These include viruses, bacteria, plants (phytoplankton) and animals (zooplankton) that drift in the sea. Most are microscopic, but some - such as the various species of jellyfish and sea-gooseberry - can be much bigger.

Diatoms and dinoflagellates dominate the phytoplankton. Although they are microscopic plants, diatoms have hard shells and dinoflagellates have little tails that propel them through the water. Phytoplankton populations in the Irish Sea have a spring "bloom" every April and May, when the seawater is generally at its greenest.

Crustaceans, especially copepods, dominate the zooplankton. However, many animals of the seabed, the open sea and the seashore spend their juvenile stages as part of the zooplankton. The whole plankton "soup" is vitally important, directly or indirectly, as a food source for most species in the Irish Sea, even the largest. The enormous Basking Shark, for example, lives entirely on plankton and the Leatherback Turtle's main food is jellyfish.