Ocean's Vital Plankton Sheets Form by 'Mishap'

One of the mightiest creatures in the sea is a microscopic plankton, at the mercy of fickle ocean currents, according to a new study.

Single-celled phytoplankton may look unremarkable on their own, but they form massive sheets that stretch for miles through the ocean, and their importance to life on Earth is staggering. They produce 50 percent of the oxygen we breathe. They can be the food that nourishes a thriving ecosystem, or just as easily destroy a fishery by releasing devastating toxins into the water.

Each sheet can contain trillions of organisms and last for days, but how they form and why remain mysterious. In a study published recently in the journal Science, researchers have found the sheets are probably accidents of ocean circulation -- the tiny swimmers get tumbled and trapped between layered currents of water.

"We think it's a mishap that occurs on their way to the surface," co-author Roman Stocker of the Massachusetts Institute of Technology said. "But it's possible that there is some advantage to being in these layers."

For instance, plankton may only release the lethal poisons found in harmful algal blooms when they are packed in close together.

"Once in high enough concentrations, the cells say 'I know that we are many, so I can start releasing toxins," Stocker said. "You may need a threshold of concentration to start their chemical defense from predators."

From the plankton's perspective, overpacked layers, also known as red tides, may be the equivalent of fetid, overcrowded prisons.

"When we see large populations of organisms aggregated in one place, we think it's a great success story," Daniel Grunbaum of the University of Washington said. "But it could be a catastrophic failure. I bet the mortality rate is very high in these layers. They could be very important for predators to feed off of, and for spreading pathogens."

Still, the layers provide a cornerstone for rich and diverse ocean ecosystems. With their swimming abilities overwhelmed, the plankton are easy prey for zooplankton. That in turn attracts small fish, then bigger fish and so on up the food chain, supporting a thriving metropolis of biodiversity.