Marine heatwaves often make news when corals bleach or fish wash up dead. A new global study led by researchers at UBC and L’Institut Agro shows that bursts of extreme ocean heat are also reshaping the entire structure and function of marine food webs, with consequences that can last years after the water cools.
Image by Dimitris Vetsikas from Pixabay
“Marine heatwaves are strongly and persistently reshaping marine food webs,” said Dr. William Cheung, professor in the Institute for the Oceans and Fisheries (IOF), and co-author of the study. “Even though these events can last only weeks to months, their effects on the ecosystem can linger for years, especially for large predators that recover slowly. It really highlighted that extreme events, not just gradual warming, can drive long term ecological change.”
Dr. Vianney Guibourd de Luzinais, first author, and a Ph.D. student in the IOF and L’Institut Agro at the time this study was prepared, was struck by how deeply these short, intense events reverberate through marine ecosystems. “What surprised me most was how profoundly marine heatwaves alter the structure and functioning of ecosystems,” he said. “Although these events are short-lived but extremely intense, their effects ripple through the entire food chain and persist long after the heatwave has ended, leaving many ecosystems too little time to fully recover before being hit again, resulting in long-term ecological modifications.”
Published in the journal Biogeosciences, the study uses a trophodynamic ecosystem model, called EcoTroph-Dyn, to ask a key question: what extra damage do marine heatwaves cause on top of the slower, background warming of the ocean?
The global picture that emerged was stark. In a world with only gradual warming, total consumer biomass from trophic levels were projected to decline by about 0.07 percent per year between 1998 and 2021. When marine heatwaves were included, that average decline nearly doubled to about 0.12 percent per year. Standard projections based only on mean temperature trends would underestimate how much biomass is being lost from marine ecosystems.
Declines would be most pronounced in tropical systems, which support many of the world’s most productive fisheries. High trophic levels, representing larger predators, lost a greater fraction of their biomass than lower levels and remained depressed for longer. The model also showed that including heatwave driven mortality led to an additional biomass loss of roughly 3 percent. In several places, such as the California current and Alaska coastal waters, total biomass has still not returned to pre-Blob levels by 2021.
The study also shows that heatwaves do more than simply reduce biomass. They change how energy moves through the food web. During events like the Blob, flow kinetics, the speed at which biomass moves from prey to predators, increased, while transfer efficiency, the fraction of energy passed between trophic levels, decreased. Warmer water raises metabolic costs, so organisms burn more energy just to maintain basic functions, leaving less to support growth and reproduction and less to pass up to larger predators. Over time, that favours communities of smaller, faster growing species and erodes the biomass of slow growing top predators, including commercially important fish such as tuna and salmon.
“Our use of the model was deliberately simple and did not resolve individual species or seasonal life cycles,” said Guibourd de Luzinais. “It still captured broad patterns that match observations and other ecosystem models, suggesting that in many regions, especially in the tropics, marine heatwaves are arriving more often than ecosystems can fully recover, ratcheting down biomass at higher trophic levels over time.”
For fisheries and conservation planning, the message is clear. “Management strategies that only account for gradual ocean warming risk underestimating future pressures on fish stocks and ecosystem services,” said Cheung. “As extreme events such as marine heatwaves continue to intensify, they will drive the trajectory of marine ecosystems, and we will need to develop adaptation strategies that recognise the growing role of climate extremes in shaping life in the sea.”
Marine heatwaves deeply alter marine food web structure and function was published in Biogeosciences.