The oceans are the lifeblood of our planet and plankton its red blood cells. Those vital “red blood cells” have declined more than 40 per cent since 1950 and the rate of decline is increasing due to climate change, scientists reported this week.

“Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries,” said Boris Worm of Canada’s Dalhousie University and one of the world’s leading experts on the global oceans. “An ocean with less phytoplankton will function differently,” said Worm, the co-author of a new study on plankton published this week in Nature.

Plankton are the equivalent of grass, trees and other plants that make land green, says study co-author Marlon Lewis, an oceanographer at Dalhousie. “It is frightening to realise we have lost nearly half of the ocean’s green plants,” Lewis told IPS. “It looks like the rate of decline is increasing,” he said.

Climate change is warming the oceans about 0.2°C per decade on average. This warmer water tends to stay on top because it is lighter and essentially sits on top of a layer of colder water. This layering, or stratification, is a problem for light-loving plankton because they can only live in the top 100 to 200 metres. Eventually they run out of nutrients to feed on unless the cold, deeper waters mix with those near the surface. Ocean stratification has been widely observed in the past decade and is occurring in more and larger areas of the world’s oceans.

Plankton crucial for Earth

Phytoplankton or plankton are very small algae that live near the surface of oceans and form the basis of the marine food web. The unheralded plankton tribe may be the hardest-working group of organisms on the planet. Not only do they feed nearly everything living in the oceans, they absorb and sequester CO2 from the atmosphere, they also play a key role in cloud formation. Plankton give off dimethyl sulfide, a chemical which floats to the ocean’s surface, evaporates, breaking down into sulfur compounds that become the nuclei around which clouds form. Without plankton, the Earth would be a very different planet.

The researchers spent three years analysing and synthesising an unprecedented collection of historical and recent oceanographic data involving nearly half a million measurements of the transparency of sea water over the past 120 years. Previously, the “big picture” regarding plankton globally only went as far back as 1997 with the launch of special satellites. Worm, Lewis and colleague Daniel Boyce found that most phytoplankton declines occurred in polar and tropical regions, and in the open oceans where most phytoplankton production occurs. There was a direct correlation between rising sea surface temperatures and the decline in phytoplankton growth over most of the globe, especially close to the equator, they determined.

In addition to the declines in plankton, declines in the numbers of species in tropical waters and increases in the number of species in temperate waters have been observed, Worm said. As on land, some marine species are exquisitely sensitive to temperature and will move if a region becomes too warm.

Dead zones increasing

Another related mega-change in the oceans is the dramatic increase in number and size of dead zones-areas too low in oxygen to support life. Fertiliser and sewage run-off cause huge growth of plankton, which then quickly die and are consumed by bacteria that deplete waters of oxygen. The Gulf of Mexico has a 22,000 square kilometre dead zone every spring due to run-off from the Mississippi River. Ocean stratification, where warm water sits firmly on top of cold, nutrient-rich water, also creates dead zones and lowers the overall productivity of the oceans, says Worm. Such dead zones were rare 40 years ago but now number several hundred. Without urgent action, climate change will continue to warm oceans, increasing stratification and producing larger and more dead zones with a major impact on future fisheries, a 2009 study in Nature Geoscience warned.

It will take a thousand years for the oceans to cool down, so it is imperative to pull the emergency brake on global warming emissions, the study concluded. Another pressing reason to pull the emergency brake is the increasing acidification of oceans due to human emissions of CO2. Each day, the oceans absorb 30 million tonnes of CO2, gradually and inevitably increasing their acidity, which in turn reduces the amounts of calcium carbonate available to some plankton and other species that need it to form their shells and skeletons.

STEPHEN LEAHY - IPS