What evolutionary feature would distinguish phytoplankton from zooplankton?

The most significant difference between zooplankton and phytoplankton is that zooplankton are protozoans and animals, whereas phytoplankton are photosynthetic organisms, including algae (protists), blue-green algae or cyanobacteria (bacteria), and organisms such as dinoflagellates, which do not fit neatly into a single group