Dear EarthTalk

Why is the plankton in the oceans dying? And what does this mean for the health of the oceans and marine life?

Marilynn Block

Portland, OR

As the lowest link on the marine food chain, plankton — that tiny aquatic plant, animal and bacterial matter floating throughout the world’s oceans — is a vital building block for life on Earth. Besides serving as a primary food source for many fish and whales, plankton plays a crucial role in mitigating global warming.

Indeed, the ocean is the world’s largest “carbon sink”: As much as one-third of man-made CO2 emissions are stored in the oceans and therefore do not contribute to global warming. This is because its plant component, phytoplankton (its animal component is called zooplankton), pulls massive amounts of carbon dioxide (CO2) out of the atmosphere as it photosynthesizes.

But various environmental factors are taking their toll on plankton the world over. The U.S. National Oceanic and Atmospheric Administration (NOAA) reported recently that marine phytoplankton is declining across the oceans. Even Canadian cod fishermen are noticing that the plankton-feeding fish they catch are often nearly starving as a result of lack of this crucial food source.

A 2007 study published in the scientific journal Nature found that human-caused increase in CO2 pollution is altering the pH (acidity) levels in the oceans. This change in chemistry is expected to have adverse effects on the entire ecosystem. More acidic ocean water inhibits the ability of shell-forming marine organisms — from plankton to mollusks to corals — to form properly. Smaller and less healthy populations of plankton would be bad news for all the other creatures above it on the ocean’s food chain.

Higher water temperatures, also attributable to our fossil fuel addiction, can also have a devastating effect on plankton. A recent report in the Journal of the Marine Biological Association of the United Kingdom noted that, in the Adriatic Sea cooler winter conditions — which are less frequent in a warmer world — are needed for plankton production and nutrient availability. Furthermore, warmer sea temperatures can cause “blooms” of other sea life (such as happens with algae), resulting in oxygen starvation in the water, a condition that is devastating to plankton and other marine creatures and organisms.

In other situations, blooms of phytoplankton themselves — the tiny plants can gorge on the nutrients from the run-off from farms and lawns on land — can lead to oxygen starvation in the water. “The decomposition of these multitudes of phytoplankton removes oxygen from seawater, creating oxygen-poor ‘dead zones’ where fish cannot live,” reports Carly Buchwald, a researcher at Woods Hole Oceanographic Institution.

Satellite imagery shows that these “dead zones” are expanding. Some scientists are advocating “iron fertilization” — the spreading of large amounts of iron across the world’s seas — to spur plankton growth. But others worry that such tinkering with complex ecosystems could have potentially harmful effects.

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