Marine heatwaves could interfere with the ocean's ability to lock away carbon

By Sarah Anderson, PhD

In a new study published in Nature Communications, a team of researchers from institutions in Canada, China, Denmark, and the United States report that spurts of intense ocean warming can impact microbial community structure and carbon cycling dynamics in underwater ecosystems. 

“Our findings indicate that thermal stress caused by successive marine heatwave events altered the production, transformation, and transport of carbon-based organic matter in the ocean,” said Steven Hallam, a professor in the Department of Microbiology and Immunology at UBC. 

By analyzing the effects of marine heatwaves on these carbon cycling processes, the researchers provide new understanding of how episodic increases in ocean temperature can disrupt the natural mechanisms that combat greenhouse gas emissions. As marine heatwaves become more frequent with global warming, these results can help scientists to predict and prepare for the potential of these extreme weather events to exacerbate climate change and feed a vicious circle of environmental destruction.

Microscopic organisms such as bacteria and plankton form the foundation of both the food web and the carbon cycle in the ocean. As they produce carbon-based energy and are consumed and degraded, these microbes power a biological pump that shuttles carbon to the deep ocean and keeps it sequestered under the sea rather than released into the atmosphere as carbon dioxide.  

From 2007 to 2020, researchers in Hallam’s lab evaluated how these microbial communities respond to changing climate patterns by collecting water samples along the Line P transect— a 1,425-kilometer aquatic highway stretching from coastal British Columbia into the northeastern subarctic Pacific Ocean. These samples were analyzed at the Hakai Institute, where scientists sequenced environmental DNA that the microorganisms had secreted into the water to determine the microbe species present. This effort captured two distinct marine heatwaves in the region: one from 2013 to 2015 (referred to as “The Blob”) and another from 2019 to 2020.

During this period, researchers at the Monterey Bay Aquarium Research Institute had deployed robotic submarine-like structures in the area to collect data on water properties including temperature, nutrients, and particulate organic carbon (POC)— tiny particles that serve as the ocean’s carbon currency. At the same time, another group analyzing Line P water samples had quantified chlorophyll and other plant pigments to characterize the members of the phytoplankton, a family of plant-like microorganisms. 

The researchers brought together their individual pieces of the puzzle to assemble a complex portrait of the impacts of the marine heatwaves. While the effects varied over time, the team observed that both events featured interludes of exceptionally high levels of POC in the water. 

They found that these spikes in POC mapped onto fluctuations in the makeup of the phytoplankton community. Just as a shift in the number of plant-based restaurants versus hamburger joints in a town might influence how many vegetarians and carnivores live there, changes in the types of phytoplankton available as a food source affected the distribution of organisms occupying the next rung up the food chain. 

Due to differences in the way various higher-level grazing organisms process carbon, these effects ultimately culminated in changes in the composition, size, and transport of POC. Rather than rapidly sinking to the bottom of the ocean, these particles lingered in shallow zones, increasing the risk that carbon seeps into the atmosphere. 

“This research highlights the power of integrating autonomous and shipboard sampling within the context of long-term ocean observatory programs like the Line P transect,” Hallam said. “Tracking successive marine heatwave events requires a lot of coordination and effort out there in the open ocean, but it is necessary in order to detect and interpret the interplay between food web structure and carbon transport processes and the impact of thermal stress on the ocean’s role as a carbon sink.”

While Hallam’s sampling work for the line P project was interrupted by the COVID-19 pandemic, he hopes that similar sustained, multi-platform ocean monitoring initiatives will yield exciting new insights in the future. “The extent to which food web structure and biological carbon pump efficiency recover between marine heatwave events remains an ongoing area of research that requires continued support for ocean observation in the northeastern subarctic Pacific Ocean,” he said. 

This work was funded by the Tula Foundation, the G. Unger Vetlesen and Ambrose Monell Foundations, the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the Department of Fisheries and Oceans Line P program with additional support from the US National Science Foundation’s GO-GBC project, the David and Lucile Packard Foundation, the China National Science Foundation, the Fundamental Research Funds for the Central Universities, and the Danish Center for Hadal Research.