Enormous, Baffling Plankton Blooms Fueled by Hydrothermal Vents Deep in Earth's Belly

In 2014 and 2015, two massive plankton blooms that appeared off the coast of Antarctica, baffling scientists observing them.

Plankton blooms occur when populations of phytoplankton—microscopic, water-dwelling organisms—increase rapidly. Normally, their numbers are limited because of a lack of the nutrients required to survive. Under the right conditions, a population can explode, creating a bloom that can stretch for thousands of miles in just a few days.

While examining 2014/2015 data from the Southern Ocean, Stanford's Mathieu Ardyna noticed something unusual: Two enormous plankton blooms around 1,300 miles off the coast of Antarctica and 1,400 miles from Africa.

These blooms should not have existed. The regions where they formed are known to have severe iron shortages, which is a nutrient phytoplankton requires to thrive. The area also has low chlorophyll concentrations—high concentrations indicate a large phytoplankton population.

The only way these blooms could have been generated is if there had been a huge influx of iron, but most of the normal sources—such as melting sea ice—could not explain it.

Instead, Ardyna and colleagues suggested the presence of hydrothermal vents—an opening on the seafloor from which heated, mineral-rich water escaped. Scientists know there is a stretch of hydrothermal vents along a mid-ocean ridge around 750 miles from where the blooms occurred. Previously, it was thought nutrients generated from these vents tended to be confined to the local region. However, if the bloom was the result of nutrients from the vent, it would show currents could transport minerals far and wide, playing a far greater role in ocean ecosystems.

In a study published in Nature Communications, researchers say they have the first observational evidence that iron hydrothermal vents can stimulate phytoplankton blooms in the Southern Ocean, far away from its original source.

By looking at modern data from satellites and floating robots, as well as water samples taken during scientific missions during the 1990s, researchers were able to show how a powerful current around Antarctica can transport the iron east. When this coincides with the arrival of spring—and the sunlight phytoplankton needs to photosynthesize—an enormous bloom can be triggered.

Findings suggest the goings on in the deep ocean could play a bigger role with surface ecosystems than previously thought. Understanding how, when and where phytoplankton blooms form is important, as they consume carbon dioxide and produce the oxygen we breathe.

"A circumpolar analysis is clearly needed to evaluate the overall impact of hydrothermal activity on the carbon cycle in the [Southern Ocean], which appears to trigger local hotspots of enhanced biological pump activity and increase its potential as a sink for atmospheric CO2," the researchers conclude.

Ocean cave
File photo showing an ocean cave. iStock