Summit County Citizens Voice

A krill swarm in the Southern Ocean. PHOTO COURTESY NOAA.

Tiny crustaceans recycle significant quantity of iron from the depths to near-surface waters, where it helps stimulate plant growth

By Summit Voice

New research by the British Antarctic Survey shows that krill, the shrimp-like creatures at the heart of the Antarctic food chain, play a key role in fertilizing the Southern Ocean with iron by feeding on iron-rich fragments of decaying organisms on the sea floor, then releasing the iron near the surface.

That recycled iron helps stimulate the growth of phytoplankton thereby enhancing the ocean’s capacity for natural storage of carbon dioxide, the scientists reported this month in the journal Limnology and Oceanography.

Antarctic krill is the staple diet for fish, penguins, seals and whales, and is harvested by commercial fisheries for human consumption. The shrimp-like crustaceans grow to a length of up to 6cm and can live for 5-6 years. Krill feed on phytoplankton and are in turn eaten by a wide range of animals.

“We are really excited to make this discovery because the textbooks state krill live mainly in surface waters,” said lead author from British Antarctic Survey, Dr Katrin Schmidt. “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 percent of the population can be migrating up and down the water column at any one time.”

The relative rarity of iron is a limiting factor for plant growth in about 40 percent of the world’s oceans including the Southern Ocean. Iron is super-abundant on land, but it’s 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.

Until now, most scientists thought 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.

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.

“The next steps are to look at exactly how this iron is released into the water,” Schmidt said.

There are an estimated 100-500 million tonnes of krill in the Southern Ocean – similar to the weight of the world’s human population.

Filed under: biodiversity, Environment, Marine biology, water Tagged: | Antarctica, British Antarctic Survey, Environment, Iron, Iron in ocean, krill, Limnology, National Oceanic and Atmospheric Administration, Seabed, Southern Ocean, Summit County News, Surface water