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Plankton poo mops up carbon and dumps it

plankton poo

Floater or sinker? (Image: Australian Antarctic Division/Karin Beaumont/Commonwealth of Australia, 2004)

Whether zooplankton poo sinks or floats has big implications for the storage of carbon in the ocean, according to an Australian researcher.

If it sinks, the carbon stays in the ocean. If it floats, carbon dioxide is released into the atmosphere.

The research by Dr Karin Beaumont from the University of Tasmania and Australian Antarctic Division won a Fresh Science 2004 award, announced today.

The awards are presented to young scientists who can summarise their work in one minute, or as a limerick or haiku.

Plankton are a potential sink for atmospheric carbon dioxide, whose concentration is increasing, most scientists say, through human activities such as burning fossil fuels, and clearing land for agriculture.

Scientist know that plant-like phytoplankton can absorb up to 40% of atmospheric carbon dioxide.

But Beaumont's research showed that zooplankton, which feed off the phytoplankton, can send carbon dioxide back into the atmosphere if their poo is light or crumbly.

Some zooplankton such as krill, whales' favourite food, or jelly-like salp, have poo, or faecal pellets, that sink and keep carbon in the ocean.

Other zooplankton, such as the single-tailed dinoflagellates or the 12-legged copepods, have poo that float, releasing carbon back into the atmosphere.

In fact most of the microzoa plankton, zooplankton less than 0.8 millimetres in size, have floaters, Beaumont said.

"Some of the smaller animals tend to eat poo as well and will even eat the larger poo, which is sinking, and turn it into floating poo."

Microzoa represent most of the biomass in the ocean and process the most carbon, Beaumont said.

Whether the poo is a floater or a sinker depends very much on its size, as well it its density, shape and its contents, said Beaumont.

"Some of it is too light [to sink], and some of it breaks up more easily. Other poo contains whole shells and crunched up materials."

Beaumont said the poo also has an internal structure that can affect how it breaks up, such as an outer membrane, or connective membranous fibres holding the cells together.

"Some parts of it you can't identify, it just looks like crud," she said.

Around 40% of carbon dioxide from the atmosphere is transformed not just into sinking microscopic poo, but also into carbon locked into the shells of larger phytoplankton, and detritus, marine snow made of agglomerates of minerals. All this sinks into the deep ocean, Beaumont said.

To find out which zooplankton have which type of poo, Beaumont with Assistant Professor Juanita Urban-Rich from the University of Massachusetts in Boston, is compiling an internet guide to zooplankton poo.

The guide will contain images of poo from these microscopic creatures to help identify which creatures have which sort of poo.

Beaumont said marine scientists from all over the world could contribute to the guide.

If scientists know what poos the zooplankton do, they can work out how changes in temperature, salinity and ultraviolet (UV) light affect the population of the different species responsible.

For example, the ozone hole can increase UV light, favouring those organisms that thrive in high UV.

Beaumont said the next stage of her research was to look into sediment traps, funnels moored at depths of 1000 to 2000 metres.

"Lots of poo reaches the ocean floor but no-one has been able to identify it," she said.

Tags: environment, marine-biology, zoology