Increasing ocean temperature lessen the Phytoplankton

As the Earth’s oceans warm, the masses of tiny plants (usually called Phytoplankton) growing at their surface is declining, say US researchers. Their results show that the productivity of global oceans is tightly linked to climate change and has steadily decreased between 1999 and 2004.

Phytoplanktons are responsible for about half the photosynthesis on Earth. They convert light and nutrients into organic material, which is then fed right the way up through the food chain, making them key elements of life on the planet. After dying, some of the plants drift down into the deep ocean and lock up carbon for very longer periods.

The team was led by Michael Behrenfeld, at Oregon State University, US, and used a sensor on NASA’s SeaWiFS satelite to measure different shades of green in the ocean. This allowed them to watch how Chlorophyll in the oceans ebbed and flowed over the past 10 years. They looked at how these changes fitted changes in ocean temperatures and the predictions of computer models.

Their research had two phases. Between 1997 and 1998, the amount of Phytoplankton in the seas rose. At this time, the oceans were cooling after the strongest ever El Niño, which had warmed ocean temperatures. From 1999 to 2004, there was a general warming of the oceans and, the images from space revealed, a persistent decrease in Phytoplankton. In some regions, the drops in ocean productivity were often over 30%. Globally, the reductions meant that, between 1999 and 2004, about 190 million tonnes of carbon per year were not absorbed by the tiny plants and converted into organic matter. After 2004, there was a small upturn in productivity

The tiny phytoplanktons live in the top layer of the ocean, but rely on nutrients that are brought up from deeper in the ocean. This mixing relies on convective currents being able to break through the “stratification” of water. This layering arises because layers of water at different temperatures have different densities and therefore do not mix.

If the top layer of ocean water warms, as it has done, the density contrast between it and the layer below becomes greater. This makes it harder for the upwelling nutrients to reach the surface, starving the Phytoplankton. And the researchers found that drops in the amount of Chlorophyll being detected by the satelite closely corresponded to increases in surface water temperature, confirming the predictions of climate models.

Models predict that the loss of Plankton with warming in the mid-latitudes will be reversed near the poles, where researchers expect to see a rise in productivity. This rise is not predicted to counter the loss of Plankton elsewhere. The reason for the reversal is that, near the poles, the limiting factor to Phytoplankton growth is light, not nutrients. Stratification pushes the Plankton up closer to the surface, bringing them closer to light and boosting growth.

Source:  Journal Ref.:Nature (vol 444, p 752)

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