Tiny single-celled creatures known as phytoplankton form the base of the ocean’s marine food web and, together, produce half the world’s oxygen. Generally speaking, if all goes well, these creatures, of which there are many species, swim to the surface in the morning, where they conduct photosynthesis, and then to deeper waters at dusk, where nutrients are more abundant, according to William M. Durham, a doctoral student at MIT in Cambridge. Durham, along with professors Roman Stocker at MIT and John O. Kessler at the University of Arizona in Tucson, investigated how the phytoplankton form dense accumulations, called thin layers. They found that adjacent layers of water moving at various speeds produce a “shear” flow that can trap vertically migrating phytoplankton. Durham said that the fluid shear overturns the phytoplankton, preventing them from swimming vertically. The cells attempt to swim upward, but when tumbling, swim instead in many directions, leading to zero vertical movement, he explained. Researchers have discovered that thin layers of photosynthetic phytoplankton form in the ocean when strong variations in flow velocity cause the cells to overturn. The flow conditions can trap the single-celled creatures in layers. When a toxic species of phytoplankton becomes trapped in a layer, it can produce a harmful algal bloom commonly known as “red tide.” Courtesy of Glynn Gorick, William M. Durham and Roman Stocker. Because various species of phytoplankton exhibit a unique resistance to tumbling, each species could be trapped in a different level of shear, creating a marine-style layer cake for the dining pleasure of young fish or other oceanic life forms that feed on specific species. The organisms stay trapped in the layers until a shift in wind or tide changes the course of the current and releases them. Red tide When a toxic species of phytoplankton becomes trapped in a layer, it can produce a harmful algal bloom, or “red tide,” that can have a significantly negative social and economic impact on coastal areas, particularly on their fishing and recreational industries. It can sicken and kill larger animals that feed on it. Durham said there are many species of toxic phytoplankton; for example, on the West Coast of the United States, there are 33 species of phytoplankton known to cause harmful algal blooms. The layers, which may be harmful or harmless, depending on the organisms trapped inside them, form in the top 50 m of the ocean and can measure anywhere from a few centimeters to a couple of meters thick. They can span several kilometers horizontally and can last hours, days or weeks, the researchers say. Durham said that regions of elevated shear actually can be predicted, and that the information gained from their study could be essential for locating and predicting the occurrence of thin layers. Tidal currents produce zones of elevated shear at regular intervals, he said, adding that wind-driven currents that generate elevated shear zones also can be predicted. In the study, the team used video microscopy to track the movement of individual cells as they became trapped in the layers of shear. They also modeled the movement of the swimming cells mathematically and proved that, once trapped, phytoplankton cannot escape these layers. Videos related to the experiment can be seen at www.sciencemag.org/cgi/content/full/sci;323/5917/1067/DC1. They show both numerically generated and experimentally visualized cell trajectories. A report on the study was published in the Feb. 20 issue of Science.