Our planet would be unrecognizable without the ocean for myriad reasons, not least of which is the key role it plays in keeping our climate in check.
Carbon dioxide levels in our atmosphere “would jump by nearly 50 percent” without the many ecological services the twilight zone provides, according to the Woods Hole Oceanographic Institution based on Cape Cod in Massachusetts.
From housing the vast majority of marine fish by weight, to facilitating the removal of up to 6 billion metric tons of carbon from the upper ocean every year, the twilight zone’s daily operations help make life possible — within and beyond its salty waters.
Yet in the face of indiscriminate fossil fuel use, the planet is struggling to maintain its ecological order.
Sunlight illuminates a small stretch of the surface, then filters down until it completely disappears at the ocean’s pitch-dark depths, around 1,000 meters below.
Buesseler thinks of this twilight zone – also called the mesopelagic zone, from the Greek words for ‘middle’ and ‘open sea’ – like a gatekeeper between the surface and the deep.
But a warmer, more acidic ocean does us no favors when it comes to maintaining its role as one of the biggest carbon sinks on our planet.
Where the sun shines at the surface of the water, microscopic plants called phytoplankton and algae are happily soaking up those rays and taking in carbon dioxide while they photosynthesize.
Then, when the sun goes down, the countless fish and other sea creatures that dwell in the midwaters embark on a long journey, traveling up hundreds of meters to feed on those tasty aquatic plants under the cover of darkness.
When those small fish poop, or when their bigger predators poop, that waste contains some of the carbon captured by the plants.
Meanwhile, some percentage of the microscopic plant life that thrives at the surface dies and sinks fairly quickly through the water column, Raven said.
“It’s just flecks of carbon from basically the atmosphere maybe a week ago that got turned into something alive that’s sinking to the seafloor,” she added.
The daily twilight zone commute — known as the migrant pump in the report — plus the gravitational pump fueled by marine snow are two of the ways the ocean transfers carbon.
Buesseler emphasized that scientists have a lot of outstanding questions in terms of how exactly these pumps operate, and just what portion of carbon actually makes it to the bottom of the ocean without getting eaten or soaked back into the water as carbon dioxide expelled by respirating fish along the way.
Cold water is better at dissolving and absorbing gasses like CO2 compared to warmer water, which is why a large amount of it gets dissolved in the ocean’s chilliest waters, according to the report.
After years of working overtime to take in some of the excess CO2 we’ve put in the atmosphere, ocean acidification has already increased 30 percent compared to preindustrial levels, according to the report, and could increase an additional 120 percent by 2100.
Researchers are working to answer how warming and acidification affect the biological carbon pump.
“We’ve kind of had it relatively good for a while here, in that the ocean has been forgiving.
Researchers have a lot more to learn about the twilight zone, and certain developments could offer them more flexibility and opportunity.
Across the globe, scientists are also studying how to remove more CO2 from the atmosphere by harnessing the power of the ocean’s carbon pumps.
The idea of humans intervening in ecological processes to address a problem – one that we’ve created – is controversial, raising concern over the potential for unforeseen consequences.
Preventing the worst outcomes while conserving natural ecosystems in the ocean and beyond will require multi-faceted action on the part of humans — some of which might have sounded like science fiction decades ago.