Seaweed Feeds the Nearshore Food Web in the Gulf of Alaska

In the highly productive nearshore waters of the North Pacific, macroalgae (seaweeds) and phytoplankton form the base of the food web. Which matters more? A study in the northern Gulf of Alaska set out to find out.

Seaweeds in the intertidal zone of the Gulf of Alaska (U.S. Geological Survey / Sarah Traiger).

The study

At sites in four regions, researchers collected three "indicator" consumers that feed in different ways:

• mussels (Mytilus trossulus), filter-feeding shellfish of the intertidal zone;
• kelp greenling (Hexagrammos decagrammus), fish that feed on the seafloor;
• black rockfish (Sebastes melanops), fish that feed in open water.

They used carbon and nitrogen stable isotopes to estimate how much of each animal's food came ultimately from seaweed versus phytoplankton, and annual growth rings in mussel shells and fish otoliths to see whether diet affected growth. For mussels, they also looked at changes during the Pacific Marine Heatwave of 2014–2016.

What they found

• Seaweed was the main food source — more than 60% of the diet for all three consumers.
• Mussels grew better with more seaweed: the more macroalgae in their diet, the faster they grew. No such relationship showed up in the two fish.
• The heatwave hurt. During the marine heatwave, seaweed's share of mussels' diet dropped by more than 10%, matching a decline in some seaweed species at the time, and mussels grew 45% less than in other periods. The link between seaweed and mussel growth was strongest then, explaining 48% of the variation in growth, compared with 3–12% before or after.

Why it matters

The results suggest seaweed is likely more important than phytoplankton to cold-temperate nearshore food webs. Protecting and expanding seaweed habitat is likely to benefit these ecosystems under any climate scenario — and especially during marine heatwaves.

Sources

• National Park Service: "The important role of seaweed in nearshore food webs," summarizing Corliss K, von Biela V, Coletti H, Bodkin J, Esler D, Iken K (2024), "Relative importance of macroalgae and phytoplankton to nearshore consumers and growth across climatic conditions in the northern Gulf of Alaska," Estuaries and Coasts. The source's "kelp greening" is corrected here to kelp greenling.