29 July 2010

Microscopic base of marine food chain declining

Warming oceans may be killing microscopic phytoplankton. A recent study reveals worldwide phytoplankton levels are down 40% since the 1950s.
Smithsonian Environmental Research Centre Phytoplankton Guide

Why should we care? "Phytoplankton ultimately affects all of us in our daily lives," said the study's lead author Daniel Boyce. "Phytoplankton ... produce half of the oxygen we breathe. In fact, much of the oxygen in our atmosphere today was produced by phytoplankton ... over the past 2 billion years."

They also help keep Earth cool. They take carbon dioxide — the key greenhouse gas — out of the air to keep the world from getting even warmer.

They also produce half the organic matter on the planet and are the base of the marine food chain, supporting everything in the ocean from fish to dolphins to seabirds. When plant plankton plummet -- like they do during El Nino climate cycles -- sea birds and marine mammals starve and die in huge numbers.

The plankton numbers show problems that can't be seen just by watching bigger more charismatic species like dolphins or whales.

The study used an unprecedented collection of historical and recent oceanographic data from which they documented phytoplankton declines of about 1% of the global average per year. This trend is particularly well documented in the Northern Hemisphere and after 1950, and would translate into a decline of approximately 40% since 1950.

According to the study, the likely cause of the phytoplankton decline is global warming. When the surface of the ocean gets warmer, the warm water at the top doesn't mix as easily with the cooler water below. That is, warmer oceans become more stratified, creating a "dead zone" at the surface in which fewer nutrients are delivered from deeper layers. That makes it tougher for the plant plankton which are light and often live near the ocean surface to get nutrients in deeper, cooler water.

If the planet continues to warm in line with projections of computer models of climate, the overall decline in phytoplankton might be expected to continue.

David A. Siegel & Bryan A. Franz. Oceanography: Century of phytoplankton change. Nature, July 28, 2010; p569 DOI: 10.1038/466569a

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