Plankton Doom?

Climate change may be taking the bloom off the plankton. Researchers say they’ve found “unequivocal” evidence of long-term declines in the ocean’s teeming populations of microscropic algae, which form the base of the marine food web. Warming surface waters appear to be a main culprit, the researchers report in today’s Nature.

Scientists have long studied marine phytoplankton, a vast array of species that thrive in light-rich waters and create half of all the organic matter on Earth. Under the right light and nutrient conditions, phytoplankton “blooms” can carpet the sea surface with billions of individual organisms, changing the water’s color and chemistry, and providing a feast for other creatures. Phytoplankton also play an important role in global climate change, since they are a major “sink” that removes carbon dioxide from the atmosphere.

Since 1979, researchers have used satellites to keep an eye on phytoplankton productivity, and have documented ebbs and flows that were linked to regional shifts in climate and ocean conditions. But longer-term trends have been harder to see. To bring them into focus, marine scientists Daniel G. Boyce, Marlon R. Lewis & Boris Worm of Dalhousie University in Halifax, Canada, compiled nearly 450,000 measurements of plankton and water clarity that researchers made between 1899 and 2008. They then converted the varying numbers to a single measure, known as “total chlorophyll pigment concentration,” or “Chl.” Finally, they mapped Chl changes over time around the globe, and examined how the changes related to things like shifting water temperatures, nutrients, winds and currents.

Among the worrying conclusions: Phytoplankton concentrations have declined in eight of the world’s ten ocean regions and, planet-wide, have been dropping by about 1% of the global average each year. The trend is particularly noticeable in the Northern Hemisphere, where concentrations have dropped nearly 40% since 1950. Declines were also notable in tropical and subtropical seas far from land, where phytoplankton productivity is high. In general, the long-term declines were strongly linked to rising sea surface temperatures over the last century. In part, that may be because as sea waters warm, thick layers of warm and cool waters form that can prevent nutrients from reaching the surface and feeding plankton growth.

The declines have major implications, says lead author Boyce. “Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans.” An ocean “with less phytoplankton will function differently,” adds co-author Worm. “This has to be accounted for in our management efforts.” – David Malakoff

Source: Boyce, D., Lewis, M., & Worm, B. (2010). Global phytoplankton decline over the past century. Nature, DOI: 10.1038/nature09268

Image: Karl Bruun, Nostoca Algae Laboratory, courtesy of Nikon Small World.