Waking plankton from hibernation

February 15, 2012 - 04:55

They are the motor of the ecosystem in the oceans of the high North. But we don’t know much about where plankton are during the sunless winters or how they waken in spring.

Else Nøst Hegseth at the University of Tromsø grows algae that have been in winter hibernation on the seabed. These bottles contain samples from the Svalbard Archipelago. (Photo: Asle Rønning)

Spring blooms of plant plankton are the basis of the ecosystem in the Barents Sea and other ocean areas.

Plant plankton - or phytoplankton - comprises the food of animal plankton and bottom animals, which in turn are eaten by fish, birds and sea mammals.

Associate Professor Else Nøst Hegseth at the Department of Arctic and Marine Biology at the University of Tromsø is conducting research on the way phytoplankton can run on a bare minimum during the months-long polar night, and then bloom again in colossal numbers.

“Diagrams of the Barents Sea ecosystem usually start with a box labelled phytoplankton. I’m interested in what comes before that box,” says Nøst Hegseth.

Tiny organisms with glass shells

Nøst Hegseth researches diatoms, which are one of the most ecologically significant types of plant plankton and one of the most common life forms in the oceans.

Diatoms are unicellular organisms that render a shell of glass or silicate.

We are talking about very tiny organisms – five of them to a millimetre – which together comprise millions of tonnes of biomass during the spring blooms in the Barents Sea in April and May.

Lots of plankton species need light to survive. South of the Polar Circle these algae, called autotrophic algae because they synthesize their own food, get some sunlight even in the winter.

No light to live on during winter
Collecting samples from Svalbard last year. The research vessel F/F Jan Mayen has since been renamed F/F Helmer Hanssen. (Photo: Else Nøst Hegseth)

During the dark winter months of the high North, however, they have no light to live on. Apparently they survive by going into a kind of hibernation on the bottom of the ocean.

When blooming these algae can propagate both by cell division and sexual reproduction.

In addition to sunlight they require nitrates, phosphates and silicates to survive. They absorb these compounds from seawater.

But when the algae blooming ends they are transformed into so-called algal spores.  They shrink and get a thicker silicate shell. They become inactive and sink down from the surface algae.

Sinking glass balls

“They could be compared to small glass balls, which are so dense that they sink to the bottom,” says Nøst Hegseth.

Down there they can lie in waiting until the next year. The researcher compares algal spores to seeds in flowering plants, which can lie dormant for months or years before sprouting.

These miniscule glass balls cannot move about on their own power. Something else has to raise them to the surface when it’s time to wake up in the spring, enabling them to start a new bloom.

“The only process that could get them to the surface is circulation of the layers of seawater. That can be triggered by the wind, tides or currents,” says Nøst Hegseth.

Blooming in the lab
Two diatoms which have linked together, viewed through a microscope. These belong to the species Thalassiosira bioculata. (Photo: Else Nøst Hegseth)

Samples from the seabed can reveal more about the presence of various species of algal spores. But it is near-on impossible to find these algal spores in samples taken from the seabed, even through a microscope.

So Nøst Hegseth takes mud samples brought up from the bottom of the ocean and cultivates this ooze under conditions that mimic the real arctic spring. Once the algae start growing they’re easy to find.

Right now the algae research lab is the home of seabed samples from Rijpfjord, a fjord on the northern coast of Nordaustlandet. Nordaustlandet is the second largest of the islands comprising Norway’s Svalbard archipelago.

Researchers hope to find algal spores in these samples which will bloom under proper conditions.

The samples have been in the lab for a week. In another week or two an artificially early spring bloom should commence.

“We are anxious to see how much we get and which species we’ll find,” says Nøst Hegseth.

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Read this article in Norwegian at forskning.no

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Translated by
Glenn Ostling

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