Increasing Arctic Ocean temperature affects plankton growth

An international study involving Australian researchers has found that while increasing carbon dioxide has a positive effect on primary production, or phytoplankton growth, in the Artic, this benefit is lost once temperatures increase.

The paper, published in the journal Nature Climate Change on Tuesday, is the first to investigate the direct link of temperature variations on the ocean's productivity to help improve predictions about the impact of climate change on the world's oceans.

Phytoplankton, the fundamental building blocks of the world's oceans as food for larger marine organisms, also account for half of the photosynthesis on earth, capturing carbon dioxide in the atmosphere - a major greenhouse gas - to grow and produce oxygen.

It is widely held that phytoplankton will play a key role in stemming the effects of human-induced climate change.

University of Western Australia researcher Lara Grarcia-Corral told Xinhua on Tuesday scientists found that the ocean's organic matter can regulate carbon dioxide, but its effects are only noticeable while temperatures are low.

"In spring, when temperatures are still low and enough nutrients are available in the water column, the chances of phytoplankton to mitigate the consequences of climate change increases, as if they were a big forest," Garcia-Corral said.

Garcia-Corral said the retreating Artic ice shelf - which has shrunk by half in the last two decades - releasing carbon dioxide into the oceans could benefit the phytoplankton.

"This influx of CO2 could benefit primary producers, but we cannot forget that the temperature of the Arctic Ocean is also increasing almost three times faster than global temperatures," Grarcia-Corral said.

"With the advent of the summer, when the temperature increases, the fertilizing effect of carbon dioxide diminishes to nothing and the ability of phytoplankton to capture carbon dioxide decreases."

Garcia-Corrall said this change is fundamental to elaborate on the projections of future impacts of climate change on the world's oceans.

Studies dating back to 2006 have shown that the ocean's productivity is indirectly influenced by the rising sea-surface temperatures, however this is the first to directly measure the impact of temperature on phytoplankton productivity.

Temperature indirectly affects phytoplankton by restricting the movement of nutrients through the water column, from the deep to the upper layers of the ocean where they are found though events called "upwelling. "

Upwelling however relies on currents being able to break through different layers of water that have formed at different temperatures.

The layering, or stratification, arises because water at different temperatures have different densities, with warmer water being less dense.

However, as global oceans warm, the differential between the layers becomes too great, stopping the currents from breaking the layering and "mixing", therefore nutrient rich cold water from the deep cannot reach the warm water at the surface, starving the plankton of nutrients.

As a result, the plankton dies without capturing the excess carbon dioxide.

Australian scientists are now looking at ways to overcome nutrients being the limiting factor in phytoplankton growth to make the world's oceans a carbon reservoir and have phytoplankton act as a "sink" for carbon dioxide.
 Xinhua - china.org.cn
2/9/15
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