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New research shows that marine heat waves can reshape ocean food webs, which in turn can slow the transport of carbon to the deep sea and hamper the ocean's ability to buffer against climate change.

The study, published in Nature Communications, was conducted by an interdisciplinary team of researchers from MBARI, the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, the Hakai Institute, Xiamen University, the University of British Columbia, the University of Southern Denmark, and Fisheries and Oceans Canada.

To explore the impacts of marine heat waves on ocean food webs and carbon flows, the research team combined multiple datasets that tracked biological conditions in the water column in the Gulf of Alaska for more than a decade. This region experienced two successive marine heat waves during this time, one from 2013 to 2015 known as "The Blob," and another from 2019 to 2020.

"The ocean has a biological carbon pump, which normally acts like a conveyor belt carrying carbon from the surface to the deep ocean. This process is powered by the microscopic organisms that form the base of the ocean food web, including bacteria and plankton," said the lead author, Mariana Bif, previously a research specialist at MBARI and now an assistant professor in the Department of Ocean Sciences at the Rosenstiel School.

"For this study, we wanted to track how marine heat waves affected those microscopic organisms to see if those impacts were connected to the amount of carbon being produced and exported to the deep ocean."

The research team used information collected by the Global Ocean Biogeochemical (GO-BGC) Array, a collaborative initiative led by MBARI that uses robotic floats to monitor ocean health.

The GO-BGC project has deployed hundreds of autonomous biogeochemical Argo (BGC-Argo) floats, which measure ocean conditions such as temperature, salinity, nitrate, oxygen, chlorophyll, and particulate organic carbon (POC) up and down the water column every five to 10 days.

The team also looked at seasonal data from ship-based surveys that tracked plankton community composition, including pigment chemistry and sequencing of the environmental DNA (eDNA) from seawater samples collected during the Line P program carried out by Fisheries and Oceans Canada.

The study found that marine heat waves did impact the base of the ocean food web, and those impacts were connected to changes in the way that carbon was cycled in the water column. However, the changes that occurred in the food web were not consistent across the two heat waves.

Under typical conditions, plant-like phytoplankton convert carbon dioxide to organic material. These microorganisms are the foundation of the ocean food web. When they are eaten by larger animals and excreted as waste, they transform into organic carbon particles that sink from the surface through the ocean's mesopelagic, or twilight, zone (200 to 1,000 meters, approximately 660 to 3,300 feet) and down to the deep sea. This process locks atmospheric carbon away in the ocean for thousands of years.

During the 2013–2015 heat wave, surface carbon production by photosynthetic plankton was high in the second year, but rather than sinking rapidly to the deep sea, small carbon particles piled up approximately 200 meters (roughly 660 feet) underwater.

During the 2019–2020 heat wave, there was a record-high accumulation of carbon particles at the surface in the first year that could not be attributed to carbon production by phytoplankton alone. Instead, this accumulation was likely due to the recycling of carbon by marine life and the buildup of detritus waste. This pulse of carbon then sank to the twilight zone, but lingered at depths of 200 to 400 meters (roughly 660 to 1,320 feet) instead of sinking to the deep sea.

The team attributed these differences in carbon transport between the two heat waves to changes in phytoplankton populations. These changes cascaded through the food web, leading to a rise in small grazers who do not produce fast-sinking waste particles, so carbon was retained and recycled at the surface and in the upper twilight zone rather than sinking to deeper depths.

"Our research found that these two major marine heat waves 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," said Bif.

This research demonstrated that not all marine heat waves are the same. Different plankton lineages rise and fall during these warming events, underscoring the need for long-term, coordinated monitoring of the ocean's biological and chemical conditions to accurately model the diverse, and expansive, ecological impacts of marine heat waves.

"This research marks an exciting new chapter in ocean monitoring. To really understand how a heat wave impacts marine ecosystems and ocean processes, we need observation data from before, during, and after the event. This research included robotic floats, pigment chemistry, and genetic sequencing, all working together to tell the entire story. It's a great example of how collaboration can help us answer key questions about the health of the ocean," said MBARI Senior Scientist Ken Johnson, the lead principal investigator for the GO-BGC project and a co-author of the study.

Ocean observations and models suggest that marine heat waves have been expanding in size and intensifying over the past few decades. The ocean absorbs a quarter of the carbon dioxide emitted each year, thanks to the steady stream of carbon particles sinking from the surface to the deep sea.

A warmer ocean can mean less carbon locked away, which in turn can accelerate climate change. Beyond the changes to carbon transport, the shifts in plankton at the foundation of the ocean food web have cascading impacts on marine life and human industry too.

"Climate change is contributing to more frequent and intense marine heat waves, which underscores the need for sustained, long-term ocean monitoring to understand and predict how future marine heat waves will impact ecosystems, fisheries, and climate," said Bif.