Here's some interesting reading :
Krill the Messenger
A headline in the November 4, 2004 edition of The Los Angeles Times screams “Antarctic Food Chain in Peril, Study Finds.” That’s because Usha Lee McFarling’s story sounds an alarming note. “Krill — the heart of the rich Antarctic food chain nourishes whales, seals and penguins — have declined by more than 80% in the last 25 years in key ocean regions, according to a new study that links the loss to warming temperatures.” McFarling explains how higher temperatures result in “diminished ice cover in some parts of the waters surrounding Antarctica.” This is important, she writes, because “krill larvae require sea ice to survive the winter [and] without sea ice, the larvae starve.”
McFarling bases her article on a study published in the November 4th edition of Nature magazine. In his study, lead researcher Angus Atkinson of the British Antarctic Survey relates the observed decline in krill to sea ice conditions and uses a couple of figures to illustrate their relationship. The first (Figure 1) depicts the history of the krill population (as sampled from capture nets) in the southwest sector of the Atlantic Ocean, a region Atkinson defines as lying just off the tip of the Antarctic Peninsula, that thin arm of Antarctica that extends toward the southern tip of South America. It is one of the most productive regions for krill in the southern Atlantic. Krill numbers certainly appear to have declined since the mid-to-late 1970s.
Figure 1. Temporal history of krill density in the southwest Atlantic sector of the Southern Ocean, 1976-2003 (source: Atkinson et al., 2204).
In order to ascertain whether sea ice conditions are associated with the observed decline in krill, Atkinson goes on to relate krill density to the duration of ‘fast-ice’ (that is, ice connected to the shoreline) as measured the previous winter at the South Orkney Islands, a small island chain near the middle of the study region. He postulates winter ice to be prime larvae over-wintering grounds. In other words, the more fast-ice ice there is, the greater number of krill larvae will survive the winter, leading to a larger krill population the following summer. He illustrates the relationship using Figure 2.
Figure 2. Relationship between winter ice duration at the South Orkney Islands and krill density (number per square meter). The longer there is ice, the more krill there are (source: Atkinson et al., 2004).
There does appear to be strong correlation between the previous winter’s sea ice duration and krill density. Taken together, these two figures leave the impression that the declining krill population is related to declining sea ice duration. Of course, it goes without saying that the decline in sea ice duration is related to global warming. As Atkinson describes it, the region around the Antarctic Peninsula is “one of the world’s fastest-warming areas” (without a tie-in to global warming, results like these likely would not rate a banner headline in a leading newspaper).
But it requires one final figure to conclusively demonstrate the relationship between krill decline and sea ice loss — one showing the temporal history of winter fast-ice duration at the South Orkneys. But wait a minute! Where is it? Isn’t this Atkinson’s key research finding? Try as we might, we don’t find anything that illustrates a recent decline in sea ice duration around the South Orkneys. That seems odd. We searched for the missing data on the Internet and after a couple of hours, we located it and publish it as Figure 3. You’ll quickly see why Atkinson didn’t incorporate the history of fast-ice duration in his article. There was no change in the duration in ‘fast sea ice’ at the South Orkneys between 1975 and 2000. That’s the time associated with the steep decline in krill density.
How can declines in sea ice related to global warming be responsible for decimation of the krill population when winter sea ice duration is not declining at the locus of the relationship?
Figure3. The history of winter ice duration observed at South Orkney Islands, 1975-2000 (source: Clarke and Harris, 2003).
Once we scented blood in the waters off Antarctica, like orca we drove in for the kill. The record of sea ice duration at the South Orkneys begins in 1903. Figure 4 represents the complete record and shows sea ice conditions to be variable from year to year. However, since about 1960, the multi-year average is pretty constant.
Figure 4. The history of winter ice duration observed at South Orkney Islands, 1903-2000 (source: Clarke and Harris, 2003).
Contrast whatÂ’s been happening since 1960 with what took place early in the 20th century. In the early decades, winter sea ice duration averaged about 200 days per year (and even longer around 1930). It rapidly declined to the late 1950s, averaging little more than 100 days per year. If krill are as sensitive to sea ice variations as Atkinson and his colleagues believe, we canÂ’t imagine how they survived the first half of the 20th century!
But, of course, we can imagine it. Krill adapt to changing conditions. They might do so by relocating elsewhere in the Southern Ocean. Most of the Southern Ocean actually is experiencing increases in sea ice extent and duration. Perhaps the krill alter the depth at which they live in the water column. Maybe some other mechanism is involved.
Steve Nicol, a krill expert from the Australian Antarctic Division, was incredulous when McFarling got hold of him for comment. “Could we really have lost 900 million tons of krill [from a biomass that was once estimated at 10 billion tons] without anyone noticing?,” he responded. “I don’t think so. You would expect to see most of the predators in decline, and this doesn’t appear to be happening.” Interesting observation, but insufficient to ‘stop the presses’ and keep The L.A. Times from becoming the latest victim of Nature’s peer-reviewers neglecting (or at least failing) to ask the right questions. Perhaps they and the Nature editorial staff simply choose to look the other way when there’s an opportunity to publicize another “fear global warming” message. This has been a continuing and disturbing pattern at Nature.
References:
Atkinson, A., Siegel, V., Pakhomov, E., Rothery, P., 2004. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature, 432, 100-103.
Clarke, A., Harris, C.M., 2003. Polar marine ecosystems: major threats and future change. Environmental Conservation, 30, 1-25.
Cavalieri, D.J., Parkinson, C.L., Vinnikov, K.Y., 2003. 30-year satellite record reveals contrasting Arctic and Antarctic decadal sea ice variability. Geophysical Research Letters, 30, doi:10.1029/2003GL018031.
http://www.greeningearthsociety.org/wca/2004/wca_27a.html
