Acoustics under the ice: a complete story of marine life temporal cycles

Researchers lowering the Chukchi Ecosystem Observatory (CEO) innto the Chukchi Sea. University of Alaska Fairbanks
Researchers lower the Chukchi Ecosystem Observatory (CEO) into the Chukchi Sea. University of Alaska Fairbanks
Diagram of the CEO mooring. The echosounder is located at the top with multiple buoys surrounding a cage.
Diagram of the CEO mooring with the echosounder at the top. Silvana González

Only navigable four months of the year due to sea ice, the Chukchi Sea in the Arctic Ocean is one of the more challenging places on the planet to conduct research. The extreme environment has limited exploration and study of the sea’s marine life to the point where much of it still remains a mystery.

Silvana González, a PhD student at the University of Washington School of Aquatic and Fishery Sciences, uses acoustic techniques to collect data in otherwise inaccessible locations. In high-latitude marine ecosystems, like the Chukchi Sea, traditional vessel-based sampling for fish and zooplankton is only possible in seasons without sea ice. This limiting factor results in an incomplete picture of the life history of these species and the ecosystem as a whole. By utilizing remote acoustic measurements recorded throughout the year and under the sea ice, González is able to piece together a more complete picture of arctic marine life.

A new study led by González, shows the abundance and behavior of fish and zooplankton in the NE Chukchi Sea is mainly influenced by factors that vary at different time scales. The results were published March 18 in Polar Biology.

This research study is part of a broader project, the Chukchi Ecosystem Observatory (CEO), a multi-institutional, multi-investigator partnership that operates and maintains a subsurface moored observatory on the NE Chukchi shelf. The observatory records biological and physical measurements with high temporal resolution throughout the year.

The researchers combined the CEO’s continuous environmental data with new data from a scientific echosounder, a non-invasive sampling method that uses sonar to quantify zooplankton and fish in the water column.

“Our findings indicate variability in fish and zooplankton abundance and behavior occurs at distinct temporal scales: daily, seasonally, and annually,” said González. These scales are different for fish and zooplankton groups and are associated with different environmental drivers.”

Sample of acoustic backscatter under the sea ice from the CEO.
Sample of acoustic backscatter under the sea ice from the CEO. Silvana González

Having multi-year, continuous observations in an area where sampling had been limited by the presence of sea ice, the researchers were able to make the following conclude that:

  • Daily vertical migrations are present throughout the year, but are strongest in autumn when day-night cycles are pronounced.
  • Seasonal variability in zooplankton abundance and behavior occurs at scales of 3-to-4 months and is mainly associated with sea ice patterns, which may also regulate the onset of primary production, whereas seasonal variations in fish abundance and behavior are associated most closely with salinity patterns (~ 3 months) and slower changes in water temperature (~ 6 months). 
  • Annual cycles in biological characteristics are influenced by year-round variations in water temperature, sea ice concentration, light irradiance, and wind.

These novel findings indicate that extrapolating patterns observed in short summer sampling windows, where lack of sea ice makes traditional survey methods possible, may not be representative of patterns occurring at other times of the year.

“Biological patterns and their associations with environmental factors vary through time and emphasize the importance of high-resolution long-term studies for comprehensive ecosystem characterizations,” said González.

The team’s time-based characterization of biological variability in the Chukchi Sea identifies the appropriate time scales of observation that should be used in Arctic monitoring programs. This allows researchers to predict and detect biological responses to rapidly changing environments, like those impacted by climate change, with greater accuracy. Predicting the potential direction and magnitude of these changes will help scientists design or improve mitigation strategies and manage Arctic marine species.

Artwork by Alaskan artist Klara Maisch that represents the environmental changes over a one year cycle at the CEO. Art depicts many marine species such as whales, walruses, and fish.
Artwork by Alaskan artist Klara Maisch showing the environmental changes over a one-year cycle at the CEO. Klara Maisch

Research is ongoing to determine if the observations from the CEO are occurring elsewhere in the Arctic. The installation of additional research observatories enables comparison of similar temporal scales throughout the Arctic and Antarctic, adding to our understanding of the planet’s more extreme marine environments.

Co-authors include John Horne, a UW professor of aquatic and fishery sciences, and Seth Danielson, associate professor, University of Alaska Fairbanks College of Fisheries and Ocean Sciences.

Support for this research came from the North Pacific Research Board Graduate Student Research Award and the Oil Spill Research Institute Graduate Research Fellowship. The Chukchi Ecosystem Observatory receives operations and equipment funding from the North Pacific Research Board and the Alaska Ocean Observing System.

For more information, contact González at silgonz@uw.edu.

 

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