B
A A: A pteropod. B: A copepod.
Image: A www.pmel.noaa.gov B Wikimedia Commons
Left: The left side of this diagram shows the biological pump in the Southern Ocean and the right side shows the simpler physical pump. Image: ©S.Chisholm, Nature 407, October 2000
Krill Krill are tiny crustaceans (order Eupausiacea). They are found in all the world’s oceans and a very important part of the ocean food chain. They are near the bottom of the food chain because they feed on plankton and smaller zooplankton species. Krill are then eaten by larger predators. There is one species of krill in the Southern Ocean – the Antarctic krill (Euphasia superba) – that has an estimated biomass of over 500 000 tonnes. Of this, over half is eaten by whales, seals, penguins, squid and fish each year. Krill migrates vertically through the ocean layers – near the surface during the night and in deeper waters during the day.
show that the Southern Ocean (south of 40°) might account for more than 40% of the total uptake of all oceanic waters. Many uncertainties remain in these calculations, however. This means that there is a real and urgent need to improve these models. Some aspects of the biological pump are still hotly debated by scientists. For instance, what motivates biologist oceanographers is that despite the high level of macronutrients in the Southern Ocean, which feed photosynthetic processes, primary production remains low. These areas are thus called ‘high nutrient – low chlorophyll’ (HNLC) areas. One of the main hypotheses is that other micronutrients, like iron, limit photosynthesis and thus primary production. The consequence ▲ ▲
The biological pump is more complex. The biological pump refers to interactions in the upper water column where light penetrates. The pump starts with the fixation of dissolved CO2 by photosynthesis in phytoplankton. As we have already learnt phytoplankton is then eaten by heterotrophic zooplankton, which is the main food source of various predators. Part of this pool of carbon is released into the atmosphere when organisms respire, while another part sinks vertically into the deep ocean. This vertical transport is made up of various types of detritus, faecal pellets, carcasses of organisms and algae skeletons. This is called ‘marine snow’. If the export of carbon into the deep ocean is more than the emission of carbon, the area can be considered as a carbon ‘sink’. Some calculations
The Antarctic krill (Euphasia superba).
A krill swarm.
Image: Wikimedia Commons
Image: Wikimedia Commons
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