This Intrepid Robot Is the WALL-E of the Deep Sea

Yet just how much carbon gets trapped can vary from ocean to ocean and from season to season. In general, researchers just don’t have a good handle on the biological and chemical processes going on down there. “The rover helps us understand how much of that carbon might actually make its way into the sediments in the deep sea,” says MBARI marine biologist Crissy Huffard, who coauthored the new paper. “It’s our only view into how much carbon might actually get stored into the sediments, versus how much actually is consumed and probably contributing to acidification in the deep sea.” (When carbon dioxide dissolves in seawater, it forms carbonic acid.)

Here’s a tricky example of one of those seafloor carbon mysteries. In California, the land is heating up much faster than the adjacent ocean, a differential that intensifies seasonal winds. That could be driving more upwelling—wind pushes the surface water away, and water from below rushes up to fill the void. This would bring up more nutrients that feed phytoplankton, which bloom in surface waters, and then die and become marine snow. Between the years 2015 and 2020, for instance, BR-II’s fluorescence camera detected a massive increase in the amount of phytoplankton reaching the seafloor in big pulses. Simultaneously, its sensors detected a decrease in oxygen, meaning the microbes in the seafloor were busy processing the bonanza of organic material.

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That raises some questions for Huffard. “Just in general, the area’s becoming a lot more erratic in its food supply—it can be years’ worth of food coming down in a few weeks. So how is that changing the whole ecosystem?” she asks. “The response by the animal community is almost instant. They start consuming it right away, there’s no big lag. The microbes are just primed and ready to go.”

What does this mean for the carbon cycle? Theoretically, the more organic material that’s raining down, the more that’s getting sequestered away from the atmosphere. But at the same time, organisms on the seafloor that are eating this bonus buffet are also using up oxygen and spitting out carbon dioxide, which may be acidifying deeper waters. And because the ocean is constantly churning, some of that carbon may even make it back up to surface waters and into the atmosphere. “We’re showing that more and more carbon than would have otherwise been predicted is making its way to the deep sea,” says Huffard. “The rover adds the dimension to tell us that most of that carbon is actually getting eaten once it’s down there, not being stored in the sediment.”

Are these extra-large pulses of marine snow now a permanent feature of the deep waters off California, or an aberration? With the benthic rover, scientists can gather the long-term data required to start providing answers. “The deep sea is largely understudied and under-appreciated, despite the fact that it is critical to keeping the planet healthy and combating climate change,” says Lisa Levin, who studies the seafloor at the Scripps Institution of Oceanography but wasn’t involved in this work. “An army of such devices could help us better understand biogeochemical changes—critical to improving climate models, ecosystem models, fisheries models, and more.” Rovers might also help scientists study the effects of deep-sea mining operations.