Sinking particle fluxes and nitrogen loss in the eastern tropical North Pacific oxygen deficient zone measured using incubating sediment traps

In the vast, sunlit upper layers of the ocean, tiny plants called phytoplankton grow and multiply, forming the base of the marine food web. When these organisms die, or when animals eat them and produce waste, this organic material sinks like snowflakes toward the dark depths. As it falls, bacteria and other microbes break it down, a process that consumes oxygen. In most parts of the ocean, this oxygen is quickly replenished by currents and mixing. However, in specific regions near the equator, the water is so stable and the consumption of oxygen so intense that a thick layer of water forms where oxygen is nearly absent. Scientists call these oxygen-deficient zones. In these dark, airless environments, a different kind of microbial life takes over. Instead of breathing oxygen, these microbes use nitrogen compounds to survive, a process that ultimately releases nitrogen gas back into the atmosphere. This removal of nitrogen is a critical part of the ocean's chemistry, acting as a brake on how much plant life the ocean can support globally.

For decades, scientists have tried to understand exactly how much nitrogen is lost in these zones and where the fuel for this process comes from. The prevailing view has been that the nitrogen loss is driven almost entirely by organic matter sinking from the sunlit surface, like a steady rain of dead plants and animals falling from above. To measure this, researchers have traditionally used long-term moored traps that sit on the ocean floor or drift at fixed depths, collecting this falling debris over weeks or months. However, these long deployments often smooth out the short-term bursts of activity that might be crucial to the system. Furthermore, bringing water samples from these deep, oxygen-starved zones up to the surface for testing is notoriously difficult; the moment the water touches the air, it can be contaminated with oxygen, ruining the delicate chemical balance needed to study the microbes inside.

A team of researchers led by Megan Duffy set out to solve these problems by taking the laboratory to the deep ocean, rather than bringing the deep ocean to the laboratory. They deployed a new type of instrument in the eastern tropical North Pacific, home to the world's largest oxygen-deficient zone. These instruments were sophisticated, free-floating sediment traps equipped with incubation chambers. Instead of just collecting falling particles, these traps could close their gates at a specific depth, trap the water and the sinking particles inside, and then inject a special tracer to measure how fast the microbes were converting nitrogen into gas, all while remaining at the cold, dark, oxygen-free pressure of their natural home. The team ran these experiments over three separate cruises in 2017, 2018, and 2019, capturing data at various depths within the oxygen-starved layer.

The results revealed a picture of the ocean that was far more dynamic and surprising than the standard models predicted. The researchers found that the flow of sinking particles did not follow the smooth, predictable decline that had been expected for decades. Instead of simply getting thinner as they fell deeper, the amount of organic matter actually increased at certain depths within the oxygen-deficient zone. This "inverted flux" meant that fresh organic material was being injected into the middle of the dark zone from sources right there in the water, rather than just falling from the surface. The team identified two main sources for this local production. First, at the top of the oxygen-deficient zone, a secondary layer of green algae, specifically a type of cyanobacteria adapted to low light, was growing and adding fresh material to the sinking stream. Second, deeper down, migrating animals like zooplankton and small fish were moving up and down through the water column, releasing waste and fragments that added to the organic matter at those specific depths.

When the researchers measured the rate at which nitrogen gas was being produced, they found that these local sources of organic matter were driving the process more than previously thought. The experiments showed that the microbes living on the sinking particles were responsible for a massive portion of the nitrogen loss. In fact, the rates of nitrogen production measured directly inside the deep ocean were higher than those measured in previous studies that relied on bringing water samples to the surface. This suggests that the traditional methods of sampling might have missed the most active hotspots of microbial life, likely because the large particles that fuel these reactions sink too fast to be caught in standard water sampling bottles. By keeping the particles and the water together in their natural environment, the new traps captured the true intensity of the biological activity.

The study also challenged the idea that the amount of nitrogen lost is strictly tied to how much plant life is growing at the very surface of the ocean. While surface productivity does matter, the researchers found that the sinking flux of particles was often consistent even when surface growth varied wildly. This is because the system is fueled not just by what falls from above, but by what is created within the dark zone itself. The cyanobacteria and the migrating animals act as internal engines, generating fresh organic matter that keeps the nitrogen-removing microbes fed, even when the surface waters are quiet. This discovery changes the understanding of how the ocean's nitrogen budget works. It suggests that as climate change causes these oxygen-deficient zones to expand and move into new areas, the ocean may continue to lose nitrogen at high rates, driven by these internal sources of food, rather than slowing down as surface productivity might otherwise dictate.

The researchers calculated the total amount of nitrogen lost from this region over a year based on their measurements. They estimated that sinking particles alone account for the loss of about 24.0 teragrams of nitrogen per year, and when including the dissolved components of the water, the total loss reaches 40.7 teragrams. These numbers align closely with earlier estimates made using different methods, giving scientists confidence in the new approach. However, the key takeaway is not just the total amount, but the mechanism. The study proves that the ocean is not a passive recipient of surface debris; it is an active, self-sustaining system where life in the dark creates its own fuel to drive the global nitrogen cycle. By observing the ocean in its natural state, without the disturbance of bringing it to the surface, the team has provided a clearer, more accurate view of one of the planet's most important chemical processes.