Enormous iceberg threatens food chain

Icebergs do get large, but few of us might realize just how large.

Think big.

Bigger.

No, no, really big.

Think twice the size of Rhode Island.

Stanford University researchers recently announced the results of a study involving a gigantic iceberg that broke loose from Antarctica’s Ross Ice Shelf in May 2002. The tremendous block ripped free – or calved – from the face of the shelf and drifted northward until it ran aground in November near Pennell Bank, Antarctica. Lodged on the shore, the iceberg blocked the exodus of other ice chunks from the area and completely altered the base of the polar food chain.

The iceberg, named C-19 by scientists, is one of two that have broken loose from the Ross Ice Shelf over the past 26 months. When C-19 ran aground, its tremendous size left most of it extending out into the sea, forming a barrier perpendicular to the shoreline that prevented ice from flowing out of the Ross Sea and into the open ocean.

By blocking the outflow of this sea ice, the iceberg greatly reduced the amount of sunlight streaming into the Ross ocean waters. Without light coming down from above the waters, light-dependent plankton find it difficult to survive. This creates the first break in an intricate food chain.

Phytoplankton, primary producers in the Antarctic food chain, are an important food source for krill.

Larger animals, such as penguins, other seabirds and even whales dine frequently on krill. Therefore, any sharp reduction in phytoplankton populations – like the one scientists observed between November 2002 and April 2003 – can negatively affect populations of larger animals by reducing their food supply.

MELTING AWAY NATURALLY

Open water areas along the Antarctic coast, known as polynyas, are biological oases where water is shallow and easily warmed by available sunlight. Adelie penguins obtain large amounts of their food from these Antarctic hot spots, where phytoplankton blooms result in large quantities of krill.

Living near polynyas is an ideal situation for penguin populations, because they do not need to travel long distances for food and can reduce their exposure to predators. However, when an iceberg like C-19 lodge near polynas, the ideal situation becomes much less ideal.

‘Although large iceberg calvings are common in the Antarctic, the huge reduction in primary productivity isn’t,’ said Gert van Dijken of Stanford University. ‘Since the Ross ice shelf is basically a glacier floating on the water, there will always be these types of calvings.’

The important question, van Dijken said, is whether future iceberg calvings will negatively affect other organisms, as C-19 did.

Using data from NASA’s SeaStar satellite, scientists examined the presence of phytoplankton in the Ross Sea from November to April. They discovered that C-19 reduced phytoplankton growth in the area by more than 90 percent. While the decline of phytoplankton populations and the subsequent effect on populations of larger animals dependent on their presence have yet to be quantified, a phytoplankton population decline of this magnitude has never before been observed.

In eastern Antarctica, more than 90 percent of all Adelie penguin colonies live near coastal polynyas. Nearly a third of the world’s total Adelie penguin population is found in the Ross Sea. The Ross Sea, along with just a few other ocean locations, is responsible for more than 75 percent of all plankton production.

Sharp declines in phytoplankton populations could directly translate into danger for penguins and other animals.

‘Other icebergs in the last 25 years have not shown the same effect,’ van Dijken said.

SLIPPERY PREDICTIONS

While these facts are cause for concern, scientists are not really worried yet.

Some even remain skeptical about the potential cause-and-effect relationship link between global warming and iceberg calvings.

‘There is no evidence that this part of the Antarctic is warming,’ said Kevin Arrigo of Stanford. ‘It is more likely part of a natural cycle of ice advance and iceberg calving.’

Although, if for some reason global warming does begin to affect the Ross Ice Shelf and cause it to move faster, more icebergs might begin to calve and the situation would be much different, van Dijken said.

Scientists right now are focused on other areas of the Antarctic, in an attempt to develop a greater understanding of related ecosystems and create a larger picture of the polar environment.

In the case of future icebergs, van Dijken suggests letting nature run its course, rather taking any action.

‘I don’t think we should start blowing up future icebergs which might be in the way,’ he said.