Blooming Ocean: Huge Phytoplankton Explosions Explained

Huge blooms of microscopic marine plants — so big that they are visible from space — are a spectacular site, turning normally dark ocean water into a brilliant aquamarine.

Now science sleuths unraveled the recipe for these biological explosions.

The blooms of phytoplankton, as these tiny plants are called, can be found during the Southern Hemisphere's two-month summer (December through January) off the southeast coast of South America. This region supports the Patagonian Shelf marine ecosystem, which includes an important fishery.

Poorly understood phytoplankton called coccolithophores — marine single-celled plants with calcite scales — are teeming in the sunlit upper layer of the world's oceans. But exactly how they form large blooms hasn't been very clear.

"Coccolithophores are a complex group of plankton, and in many areas of the world ocean satellite-based observations provide the only information we have," said Stuart Painter of the National Oceanography Centre in Southampton, England. "We often have little direct knowledge of the environmental factors coincident with these blooms."

Recently, a large bloom near the Patagonian Shelf coincided with satellite images showing the distribution of reflective calcite from the teensy plankton. Calcite is a carbonate mineral commonly found in limestone. It also forms the microscopic plates – "coccoliths" – that surround coccolithophores, possibly for protection.

Water samples from the bloom revealed that many factors thought to trigger blooms were indeed found in the waters overlying the Patagonian Shelf and the shelf break, where the seafloor dips down to the deep seabed, north of the Falkland Islands. These conditions included a specific cocktail of nutrients and seawater temperature.

How everything is mixed together is also important, the research team discovered. They took measurements at the Brazil-Falklands Confluence to the northeast, where two major ocean currents collide. The Brazil Current carries warm, salty subtropical water southward, and the Falklands Current brings cold, fresh and nutrient-rich water up from the sub-Antarctic region. The continental shelf itself experiences strong tides and inputs from large rivers. To complicate matters further, low-salinity water also enters the Patagonian Shelf region from the Pacific Ocean through the Magellan Strait in the south.

"The marine environment of the Patagonian Shelf region is well known for its complexity, but what has been less clear until now is how this relates to the large blooms of coccolithophores in this region," Painter said.

The distribution of these waters is strongly influenced by the shelf break front, which is the focus of intense biological production. It can vary from 10 to 40 miles (20 to 200 kilometers) in width, determining the spot where conditions are right for coccolithophore blooms.

The research team identified five distinct water masses, each having different characteristics, such as temperature and nutrient concentration. They also varied in the amount of chlorophyll in their surface waters, indicating different levels of phytoplankton production.

"The complex interaction of large currents and different water masses clearly exerts strong controls over the position of coccolithophore blooms in this region," Painter said.

The study is detailed in the Oct. 31 edition of the journal Continental Shelf Research.

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