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Failing food pantry?

Seafloor organisms face possible challenges as climate changes

Scientists are well aware that climate change has come to the Antarctic Peninsula in a big way. Ice shelves are collapsing, glaciers are retreating and the duration of winter sea ice is shrinking.

The latter has serious implications for critters like the shrimplike krill, which feed on the ice algae that forms underneath the sea ice, and for Adélie penguins that prey on the krill. Less sea ice means less krill, a biomass deficit that ripples up the food chain. That story is also well known.

But hundreds of meters below the sea ice sits the continental shelf, home to a unique marine menagerie, from sea cucumbers and worms called polychaetes to sea stars and corals and even bacteria, many of which also rely on sea ice algae for dinner. How this community will respond to climate change is a story that interests scientists like Craig Smith, David DeMaster and Carrie Thomas.

“The Antarctic Peninsula is warming faster than anywhere in the world. It can give us insights, advance warning, into what’s likely to happen in polar ecosystems in general as sea ice disappears,” explained Smith, a marine ecologist at the University of Hawaii and a principal investigator for a ship-based project called FOODBANCS2 (Food for Benthos on the Antarctic Continental Shelf 2). “It’s an important time to get a handle on climate change processes and how they’re going to affect these high-latitude ecosystems.”

FOODBANCS refers to a food bank, a pantry of sorts, of the organic material that falls and accumulates on the seabed during the summer as sea ice melts and phytoplankton blooms erupt near the water surface in an area called the euphotic zone. The bottom-dwelling organisms subsist on that short burst of food that falls like breadcrumbs before winter returns, bringing with it sea ice and darkness.

But warming temperatures are disrupting that cycle, shortening the length of winter sea ice, meaning the mix, or composition, of food falling onto the shelf is changing. The question then becomes: How will the benthic fauna respond to that possible change in diet?

“Those animals that rely on ice algae for their nutrition will have a much harder time of it overall,” said DeMaster, a biogeochemist from North Carolina State University and a principal investigator on FOODBANCS2. “We’re already seeing diminished amounts of sea ice during some of our cruises. In the future, it will only get worse.”

Thomas, the project’s third PI from North Carolina State University, is an organic geochemist who is measuring rates of organic matter degradation on the seafloor, conducting feeding experiments on bottom fauna, and using biomarker analyses to unravel changes in the sources of organic matter reaching the seabed.

This month the trio are leading a team of 16 scientists for a month-long cruise aboard the ARSV Laurence M. Gould to retrieve samples from the seafloor along a 500 kilometer, north-south transect just west of the Antarctic Peninsula. This will be the third and final science cruise of the project, which started in February 2008 and included a winter expedition in July and August during the same year.

Over those five degrees of latitude, the scientists expect to see marked differences in the types of material that fall to the seabed and the structure of the benthic communities. That’s largely dependent on the duration of sea ice, which survives longer the farther south one travels, as long as nine months, according to Smith.

“We anticipate seeing a change in feeding strategies as we’re moving along that temperature and sea ice gradient,” DeMaster noted. The idea is that as the climate changes, the southern extreme of the transect will begin to mirror that at the northern end, allowing the scientists to make predictions about how the ecosystem may change.

Smith said, “We do see a substantial shift in community structure from the northern to the southern [ends].”

This project is actually a sequel to an earlier series of five cruises that Smith, DeMaster, and Thomas made along the Antarctic Peninsula from 1999 to 2001 — FOODBANCS1. It was during those five cruises that Smith, DeMaster, Thomas and colleagues developed the food bank hypothesis.

The original FOODBANCS research also clued the scientists into the idea of studying the shelf as a means for looking at the effects of climate change on the peninsula. As Roy Arezzo, an educator who participated in the FOODBANCS2 cruise last austral winter, wrote on his blog, “Due to the build-up of a nutritious food bank layer on the seafloor, the benthic community responds less to seasonal fluctuations, serving as a ‘low-pass filter’ and making it possible to monitor long-term changes in Antarctic production cycles.”

Tools and techniques

The team will use a variety of tools on board the vessel to collect the samples needed for experiments and analyses. The transect includes five stations, or sampling sites, where the vessel holds position for the scientists to deploy their instruments.

One of the more impressively named tools is something called a megacore, a coring device capable of extracting 12 separate, shallow cores at one time. A frame holds a rack of trigger-controlled coring tubes mounted on a weighted slide. When the base of the frame hits bottom, the weighted slide drives the core tubes down into the sediment.

“Some samples are so pristine we find organisms grazing on top of the sediment or boring tunnels through the middle sections,” Arezzo wrote.

DeMaster said the cores offer the scientists an “extremely good” look at the interface between the sediment and water. They can use the samples to learn how organic matter is cycled and degraded.

The deployment and recovery of the megacore takes a couple of hours, depending on the roughness of the seas. “When the ship is heaving 10 or 12 feet during a pretty good blow, the sediment-water interfaces aren’t as pristine as they are during winter conditions when sea ice dampens the wave field,” DeMaster said.

Other methods to collect samples include sediment traps, trawls and other coring tools, such as a box core that takes a 50- x 50- x 50-centimeter cube of mud to measure benthic biomass and diversity. This cruise will also recover two time-lapse cameras that have been operating on the seafloor at both ends of the transect since the winter cruise.

Some measurements occur onboard the ship, but most of the work involves preserving the samples for later study. Smith said the team will certainly study the photographs from the cameras as soon as possible. “We’ll be too excited to see what happens over time to let those sit around too long,” he said.

Another analysis that occurs at sea involves using a special dating technique to determine the nature of seafloor organic matter deposition, which helps the scientists determine feeding strategies. One marker is thorium-234, a naturally occurring radioisotope with a short 24-day half-life. The presence of thorium-234 atoms in a sample indicates it recently fell to the seafloor because of the rapid radioactive decay.

DeMaster will also use carbon-14 data to track the age of the seabed organic material. Some of the bottom-dwellers, he noted, are fussy and only choose from the surface floc layer of freshest food, while others take in bigger gulps of bulk organic material from the upper sediment column. The latter organisms have special digestive enzymes that preferentially digest only the fresh organic material.

“For being dumb animals, they are quite proficient at getting the energy-rich fresh plankton that’s settling on the seabed, either by extracting it from the upper millimeter or preferentially digesting it in their gut,” DeMaster said.

The scientists collect literally hundreds of organisms for their experiments, many of them new species, according to Smith. “One of the most important, abundant species in Antarctica was just described a month ago,” he said.

The new species, Aurospio foodbancsia, a polychaete worm, was just described in the journal Marine Ecology. “It’s remarkable that you can go out and collect specimens, and the most abundant organism in your samples is new to science and distributed over a large area of the Antarctic continental shelf,” Smith said.

Winners and losers

But how will Aurospio foodbancsia and the hundreds of other types of critters on the seafloor fare in the coming years? Smith said he sees the impending changes to the peninsula climate as a serious threat to many of today’s species. As conditions warm, he said, the less extreme environment will welcome invasive species, particularly from South America.

“I think most of the endemic Antarctic fauna will be losers,” he said. “It’s a system that’s highly structured by sea ice and by the very cold temperatures that result from sea ice formation and somewhat higher salinity.”

He said one aggressive invading predator, a stone crab, has been already documented at deeper depths off Antarctica, and it may eventually move up onto the shelf. The bottom-dwelling critters evolved without such a predator and they would likely be vulnerable to predation.

“I see a lot of doom and gloom for the Antarctic fauna, quite frankly,” Smith said.

DeMaster noted that some species that are able to adapt to changes in diet, such as switching to a common type of phytoplankton called diatoms, will be favored. “There will be some success and some failures in terms of competition for the nutrition that makes it to the seabed. Some may adapt. We don’t know if some of these animals may successfully transfer to freshly fallen diatom organic matter versus ice algae as a source of nutrition.”

But the scientists also worry about the fate of a terrestrial species — humans. DeMaster said that the same climate changes along the peninsula, West Antarctica and Greenland are speeding the discharge of ice into the oceans. Some estimates say sea level could be up by a meter at century’s end.

“A sea level rise [like] that is considerable when you consider 50 percent of the world’s population lives within 100 kilometers of the coast,” DeMaster said.

NSF-funded research in this story: Craig Smith, University of Hawaii, Award No. 0636806; and David DeMaster and Carrie Thomas, North Carolina State University, Award No. 0636773.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs