New research reveals that prolonged periods of extreme warmth in the ocean can disrupt the very foundation of marine food webs, significantly impairing the ocean’s ability to absorb and store atmospheric carbon. A multi-year study in the Gulf of Alaska found that marine heat waves alter microscopic plankton communities, which form the base of the food chain. These changes effectively “jam” the biological processes that transport carbon from the surface to the deep sea, where it would otherwise be locked away for centuries.
The findings, published in Nature Communications, underscore a troubling feedback loop in the planet’s climate system. The ocean absorbs about a quarter of the carbon dioxide that humans emit annually, acting as a crucial buffer against accelerating climate change. However, as the climate warms, the increasing frequency and intensity of marine heat waves weaken this natural carbon sink. This disruption not only has profound implications for global carbon cycles but also threatens the stability of marine ecosystems and the fisheries that depend on them.
Decade of Observation in the Gulf of Alaska
To understand the impacts of these warming events, an international team of researchers analyzed over a decade of data from the Gulf of Alaska, a region that experienced two major marine heat waves. The study integrated extensive datasets from multiple sources, including advanced robotic floats and traditional ship-based plankton surveys. This comprehensive approach allowed scientists to monitor the ocean’s physical and biological conditions before, during, and after the heat waves, providing a rare window into the ecosystem’s response.
The Blob and its Successor
The first event studied was the infamous marine heatwave known as “The Blob,” which persisted from 2013 to 2015. A second, distinct heatwave followed from 2019 to 2020. Researchers were surprised to find that the ocean’s biological response was significantly different during each event, even though both involved substantial warming of surface waters. This highlights the complex and often unpredictable nature of ecosystem responses to climate stressors. The long-term monitoring was critical for capturing these distinct reactions and understanding the underlying mechanisms.
Shifting Foundations of the Food Web
The core of the disruption lies at the microscopic level with phytoplankton, the plant-like organisms that underpin the entire oceanic food web. These organisms use photosynthesis to convert carbon dioxide into organic matter, forming the first link in a chain that feeds everything from tiny zooplankton to large whales. The study found that both heat waves altered the composition and dynamics of these essential plankton communities.
These shifts have cascading consequences that travel up the food chain. Changes in the type and abundance of phytoplankton can affect the health and populations of fish, seabirds, and marine mammals. The stability of commercial fisheries is also at risk when the foundational layer of the food web is altered. The research underscores how climate-driven changes in the ocean can have far-reaching ecological and economic impacts.
The Biological Carbon Pump Under Strain
Normally, the ocean’s “biological carbon pump” acts like a conveyor belt, transporting carbon from the atmosphere into the deep ocean. This process begins when phytoplankton die and sink, or are consumed by other organisms whose waste products then sink. This falling organic material, often called “marine snow,” carries carbon to the deep sea, where it can remain sequestered for millennia. A healthy biological pump is essential for the ocean’s role in regulating the global climate.
Two Different Kinds of Malfunction
The study revealed that the two heat waves disrupted this carbon conveyor belt in different ways. During “The Blob” from 2013–2015, surface phytoplankton were highly productive, but the resulting carbon particles were small and did not sink efficiently. Instead, they accumulated at intermediate depths of around 200 meters, failing to reach the deep ocean for long-term storage.
The 2019–2020 heatwave presented a different scenario. An unprecedented amount of carbon built up at the surface, not just from new phytoplankton growth but also from the recycling of carbon and waste from marine life. This mass of organic material eventually sank into the ocean’s “twilight zone” but remained at shallower depths between 200 and 400 meters, never completing the journey to the abyss.
Advanced Technology Reveals New Insights
This detailed understanding of ocean processes was made possible by a combination of cutting-edge and traditional research methods. The study relied heavily on data from robotic floats deployed by the Global Ocean Biogeochemical Array project. These autonomous instruments move through the water column, collecting continuous data on temperature, salinity, oxygen, and chlorophyll levels. This high-frequency data provided a real-time look at how the ecosystem was changing.
The robotic float data was complemented by ship-based surveys that collected water samples to analyze plankton populations through their pigments and DNA. According to Mariana Bif, the study’s lead author and now an assistant professor at the University of Miami, “Our research found that these two major marine heatwaves altered plankton communities and disrupted the ocean’s biological carbon pump. The conveyor belt carrying carbon from the surface to the deep sea jammed, increasing the risk that carbon can return to the atmosphere instead of being locked away deep in the ocean.”
Urgent Need for Continued Monitoring
The findings demonstrate that a warmer ocean may be less effective at storing carbon, which could in turn accelerate climate change. As marine heat waves become more frequent and intense, the disruptions observed in the Gulf of Alaska could become more widespread. Researchers emphasize the critical need for sustained, long-term ocean observation to monitor these changes.
Understanding how different marine ecosystems respond to heat waves is essential for accurately modeling and predicting the future of the ocean’s role in the global carbon cycle. The collaborative, multi-platform approach used in this study serves as a model for future research aimed at addressing critical environmental questions in a rapidly changing world. The health of marine ecosystems and the stability of the global climate depend on this continued scientific effort.