How Does Energy Reach the Top of the Food Web?

A paper coauthored by PIFSC scientists looks at how energy makes its way to the top of the central North Pacific food web. The scientists used information on what marine organisms across the food web eat to build a food web model, diagrammed below. This model shows how energy produced by phytoplankton moves up through the food web, all the way to apex predators like tuna, billfish, and sharks. Results from this study deepen our understanding of how energy reaches commercially important species such as bigeye tuna and swordfish.

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Schematic model of the central North Pacific food web used in this study (from Choy et al. 2016)

Ecosystem models are one tool scientists use to study the food web. PIFSC scientists used an ecosystem model to improve their understanding of the middle of the food web, which is occupied by small organisms called micronekton. Micronekton are 2 – 10 cm in length, small enough to fit in the palm of your hand. They include marine animals like small fish, squid, crustaceans, and jellyfish. These animals are a critical part of the food web; they link energy produced at the base of the food web to apex predators at the top of the food web. Yet, compared to the top and bottom of the food web, little is known about these mid-trophic micronekton. They’re challenging to study for two main reasons. One, they live throughout the water column at depths ranging from the ocean’s surface to the ocean floor. Two, they’re pretty good at avoiding the nets scientists use to capture them.

This modeling study showed that some types of micronekton play a bigger role in the food web than others. Much of the energy that reaches apex predators flows through micronekton crustaceans and mollusks – organisms like the shrimp and squid shown here. Mid-trophic fish and jellyfish, on the other hand, transfer comparatively little energy to the top of the food web.

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Micronekton crustaceans with a 6-inch ruler for scale

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Micronekton mollusks with a 6-inch ruler for scale

Understanding how energy flows through the food web, and which organisms transfer the most energy, helps both scientists and fishery managers understand how apex predators may be impacted by future ecosystem change. This knowledge also highlights areas for future research. For example, there’s currently little known about how crustaceans and mollusks in the central North Pacific may be impacted by ocean acidification. Learning how important these organisms are in the food web underscores the need for investigating questions like this.

You can read more about micronekton and their role in the central North Pacific food web here: http://www.int-res.com/articles/meps_oa/m549p009.pdf

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