Iron critical to ocean productivity, carbon uptake
CORVALLIS, Ore. – A new study has found that large segments of the Pacific Ocean lack sufficient iron to trigger healthy phytoplankton growth and the absence of the mineral stresses these microscopic ocean plants, triggering them to produce additional pigments that make ocean productivity appear more robust than it really is.
As a result, past interpretations of satellite chlorophyll data may be inaccurate, the researchers say, and the tropical Pacific Ocean may photosynthesize 1-2 billion tons less atmospheric carbon dioxide than was previously thought. Global ocean carbon uptake is estimated at 50 billion tons, so the reduction in the estimate of the uptake is significant – about 2 to 4 percent.
Results of the study were published this week in the journal Nature.
When stressed by a lack of iron, phytoplankton create additional pigments that fluoresce, or light up, unlike normal pigments, according to lead author Michael J. Behrenfeld, a research scientist in Oregon State University's Department of Botany and Plant Pathology. Unfortunately, he added, satellite imagery could not readily distinguish that difference.
"It's really a fascinating process," Behrenfeld said. "When phytoplankton species make these extra pigments, they don't use them right away – they bank them. Then when they get an infusion of iron, they just take off. They don't have to wait to begin dividing and growing. But that green color wasn't an indication of health, it was an indication of stress from a lack of iron."
The study is also important because it looked at the availability of iron throughout the tropical Pacific Ocean instead of small portions of it. Behrenfeld and his colleagues looked at 12 years of fluorescence data taken along 36,000 miles of ship tracks throughout the tropical Pacific. They now have a "fluorescence fingerprint" of which parts of the ocean are iron-stressed, as well as which parts suffer from lack of nitrogen – another key element to ocean productivity.
"Nitrogen and phosphorus are nutrients that come up from the ocean bottom to feed the upper water column," Behrenfeld said. "Iron, on the other hand, can come from the deep or from the air, but it also enters the ocean through dust deposited by the wind. Windstorms blowing sand and dust off large deserts are a major source of iron for the world's oceans.
"It's like dumping a load of Geritol or some other iron supplement into the water."
Three large areas appear limited by a lack of iron, the researchers say – the southern ocean around Antarctica, the sub-arctic north Pacific below Alaska, and a huge area in the tropical Pacific centered on the equator. With their newfound knowledge of fluorescence, the scientists believe they now can use satellite imagery to identify specific areas that are iron-stressed – and how they respond to changes such as the sudden influx of iron from a windstorm.
"It turns out different places in the ocean are missing different nutrients," said Robert Sherrell, a scientist from Rutgers University and a co-author on the study. "The new fluorescence technology now allows us to tell which combination of nutrients is stressing the phytoplankton."
Behrenfeld said the presence of iron stress in the ocean links phytoplankton to the climate through changes in terrestrial-based dust deposited in the ocean, but it is too early to tell if there is an impact of recent climate change on iron-stressed populations because the satellite data record is too short.
"But now we have the tools to determine that," he emphasized.
The northern portion of the tropical Pacific is more nitrogen-stressed and doesn't have the "false greenness," according to Behrenfeld.
The researchers are creating new models of carbon cycling using NASA satellite imagery which they have calibrated using their ship-based measurements of fluorescence.
The role of the ocean in the global carbon cycle is critical – and nowhere is it more pronounced than the tropical Pacific Ocean. As phytoplankton plants grow, they suck carbon dioxide out of the atmosphere to build new cells.
A better understanding of this carbon cycle is a key to studying global climate change. Iron fertilization of phytoplankton is also a key to a healthy marine food chain.
Both Behrenfeld and Peter Strutton, an assistant professor of oceanography at OSU and a co-author on the Nature paper, have been involved in experiments in which iron is introduced into the ocean in an attempt to boost productivity. Those studies found that introducing iron into small portions of the Pacific did indeed trigger phytoplankton growth, but it wasn't as robust or as sustained as models predicted.
"It wasn't the silver bullet that scientists originally hypothesized," Strutton said. "The carbon export was slower than we thought. It could be the scale was too small, and it could be that the (biological) response was too slow and we didn't wait long enough."
Behrenfeld said introducing iron into the ocean system is complex because the mineral isn't water-soluble and requires repeated infusions.
"When you first do it, there's an explosion of growth and then it plateaus," he said. "Then you add a bit more iron, and the phytoplankton respond a bit more. Then you add a third shot, and it triggers some more modest growth. But at the same time you're promoting phytoplankton growth, the grazers that feed on them come to life because they suddenly have a more abundant food supply.
"So the plankton can disappear as fast as you've made them grow."
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