Reporting this month in the journal Limnology and Oceanography, an international team of researchers describe how Antarctic krill (Euphausia superba), once thought to live mostly in surface waters, regularly feed on iron-rich fragments of decaying organisms on the sea floor. They swim back to the surface with stomachs full of iron, releasing it into the water.
Antarctic krill is the staple diet for fish, penguins, seals and whales; and is harvested by commercial fisheries for human consumption.
Lead author from British Antarctic Survey, Dr Katrin Schmidt says, "We are really excited to make this discovery because the textbooks state krill live mainly in surface waters. We knew they make occasional visits to the sea floor but these were always thought as exceptional. What surprises us is how common these visits are -- up to 20% of the population can be migrating up and down the water column at any one time."
The scientists painstakingly examined the stomach contents of over 1000 krill collected from 10 Antarctic research expeditions. They found that the krill, caught near the surface, had stomachs full of iron-rich material from the seabed. The team also studied photographs of krill on the sea floor, acoustic data and net samples. All these provided strong evidence that these animals frequently feed on the sea floor.
This finding has implications for managing commercial krill fisheries and will lead to a better understanding of the natural carbon cycle in the Southern Ocean.
Schmidt continues, "The next steps are to look at exactly how this iron is released into the water."
This research was carried out by British Antarctic Survey, Southampton University, Australian Antarctic Division and Oslo University.
About Antarctic krill
Antarctic krill (Euphausia superba) is one of the most important animals in the Southern Ocean. These shrimp like crustaceans grows up to a length of 6cm and can live for 5-6 years. Krill feed on phytoplankton and are in turn eaten by a wide range of animals including fish, penguins, seals and whales. They are also a potentially valuable source of protein for humans and can be fished easily with large nets.
Antarctic krill are a key species in the Southern Ocean food web, channelling energy from phytoplankton (microscopic plants) efficiently to the large populations of penguins, seals and whales that make Antarctica unique. There is an expanding krill fishery in Antarctica, managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). CCAMLR requires research on the role of krill in the wider food web, in order to manage this expanding fishery in a precautionary manner.
There are an estimated 100-500 million tonnes of krill in the Southern Ocean -- similar to the weight of the world's human population.
Primary production (pelagic plant growth) is limited by the availability of iron in about 40% of the Worlds' Oceans including the Southern Ocean. Whilst iron is super-abundant on land, is only present in minute concentrations in the upper productive layers of the ocean. In large parts of the Southern Ocean small additions of iron dramatically increase plant growth and the drawdown of atmospheric CO2 into the ocean.
The supply processes of iron into the ocean are therefore important in the Antarctic carbon cycle, where CO2 uptake is stimulated by plant growth, and the carbon is subsequently stored in the ocean interior due to the settling of dead plant cells. The supply of iron is mainly thought to be due to upwelling from deep sediments, run-off from land and glaciers, melting of icebergs and the settling of wind-blown dust. All of these processes are purely physical, so this work suggests a novel route via biological processes: with krill returning significant quantities of iron in their stomachs after visiting the seafloor to feed.