Is there a shade flora in the marine plankton?

Since the discovery of a “shade flora” at the turn of the century, many species of marine phytoplankton have been reported by one author or another as occurring preferentially or exclusively at some depth below the surface (usually, around 100 m depth). Collation of the available data results in a critical list of some twenty reliable species belonging to a dozen genera and five classes. These organisms range in size from 1 μm to 1 mm and they cover virtually the whole spectrum of phytoplankton shapes and cell coverings. Thus, although a definite algal community seems to accommodate itself to shade conditions in the sea, no common morphological adaptation is evident.