21/04/2018 Oases of life close to enormous methane plumes on the otherwise barren Arctic Ocean seafloor

Expedition to Arctic Ocean reveals images of cold seeps Collection of a carbonate crust with abundant organisms attached at a methane seep. Photo: Screenshot

New PhD dissertation from CAGE shows that methane derived carbon is a crucial part of the food web of the Arctic Ocean. It sheds new light on unique animal assemblages discovered thriving in the darkness close to cold methane plumes  

Text: Maja Sojtaric

PhD student Emmelie Åström investigated the cold methane seeps offshore Svalbard and in the Barents Sea. These methane seeps are found within depth range of 85 to 1200 metres. Her aim was to  describe the animal assemblages and surrounding habitats related to the seeps.

Until now, the biological communities associated with methane seep localities in the high- Arctic, known as cold seeps, have remained unexplored. “We have, for the first time, examined the habitats and mapped the life forms at some of these cold seeps”, says Åström.

Seafloor at 500 m water depth. Carbonate blocks are colonized by anemones and sponges. Tube worms are abundant at the seafloor. In the upper part of the image, we see patches of bacterial mats. All of these features are indigenous to sites of methane release. Photo: CAGE

 

“We studied the ecological structure, diversity and food web dynamics at high-Arctic cold seeps and documented how they compared with conventional, non-seep communities. A part of the study was to examine the interaction between seep and non-seep communities where they co-occur.”

Åström successfully defended her PhD dissertation on the topic Friday, April 20 2018.

Successful PhD defence by CAGE student Emmelie Åström.

She had previously published several results on the findings.

Biological oasis with plenty of food

One study published in Limnology and Oceanography  (Open access) shows that at a thousand meters depth on Vestnesa Ridge, cold seeps are an important local source of energy to sustain life. In the perpetual darkness at great depths in the ocean, there is little available food.

The absence of sunlight precludes photosynthesis as local food in the deep ocean, and organic material falling from the surface waters is usually highly degraded by the time it reaches the deep seafloor.

“But at methane cold seeps, energy to sustain life can be produced at the seafloor, and independent of the sun.” says Åström.

This process, known as chemosynthesis, involves microbes living at methane-rich cold seeps converting methane and hydrogen sulphide into energy usable by animals. The animal community flourishes at these Arctic cold seeps. Many different animals are observed and they are far more abundant compared to sites where no methane is released.

“Among the most common animals living in the sediment, we find chemosymbiotic worms, small crustaceans and clams.”

Plaice in a mat of chemosymbiotic worms that thrive close to methane seeps. Scale bar 20 cm. Photo: CAGE

In fact two previously unknown species if clams were discovered by Åström and collegues at one of these seeps.

Cold seeps have a strong, localized effect over small areas, supporting dense communities of small chemosymbiotic worms that play an important role in structuring the seabed animal community.

 

“We found that carbon from these cold seeps is suporting the ‘normal’ food web of the Barents Sea ecosystems.”.

 

Shelter from predators

One important feature of the sea floor close to the methane seeps are carbonate rocks generated by methane seeping from the seafloor. These carbonate rocks create complex 3D structures on the otherwise barren seabed where animals hide from predators and wait for prey. It is in many ways an environment similar to coral reefs.

Expedition to Arctic Ocean reveals images of cold seeps

“The rugged and quite large features of the carbonate rocks are part of the reason why we are seeing an oasis effect in some of these areas. These seabed structures provide spatial complexity and heterogeneity to the deep sea, which would otherwise appear as a sandy desert,” says Åström. These formations on the seabed allow animals to congregate around them or to attach themselves to the hard substrate.

 “We were surprised to find that cold seeps in the high-Arctic support such varied animal communities and that we actually can trace the chemosyntethic produced carbon further into the food web and to conventional non-seep animals ” concludes Åström.

 

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