The Ocean Is a Carbon Toilet. Marine Heat Waves Are Clogging It.

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The planet would be a whole lot hotter if it weren’t for fecal pellets. Across the world’s oceans, tiny organisms known as phytoplankton harvest the sun’s energy, gobbling up carbon dioxide and releasing oxygen. They’re eaten by little animals called zooplankton, which poop out pellets that sink to the seafloor. What is essentially a giant toilet, then, flushes carbon at the surface into the depths, where it stays locked away from the atmosphere, thus keeping the amount of CO2 up there in check.

But as humans pump ever more carbon into the sky, relentlessly raising ocean temperatures, worrying signals are flashing that this commode could be changing in profound ways. Consider the northeastern Pacific, off the coast of Alaska, where two major heat waves took hold of the sea, one from 2013 to 2015 and the other from 2019 to 2020. A new study found the two events transformed the composition of phytoplankton and zooplankton, essentially clogging the toilet and preventing the downward transport of carbon into the depths.

“These long-term studies help put everything into context and also really sound the alarms,” said Anya Štajner, a PhD candidate in biological oceanography at the Scripps Institution of Oceanography, who wasn’t involved in the research. “The ocean is changing. And not only is it going to affect the ocean — it’s going to affect the life in the ocean. And eventually that’s going to affect us, because we rely on the ocean for our air, our food, our climate regulation.”

Of course, each bit of the world’s oceans has its own unique chemistry, biology, and ecology, so what happens there might not happen everywhere. But with these bursts of heat, this swath of the sea saw declines in its ability to sequester the gas that’s heating the planet. That’s a precarious situation, given that the oceans capture a quarter of humanity’s CO2 emissions. “While we can generalize that maybe what we saw here would happen in general across other marine heat waves in the ocean, like the carbon accumulation, I think it’s important to assess that regionally as well,” said Colleen Kellogg, a microbial oceanographer at Canada’s Hakai Institute and co-author of the paper, which published today in the journal Nature Communications.

The researchers tapped a decade of data from Biogeochemical Argo floats, which autonomously wander up and down the water column taking readings of ocean chemistry. When they reach the surface, they ping that data to a satellite. In this way, the scientists got a 10-year stream of readings without having to constantly be on a boat in the northeastern subarctic Pacific Ocean, which is not known for hospitable winters.

The two ocean heat waves started like those we experience on land, with the atmosphere warming things up. Indeed, the ocean has absorbed 90 percent of the additional heat that humans have created. Accordingly, while in the 19th century just 2 percent of the ocean surface experienced bouts of extreme temperatures, that figure is now well over 50 percent. Such events will only grow more common and more intense unless humanity dramatically reduces its greenhouse gas emissions, and fast. As it happens, the northern Pacific has once again been smashing records of late, perhaps in part due to regulations in 2020 cutting the amount of aerosols generated by ships, which usually cool the planet by reflecting the sun’s energy back into space.

Like our most ferocious atmospheric blasts of heat, a lack of wind during the two events made matters even worse. Typically, after the seawater warms in the spring and summer, winter winds blow across the surface, pushing it along. This forces deeper, cooler waters to race upward to fill the void, keeping the water column more uniform, temperature-wise. This didn’t happen during both heat waves, and the sea remained more stagnant, as it normally does later in the year.

Because warmer water is less dense, it remains at the surface, creating a sort of cap. “Then in the subsequent spring and summer, that water is even warmer, because it didn’t cool the winter before,” said Mariana Bif, a marine biogeochemist at the University of Miami and lead author of the paper. (Bif conducted the research while at the Monterey Bay Aquarium Research Institute.) “So the impact of marine heat waves starts in the atmosphere, and then it’s transferred into the ocean.”

The two heating events were not created equal, though. The first coincided with an El Niño — a band of warm water off the coast of South America — that raised temperatures in the northeast Pacific even higher. The second saw a marked decrease in salinity due to changes in ocean circulation. Because water with lower salinity is less dense, it hangs around the surface, as the saltier stuff sinks. This further strengthened the warm cap.

The lack of winter churning also meant the nutrients typically drawn from deeper waters were cut off, denying the phytoplankton in that cap of the elements they needed to grow. Together, the high heat and low nutrients at the surface totally changed the environment for the organisms living and processing carbon there.

That transformed the ecosystem. Like plants on land, different types of phytoplankton need different amounts of nutrients, and in different proportions. “Usually, for example, in areas where you have this great mixing and great nutrients, you have a bunch of large phytoplankton that produce a lot of carbon — a lot of biomass,” Bif said.

As conditions changed during the heat waves, it was the littlest of phytoplankton species that benefited. These needed less nutrients to bloom, so they proliferated as larger species declined. And because different species of zooplankton dine on differently sized phytoplankton, the smaller ones that ate the smaller species suddenly had much more sustenance. “Those guys are going to make smaller fecal pellets, which would kind of float in the water more than sink,” Kellogg said. “So that could be contributing to the reduction in carbon moving from the surface to the deep ocean.”

Because the researchers had access to that data up and down the water column, they could monitor how all that carbon was sinking during the heat waves. Or rather, how it wasn’t — because the ocean’s carbon toilet was malfunctioning. In the first event, carbon particles were piling up 660 feet deep, and in the second, between 660 and 1,320 feet. In these zones, zooplankton grazers continued to chew on the particles, breaking them into smaller bits that couldn’t sink. In the second marine heatwave, an increase in particularly small zooplankton meant more production of tinier, non-sinking fecal pellets.

Not only was the toilet not properly flushing carbon, but more and more waste was being added to these waters as the heat waves rolled on. This gave bacteria lots of organic matter to break down, adding CO2 back into the sea. Eventually, currents would bring that CO2-rich water back to the surface, where the gas can be released back into the atmosphere.

Now scientists will have to monitor more heat waves in other parts of the world’s oceans to see if the same dynamics are at play, and how much that might be hobbling the sea’s ability to sequester carbon. At the same time, phytoplankton and zooplankton are suffering through crises other than heat, like ocean acidification potentially interfering with some species’ ability to grow protective shells.

If there’s less phytoplankton, there will be less oxygen coming out of the oceans, and less food for the zooplankton that feed all manner of other animals in the sea, including whales. “Paying attention to what’s happening at the base of the food web is going to give us a lot of information,” Štajner said, “both about how things are going to trickle up to these larger marine animals that we care about, but also insights about our climate.”

Luckily, with thousands of Biogeochemical Argo floats collecting data around the planet, researchers are getting an ever-clearer picture of how seas are changing, and phytoplankton along with them. “The oceans are very under-sampled, very understudied,” Bif said. “But they play a central role in climate. We can’t understand what we can’t observe.”

This article originally appeared in Grist at https://grist.org/climate/the-ocean-is-a-carbon-toilet-marine-heat-waves-are-clogging-it/.

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