Every spring, the North Atlantic blooms in a massive explosion of phytoplankton, covering the sea with green masses of organisms. A team of oceanographers at the UW recently released a study chronicling the emergence of algae populations in the region in an effort to better understand the environmental effects of such blooms.
The growth has an immense impact; the North Atlantic Bloom extends from the Caribbean to the Arctic, is visible from space to satellites with particular color-sensors, and absorbs 20 percent of the entire oceanic uptake of carbon dioxide. The North Atlantic Bloom alone absorbs almost 7 percent of all carbon dioxide emitted each year, the tiny plankton gorging on the carbon dioxide and releasing oxygen via photosynthesis.
UW oceanographers discovered that oceanic eddies trigger phytoplankton growth by switching water-density gradients, enabling the plankton to reach sunlight on the surface that offers nutrients plankton couldn’t find in its depths. A study of the eddies was first proposed in 2006. “It was one of the more memorable projects I have been involved in,” said Eric D’Asaro, principal investigator and senior principal oceanographer of the ocean physics branch of the UW Applied Physics Laboratory (APL).
In 2008, a team of investigators and students from the APL, along with colleagues from the University of Maine and Dalhousie University in Nova Scotia, set out to study the bloom near Iceland, attempting to study the bloom’s attributes through its entire duration. “[The study] is just a long-standing body of work that … Eric and I have been pursuing,” said Craig Lee, an associate professor of oceanography who is also affiliated with the ocean physics branch of the UW APL. “We would take days of observations, and we’ve been analyzing the data for years.”
The team began observations in early April, employing autonomous underwater robotic Seagliders and Lagrangian mixed-layer floats that were developed at the APL. The observations were then followed by a month-long cruise, and the researchers finally finished in June as the bloom ended.
“[The gliders] have the longest range and longest endurance of any autonomous underwater vehicle,” said Charles Eriksen, a professor of oceanography who helped develop the gliders. “They lasted nine-and-a-half months. … We had to tweak them a little, changing an aspect to get it more efficient by a certain percentage.”
According to the project summary, each glider was also equipped for the project with new sensors in order to measure every aspect of the bloom, which were calibrated with shipboard sensors and water samples in order to ensure functionality. “We knew that this technology of floats and gliders had the potential to be revolutionary, and we wanted to try it out on an important problem,” D’Asaro said.
Focusing on diatoms, a common type of phytoplankton, the ship followed the movement of the bloom. The gliders dove to depths of up to 1,000 meters and transmitted data back to the ship.
The bloom began on April 20 with increased oxygen concentration and peaked in mid-May, with large phytoplankton sinking — burying carbon, removing it from the atmosphere for the foreseeable future — necessitating even deeper dives by the gliders in order to gather a complete picture. Despite the appearance of wintry conditions, the data made it clear that the bloom began due to the eddying. “It’s only recently that data transmission in that significant of amounts was possible, with advances in antennae that could use cell towers as satellites,” Eriksen said. “The gliders are able to transmit far more information … and also survive rough conditions.”
Sophisticated computer modeling was required in order to definitively formulate the mechanism. Amala Mahadevan, an oceanographer at the Woods Hole Oceanographic Institution (WHOI) and lead author of the study, helped model all the information, generating a virtual rendition of the bloom with and without eddies. The results conclusively demonstrated the model with eddies to be correct. “This has implications for other animals, the bloom happening earlier,” Lee said. “The changes in circulation can change the timing of the bloom.”
The bloom’s timing and range are an intrinsic part of the carbon cycle, and are also critical to other oceanic life. The plankton serve as not only a carbon sink, which affects the global climate, but as fuel and food for other plants and fish. The North Atlantic Bloom occurs only in spring, but its ramifications are far-reaching, the research summary stated. “We will continue to study the existing data, but hope to make measurements for a much longer time, over an entire year, in order to see how the spring bloom fits into the whole annual cycle,” D’Asaro said.
External link: http://dailyuw.com/news/2012/jul/24/new-bloom/