By Sid Perkins
Phytoplankton sometimes come together in the ocean because they can’t tell which way is up, new research suggests.
![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2009/02/9810.jpg?resize=300%2C300&ssl=1)
Oceanographers have long known that certain species of phytoplankton often form kilometer-wide layers only a few centimeters thick. Researchers have sought an explanation because these layers are often the source of toxic algal blooms known as red tides.
Now researchers have one idea for how the layers form: Lab experiments hint that conditions inside the thin layer of water separating a surface current from a deeper one flowing in a different direction can disorient the phytoplankton, disrupting their swim to the surface and causing them to accumulate in profusion at a single depth.