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For myriad ocean residents, such as jellyfish, crabs, urchins, fish, and prawns, giant kelp is an important source of food and shelter. A new study, however, shows that this dietary staple is becoming less nutritious.

Research led by coastal biogeochemist Heili Lowman while at the University of California, Santa Barbara (UCSB), shows that in California’s Santa Barbara Channel giant kelp’s nutritional nitrogen content has fallen by 18 percent as ocean temperatures rose over the past 19 years, even as the amount of kelp stayed the same.

Catherine Pfister, a marine ecologist at the University of Chicago in Illinois who was not involved in the research, cautions that the finding is a correlation and doesn’t necessarily mean that warming oceans are directly causing the decline in the kelp’s nitrogen content. Still, she praised the study for illuminating a trend that points the way for future investigations. Showing a link between warmer ocean temperatures and a decline in kelp’s nutritional value, for example, raises the question of how the change occurred and exactly which factors led to the shift.

The researchers showed both seasonal and year-to-year changes in kelp’s nutritiousness by analyzing data from kelp blades sampled in California’s Santa Barbara Channel by the Santa Barbara Coastal Long-Term Ecological Research program between 2002 and 2021. Kelp’s dwindling nutritional value means things that eat it have to expend more energy foraging to get the same benefit, with consequences that may ripple up the food chain.

Some seasonal changes in nutritiousness are expected because colder water tends to have a higher nutrient content. Cool temperatures slow marine organisms’ metabolisms causing them to consume fewer nutrients. Upwelling of cold water from the ocean’s depths also brings nutrients from the seafloor.

Kyle Emery, a coastal ecologist who worked on the project while a graduate student at UCSB, says he hopes future research might dig into the issue further by measuring kelp in other locations along the west coast of North America. “There could be other dynamics at play in different places along the coastline,” where water temperatures, currents, upwelling, and other conditions vary, he says.

The research is an important reminder that the impacts of climate change may not always be obvious. Past research focusing on measuring the amount of kelp hasn’t always taken into account the health of remaining forests. Yet understanding why kelp is becoming less nutritious is important to maintaining it for the many species that rely on it for sustenance and survival.

“I am hoping that this study serves as an advocacy moment for studying not only the quantity of kelp but the quality of it,” says Lowman.