Research cruise tracks down sources of trace element key to phytoplankton blooms
Posted May 25, 2012
Iron deficiency in humans is a common condition that can cause fatigue or weaken the immune system. In the surface waters of the ocean, where microscopic plants called phytoplankton live, the lack of iron limits the productivity of these organisms that form the base of the food web.
For people, the source of iron for a healthy diet is no secret. In the frigid waters of the Southern Ocean, the sources of iron that fuel blooms of phytoplankton, which serve as food for critters like the shrimp-like krill, are less clear.
Until now. A team of scientists on a seven-week cruise to the Ross Sea aboard the research vessel Nathaniel B. Palmer earlier this year investigated at least four sources of the trace element.
The expedition took the researchers to Antarctica’s largest ice shelf and a biologically productive polynya, an area of open water surrounded by sea ice, among other locations. Using a variety of instruments — from a high-tech camera towed through water to a specially designed conductivity, temperature and depth (CTD) rosette system used to sample water — the scientists mapped water masses, measured iron content and collected biological specimens.
“It was a truly interdisciplinary project involving physics, biology and chemistry,” said Dennis McGillicuddy, chief scientist for the Processes Regulating Iron Supply at the Mesoscale (PRISM) project.
The mesoscale refers to the physical scale of processes studied — in this case, ocean features that are a few tens of kilometers in size. The rest of the title requires a bit more background.
There are a number of microscopic plants and animals in the surface waters of the ocean that are collectively known as plankton. Phytoplankton are the plant portion of that complex biological assemblage, mostly consisting of single-celled algae.
Like terrestrial plants, phytoplankton use a process called photosynthesis to capture light from the sun and convert it into chemical energy stored in the form of carbohydrate. They also rely on nutrients in the water for growth, also not unlike how garden plants use nutrients in the soil. The Ross Sea is particularly rich in the macronutrients nitrate, phosphate and silicate.
In fact, based on the high abundance of such nutrients and the 24-hour sunlight of the Antarctic summer, scientists were puzzled as to why the Southern Ocean wasn’t productive enough to utilize all the nutrients. Back in 1988, John Martin and Steve Fitzwater finally offered an explanation in the journal Nature — iron-deficient waters.
“It’s kind of ironic that iron is limiting for phytoplankton. Iron is the fourth most abundant element in the Earth’s crust, but it’s in very low supply to surface waters of the ocean,” noted McGillicuddy, a senior scientist at Woods Hole Oceanographic Institution.
The problem is particularly acute in the Southern Hemisphere due to geography — an absence of landmasses. Dust deposition is one of the key pathways for how iron makes its way to the ocean surface. Glance at a globe: The Northern Hemisphere is heavy with continental landmasses, unlike the flipside of the equator.
“That’s what makes the Southern Hemisphere so different in terms of its iron cycling,” McGillicuddy said.
Still, iron is making its way into the surface waters of the Southern Ocean, sparking phytoplankton blooms so large that the greenish color of the chlorophyll can be seen by satellites from space.
The PRISM scientists hypothesized four possible sources of iron: circumpolar deep water intruding onto the continental shelf; sediments on shallow banks and near-shore areas; melting sea ice around the perimeter of the Ross Sea polynya; and glacial meltwater from the Ross Ice Shelf.
“The bottom line is that all four of our hypothesized sources are contributing to the iron budget,” McGillicuddy said.
John Klinck, director of the Center of Coastal Physical Oceanography at Old Dominion University, and his team on the ship were responsible for collecting data from the CTD rosette, an instrument that measures various physical ocean properties and collects water samples from different depths. Such measurements help the scientists map the currents, eddies and other structures of the water column that may be moving iron to the surface of the ocean.
“The combination of temperature, salinity and dissolved oxygen is indicative of where the water comes from,” Klinck explains.
For example, low levels of oxygen indicate a water mass that was below the surface for a long time, perhaps centuries. A temperature of minus 1 degree Celsius suggests ocean water that has been on the continental shelf for a while.
“The measurements [we took were] much closer together than traditional measurements, and we found small eddies and structures that had been missed before,” Klinck said. “We got a much higher resolution picture of what is there, and the iron measurements were clearly the new part of what we were doing.”
Those iron measurements were made by a different CTD rosette system, a so-called trace metal CTD, which does not contain any exposed iron in its construction, which could skew the results.
“Having that trace-metal clean [CTD] was an absolute necessity,” McGillicuddy said.
A different sort of water column profiling device called a SeaHorse also proved invaluable, according to McGillicuddy.
While the SeaHorse makes similar measurements to a CTD and other shipboard instruments, it can be deployed independently from the research vessel, left to make continuous measurements in one vertical column in the water. A cable descends from a surface buoy, with weights at the bottom to keep it anchored in place. Ocean wave action propels the instrument package down the tether to the bottom, where positive buoyancy drives it back up the line.
“It’s a really, really nice instrument,” McGillicuddy said. “We got a nice time series in both drifting mode and also moored on top of the Ross Bank.” The SeaHorse was provided and operated by a team led by Blair Greenan, a senior scientist at Fisheries and Ocean Canada.
An altogether different instrument called a video plankton recorder (VPR) “flew” behind the Palmer for hours a time. A digital camera in the nosecone of the VPR, which sports a strobe on the starboard wing, took 30 photographs per second. Its computer can identify organisms in the water. Other sensors aboard the VPR measure the physical ocean properties of the water and plankton.
The VPR measurements were of particular interest to the PRISM biologists, led by Walker Smith, a professor at the College William & Mary’s Virginia Institute of Marine Science. His team also collected samples of phytoplankton from the bottles carried on the CTD.
“We will be working very hard to merge those datasets to understand the mesoscale distribution of phytoplankton in these areas,” Smith said.
The biologists also conducted net tows to capture a larger class of marine organisms called zooplankton, which includes krill, to assess their distribution and biomass in the Ross Sea.
“We collected a whole series of zooplankton, which may represent one of the few broad scale attempts at understanding the distribution of zooplankton in different forms and therefore understand the food web a little better,” Smith said.
Data on iron distribution and biological productivity from the PRISM project are particularly important as scientists build a baseline of the current conditions in the Ross Sea. The region is expected to change as the global climate shifts.
McGillicuddy said it’s too early to offer any specific predictions on how climate change might affect primary productivity in the Ross Sea at this time.
“We’re just now trying to come up with a better understanding of what’s providing iron in the Ross Sea,” he said. “I think understanding those mechanisms will provide us a basis on which to make predictions on how things will change in the future.”
NSF-funded research in this article: Dennis McGillicuddy, Woods Hole Oceanographic Institution, Award No. 0944165; Peter Sedwick, Eileen Hofmann, John Klinck and Michael Dinniman, Old Dominion University, Award No. 0944174; and Walker Smith, College of William & Mary Virginia Institute of Marine Science, Award No. 0944254.
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