New Study Using Ocean Gliders Uncovers Clues About Carbon Cycling in the Sargasso Sea

Hourly measurements show when and how the ocean produces, recycles, and exports carbon — and why salps matter more than we thought

A new study published in Progress in Oceanography shows how autonomous underwater gliders are transforming our understanding of carbon movement in the ocean. By collecting continuous data over an 18-month period, the gliders captured processes with much greater detail than has been possible in the past and helped clarify puzzling aspects of the annual carbon cycle near Bermuda.

“These gliders dive to 900 meters and return to the surface every two hours, giving us a level of detail we simply can’t get from once-a-month sampling alone,” said Ruth Curry, program manager of the glider laboratory at the Bermuda Institute of Ocean Sciences, a unit of the Julie Ann Wrigley Global Futures Laboratory at Arizona State University. The long-running Bermuda Atlantic Time-series Study (BATS) program provides extensive knowledge of the region, and the glider research builds on it by supplying continuous observations that complement monthly measurements.

One standout finding involved salps — transparent, fast-growing zooplankton that can bloom briefly in huge numbers. They migrate nightly to the ocean surface to feed, then return to deeper waters before dawn, releasing “dense, fast-sinking fecal pellets” that rapidly transport carbon to depths greater than 1,300 feet (400 meters). Their intense feeding rates and sheer abundance during blooms can dramatically boost carbon export from the surface 328 feet (100 m) of the water column below the depth of winter mixing.

According to Curry and the team from Bigelow Laboratory for Ocean Sciences and ASU School of Earth and Space Explorartion, the gliders also helped resolve a long-standing puzzle: why biological production in the Sargasso Sea remains high in summer despite extremely low surface nutrients. “Relatively high oxygen production rates in the absence of nutrients was for decades at odds with our understanding of the marine carbon cycle,” Curry said. Continuous measurements showed that cyanobacteria types that dominate in summer and fall produce carbon-rich, nutrient-poor organic matter known as GLOM (gel-like organic matter). The team hypothesizes that GLOM material resists recycling near the surface, then slowly sinks and is eventually broken down and respired; thus, consuming oxygen in deeper layers.

High-resolution temporal observations collected by the gliders over a full year allowed researchers to constrain and estimate how much carbon is produced and when and at what depths it is exported to and recycled. Their results suggest that the Sargasso Sea may be more important to long-term carbon storage than previously believed. The study found that about 25% of the carbon produced at the surface is exported to depth, much higher than the widely cited global average of about 10%.

“How resilient are the oligotrophic gyres, and how will they respond to changing conditions such as warming and increased stratification?” Curry asked. “These questions are increasingly relevant to anticipating future trends in carbon sequestration and oxygen production in the context of globally changing climate conditions.”

Curry emphasized the broader potential of autonomous platforms: “These results showcase the value of using autonomous technologies to quantify ocean processes over long periods and in cost-effective ways. Continuous glider observations can reveal both physical and ecosystem variability, including the role of salps in the carbon cycle.”

Bermuda’s central location in the North Atlantic gyre, along with ASU BIOS’s research programs, including the long-running BATS program, make the work possible. BATS’s monthly shipboard measurements helped calibrate the gliders, while the gliders filled in the gaps between cruises. Additional shipboard contributions, such as zooplankton surveys identifying salps as key drivers of springtime mixing, further strengthened the study.