Phytoplankton greens the Arctic oceans

Arctic sea-ice has thinned so much over the past 30 years that even sun-dependent phytoplankton can now bloom beneath it, according to new research.

A paper published in the journal Science suggests thinning sea-ice accounts for a surprising Arctic bloom of phytoplankton – a group of organisms that exist in the well-lit surface water of the world’s oceans, lakes and rivers.

Phytoplankton is responsible for half of all photosynthesis that occurs on earth, effectively converting carbon dioxide into organic compounds that help sustain oceanic life. It is known for colourful blooms, prompted by periods of rapid growth once or twice a year, generally observed only in ice-free water.

However, in 2011, a massive bloom was observed underneath a sea-ice-covered region of the Chukchi Sea, on the edge of the Arctic Ocean off the northwest coast of Alaska, thought to be inhospitable to phytoplankton.

In the wake of this surprising event, a research team led by Christopher Horvat at Harvard University in Massachusetts, US, set about investigating the evolution of phytoplankton events in the Arctic over the past 30 years.

The researchers suggest thinning sea-ice is the reason behind recent blooms.

“The thinner summertime Arctic sea-ice is increasingly covered in melt ponds, which permit more light penetration than bare or snow-covered ice,” they explain.

Phytoplankton is difficult to observe in the Arctic because the chlorophyll responsible for its vibrant colouring can’t be seen from the surface due to the intervening sea-ice. To combat this, the team modelled melt pond formation, adding in data on the light-transmitting properties of different melt ponds, to ascertain the likelihood of bloom events in recent decades.

The results show that over the past 10 years light conditions may have permitted annual phytoplankton blooms in as much as 30% of the Arctic.

“Our model results indicate that the recent thinning of Arctic sea ice is the main cause of a marked increase in the prevalence of light conditions conducive to sub-ice blooms,” the paper reads. “The modern Arctic is undergoing a major ecological shift because of climate change.”

Although phytoplankton helps absorb carbon dioxide, any changes to its distribution could impact the ecology of the entire ocean. The consequences of this ecological shift on carbon levels is unknown, and the researchers underscore the need for further research into this issue.

The team says this study will make way for more detailed analysis of blooms in specific areas of the Arctic, particularly as sea-ice is projected to thin even more in coming years.