Answer:
The key factor for zooplankton survival and existence is their food supply, primarily phytoplankton. Phytoplankton are tiny marine plants that form the base of the marine food web. Zooplankton feed on phytoplankton, so their abundance and health are directly tied to the availability and quality of phytoplankton in their environment. Other factors like temperature, salinity, and oxygen levels also play roles in their survival, but food availability is paramount.