Important algae not always where we expect it to be
Movements influence ocean carbon cycle, study shows.
A globally important ocean algae is mysteriously scarce in one of the most productive regions of the Atlantic Ocean, research shows.
In fact, a new dataset reveals that coccolithophores both struggle and thrive in unexpected places. They are common in subpolar and temperate waters yet scarce around the equator, where an abundance of nutrients and sunlight create a biologically productive region.
Research led by Bigelow Laboratory for Ocean Sciences, US, also discovered that some coccolithophore species thrive deep below the surface near the farthest reaches of sunlight – within or just above a layer known as Sub-Antarctic Mode Water.
This distinct feature flows north from the Southern Ocean and provides nutrients to much of the global ocean, including the northern hemisphere.
William Balch, lead author of a paper published in the journal Deep Sea Research I, suspects that booming coccolithophore populations in the Southern Ocean are depleting the water layer's nutrient supply and altering its chemistry, potentially making it inhospitable for coccolithophores by the time it reaches the equator.
"Sub-Antarctic Mode Water exerts a staggering level of control on much of the global ocean," he says. "If coccolithophores are changing its essential properties, then they could be influencing which species grow in food webs as far away as the equator or even in the northern hemisphere."
Coccolithophores build protective crystalline plates from chalk minerals by extracting dissolved inorganic carbon from seawater. The way a species' plates are shaped influences how those plates scatter light in the surface ocean, especially after they become detached and begin to sink towards the seafloor.
Balch and colleagues discovered that not all coccolithophores drop their plates, and that the plates found throughout the water column come from just a few species.
This finding vastly simplifies the calculations needed to measure the carbon that coccolithophores contain from satellite reflectance data, they say. Coccolithophores play a major role in the global carbon cycle, and understanding where they live and how they scatter light is essential to quantifying how this important element moves between the surface ocean and seafloor.
"Understanding these large-scale patterns helps us understand ocean productivity in the entire Atlantic basin," says Balch.
The vast new dataset was created from measurements collected over 10 45-day research cruises as part of the Atlantic Meridional Transect program, which crosses the Atlantic Ocean between the UK and the tip of South America.
The researchers say their findings also have important applications to observations that rely on NASA Ocean Color satellites. These powerful oceanographic tools allow scientists to detect coccolithophore populations by measuring the light they reflect back into space, but they require on-the-water measurements to ground-truth the satellite data. NASA was the primary funder of this work.