The zooplankton in my journal are all on the scale of millimeters to perhaps a centimeter- some are smaller than that- sometimes only a few micrometers big. All of them need to be viewed under a microscope in order to see their features- but we have some collected on the boat that you can see with the naked eye. The chaeotgnaths are about 1-2cm long! I've never seen them get so big before- I've only seen teeny tiny ones.
Zooplankton eat phytoplankton (as you can see, the copepod with the diatoms above and him and some in his tummy!). Zooplankton get eaten by large animals- often fish. Krill are considered a zooplankton- and they get eaten by HUGE whales- so sometimes the food chain is very short from phytoplankton to zooplankton to whales, and sometime it is very long, from phytoplankton to zooplankton to a small fish to a bigger fish...etc etc. Sometimes zooplankton eat other zooplankton- the chaeotgnath is a predatory zooplankton- they'll eat other zooplankton (usually smaller than themselves)
They are very interesting creatures don't you think? Thanks for the great question!
Yes other animals feed on them, and they are eaten by anything bigger than them.