Hurricanes stir up dramatic changes in the deep ocean

Most people know hurricanes as destructive giants. On land, they tear through cities, uproot trees, and flood entire regions. But when these storms pass over the ocean, something else happens that is more subtle, but not less important.

The violent winds stir the sea, pulling up cold, nutrient-rich water from the depths and setting off a chain reaction of biological changes.

In 2018, a group of researchers led by Professor Michael Beman from UC Merced found themselves in the midst of this transformation.

While studying oxygen minimum zones (OMZs) in the Eastern Tropical North Pacific, they encountered Hurricane Bud. Instead of halting their expedition, they took a chance.

The team was presented with a rare scientific opportunity: observing how a powerful hurricane could rapidly change the ocean’s chemistry and biology.

Hurricane leads to rare ocean sampling

The team launched their expedition from Mazatlán, Mexico, and set a course toward San Diego. The goal was to study naturally occurring OMZs, which are regions in the mid-depth ocean that contain very little oxygen.

As Hurricane Bud intensified into a Category 4 cyclone, the researchers monitored weather forecasts closely. “We were very careful, and we had plans A, B, C and D in place,” Beman said. “The forecasting was extremely accurate, and we knew the storm rapidly intensified.”

Rather than retreat, the team navigated between research sites and found shelter behind islands. Once the storm passed, they sailed directly into the region it had just churned.

“There was some skill involved, but definitely some luck, too, and we ended up adding a sampling location right where the storm was at maximum power,” said Bernan.

Hurricane sparks massive ocean changes

The ocean’s surface was bright green, stained with chlorophyll from a massive phytoplankton bloom. These microscopic plants thrive when nutrients are abundant – and the hurricane had delivered exactly that.

Cold water from the depths had risen, carrying nitrogen, phosphate, and other essential elements to the surface.

“When we got there, you could actually see and smell the difference in the ocean,” said Beman. “It was green from all the chlorophyll being produced. There were totally different organisms there, and they were going nuts in the wake of the storm.”

These blooms feed entire marine food webs. Bacteria, zooplankton, shellfish, fish, and even whales take advantage of the sudden buffet.

“We were doing this at a time of year when there’s not a lot going on biologically in these areas of the ocean, so these hurricane-generated blooms are like oases for ocean organisms,” said Bernan.

Water with low oxygen levels

Unfortunately, the same deep waters that delivered nutrients also carried low oxygen levels. These oxygen minimum zones, usually found over 100 meters below the surface, rose rapidly.

At one site, Station 3.5, the OMZ had shoaled to just 41 meters deep. At other points, the oxygen-deficient zone (ODZ) reached as shallow as 50 meters. This shoaling poses a threat to fish and invertebrates that cannot survive in such conditions.

For Beman, the data was a revelation. “I’ve never seen measurements like that in those areas of the ocean, ever,” he said. The change was not just physical. The hurricane altered the ocean’s chemistry and biology as well.

Anaerobic processes – those that do not rely on oxygen – were suddenly active closer to the surface. The researchers observed signs of nitrogen cycling and sulfate reduction, both of which affect global nutrient balances.

Storm leaves chemical traces

The researchers gathered water samples from just below the surface. These revealed high levels of nitrate, phosphate, and ammonium at around 11 meters depth. These nutrients are essential for phytoplankton growth.

Dissolved organic carbon (DOC) levels were unusually low, likely due to the introduction of deep ocean water. In contrast, particulate organic carbon (POC) was high, supporting the idea that a bloom was in full swing.

Further chemical analysis uncovered more than 1,600 distinct compounds. Many were known phytoplankton markers like fucoxanthin and MGDG lipids. Others, such as lumichrome and tryptophan, are believed to mediate communication between microbes and algae.

These chemical fingerprints offered a detailed look into the biological activity that followed the storm.

Microbes respond with adaptation

RNA and DNA analysis showed microbial shifts. Flavobacteria dominated near the surface, breaking down phytoplankton. Cyanobacteria and Proteobacteria thrived too. Deeper in the ODZ, low-oxygen specialists like Desulfobacteria and Nitrospina took over.

“Margot noticed the subsurface changes from the hurricane when she was preparing her thesis chapters, especially the fact that the oxygen minimum zone had rapidly shoaled,” Beman said.

“Irina searched her unique organic matter data to look for changes driven by the hurricane, which turned out to be very clear and dramatic.”

Hurricanes trigger similar ocean changes

Twenty years of satellite data showed storms near Station 3.5 often triggered phytoplankton blooms. Bud was part of a recurring pattern.

According to the study, these storms account for 7 to 15 percent of the region’s total annual carbon export. While short-lived, the impact of each storm lingers as carbon-rich material sinks into the deep sea. These events contribute meaningfully to oceanic carbon cycling.

The Eastern Tropical North Pacific is already one of the planet’s major sites for fixed nitrogen loss. Beman and his colleagues will continue to analyze the samples and share their data with others.

“We’ve met many, many times to analyze the data and figure out what effects the hurricane had and why,” he said. The team believes there is still much to uncover.

“We are just scratching the surface of what these storms do,” Beman added. “It was a rough few days at sea. I hope we continue to learn as much as we can about what actually happens during and after hurricanes.”

The study is published in the journal Science Advances.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–