Study Probes Seismic Blast Impacts on Zooplankton

06-26-2020

Every day, waves of manmade sound boom through the global ocean. Underwater seismic blasting is a useful tool for fields from oceanography to oil exploration, but the sound and physical waves it generates can harm ocean life. Researchers have extensively studied how this practice impacts whales and other large animals – but the effects on the ocean’s smallest animals remain largely unknown.

“Every animal in the ocean, from swordfish to lobsters, begins life as a tiny planktonic being,” said Senior Research Scientist David Fields. “The world has been widely using seismic blasting for 60 years, so it’s surprising that we don’t yet understand how it impacts a huge diversity of sea life.”

Fields and a team of Norwegian collaborators aim to develop that understanding. In seismic blasting, a vessel tows an array of airguns that shoot compressed air toward the seafloor. The reverberations that return to the vessel contain information about the floor structure and sediments below. The research team is evaluating how these blasts impact the ocean’s smallest animals, called zooplankton.

This effort began in 2009, when Fields first tested how blasting affects a type of tiny crustacean called copepods. He found that the acoustic waves impact animals within close range of the source, killing some copepods within five meters – but that this effect quickly tapers off, and it is gone 10 meters from the blast.

In 2017, however, a paper in the journal Nature published a very different conclusion about the threat that blasting poses zooplankton. A team of researchers based in Australia found that seismic blasts killed animals over a kilometer away from the airgun array – a result so different from their own that it surprised Fields and his colleagues.

“When we became aware of this paper, we knew it was important to share the data that led us to our alternative interpretation of what blasting does to zooplankton,” Fields said. “This kind of dialogue is a key part of the process of science, and it is essential for arriving at the robust conclusions that are so important for the global community and decision-makers.”

After publishing their own results in the ICES Journal of Marine Science, Fields and his colleagues decided to explore the issue further. In February, they began a new set of experiments to examine how seismic blasting impacts a broad range of zooplankton species in their natural ocean environment. These experiments are part of a large, interdisciplinary project studying how seismic blasting affects cod and the zooplankton that are critical prey for this important fishery species.

The experiments are designed to capture both the acoustic and physical effects of seismic waves. In some cases, the design of a ship’s seismic array means that multiple gun ranges overlap, creating even more intense emissions. Large seismic blasts may suddenly create cavitation bubbles – zones of such low pressure that the water near the gun turns to vapor, and sometimes it actually freezes. Blasts can also incite powerful waves that may break zooplankton appendages.

“Just as we have guidelines that keep people off an island where an endangered bird is breeding, informed policies can help us minimize disturbances to cod and other animals during their brief breeding periods,” Fields said. “Knowing exactly how this activity affects fish and zooplankton is crucial to planning seismic blasting surveys with minimal impacts.”

Fields is also investigating the swimming behavior of individual zooplankton after they are exposed to seismic blasts – an important piece of information for understanding the longer-term effects of this practice. Safety measures due to COVID-19 have canceled the fieldwork he had planned for the summer, but Fields hopes to conduct the next round of experiments during the spring of 2021.

Ultimately, his findings will be used to help mitigate the impacts of seismic surveys – particularly in vulnerable ecosystems. In the Arctic, for example, the rapid decrease in sea ice cover is creating new opportunities to map the seafloor, look for oil, and identify places to build infrastructure like docks. Seismic blasting activities must take place during the ice-free summer season – which is also the critical time when fish spawn and food webs flourish.

“The effects that all this manmade aquatic sound has on the essential animals at the base of the food web is really unknown,” Fields said. “We need to understand the full ecological implications of seismic blasting so we can use this tool safely.”