Icebergs become source of nutrients for biological communities as they melt
Posted May 16, 2008
In 2005, scientists aboard the ARSV Laurence M. Gould went on what pelagic biologist Bruce Robison said some in the scientific community characterized as a quixotic quest to prove icebergs are a hotspot for life in the deep ocean.
The team of geochemists, oceanographers, biologists and others indeed found a host of animals and organisms above, below and on the icebergs, including seabirds, phytoplankton and shrimp-like krill. But they also believe these floating chunks of ice may play a significant role in removing carbon dioxide from the atmosphere thanks to the associated biological community. The research culminated with the publication of the team’s paper in the journal “Science” last year.
Now many of the same members of that group are returning to the Weddell Sea at the end of May for a one-month science cruise aboard the RVIB Nathaniel B. Palmer to further study how icebergs affect the marine ecosystem. A second cruise is planned for March 2009.
So how can a block of ice, even one as big as Manhattan Island, give birth to a diverse biological community, one that can extend as much as 4 kilometers from the iceberg?
It turns out these icebergs, made of glacial ice that once ground against bedrock, hold terrestrial materials that contain traces of micronutrients including iron, which it drops like breadcrumbs into the sea as it melts. Iron helps create blooms of phytoplankton, mostly composed of algae, which absorb carbon dioxide (CO2) from the atmosphere.
Some of that carbon will return to the atmosphere, but much of it will sink into the deep ocean, when the algae die, removing carbon from the system for hundreds if not thousands of years. CO2 is a key greenhouse gas.
“We’re trying to look at how much organic carbon is basically produced by the phytoplankton and how much escapes into the deep ocean,” explained oceanographer Ken Smith, the project’s principal investigator from California-based Monterey Bay Aquarium Research Institute (MBARI).
The researchers also want to determine how much iron the icebergs deposit in the ocean as they migrate, as well as how the micronutrient promotes phytoplankton growth and photosynthesis.
A well-known theory called the Iron Hypothesis, put forward by oceanographer John Martin in the 1980s, says that some areas of the oceans contain the major nutrients required for aquatic plant growth — nitrate and phosphate. What’s lacking are the micronutrients like iron.
Scientists call these areas of the ocean, including the Southern Ocean, high nutrient low chlorophyll regions. One save-the-planet theory, put forward by Martin and others, suggests seeding the ocean with iron to increase phytoplankton blooms, which would soak in more CO2, presumably lowering the amount of the greenhouse gas in the atmosphere and subsequently lowering temperatures. It’s sort of like adding Miracle Grow® to your garden to improve plant growth.
Timothy Shaw, a geochemist from the University of South Carolina, said it appears the icebergs can provide iron and other micronutrients from the terrestrial material they carry. On the 2005 cruise, he and his group measured isotopes of radium, including radium 224, that they collected from the seawater around two icebergs.
Radium is ubiquitous in terrestrial material, he explained, so the scientists used it as a proxy for determining the presence of iron and other micronutrients like zinc, which would also be present in terrestrial material. The radioisotope radium 224 has a half-life of 3.6 days, Shaw said.
“So, if we could measure it around icebergs, we were sure that it wasn’t a long-distance source,” he said. “The half-life is so short that you can’t have it transported long distances from continents.” The half-life is the amount of time it takes for half of the atoms in a sample to decay.
The icebergs become an intermediary between the terrestrial and ocean systems, what Shaw called a Lagrangian estuary, a moving source of biological diversity. “This kind of fits the model of a moving estuary,” said Shaw, a co-principal investigator on the project.
On the two trips to the Weddell Sea, Shaw and his group will “map the inventories of particulate material, radioisotopes and iron to validate our contention that icebergs deliver a tremendous amount of terrestrial material.”
To make that and other determinations about iceberg characteristics, the researchers will use a number of tools, including a device developed by Scripps Institution of Oceanography called a SOLO float. The SOLO float, which stands for Sounding Oceanographic Lagrangian Observer, can dive up to 2 kilometers deep in the upper ocean, taking temperature, depth and salinity readings as it moves up and down in the water based on a set of instructions encoded in its electronics package.
Thousands of these floats have been deployed in the oceans. For the Palmer cruise in June, the researchers will send several SOLO floats underneath the icebergs they study.
“What we’ve done is just borrow that technology,” Smith said. “We got people at Scripps to build us several of these SOLO floats, and we secured inverted cones on them, so they’re like rain gauges. They just collect material as it settles through the water column.
“The intent of that is to collect the carbon that is being produced by the community,” he added. “This gives us a handle on how much carbon is escaping the upper water system. We’re hopefully looking at how much carbon is being consumed by the enriched community, and how much of that carbon as organic matter is being exported out of the photic zone to the deep sea.”
It’s in the photic zone where photosynthesis occurs, when sunlight penetrates the upper ocean. The high amount of phytoplankton biomass found around the icebergs in 2005 was similar to that found near the edge of seasonal pack ice or during iron enrichment experiments like that proposed by Martin, the scientists reported in the “Science” paper.
Another key tool in the team’s arsenal is a remotely operated vehicle (ROV) tethered to the Palmer. Robison, the pelagic biologist, also from MBARI, is the primary ROV pilot. He said engineers have substantially modified the robot from when the scientists used it three years ago.
They added two thrusters and a tool sled that allows the ROV to carry more instruments. They also upgraded its cameras, and outfitted it with a new, longer tether so the versatile little robot can explore the bottom of icebergs while the ship maintains a safe distance, about 300 meters.
“It’s a little hairy getting too close to these big icebergs, because they calve and thousands of tons of ice can come falling into the ocean unannounced,” Robison said. “For those wanting to work in close, the only way to get in there, to see things and make measurements and collections, is with a remotely operated vehicle.
“We’ve tricked it out with all types of gear,” added Robison, a co-principal investigator on the project. “When we got a peek around the corner [in 2005], so to speak, we couldn’t do any exploring underneath, and that’s something we’re looking forward to this time with the longer tether.”
But thanks to the ROV in 2005, the team made another particularly provocative discovery, despite the robot’s limited range. They found “tufts” of algae adhering to the iceberg where sand-grain-sized volcanic rocks were embedded in the ice. In addition, the surface of the iceberg resembled a uniformly flattened golf ball, with the algae growing around the edges of the dimples, which were about 6 to 8 centimeters wide and about 2 centimeters deep.
“It was a significant discovery. Something no one had seen before,” Robison said. “Krill were feeding on [the tufts of algae] extensively.” Added Smith, “There are huge fields of these things in the light zone.”
The algae, made up mostly of diatoms, resembled that found in benthic communities in shallow subtidal zones. The scientists estimated that these algae might inhabit as much as 25 percent of the submerged portion of an iceberg. That represents a significant source of primary plant production — and another source for sucking CO2 out of the atmosphere — considering the number of icebergs floating along in the Southern Ocean.
In fact, using satellite images, the researchers counted nearly 1,000 icebergs in an 11,000-square-kilometer area of ocean. They calculated that in 40 percent of the Weddell Sea the icebergs are raising biological productivity.
It’s not unreasonable, Robison said, to hypothesize that icebergs are spawning similar productivity in other areas around the Southern Ocean. The idea seems incredible — that there are literally thousands of these floating estuaries sucking CO2 out of the air like a straw — until one considers what the researchers have already discovered.
It all began with a simple observation, according to Robison.
“One thing that you can’t help but notice is that anytime anything floats in the ocean — whether it’s a clump of seaweed or an old bottle or a wooden raft — eventually animal communities build up around it. Barnacles attach, some plankton goes there for shelter, and big fish hang out there to feed on the plankton,” he explained.
“With the icebergs increasing, chances are that would have an effect on these pelagic communities,” Robison added. “These ideas come from lots of experience of looking at the ocean, scratching your head, and trying to figure out what’s going on and how does this work? And what am I missing?”
Perhaps the better question to ask: What are they going to find next?
NSF-funded research in this story: Ken Smith and Bruce Robison, Monterey Bay Aquarium Research Institute; and Timothy Shaw, University of South Carolina.
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