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North Atlantic Phytoplankton Bloom Triggered By Eddies

July 6, 2012
Image Caption: A satellite image shows the different kinds of phytoplankton, some blue and some green, that bloom and get carried around the North Atlantic by swirling eddies. Credit: NASA

redOrbit Staff & Wire Reports – Your Universe Online

Scientists studying the annual growth of tiny plants in the North Atlantic Ocean have discovered that this year´s growth spurt began before the sun was able to offer the light needed to fuel the yearly phenomenon.

The annual growth spurt of plankton, which is known as the North Atlantic Bloom, takes place each Spring, and results in an immense number of phytoplankton bursting into existence — first “greening,” then “whitening” the sea as one or more species take the place of others.

In winter, cooling and strong winds generate mixing that pushes phytoplankton into deeper waters, robbing them of sunlight, but drawing up nutrients from depths. As winter turns to spring, days become longer. Phytoplankton are then exposed to more sunlight, fueling their growth.

But this year´s discovery initially puzzled researchers, since for decades the phytoplankton have required the longer days and calmer seas of Spring to fuel their growth.

However, the current study suggests evidence of another trigger.

The researchers found that whirlpools, or eddies, that swirl across the North Atlantic sustain phytoplankton in the ocean’s shallower waters, where the plankton can get plenty of sunlight to fuel their growth even before the longer days of spring begin.

The eddies form when heavier, colder water from the north slips under the lighter, warmer water from the south.

The study revealed that the eddies cause the bloom to begin about three weeks earlier than would otherwise be the case if the growth was spurred only by the longer days of Spring.

“That timing makes a significant difference if you think about the animals that eat the phytoplankton,” said Eric D’Asaro of the University of Washington (UW), one of the study´s co-authors.

Many small sea animals spend the winter dozing in the deep ocean, emerging in the spring and summer to feed on the phytoplankton.

“If they get the timing wrong, they’ll starve,” Craig Lee of UW’s Applied Physics Laboratory and School of Oceanography said.

Since fish eat these tiny sea animals, any reduction in their number could harm the fish population.

The North Atlantic Bloom is also important to the global carbon cycle, since Springtime blooms of microscopic plants in the ocean absorb enormous quantities of carbon dioxide, emitting oxygen via photosynthesis.

Their growth contributes to the oceanic uptake of carbon dioxide, amounting globally to about one-third of the carbon dioxide we put into the air each year through the burning of fossil fuels. Indeed, the North Atlantic is critical to this process, and is responsible for more than 20 percent of the ocean’s uptake of CO2.

“Our results show that, due to eddies, the bloom starts even before the sun begins to warm the ocean,” said Amala Mahadevan, an oceanographer at the Woods Hole Oceanographic Institution and lead author of a paper about the study published in the journal Science.

“Every undergraduate who takes an introductory oceanography course learns about the ecological and climate significance of the North Atlantic Bloom – as well as what causes it,” said Don Rice, program director in National Science Foundation´s Division of Ocean Sciences, which funded the research.

“This study reminds us that, when it comes to the ocean, the things we think we know hold some big surprises.”

The newly discovered mechanism helps explain the timing of the spring and summer bloom, known to mariners and fishermen for centuries and clearly visible in satellite images. It also offers a new look at why the bloom has a patchy appearance: it is shaped by the eddies that modulate its formation.

But making the discovery was not without its challenges.

“Working in the North Atlantic Ocean is challenging, but we were able to track a patch of seawater off Iceland and follow the progression of the bloom in a way that hadn’t been done before,” said study co-author Mary Jane Perry of the University of Maine.

“Our field work was set up with floats, gliders and research ships that all worked tightly together,” said D’Asaro.

“They were in the same area, so we could put together a cohesive picture of the bloom.”

The scientists focused on phytoplankton known as diatoms, which live in glass houses – walls made of silica.

“When conditions are right, diatoms blooms spread across hundreds of miles of ocean, bringing life-sustaining food to sometimes barren waters,” said Lee.

In April, 2008, Lee, Perry and colleagues arrived in a storm-lashed North Atlantic aboard the Icelandic research vessel Bjarni Saemundsson. They launched custom-designed robots in the rough seas, along with a float that hovered below the water’s surface to follow the motion of the sea.

Lurking alongside the float were six-foot-long, teardrop-shaped gliders that dove to depths of up to 3,000 ft. After each dive, the gliders, working in areas up to 30 miles around the float, rose to the surface, pointed their antennas skyward and transmitted their stored data back to shore.

The float and gliders measured the temperature, salinity and velocity of the water, and gathered information about the chemistry and biology of the bloom itself–oxygen, nitrate and the optical signatures of the phytoplankton.

In total, scientists aboard two ships, the WHOI-operated research vessel Knorr and the Bjarni Saemundsson, visited the area four times.

Soon after the measurements from the floats and gliders started coming in, the scientists perceived that the bloom had started even though surface conditions still looked winter-like.

“It was apparent that some new mechanism, other than surface warming, was behind the bloom initiation,” said D’Asaro.

Mahadevan used sophisticated three-dimensional computer modeling to analyze the information collected at sea by Perry, D’Asaro and Lee.  She generated eddies in the model from a south-to-north variation of temperature in the ocean. Without these, the bloom happened several weeks later, and didn’t have the space and time structures observed in the North Atlantic.

In the future, the scientists hope to put the North Atlantic bloom into a wider context. They believe much can be learned by following the phytoplankton’s evolution across an entire year, especially with gliders and floats outfitted with new sensors. The sensors would also look at the tiny animals that graze on the phytoplankton.

Ocean physics, particularly what we are learning about eddies, is intrinsic to life in the ocean, the researchers said.  They shape the oceanic ecosystem in numerous ways. For instance, no phytoplankton means no zooplankton, and no zooplankton means no fish.

Furthermore, eddies and phytoplankton play an important role in the oceanic cycling of carbon, without which we would have a different climate on Earth, the researchers noted.

“We envision using the gliders and float to make measurements–and models–of ocean physics, chemistry and biology that span wide regions of the world ocean,” said D’Asaro.

That would spark a new understanding of the sea, all from a tiny plankton that each spring and summer blooms by the millions and millions, Lee added.

“What we’re learning about eddies is that they’re a critical part of life in the ocean,” said Perry.

“They shape ocean ecosystems in countless ways.”


Source: redOrbit Staff & Wire Reports - Your Universe Online