I found a troubling study published back in July 29, 2010 in Nature reporting that phytoplankton, the tiny microscopic plants that form the foundation of the ocean’s food webs, were declining.
Phytoplankton is not only vital as a food source, but is also a key player in the earth’s carbon cycle, reducing carbon dioxide and producing half of the planet’s oxygen output. The worrying thing is that numbers have been in a decline since the beginning of the 20th century. The researchers at Dalhousie University stated the global phytoplankton population has dropped around 40% since the 1950s. This is extremely unsettling as the consequences of the declines for the marine ecosystems and to the carbon cycle are largely unknown. What we do think is that rising sea surface temperatures (SST) due to climate change are to blame.
The scientist examined the historical record, well before 1997 when continuous satellite measurements began. Records prior to this, a collection of data points exceeding half a million points, collected using a Secchi disk, in addition to chorophyll measurements, were used to estimate phytoplankton decline. The Secchi disk was developed by a Jesuit astronomer, Father Pietro Angelo Secchi, in the 19th century when the navy requested him to map the transparency of the Mediterranean Sea.
The disk itself was a white dinner plate in size that was lowered into the water until it can no longer be seen. The depth the disk reaches just prior disappearing provides a measure of water clarity. It is acceptable for this measure to be used as a proxy for phytoplankton population in a given area, since the organisms live close to the ocean's surface, where they are exposed to sunlight, use it to produce energy.
Combining the Secchi disk data, chlorophyll measurements and satellite data the researchers found that the most significant phytoplankton declines appeared in the tropics and closer to poles. They hypothesize the rising SSTs are driving the phytoplankton decline. As the surface waters warm, it forms a definitive layer that does not mix with the lower, cooler nutrient rich waters. Subsequently, the phytoplankton are deprived of the vital materials required to fix carbon dioxide and turn sunlight into energy.
Only time will tell what will happen with continued SST rise and only longer-term study can elucidate what is going on. If the oceans continue to lose these microscopic primary producers it will not only drastically affect marine ecosystems and the organisms with them, but also profound effects on the ability of the planet to reduce CO2. Close to a decade on from the publication of this study, how much worse has it gotten?
For the 2010 publication please visit http://www.nature.com/nature/journal/v466/n7306/abs/nature09268.html
