Harbor seal skulls provide clues to Puget Sound’s past food webs

Undergraduate laboratory assistant Hyejoo Ro holding a harbor seal skull while sampling specimens at University of Puget Sound Slater Museum in Tacoma, WA. Megan Feddern

The adage “you are what you eat” generally turns out to be true. Foods we ingest are broken down into amino acids and absorbed into our bodies, leaving trace elements in our bones. In turn, these amino acids can be traced back to their source like a biological receipt, revealing information about the environment.

Using this knowledge, researchers are conducting isotope analysis of amino acids in harbor seal skulls to determine the composition of historical marine food webs. Their results were published on March 22 in Global Change Biology.

Megan Feddern, the study’s lead author and a graduate student at the University of Washington School of Aquatic and Fishery Sciences, is analyzing harbor seal skulls from museum collections to piece together what food webs looked like in the past. The museum specimens—some dating back nearly 100 years—were collected from coastal Washington and the Salish Sea north to the Gulf of Alaska and the eastern Bering Sea.

Harbor seal specimens stacked on the workbench and ready for sampling at the Royal British Columbia Museum in Victoria, British Columbia. Megan Feddern

Feddern explains that museum specimens like these can help create historical datasets. The composition of the bone provides researchers with a window into the seals’ diets for roughly the last year of their life. The different environmental conditions at the time could influence nutrients available to the food chain where harbor seals were feeding.

“We know that harbor seals eat a variety of fishes, making them good indicators of a general view of the whole food web, which allows us to measure many different food web pathways,” said Feddern.

As predators, harbor seals are found at the top of their aquatic food chain, whereas primary producers, like phytoplankton, are at the bottom. The environmental conditions present at a given point in time can dictate the abundance and variety of prey items available. Based on signatures from the seals’ diet, researchers can characterize nutrients and primary producers present and compare it to the environmental conditions at that time.

Environmental conditions, such as freshwater discharge, upwelling, and Pacific Decadal Oscillation (PDO), impact nutrients—like nitrogen and carbon—that are available for use by the food web’s primary producers. These nutrient levels are passed up the food chain and imprinted into the seals’ bone.

“What we’re trying to do is link the environmental conditions to the top predator’s food web and that’s something that is not always considered,” said Feddern. “Some studies look at the environmental conditions and compare them to species abundance, but they don’t usually factor in the productivity of primary producers or what links that environment to that species.”

Mechanisms of environmentally induced changes in nitrogen and carbon resources (a–d) which are assimilated into the stable isotope ratios of primary producers (1 and 2), and the stable isotope ratio is conserved when assimilated into consumers at higher trophic levels in the food web. Megan Feddern

By measuring values of source amino acids in these specimens, the researchers derive indices of primary production and nitrogen resources that were assimilated into food webs. The strength of the indices’ response to various conditions over the past century differs widely across the northeast Pacific’s ecosystems.

Primary production and nitrogen availability in the Gulf of Alaska were dependent on regional climate events like the North Pacific Gyre Oscillation and upwelling. By contrast, the coastal Washington and Salish Sea food webs were associated with local indices of freshwater discharge. Feddern believes this is due to fewer anthropogenic factors like agricultural runoff in Alaska, where the food web is driven more by nutrients from natural upwelling.

The results also showed that since 1975, above average ocean conditions in the Gulf of Alaska have altered the composition of primary producers, whereas a similar change was not detected in the Puget Sound region.

“When we get these large-scale climatic changes in something like the PDO, we’re going to see different responses in nutrients and primary production in the Gulf of Alaska food web compared to the Washington food web,” said Feddern. “There are really big regional specific responses in these food webs that make it important to consider some of the spatial complexity when we’re trying to think about how PDO or sea surface temperature might be impacting marine life.”

Looking ahead, researchers know that climate change will influence nutrient cycling and primary production in the world’s oceans, dictating variable impacts on coastal food webs. Using museum specimens as record keepers of the past can provide insight into the future.

“By understanding how the environment impacted nutrients important to coastal food webs in the past, we can anticipate how future environmental changes may impact food webs,” said Feddern.

Additional UW co-authors include Gordon Holtgrieve, a professor of aquatic and fishery sciences, and Eric Ward, an affiliate faculty at the School of Aquatic and Fishery Sciences.

This research was funded by Washington Sea Grant and a Joint Institute for the Study of the Atmosphere and Ocean Grant.

For more information, contact Feddern at mfeddern@uw.edu.

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