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The raised stone at the apex of the arch pictured above is the keystone, and in historical architecture is considered so important that it is often decorated with a lion’s head, family seal or other ornament. Without this piece the whole arch will crumble.

Likewise, a keystone species is one that plays an overly important role in a biological ecosystem. Remove this one animal and every other member of the community disappears as well. In Antarctic waters the role of the keystone species is played by a tiny shrimp-like creature called a krill.

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At the risk of my analogy crumbling like a keystone-less arch, I should point out that removing any of the voussoirs, those stones on either side of the apex, will cause the structure to collapse as well. But we’ll ignore this architectural technicality so we can move on to talk about the crustaceans.

Krill are related to crabs, shrimp and lobsters. But instead of crawling around on the bottom, krill are pelagic. One common misconception is that they are plankton, and because many types of whales feed on krill, this leads to the also common misconception that whales feed on plankton. Plankton are drifters, at the mercy of the tides and currents, while krill are nektons, like fish, and are able to control both their vertical as well as lateral movements in the water. (For more on how whales feed, see my diary on Baleen.)

Krill are herbivores, feeding on algae and phytoplankton that are suspended in the water column. In turn they are preyed on by nearly every Antarctic predator that exists. And those few predators that don’t feed directly on them feed on the ones that do. For some species, like penguins, krill provide nearly 100% of their calories. For others, like blue whales, they seasonally provide such a huge amount of nutrition that the whales could not survive the rest of the year without them. In other words, as mentioned above, krill is the Antarctic’s keystone species. If the krill were to disappear they would take every other species in the ecosystem down with them.

Worldwide there are about 80 species of crustaceans known as "krill". The largest is the 2-inch long Antarctic krill (Euphausia superba) and they exist in mind-numbing concentrations in the cold southern seas. At any given time there are four or five billion individuals, and when they congregate for spawning the pink swarms can be seen from space.  Krill swarms move in a diurnal migration, spending the day in deep water and rising to the surface each night to feed. Below is a great shot of a pair of blue whales gorging on a large swarm of krill (image by the Australian Antarctic Division).

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The reason Antarctic krill can exist in such dense concentrations is because of upwelling that occurs along the Antarctic Convergence. This is a line which encircles the Antarctic continent and marks the place where cold polar currents meet with warmer northern waters. This results in a massive upwelling zone along the convergence as the cold water sinks below the warm water. As the warm water rises to the surface it pulls up nutrients, especially nitrate and phosphate, which encourage the growth of microalgae. During the summer months this algae-growing effect is magnified by nearly 24 hours per day of sunlight, allowing the algae to photosynthesize around the clock. This phytoplankton-enriched water in turn provides food for massive numbers of krill. The Antarctic Convergence is illustrated below (via Wikipedia).

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Population studies have shown an alarming decrease in the number of krill in the Antarctic recently, a decline of nearly 80% in just the past thirty years. Like many of today’s disturbing biological trends, this decline is attributed to climate change. One important source of the krill’s food is "ice algae", which grow on the very edges of the rapidly shrinking glaciers surrounding the Antarctic continent. As the ice melts, retreating back towards land, the surface area available for ice algae to grow shrinks as well. In general, phytoplankton suspended in the water is found in greater concentrations than algae growing on ice. However, planktonic algae go through seasonal cycles, abundant during summer "blooms" and nearly vanishing during seasons of darkness. During these non-bloom times the ice algae becomes a vital food source for the area’s herbivores. Below is a photo of krill scraping this ice algae from the underside of a glacier.

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Another threat to the krill populations, and thus to the entire ecosystem as a whole, is commercial harvesting. Fortunately this has so far been limited to Japanese fleets and to a smaller extent those of Russia and Norway. This is mainly because we have yet to find a market on a large scale. Being rich in Omega-3 fatty acids, krill is netted to some degree and sold in pill form as dietary supplements. Once this changes, as new markets for the flesh are found, wholesale harvesting will be devastating to whales, penguins and other predators who will have to compete with fishing boats for what is for many the only source of protein available in the polar seas.

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Bonus: Richard Seaman has a good photo essay on New Zealand’s krill at his blog The Flying Kiwi.

Other diaries in this series can be found here.

Originally posted to Mark H on Thu Mar 12, 2009 at 05:06 PM PDT.

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