How krill helps fertilise Southern Ocean with iron
Small and large marine creatures make their way to the Southern Ocean each year to feed on the abundant number of krill, the shrimp-like creature that plays a crucial role in the Antarctic food chain. Besides being an important source of food for penguins, fish, seals and whales, commercial fisheries also harvest krill for human consumption. New international research shows that krill could influence the fertilisation of the ocean with iron. This would promote the growth of phytoplankton, which are microscopic plant-like organisms. Presented in the journal Limnology and Oceanography, the findings reveal that this process boosts the ocean's capacity for natural storage of carbon dioxide.
Researchers led by the British Antarctic Survey in the United Kingdom reveal how surface-living Antarctic krill (Euphausia superba) feed on iron-rich fragments of decaying organisms found on the seabed, releasing the iron-rich contents of their stomachs back into the water when they swim back to the surface.
'We are really excited to make this discovery because the textbooks state krill live mainly in surface waters,' explains lead author Dr Katrin Schmidt of the British Antarctic Survey. '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.'
A thorough assessment was made of the stomach contents of more than 1 000 krill gathered from 10 Antarctic research expeditions. According to the team, the stomachs of krill caught near the surface were full of iron-rich material from the sea floor. The researchers used acoustic data, net samples and images of krill on the seabed in their evaluation as well. The result is that these animals frequently make their way down to the sea floor in order to feed.
Their discovery could impact the management of commercial krill fisheries, potentially providing fresh insight into the natural carbon cycle in the Southern Ocean. The researchers say that tiny additions of iron significantly boost plant growth and the drawdown of atmospheric carbon dioxide in the ocean.
Antarctic krill can grow up to a length of six centimetres, and can live for five or six years. Experts say the krill fishery in the Antarctic is growing, managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). To ensure that the fishery's expansion is conducted in a precautionary manner, CCAMLR needs to have research on the role of krill in the wider food web.
Current estimates say there are between 100 and 500 million tonnes of krill in the Southern Ocean. The researchers say this figure is similar to the weight of the human population.
'The next steps are to look at exactly how this iron is released into the water,' says Dr Schmidt.
Scientists from Southampton University in the United Kingdom, Oslo University in Norway and the Australian Antarctic Division contributed to this study.