Kevin Arrigo, a biological oceanographer at Stanford University, has spent nearly two decades studying remote sites in Antarctica that experts like to call an “oasis in a desert of ice.” They are so inaccessible that even scientists who study them may not have laid eyes on them.
The handful that Arrigo visited smelled like rotten eggs. They are called polynyas, formations that derive their name from the Russian word for “hole in the ice,” and are typically an expanse of open seawater along the coast that is enclosed by floating sea ice and the continental shelf.
New research led by Arrigo found that the most important determinant of productivity—or the abundance of phytoplankton—in these polynyas is iron that is being pumped into them by glacial meltwater. With a warming planet and intensification of glacial melting, coastal Antarctica, where these polynyas are formed, is likely to become more productive, according to the researchers. That could create ripple effects for the food chain.
“If you were standing on the shore, you would see an area of open water, and beyond the open water a bunch of sea ice,” Arrigo said. The pungent odor that hangs over some polynyas, Arrigo explained, is from emissions produced by certain varieties of phytoplankton—tiny microorganisms that form the basis of the marine food chain. The ocean waters that are cleared of sea ice by strong winds blowing from the coast carve out a suitable enclave where marine organisms can thrive, unlike the rest of the icy cold Antarctic region.
“When you look at satellite images of phytoplankton abundance around Antarctica, you see these areas that just light up,” Arrigo said. “They are flaming red because there is just so much phytoplankton.” The red in satellite images is indicative of high levels of chlorophyll.
“I liked how they combined traditional oceanographic measurements like chlorophyll levels and primary production, and recent satellite work by other groups that looked at glacial melt,” said Scott Doney, a scientist at the Woods Hole Oceanographic Institution.
A melting continent filled with iron
Glacial meltwater enriches the waters of the polynya with iron, scientists have hypothesized. “Whenever we stimulate any water with nutrients, in this case iron, the productivity always goes up,” Arrigo said. This is because in this region, iron is a limiting factor for phytoplankton growth. “The fertilizer you use in your garden has nitrogen and phosphorus in it,” he said. “There is tons of that stuff down there; the first thing they run out of is iron.”
The meltwater becomes a supplier of this iron because as melting ice crawls towards the ocean, it grinds up bedrock along the way, and bedrock is full of iron. Glaciers also have iron trapped in their icy mass from the snow that has been accumulating on ice sheets over thousands of years. Falling snow entraps dust that is rich in iron. When a melting glacier saunters into the ocean waters, it introduces this iron into the system.
In the paper, the scientists also suggested that the more productive the ecosystem is, the more food is available for organisms higher up the food chain. “If I was a krill or a whale, then it is a good thing,” Arrigo said. “The largest density of penguins and seals are in the areas that polynyas are most productive.”
Creating new carbon sinks
“There is a bottleneck in the food chain,” Ainley said, “and it might not matter if the coastal polynyas are more productive—at least not for birds and mammals.” Ainley noted that there are many gaps in what we know about food webs in Antarctic ecosystems.
For Arrigo, who has extensively studied carbon cycles, an important implication of higher productivity in polynyas is for their role as carbon sinks. Because these bodies of water are sites where photosynthesis occurs, they also act as carbon stores. “Polynyas, for their size, disproportionately suck atmospheric CO2,” Arrigo said. If you increase their productivity, it is going to increase their ability to store carbon, he said.
But because even the largest polynyas are only a few hundred thousand square kilometers in size and their extent varies as the season changes, this impact is limited. “It is not going to solve any climate change issues, for sure,” Arrigo said, but the research is important in understanding the impacts of glacial melt on marine ecosystems.
Since the current work used satellite data and statistical modelling to narrow down on the most important factors that govern these systems, “the obvious next thing to do,” Doney said, “is to go into the field and make measurements to test this hypothesis.”
www.eenews.net
see also:
No comments:
Post a Comment