food web. After a spill occurs, response efforts must move quickly to minimize the inevitable damage to surrounding ecosystems. One proven method is to apply chemical dispersants, which act like soap to break the oil into small droplets that can be mixed into the water column, diluted, and consumed by microbes. However, the complexity of adding an additional chemical to the environment makes dispersants controversial. In addition, it is often difficult to translate small-scale laboratory studies of dispersed oil to what actually happens in the ocean, which makes it hard to assess this approach's full impact on an ecosystem.

Many of the compounds that compose oil have toxic effects on marine life. When dispersants are applied after a spill, these compounds are no longer relegated to an oil slick on the surface ocean. They become able to travel down into the water column, where they interact with organisms that live there, including copepods.

Copepods migrate hundreds of meters during a daily cycle that moves them between the sea's safe depths and the food-rich surface ocean. It's not yet clear what happens when they come into contact with spilled oil. Aeppli and Fields are testing whether copepod's rich sensory system can help them alter their swimming behavior to avoid it.

"Animals are constantly making decisions to help them survive in their environments, and that makes the question of how something like an oil spill will impact them really nuanced," Fields said. "Understanding the interplay between oil toxicity and animal behavior is essential to assessing whether dispersants are the right choice after a spill."

Copepods are intensely fatty, which makes them prime prey for predators like fish. This dense energy fuels the entire food web, eventually reaching top predators like polar bears and walruses. Fields and Aeppli believe that oil toxins could be stored in the fats that compose copepods' bodies – which could have profound effects on the entire ecosystem.

During the scarcity of food in winter, copepods hibernate in the deep ocean, before migrating back up to the surface to feast on blooming algae in spring. If they accumulate toxins in their bodies, copepods could effectively act as a reservoir of toxins that reappear on the scene a year after a spill has occurred, prolonging its effects.

"The answers to these questions will help us understand the true impact of oil spills in the Arctic, as well as other environments where copepods are a key link in the food web," Fields said. "In addition to addressing this important, concrete question, this project gives us a unique opportunity to learn more about copepods and how they make choices to survive in dynamic ocean ecosystems."

Through a series of experiments, the researchers are testing how copepods respond to oil and dispersants in large tanks that simulate the ocean environment. Their work expands upon related experiments, including one conducted by a Research Experience for Undergraduates (REU) student at Bigelow Laboratory in 2010 that tested the concentrations at which oil is lethal to copepods, and how their behavior changes in the face of toxicity. During the summer of 2020, two more REU students will join the project team, conducting independent research under the mentorship of Aeppli and Fields.

Funded by the Department of Homeland Security's Arctic Domain Awareness Center, this project leverages Bigelow Laboratory's unique seawater facilities to circumvent the limitations of traditional laboratory experiments, which don't allow for the copepods' movement and physical differences between the layers that form the water column. Their experiments take place in specially built tanks that allow the copepods room to swim vertically, and are equipped with cameras and sampling ports that enable the scientists to observe their behavior in response to varying oil concentrations.

"This is exactly what I think science should do – give information so that people can make the best possible choices," Aeppli said. "An essential part of what we do here at Bigelow Laboratory is to provide unbiased science that can help solve real-world problems, and to learn more about how our planet works in the process."