scattering layer

1. A concentrated layer of marine organisms found in most oceanic waters that reflects and scatters sound waves, as from sonar. The layer is of varying composition and can include both plankton and nekton (free-swimming organisms such as copepods, krill, and small fish). Scattering layers, which may occur at more than one depth in the same location, typically move upward at night to feed on phytoplankton and downward during the day, as deep as 1,000 m (3,280 ft), probably to escape predators.
2. Also called deep scattering layer